Dissertations / Theses: 'Rewetting'

1

Ilyas, Muhammad. "Rewetting processes during PWR reflood." Thesis, Imperial College London, 2011. http://hdl.handle.net/10044/1/6378.

Full text

Abstract:

Rewetting of heated surfaces is important in many physical processes and has important technological applications. Understanding of this phenomenon is required in many engineering and scientific fields. It is one of the most crucial phenomena to be considered for the safety analysis of the design basis Loss‐of‐Coolant Accident (LOCA) in light water reactors (Pressurized Water and Boiling Water Reactors). To mitigate the consequences of LOCA, water is fed into the reactor core via an emergency core cooling system; in the PWR, this water is fed to the core via the lower plenum (“bottom reflooding”) and in the BWR, this water is sprayed onto the top of the core (“top reflooding”). In both the cases, a quench front is formed which moves rather slowly. Ahead of quench front, complex and chaotic processes are occurring over a very small axial region where high temperature gradient exists. The heat transfer mechanism is not well known in this region. In this work, the detailed physics of the rewetting processes has been investigated both theoretically and experimentally. The thermal hydraulic behaviour of hot vertical channels during emergency core cooling conditions would be expected to be flow direction‐dependent, it was important to consider the two cases (top reflooding and bottom reflooding) separately. It was possible for the first time, to the author’s knowledge, to apply the fast response infra‐red thermal imaging system to study the rewetting process during top and bottom reflooding of heated vertical surfaces. The important contribution of this work was the use of this new technology to sense the variation of temperature with time at multiple nearby locations at the quench front. In the top reflood experiments, a heated stainless steel pate was quenched by a falling film flow. Through an infrared‐transparent substrate embedded in the plate and coated with platinum, temperature measurements at a location near the rewetting front were achieved using infrared thermal imaging system. The temperature/time traces showed fluctuations in temperature indicating occurrence of intermittent contacts at the quench front. A high speed video camera was also employed to capture rewetting processes by the visual observation of the rewetting front from the top surface. In the visual observations, the liquid film has been seen making intermittent contacts with the hot surface. In these experiments, the effect of the flow rate and the degree of sub‐cooling of the feed liquid has been studied. The rewetting temperature and the characteristic length of the intermittent contact region have been deduced from the experimental results. Experiments were also done to measure temperature changes at the rewetting front for the case of bottom reflooding of a heated tube using a similar technique to that employed for the studies of top reflooding. The results suggested that the rewetting behaviour was different depending on whether the reflood rate was high or low. For high reflood rate, the observations are consistent with the regime above the rewetting front being of the inverted annular type and, for lower reflooding rates, the results are consistent with the rewetting front corresponding to a film dryout in annular flow. An important finding from these experiments is the identification of transient temperature fluctuations in the transition region for the high flooding rate case. These are similar to those observed in the top reflood case and it seems very likely that these fluctuations are associated with intermittent wetting of the surface in this region. An attempt has been made to model rewetting phenomena in which the mechanism of heat transfer at the quench has been proposed. The postulated mechanism is transient near‐surface cooling resulting from intermittent solid‐liquid contacts, followed by recovery of the surface temperature of the metal substrate, with explosive vaporization occurring when the homogeneous nucleation temperature is restored at the metal‐water interface. A one‐dimensional rewetting model was constructed to explain the cyclical process; this model predicted the cyclical behaviour, with the expected qualitative dependence on system parameters. Its predictions are quantitatively consistent with experimental observation, in that the unsteady model analysis brackets the experimentally observed periodicity of the quasi‐steady actual process. The one‐dimensional model of the process has been complemented by twodimensional simulations using a commercial finite element code (ANSYS). In these simulations, an intermittent contact region has been modelled by imposing a heat transfer coefficient over a certain length between dry and wet regions. A parametric study was performed to see the effect of the rewetting velocity, the wet side heat transfer coefficient, intermittent contact heat transfer coefficient, and the length of intermittent contact region.

APA, Harvard, Vancouver, ISO, and other styles

2

Mao, Chris Qun. "Local rewetting and distortion of paper." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 2001. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp05/MQ62919.pdf.

Full text

APA, Harvard, Vancouver, ISO, and other styles

3

Pereira, Paulo Manuel Sendim Aires. "Heat transfer in rewetting of hot surfaces." Thesis, Imperial College London, 1998. http://hdl.handle.net/10044/1/7284.

Full text

APA, Harvard, Vancouver, ISO, and other styles

4

Engelhardt, Ilonka. "Impact of changing precipitation patterns on the plant-microbial response to rewetting." Thesis, Bourgogne Franche-Comté, 2018. http://www.theses.fr/2018UBFCK017/document.

Full text

Abstract:

La disponibilité en eau exerce un contrôle majeur sur les cycles des nutriments terrestres, à travers ses impacts sur le fonctionnement des plantes et des microorganismes du sol. Les changements de magnitude et de fréquence des épisodes de pluie (c’est-à-dire les régimes de précipitations) prédits par les modèles et associés au changement climatique vont ainsi avoir des conséquences importantes sur le fonctionnement des écosystèmes. Les écosystèmes arides et semi-arides sont particulièrement vulnérables à des changements de régime de précipitations, car ils sont déjà contraints par la disponibilité en eau. Cependant, des systèmes plus tempérés peuvent aussi être soumis à des périodes sèches qui peuvent affecter le fonctionnement plante-sol. Dans la présente thèse, les effets d’un historique de régimes de précipitations contrastés ont été étudiés dans des systèmes sol seul et plante-sol, afin de déterminer dans quelle mesure plusieurs semaines de régime hydrique peuvent moduler la réponse des écosystèmes à une réhumectation lors d’un événement pluvieux important. Premièrement, nous avons évalué les effets de régimes de précipitations contrastés dans des mésocosmes de sol seul, sur les communautés bactériennes et fongiques actives et inactives dans le sol, 2 et 5 jours après réhumectation. Nous avons employé une approche de 18O-SIP (stable isotope probing), en réhumectant le sol avec H218O puis en utilisant la métagénomique ciblée sur les bactéries et champignons du sol. Deuxièmement, nous avons mis en place deux expériences séparées en mésocosmes plante-sol avec couvert de blé. La première expérience sol-plante s’est intéressée à la profondeur de sol. Nous avons évalué les effets de régimes de précipitations contrastés sur le flux de C depuis les plantes vers les microorganismes du sol ainsi que la la réponse des microorganismes à différentes profondeurs de sol (de 0 à 35 cm) en utilisant des approches de traceur isotopiques stables (13C-CO2) et 18O-SIP, respectivement. La deuxième expérience plante-sol a évalué les effets de régimes de précipitations contrastés sur la dynamique temporelle (durant 29h) de la réponse du système plante-sol à la réhumectation. En outre, deux niveaux de fertilisation azotée ont permis de déterminer l’éventuelle modulation de la réponse par la disponibilité en N dans le sol. La réponse des communautés bactériennes et fongiques potentiellement actives dans le sol a été évaluée par métagénomique ciblée. La réponse de cycles biogéochimiques a été évaluée à l’aide de traceurs isotopiques stables (13C-CO2 et 15N- NO3-) pour quantifier le flux de C des plantes vers les microorganismes du sol et déterminer la compétition plantes-microorganismes du sol au cours du temps après réhumectation.Nos résultats ont montré un contrôle du régime de précipitation sur la morphologie et physiologie des plantes, les communautés microbiennes du sol ainsi que sur le cycle de l’azote du sol dans nos systèmes. En particulier, des régimes de précipitations peu fréquentes (cycles de périodes sèches longues suivies de périodes de pluie plus importantes) se sont traduits par une augmentation des potentiels de transformation de l’azote dans le sol et une réduction des stocks d’azote minéral dans le sol. Ceci a façonné l’environnement de la réponse de nos systèmes à la réhumectation, que nous avons évaluée en déterminant les dynamiques du C (couplage plantes-microbes et émissions de CO2 du sol), de l’azote du sol (compétition plantes-microorganismes du sol pour le N et émissions de N2O) et de la composition des communautés microbiennes du sol (bactéries et champignons actifs et potentiellement actifs) après réhumectation (...)
Water availability governs terrestrial nutrient cycles by impacting the functioning of both plants and of soil microorganisms. The predicted changes in precipitation patterns (i.e. the magnitude and frequency of precipitation events) associated with climate change, will thus likely have important consequences on ecosystem functioning. Dry and seasonally dry ecosystems are particularly vulnerable to changes in precipitation patterns, as they are already constrained to a large extent by water availability. However, more mesic systems may also experience dry periods that may impact plant-soil functions. In this thesis, experiments in soil-only systems and plant-soil systems were used to gain insight into how the legacy effects of several weeks of exposure to contrasted precipitation patterns set the scene for the rewetting response of the system. First, in an experiment using soil-only mesocosms, we evaluated the effects of contrasting precipitation regimes on the actively growing as well as the inactive bacterial and fungal communities 2 and 5 days after rewetting, using an 18O-SIP (stable isotope probing) approach by applying H218O followed by metagenomics targeting soil bacteria and fungi. Second, we performed two separate and complementary experiments using plant-soil mesocosms with wheat plant cover. The first plant-soil experiment focused on soil depth. It determined the effects of contrasting precipitation patterns on the flux of C from plants to microbes and the microbial response to rewetting at different soil depths, using a heavy isotope tracer approach (13C-CO2) and 18O-SIP with metagenomics respectively. The second plant-soil experiment evaluated the effects of a history of contrasting precipitation patterns on the dynamics of the rewetting response of the plant-soil system over time (over 29 hours post-rewetting). In addition, two levels of N inputs allowed to determine how N availability modulated plant-soil responses. The response of the potentially active soil bacterial and fungal communities to rewetting was assessed using targeted metagenomics. The responses of biogeochemical cycles were evaluated using heavy isotope tracers (13C-CO2 and 15N-NO3-) to quantify C flux from plants to soil microorganisms and plant-microbial competition for N over time post-rewetting.We found that precipitation patterns shaped plant morphology and physiology, microbial community composition as well as soil N cycling in our systems, which set contrasting scenes for the rewetting responses in our systems. In particular, infrequent precipitation patterns (cycles of longer dry periods followed by larger magnitude rain events) resulted in increased microbial N transformation potentials and smaller inorganic N pools. The rewetting responses were determined by evaluating C dynamics (plant-microbial coupling and soil CO2 efflux rate), N dynamics (plant-microbial competition for N and soil N2O efflux rate) and microbial dynamics (composition of active and potentially active bacterial and fungal communities after rewetting). First, we found that plant-microbial coupling (i.e the microbial assimilation of C from fresh photosynthate) may be reduced under more infrequent precipitation patterns, especially near the soil surface, and under conditions of low N availability. Our findings also suggest that whilst in soil-only systems, dead microbial cells appear to be a major source fuelling soil CO2 efflux pulse upon rewetting, in plant-soil systems root respiration plays an important role in the magnitude of the CO2 efflux upon rewetting. Second, concerning soil N dynamics, we found, in concurrence with previous studies, that soil microorganisms were the stronger competitor for N over short time scales, likely due to their overall fast response rates and high affinity for substrate, whilst plants outcompeted soil microbes for soil N assimilation, over longer time scales likely taking advantage of the fast microbial turnover (...)

APA, Harvard, Vancouver, ISO, and other styles

5

Lee, Sung-Ching. "Annual greenhouse gas budget for a bog ecosystem undergoing restoration by rewetting." Thesis, University of British Columbia, 2016. http://hdl.handle.net/2429/58985.

Full text

Abstract:

Many peatlands have been drained and harvested for peat mining, which turned those landscapes into carbon (C) emitters. Rewetting of disturbed peatlands facilitates their ecological recovery, and may help them revert to carbon dioxide (CO₂) sinks. However, it may also cause substantial emissions of the powerful greenhouse gas (GHG) methane (CH₄). Our knowledge on year-round measurements of CO₂ and CH₄ fluxes in restored peatlands is currently very limited. This study quantifies C exchanges in a disturbed and rewetted area located in the Burns Bog Ecological Conservancy Area (BBECA) in Delta, BC, Canada. The BBECA is recognized as the largest raised bog ecosystem on North America’s West Coast. It was substantially reduced in size and degraded by peat mining and agriculture. Since 2005, the bog has been declared a conservancy area, and restoration efforts focus on rewetting disturbed ecosystems to recover Sphagnum and suppress fires. Year-round (16th June 2015 to 15th June 2016) turbulent fluxes of CO₂ and CH₄ were measured from a tower platform in an area rewetted in 2007 using the eddy-covariance (EC) technique. The annual CO₂ budget was -179 g CO₂-C m-² year-¹ and the annual CH₄ budget was 16 g CH₄-C m-² year-¹. Gross ecosystem productivity (GEP) exceeded ecosystem respiration (Re) during summer months (June-August), causing a net CO₂ uptake. In summer, high CH₄ emissions were measured. In winter (December-February), substantially lower CH₄ emissions and roughly equal magnitudes of GEP and Re made the bog C neutral. Oxygen limitation due to the high water table caused by ditch blocking suppressed Re. With low temperatures in winter, CH₄ emission was more suppressed than Re. The key environmental factors controlling the seasonality of these exchanges in the study area were downwelling photosynthetically active radiation and 5-cm soil temperature. Annual net GHG flux from CO₂ and CH₄ during the study period totaled to -23.48 g CO₂e m-² year-¹ and 1242.04 g CO₂e m-² year-¹ by using 100-year and 20-year global warming potential (GWP) values, respectively. Consequently, the ecosystem was almost CO₂e neutral annually on a 100-year time horizon but was a significant CO₂e source on a 20-year time horizon.
Arts, Faculty of
Geography, Department of
Graduate

APA, Harvard, Vancouver, ISO, and other styles

6

Silva, Ana Beatriz de Albuquerque Morais. "Phosphorous cycling in dry and rewet incubated soils subjected to different fertilization histories." Master's thesis, ISA/UTL, 2011. http://hdl.handle.net/10400.5/3919.

Full text

Abstract:

Mestrado em Engenharia do Ambiente - Instituto Superior de Agronomia
Phosphorus continuous application in fertilizers, in quantities superior to those required by plants causes a P buildup in soils, potentially increasing nutrient losses and compromising water quality by eutrophication process. This work was aimed at assessing the effect of DRW cycles, in two soils with different fertilization histories subjected to organic and mineral fertilization, over P availability. Soils were collected from a long-term fertilizer management platform (NW Italy) divided in two plots experiencing since 1996 different fertilization management (one plot fertilized with low N and K levels and no P and another with fertilization high on N and P levels), and each was divided in 3 subplots treated with mineral, compost and without fertilization, since 2010. The soils were subjected, under laboratory conditions, to four DRW cycles and compared with another set at constant moisture. Weekly the soils were sampled and analyzed for some of their P forms. Soluble P increased in compost fertilized soil, especially after the third dry and rewet cycle, following microbial decay. Yet, this was more pronounced in the overfertilized soils. From these findings it is possible to conclude that the availability of P may be further enhanced by water content variations as by compost addition.

APA, Harvard, Vancouver, ISO, and other styles

7

Henstra, Steven W. "Vegetation responses to ecological restoration (rewetting) of abandoned block-cut peatlands in eastern Québec." Thesis, University of British Columbia, 2012. http://hdl.handle.net/2429/40829.

Full text

Abstract:

Abandoned block-cut peatlands are typified by an alternating topography of mined trenches and raised baulks. Although these sites re-vegetate densely with native bog species, Sphagnum species characteristic of undisturbed bogs are conspicuously absent and peat accumulation has not recommenced. Ecological restoration of abandoned block-cut peatlands involves blocking remnant drainage ditches, “rewetting” the residual peat body to: 1) re-establish vegetation assemblages dominated by Sphagnum, and 2) re-establish the upper, hydrologically active layer (acrotelm) that characterizes intact peatlands. This study evaluates the progress towards achieving these goals in three rewetted block-cut sites in eastern Québec. Plant species composition, above-ground biomass, and accumulated organic matter data were collected from mined trenches within rewetted and non-rewetted sectors of the study sites and used for comparison with nearby undisturbed reference bogs. Comparisons of current patterns of community vegetation structure in rewetted and non-rewetted sectors of the study sites indicated that vegetation assemblages are strongly influenced by water table position. In areas where rewetting resulted in a water table position at or just above the peat surface, rapid community scale vegetation change occurred, with widespread mortality of vascular vegetation followed by recolonization by hydrophytic ericaceous and herbaceous species, and hollow/lawn Sphagnum species (<4 years following rewetting). Despite this positive change, vegetation assemblages within rewetted sectors still differ significantly from those found in natural reference sites up to 17 years following rewetting. Changes in above-ground biomass indicated a significant reduction in tree biomass, a significant portion of which is present as dead-standing tree biomass 4 years following rewetting. Shrub biomass initially decreased, but then increased 10 years following rewetting as species compositions shifted. Accumulated organic matter shifted from a predominance of ericaceous litter to fibric peat in rewetted sectors, the depth of which is increasing as a function of time. These findings provide initial evidence of re-establishment of the acrotelm; however, additional research is required to determine whether the newly accumulated peat will provide the hydrological functions perceived of the acrotelm in undisturbed bogs. Continued detailed observation of the biotic recovery of these ecosystems will provide valuable information for future restoration endeavors.

APA, Harvard, Vancouver, ISO, and other styles

8

Sjöstrand, Björn. "Dewatering aspects at the forming section of the paper machine : Rewetting and forming fabric structure." Licentiate thesis, Karlstads universitet, Institutionen för ingenjörs- och kemivetenskaper (from 2013), 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:kau:diva-47640.

Full text

Abstract:

The underlying motives of the research undertaken here are twofold: to obtain a deeper understanding of the dewatering mechanisms at the forming section of a papermaking machine and to develop numerical models that describe the flow through forming fabrics. More comprehensive knowledge of dewatering in the forming section allows suggestions to be made for improvements that reduce the amount of energy used in the process without affecting the quality of the end product. The objective of this thesis is to answer the following questions: How and why does rewetting occur at the high vacuum suction boxes? How does the structure of the forming fabric affect dewatering at the forming section? Is it possible to create accurate numerical models for forming fabrics, and can these be used to predict the dewatering behaviour of new types of fabrics? Laboratory and pilot studies simulating high vacuum suction boxes were performed together with numerical modelling of the flow of air and water through both the forming fabric and the paper sheet. The conclusion drawn from the pilot study is that rewetting significantly lowers the dryness of the paper sheet exiting the suction boxes. The phenomenon is extremely rapid and is most likely driven by capillary forces. The high speed at which this rewetting occurs makes it difficult to impede by placing the suction boxes closer to the couch pick-up: the solution is more likely to be the use of new and improved designs of the forming fabric. The structure of the forming fabric has been shown to affect the dewatering rate at certain conditions of vacuum dewatering, and can possibly be connected partly to the fact that fibres penetrate the surface of the fabric to varying degrees and partly to the flow resistance of the different fabric structures. Numerical models of high accuracy can be constructed and used to predetermine how new fabric designs would affect dewatering at the forming section. This thesis quantifies aspects of dewatering such as rewetting and the influence of the forming fabric. Understanding these dewatering aspects further provides for the potential enhancement of energy efficiency in the forming section, and thereby the entire papermaking process. The forming fabric can play an important role in improving energy efficiency: rewetting after the high vacuum suction boxes occurs more rapidly than was previously known, so its design might be the only possible way of impeding it. The forming fabric can also improve the rate of dewatering: it is therefore likely that its design will be important in the next stage of developing energy efficiency and thereby play a part in achieving a more sustainable future.
This thesis quantifies aspects of dewatering such as rewetting and the influence of the forming fabric. Understanding these dewatering aspects further provides for the potential enhancement of energy efficiency in the forming section, and thereby the entire papermaking process. The forming fabric can play an important role in improving energy efficiency: rewetting after the high vacuum suction boxes occurs more rapidly than was previously known, so its design might be the only possible way of impeding it. The forming fabric can also improve the rate of dewatering: it is therefore likely that its design will be important in the next stage of developing energy efficiency and thereby play a part in achieving a more sustainable future.

APA, Harvard, Vancouver, ISO, and other styles

9

Dinh, Mai-Van [Verfasser], and Egbert [Akademischer Betreuer] Matzner. "Soluble phosphorus in forest soil : Effects of drying and rewetting / Mai-Van Dinh ; Betreuer: Egbert Matzner." Bayreuth : Universität Bayreuth, 2017. http://d-nb.info/1143009843/34.

Full text

APA, Harvard, Vancouver, ISO, and other styles

10

Gordon, Helen. "Drying and rewetting effects on microbial community composition and nutrient leaching form soils of contrasting management." Thesis, Lancaster University, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.533122.

Full text

APA, Harvard, Vancouver, ISO, and other styles

11

Peacock, Michael Anthony. "The effect of peatland rewetting on gaseous and fluvial carbon losses from a Welsh blanket bog." Thesis, Bangor University, 2013. https://research.bangor.ac.uk/portal/en/theses/the-effect-of-peatland-rewetting-on-gaseous-and-fluvial-carbon-losses-from-a-welsh-blanket-bog(648921f3-152a-43e8-9d13-29f56fb4b965).html.

Full text

APA, Harvard, Vancouver, ISO, and other styles

12

Pettersson, Emelie. "A study in how rewetting can be reduced in the paper machine with focus on the forming section." Thesis, Högskolan i Borås, Akademin för textil, teknik och ekonomi, 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:hb:diva-10827.

Full text

Abstract:

This master thesis provides an overview of the paper machine with focus on the forming section. The forming section is the first part in the paper machine where the paper pulp is injected through a head box. The paper pulp contains about 99.5% of water and 0.5% fiber. The water as content is reduced by vacuum and gravity. The problem to be studied in this project is related to external rewetting. This is water going back to the paper web from the forming fabric after the dewatering zone. The dewatering is based on vacuum slots under the forming fabric. The vacuum slots absorb water from the soaked paper pulp through the forming fabric. External rewetting causes problem, hence the paper will have a higher dry content when leaving the forming section. The paper should have as low dry content as possible in the end of the forming section. Three different forming fabrics from Albany International were chosen for the project. The structures of the forming fabrics were two different double layers and one plain weave. The performance of the fabrics was studied by 4 different methods. The methods used were 2 different wicking tests, a vacuum dewatering trial and one foulard test. Also micro tomography was done to understand the structure of each design. The main test was a foulard test where the aim was to see in what way the rewetting got affected by different pores sizes. The results showed higher water content for the paper that was on top of the forming fabric with the larger pores.

APA, Harvard, Vancouver, ISO, and other styles

13

Kwari, J. D. "Effects of straw burning and drying-rewetting cycles on the fertility of some soils of North-East Nigeria." Thesis, University of Aberdeen, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.379809.

Full text

Abstract:

The effects of straw burning and drying-rewetting cycles on nutrient availability were studied in an attempt to assess the practical importance of the yearly precropping practice by farmers of burning off excess crop residues and of the seasonal climate of North-East Nigeria. Changes in soil properties resulting from heating three soils up to 700°C and from field burning of straw on three soils were determined. Heating to 250° did not affect soil texture but increased exchangeable NH_4⁺-N, available P, Fe, Mn and phosphate adsorption. A peak of NH_4⁺-N representing decomposed organic N, retained by cation exchange sites was observed after heating at 250°. Soil physical and chemical properties were changed by heating to ≥500^o, resulting in fusion of clay particles into sand-sized particles, decomposition of soil organic matter and alteration of soil minerals with a corresponding increase in non-exchangeable cation status of the soils. The changes observed after field burning of straw were mainly due to deposition of ash; these include a dramatic increase in phosphate adsorption and addition of considerable amounts of inorganic P and exchangeable cations. The availability to maize of the inorganic P returned in ash after field burning or released as a result of laboratory heating of soils was assessed over a 7 day period. The increased P status of the soils after straw burning or heating was not significantly reflected in plant uptake of P but K adsorption was enhanced from the ash. Fertilization of heated and field burnt + overlying ash soil samples resulted in adsorption of P with a corresponding reduction of P concentration in the soil solution and P uptake. The N mineralized after rewetting air-dried clay soils or those containing very low organic matter was insignificant. Investigations also showed that, when dried soils were rewetted, the flush of mineral N was derived mainly from soil organic matter and not from killed microbial cells.

APA, Harvard, Vancouver, ISO, and other styles

14

Mamalis, Dimitrios. "Phase change and complex phenomena in drops and bubbles of pure and binary fluids." Thesis, University of Edinburgh, 2016. http://hdl.handle.net/1842/25477.

Full text

Abstract:

Evaporation, wetting and multiphase flows of drops and bubbles are everyday life phenomena with potential impact in many industrial, biological, medical or engineering applications. The understanding and controlling of the physical and chemical mechanisms governing these phenomena have become of paramount importance. This thesis encompasses three topics: evaporation of sessile droplets of polymer solutions, the role of thermocapillarity on self-rewetting fluid dynamics and migration of bubbles in liquid flows. Firstly, the evaporative behaviour of sessile droplets of aqueous polymer solutions and the effect of different molecular weights on the drying process has been studied. Drop shape analysis allowed monitoring the evolution of all stages during drying and indicating the transitions between stages. The mechanisms taking place during the crucial stages of pinning and depinning were illustrated, revealing the effects of adhesion and contact line friction forces on the final morphology of the dried polymeric deposits. Additionally, the effect of varying substrates from hydrophilic to hydrophobic was examined demonstrating the importance of interfacial interaction phenomena. The initial spreading dynamics of binary alcohol mixtures (and pure liquids) deposited on different substrates in partially wetting situations, under non-isothermal conditions was systematically investigated. Moreover, the temporal and spatial thermal dynamics within pure droplets and alcohol mixtures using IR thermography revealed the existence of characteristic thermal patterns due to thermal and/or solutal instabilities. The contribution of the Marangoni effect as an important heat transport mechanism within the evaporating droplets was investigated. The motion of buoyancy-driven bubbles in a vertical microchannel and the significant role of thermocapillarity was reported in this series of experiments. The behaviour of the bubbles in self-rewetting fluid flows departed considerably from that of pure liquids flows. Furthermore, heat transfer coefficient calculations in the single and two phase flows demonstrated that the presence of Marangoni (surface tension) stresses resulted in the enhancement of the heat transfer distribution in the self-rewetting fluid flows compared with the pure ones.

APA, Harvard, Vancouver, ISO, and other styles

15

Harrison-Kirk, T. "The effects of drying and rewetting cycles on carbon and nitrogen dynamics in soils of differing textures and organic matter contents." Lincoln University, 2008. http://hdl.handle.net/10182/656.

Full text

Abstract:

Many researchers have reported differences in soil C and N dynamics between soils of different textures and/or soil organic matter contents. However, it has proven difficult to determine the exact relationships and mechanisms between C and N dynamics and soil texture/SOM. There are few studies that consider how these soil physical and chemical conditions influence the effects of drying and rewetting on the mineralisation of C and N and the microbial transformations that follow. The objectives of this study were: 1) To determine the effects of repeated drying and rewetting cycles on C and N dynamics in soils of differing textural class and organic matter levels. 2) To use C & N mineralised at constant moisture contents to calculate mineralisation during dry/wet cycles for comparison with actual mineralisation. Two soil types with contrasting textures were chosen and 6 paddocks on each soil type were selected to produce an OM gradient for each soil. Three moisture treatments were chosen to simulate moist (field capacity at -0.01 MPa), moderately dry (120% of wilting point at -1.5 MPa) and very dry (80% of wilting point at - 1.5 MPa) field conditions. The dry moisture treatments were then combined with a rewet treatment where they were either rewet or maintained dry (+ or – rewet), resulting in a total of five dry/rewet treatments. Soils were packed into funnel tops to a BD of 1.1 g/cm³ and sealed in glass jars fitted with septa to allow gas sampling. Drying was achieved using silica gel which allowed continued gas measurement during drying periods. Gas samples were collected throughout the experiment and analysed for CO₂ by IRGA and N₂O by GC. At the start and end of the study, soils were analysed for Min N, MBC, MBN, HWC, DOC, POM, total C and total N. The correlation between calculated and actual C mineralisation data indicates that the intercept is not consistent with the origin and that the slope is not consistent with the 1:1 line. While those paddocks with high %C had high cumulative C mineralisation, there didn’t appear to be any strong relationship between soil texture or OM content and the difference between actual and calculated C mineralisation. A plot of calculated C mineralisation rates against the actual C mineralisation rates shows that much of the error in the calculated cumulative data arises from an underestimation of the mineralisation flush when the dry soil is rewetted, especially during the first dry-rewet cycle, and an over estimation of the rate at which respiration decreases as the soil dries. In order to use C mineralisation data from soils held at constant moisture contents to accurately predict C mineralisation in soils exposed to dry-rewet cycles, knowledge of the stress history for the soil would be required e.g. size, duration and frequency of rainfall events, dry rates etc. The N₂O-N emission data is inherently more variable than the C mineralisation data. The fine-textured soils tend to have much higher N₂O-N emissions than the coarser soils, probably due to the creation of anoxic sites upon rewetting in the fine-textured soils. The data indicates that prediction of N₂O-N emissions in soils exposed to dry-rewet cycles using emission data from soils held at constant moisture contents would be very inaccurate, primarily due to the inherent variability of N₂O-N emissions in soils.

APA, Harvard, Vancouver, ISO, and other styles

16

Mälson, Kalle. "Plant responses after drainage and restoration in rich fens." Doctoral thesis, Uppsala University, Department of Ecology and Evolution, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-8882.

Full text

Abstract:

Rich fens are an important, but threatened, habitat type in the boreal landscape. In this thesis I have examined responses of rich fen vascular plants and bryophytes after drainage and restoration.

The effects of drainage on the rich fen flora were observed in a long time study and the responses were rapid and drastic. During an initial stage a rapid loss of brown mosses was observed, followed by increases of sedges and early successional bryophytes, and later by an expansion of dominants. Initial effects of hydrological restoration showed that rewetting can promote re-establishment of an ecologically functional rich fen flora, but has to be combined with other treatments, such as mowing or surface disturbance.

After restoration, re-establishment of locally extinct species may be hampered by dispersal limitations. To test if reintroductions could help to overcome dispersal limitations I performed transplantation studies with four common rich fens bryophytes to a rewetted site. The results showed that the species were able to establish, and that survival and growth were promoted by desiccation protection and liming.

I further examined competition among three of the most common bryophytes in natural boreal rich fens that usually occur mixed in a mosaic pattern but show small but important microtopographical niche separation. The results indicate similar competitive abilities among the species, and no case of competitative exclusion occurred. The results help to explain the coexistence of these species under natural conditions with microtopographic variation and repeated small scale natural disturbances.

Restoring a functional flora in drained rich fens is a complex task, which requires understanding of underlying causes of substrate degradation in combination with suitable restoration measures. The thesis suggests how the results can be used in practical restoration work, and also stresses the need for monitoring of restoration experiments over longer time.

APA, Harvard, Vancouver, ISO, and other styles

17

Marques, Adriana. "Estudo do efeito da adição de diésteres em fluidos industriais de origem vegetal e suas propriedades." Universidade de São Paulo, 2012. http://www.teses.usp.br/teses/disponiveis/88/88131/tde-06052012-155617/.

Full text

Abstract:

Os óleos vegetais vêm sendo estudados como substitutos aos minerais em várias aplicações industriais, em particular no resfriamento de aços, no processo de tratamento térmico de têmpera. Este trabalho trata do estudo das características de viscosidade e propriedades de resfriamento de óleos vegetais e de formulações feitas a partir desses óleos com diésteres: Bis (2-etilhexil) adipato DOA, Bis (2-etilhexil) sebacato DOS e Diiso Tridecil adipato DITA, nas proporções de 2,5% e 5,0% (m/m). Realizou-se também nos óleos vegetais, sem aditivos, uma avaliação prévia da estabilidade oxidativa feita por RMN, baseando-se nas proporções de ácidos graxos saturados e insaturados presentes na cadeia. Avaliações foram também feitas para formulações com óleo de soja após o processo de envelhecimento acelerado. Os resultados indicaram como sendo o óleo de coco, o de maior estabilidade oxidativa. Os óleos de amendoim e soja apresentaram menores variações de viscosidade com a temperatura, caracterizados pelos menores valores do coeficiente de Walther (B). Das formulações estudadas, no estado não oxidado, a adição de 2,5% de DOS ao óleo de soja foi a que proporcionou maiores taxas máximas e camada de vapor menos estável. Após a oxidação de 48 horas, a composição de óleo de soja adicionado a 5,0% de DOA foi a que sofreu menores alterações comparadas do estado não oxidado.
Vegetable oils have been studied in the replacement of mineral oils in many industrial applications particularly on the quenching steel in the heat treatment process. This work presents studies conducted to vegetable oils in terms of viscosity characteristics and cooling behavior. Such studies incorporate also formulations on vegetable oil based by diesters additions [Bis (2-etylhexyl) Adipate - DOA, Bis (2-etylhexyl) Sebacate DOS, Diiso Trydecil Adipate DITA] in the proportions of 2.5 and 5.0 % (m/m). Oxidation stability was evaluated by NMR performed in the vegetable oil without diesters additions. This evaluation was based in the satured and insatured compounds presented in the vegetable oil chain. Similar evaluations were also performed for formulations based on the soybean oil, before and after the accelerated oxidation tests. Results indicated coconut oil as the most stable in terms of oxidation. Peanut oil and soybean oil shown minor alterations on the viscosity values when the raise of temperature, since they presented lower Walther coefficient (B). Comparing the formulations soybean oil based in the not oxidized condition, the addition of 2,5% DOS was that shown greater maximum cooling rates (CRmax) and less stable vapour phase. After accelerated oxidation tests (48 hours), formulation with 5% DOA (soybean oil based) was that presented minor variations in terms of cooling parameters compared with not oxidized condition.

APA, Harvard, Vancouver, ISO, and other styles

18

Åslund, Peter. "On Suction Box Dewatering Mechanisms." Doctoral thesis, KTH, Fiber- och polymerteknik, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4780.

Full text

Abstract:

In previous studies on suction box dewatering, three mechanisms were identified that determine the dry content of a web, viz. web compression, displacement of water by air and rewetting. In the present work, the relative importance of the three mechanisms was investigated through direct measurement of the web deformation, the dry content changes during and after the suction pulse, the air flow through the fibre network and the saturation of the web after the suction pulse. Suction pressure, suction time and rewetting time were varied. The experiments were done with chemical and mechanical pulp webs of various grammages. It was found that a large web deformation took place during the suction pulse, particularly at its beginning. Compression dewatering was found to be the most dominant dewatering mechanism. Displacement dewatering started after most of the web compression had occurred. Its contribution to the increase in dry content was most pronounced for higher suction pressures, longer suction times and for chemical pulp webs. A surprisingly large expansion of the web was observed immediately after the suction pulse. This expansion was the effect of rewetting. This rewetting strongly reduced the dry content of the web if the web had not been immediately separated from the forming fabric at the end of the suction pulse. Under the conditions studied, the decrease in dry content amounted to the order of 3 to 6 %. Rewetting was smaller for longer suction times and higher suction pressures. A considerable air flow through the web occurred under these conditions. This air flow apparently moved water from the forming fabric into the suction box, thus making less water available for rewetting. Rewetting for mechanical pulp webs was more pronounced and took place faster than for chemical pulps. The use of a membrane on top of the web during suction box dewatering proved to be advantageous for reducing the air flow through the web. However, under the conditions investigated, the dry content could not be improved. Although the web compression was increased when using a membrane, especially at a higher suction pressure, rewetting after the suction pulse had an even larger negative impact on the dry content, which, as a result, was lower.
QC 20100924

APA, Harvard, Vancouver, ISO, and other styles

19

Schwieger, Sarah [Verfasser], Jürgen [Akademischer Betreuer] Kreyling, Gesche [Akademischer Betreuer] Blume-Werry, Jürgen [Gutachter] Kreyling, Vicky [Gutachter] Temperton, and Scott [Gutachter] Wilson. "Invisible to the eye - Root dynamics in temperate minerotrophic peatlands under drainage and rewetting / Sarah Schwieger ; Gutachter: Jürgen Kreyling, Vicky Temperton, Scott Wilson ; Jürgen Kreyling, Gesche Blume-Werry." Greifswald : Universität Greifswald, 2021. http://d-nb.info/123669368X/34.

Full text

APA, Harvard, Vancouver, ISO, and other styles

20

Maigrat, Guillaume. "Caractérisation expérimentale du remouillage des aciers." Thesis, Université de Lorraine, 2016. http://www.theses.fr/2016LORR0098/document.

Full text

Abstract:

Sur la table de sortie d’un laminoir à chaud, le refroidissement des bandes d’acier est principalement assuré par des rampes de jets d’eau gravitaires qui vont impacter directement la surface supérieure de la bande en défilement. La température de la bande est initialement entre 800 et 900 °C et on cherche à la refroidir à des températures nettement inférieures mais précises avant son bobinage, cette température allant de la température ambiante à 700 °C suivant les propriétés voulues pour l’acier. Ce refroidissement assuré par les rampes de jets est transitoire et la vitesse de refroidissement est variable suivant la nature du régime de refroidissement. Le point de remouillage est défini par la température à laquelle le régime d’ébullition en film stable se termine et s’accompagne généralement d’une hausse significative des flux de refroidissements. Dans ce travail de thèse, nous avons cherché à caractériser ce point par des mesures d’épaisseur de vapeur effectuées à l’aide de sondes optiques. Les sondes optiques permettent de savoir si leur zone sensible est majoritairement dans le gaz ou le liquide et sont généralement exploitées pour faire des mesures de taux de vide. L’utilisation qui en est faite ici, à savoir une mesure précise de position d’interface, a demandé une calibration expérimentale précise afin de connaitre la position exacte de l’interface sur la zone sensible. Afin de disposer d’un outil fonctionnel, nous avons également cherché à modéliser la réponse des sondes à l’aide d’une méthode de lancer de rayons et en prenant en compte la formation du ménisque sur la pointe. Enfin, cette calibration et ce modèle ont été confrontés à des mesures expérimentales dans le cas du refroidissement d’une zone hémisphérique porté à haute température et donnant lieu à un régime d’ébullition en film stable
On a run out table of a hot rolling mill, the cooling of steel strips is mainly provided by ramps of water jets falling down that will directly impact the upper surface of the strip. The temperature of the strip starts between 800 and 900 °C and it is intended to cool at temperatures much lower than the start but still accurate before its winding, the temperature ranges from ambient temperature to 700 °C depending on the required properties for the steel product. This water-based process ensures a transient cooling at an uneven rate depending on the nature of the boiling regime. The rewetting point is defined by the temperature at which the stable film boiling regime ends. In general, the cooling fluxes show a significant increase at this particular transition. In this thesis, we sought to characterize rewetting by vapor thickness measurements and analysis using optical probes. The optical sensors inform us whether their sensitive area is mainly in the gas or in the liquid and are generally used to make void fraction measurements. The use that is made here, namely a precise measurement of the position of the interface, required a precise experimental calibration in order to know the exact position of the interface on the sensitive area. In order to have a working tool, we also modeled the response of the probes by using a ray tracing method and taking into account the formation of the meniscus on the tipoff the probe. Finally, the calibration and model were compared with experimental measurements in the case of the cooling of a hemispherical area initially at high temperature that provides a stable film boiling regime when it is partially immersed

APA, Harvard, Vancouver, ISO, and other styles

21

Tzevelecos, Wassilis. "Contribution to Heat and Mass Transfer for Space Experiments." Doctoral thesis, Universite Libre de Bruxelles, 2018. https://dipot.ulb.ac.be/dspace/bitstream/2013/269864/6/contratWT.pdf.

Full text

Abstract:

This manuscript has been realized in the frame of SELENE experiment research activities. SELENE is the ac-ronym of Self-rewetting fluids for ENErgy management and consists of a space project aiming to investigate heat and mass transfer phenomena in mono-groove configuration with self-rewetting fluids (SRFs). Self-rewetting fluids are mixture showing an anomalous trend of surface tension with temperature, an inversion of the surface tension slope after certain temperature. As consequence, when the minimum in surface ten-sion is crossed, surface tension gradient at the meniscus interface pulls the liquid towards the warmest region, preventing hot spots. This mechanism is completely spontaneous and has an interesting potential when applied to heat transfer applications as heat pipes (HPs). In HPs heat is removed by the liquid at the warmest region (the evaporator) and transported at the coldest zone (the condenser) by phase change; here, heat is removed by the pipe and dissipated outside through a radiator. To operate correctly, liquid is supplied to the evaporator by capillarity and the liquid vapour is allowed to flow back to condenser from a dedicated pipe region where liquid is not allowed. Vapour condensation releases at the condenser the heat to be dissipated. When SRFs are replacing working fluid in HP applications and temperatures are higher than the characteristic minimum in surface tension, capillary force is assisted by inverse Marangoni flow at the vapour-liquid interface.Since heat pipe performances are related to liquid supplied at the evaporator, in order to compare SRFs and not SRFs working fluids, it is needed to split the contribution of Marangoni and capillary force in the liquid flow. Marangoni effect is related to surface tension gradient that, in a mixture as SRF, is dependent on temperature and local composition at the liquid interface. For all these reasons, SELENE is designed to be the link between scientific research on HPs and heat transfer applications using SRFs. SELENE consists of a mono-groove with trapezoidal section that can be considered as a “clump” of an Inner Grooved Heat Pipe (IGHP) and, in order to split capillary and Marangoni contribution, it is integrated dedicated tools providing the required data in terms of concentration and liquid meniscus shape. Experimental data are used to build a simplified thermo-soluto-fluido dynamic model describing the thermo-mechanic mechanisms between the liquid bulk and the vapour flow. In the manuscript here presented it has been carried on a technology development of the required diag-nostics for the SELENE space project. The diagnostics have been designed to work in microgravity condi-tions even if they are tested on ground. As concentration diagnostic, in the text are proposed several tech-niques and more interest is spent on the adaptation of I-VED (In vivo Embolic Detection) technology meas-uring fluid AC impedance to retrieve composition information; the technology is not yet mature to be inte-grated in SELENE but it presents interesting features to be investigated in microgravity conditions. As me-niscus reconstruction technique it is proposed a new and innovative technology developed in the frame of the presented thesis and it consists of a non-intrusive optical technique aiming to retrieve liquid meniscus shape (and so curvature) from a single visualization window mounted at the top of the SELENE breadboard.An analytical approach aiming to retrieve a simplified mathematical model of the transfer mechanisms is also provided in the text. The analytical analysis clearly shows the relations between the experimental measured data and the velocity profiles in the liquid and vapour regions. In addition, since in SELENE exper-iment the heat conduction across the groove itself is not negligible, in the text it is provided a semi-empirical thermal model based on the Multi Lumped Model (MLM) theory and able to retrieve local heat exchanged information along the pipe length. The model is used to compare experiments with different working fluids at different operational regimes.
Doctorat en Sciences de l'ingénieur et technologie
info:eu-repo/semantics/nonPublished

APA, Harvard, Vancouver, ISO, and other styles

22

Velty, Silke. "Einfluss von Wiedervernässungsmaßnahmen auf den Stoffhaushalt degradierter Niedermoore." Doctoral thesis, Humboldt-Universität zu Berlin, Landwirtschaftlich-Gärtnerische Fakultät, 2008. http://dx.doi.org/10.18452/15752.

Full text

Abstract:

Zur Einschätzung der Auswirkungen einer Wiedervernässungsmaßnahme auf den Stoffhaushalt eines degradierten Niedermoores wurde ein ehemals entwässertes und seit 1996 wieder vernässtes ca. 10 Hektar großes Areal im Randow-Welse-Bruch im nordöstlichen Brandenburg über mehrere Jahre untersucht. Stoffkreisläufe wurden vor allem in Abhängigkeit der sich durch die Wiedervernässung ändernden Redoxbedingungen modifiziert. Obwohl Phosphor, ein die Eutrophierung auslösendes Element, selbst direkt nicht durch einen Wechsel von aeroben zu anaeroben Bedingungen durch die Wiedervernässung beeinflusst wird, änderte sich dessen Mobilität auf Grund der Rücklösung seines Sorptionspartners. Infolge der anhaltend reduzierenden Bedingungen fand nach mehrjähriger Wiedervernässung der Prozess der Rücklösung amorpher Eisen(hydr)oxide statt, wobei der ehemals an diese Verbindungen fixierte Phosphor freigesetzt wurde. Bei Niedermooren, die infolge hoher Gehalte an Eisen(hydr)oxiden ein erhöhtes Phosphor-Freisetzungspotenzial besitzen, sollte eine Wiedervernässung mit gereinigtem kommunalen Abwasser nicht in Erwägung gezogen werden. Die Erfassung der gasförmigen Emissionen ermöglichte eine Berücksichtigung innerhalb der Stoffbilanzierung und eine Bewertung der Wiedervernässungsmaßnahme hinsichtlich ihrer Klimarelevanz. Auf Grund des anaeroben Milieus durch die Wiedervernässung wurde neben Lachgas auch molekularer Stickstoff als gasförmige Stickstoffverbindung gebildet. Gegenüber der entwässerten Lysimetervariante zeigte sich ein positiver Beitrag zur Reduzierung des Treibhauseffektes durch die Wiedervernässung. Jedoch ist die Qualität des Zusatzwassers, das zur Wiedervernässung der Torfprofile des Niedermoores Havelländisches Luch eingesetzt wurde, entscheidend. In eutrophen Niedermoorökosystemen sind nährstoffreiche Wasserzuflüsse aus kommunalen Abwasserbehandlungsanlagen als Wiedervernässungsmaßnahme geeignet, so dass gereinigtes Abwasser den Landschaftswasserhaushalt stützen könnte.
A formerly drained, and since 1996 rewetted, 10 ha fen site within the Randow-Welse-Bruch fen area in northeastern Brandenurg (Germany) has been investigated for several years to evaluate the impacts of rewetting on nutrient dynamics of the degraded fen. Modifications of nutrient cycles were attributed to changes in redox conditions after rewetting. Although phosphorus which causes eutrophication is not directly influenced by a change from aerobic to anaerobic conditions following rewetting its availability increased due to the dissolution of reactive iron (hydr)oxides where phosphorus was fixed before.With prolonged water-logging phosphorus was released from Fe(III) (hydr)oxides. A lysimeter study was carried out to promote the reuse of purified municipal wastewater as an alternative solution for water supply required during rewetting of fens. Fen soil lysimeters were irrigated with wastewater of the secondary treatment of a municipal wastewater treatment plant over the growing seasons. The objective of this study was to assess how restoration of a degraded fen by means of wastewater usage affects input/output nutrient mass balances. Furthermore, we assessed the effect on trace gas emissions and discussed potential changes in the greenhouse potential. No adverse impact of wastewater utilisation for rewetting the fen soil lysimeters on soil and water quality was found over the period of study with regard to the investigated nutrients and heavy metals. Gaseous fluxes during alternative rewetting were estimated for a consideration within nutrient balances and to evaluate the global warming and cooling, respectively. Besides nitrous oxide dinitrogen was formed and released under anaerobic conditions following rewetting. A potential increase of the nitrate concentration in wastewater would be harmless because a high denitrification of the fen soil would cause a major share of dinitrogen emissions of the total gaseous nitrogen loss.

APA, Harvard, Vancouver, ISO, and other styles

23

LIMA, RITA de C. F. de. "Comportamento termoidraulico de vareta aquecida eletricamente durante transitorio de fluxo critico de calor." reponame:Repositório Institucional do IPEN, 1997. http://repositorio.ipen.br:8080/xmlui/handle/123456789/10640.

Full text

Abstract:

Made available in DSpace on 2014-10-09T12:42:40Z (GMT). No. of bitstreams: 0
Made available in DSpace on 2014-10-09T13:57:16Z (GMT). No. of bitstreams: 1 05031.pdf: 4962096 bytes, checksum: 39c12c06c0063abb20c1c82005ecef33 (MD5)
Tese (Doutoramento)
IPEN/T
Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

APA, Harvard, Vancouver, ISO, and other styles

24

Bejarano, Roberto Villa. "An Investigation Into| I) Active Flow Control for Cold-Start Performance Enhancement of a Pump-Assisted, Capillary-Driven, Two-Phase Cooling Loop II) Surface Tension of n-Pentanol + Water, a Self-Rewetting Working Fluid, From 25 ?C to 85 ?C." Thesis, University of Nevada, Reno, 2014. http://pqdtopen.proquest.com/#viewpdf?dispub=1565813.

Full text

Abstract:

Cold-start performance enhancement of a pump-assisted, capillary-driven, two-phase cooling loop was attained using proportional integral and fuzzy logic controls to manage the boiling condition inside the evaporator. The surface tension of aqueous solutions of n-Pentanol, a self-rewetting fluid, was also investigated for enhancing heat transfer performance of capillary driven (passive) thermal devices was also studied. A proportional-integral control algorithm was used to regulate the boiling condition (from pool boiling to thin-film boiling) and backpressure in the evaporator during cold-start and low heat input conditions. Active flow control improved the thermal resistance at low heat inputs by 50% compared to the baseline (constant flow rate) case, while realizing a total pumping power savings of 56%. Temperature overshoot at start-up was mitigated combining fuzzy-logic with a proportional-integral controller. A constant evaporator surface temperature of 60°C with a variation of ±8°C during start-up was attained with evaporator thermal resistances as low as 0.10 cm2–K/W. The surface tension of aqueous solutions of n-Pentanol, a self-rewetting working fluid, as a function of concentration and temperature were also investigated. Self-rewetting working fluids are promising in two-phase heat transfer applications because they have the ability to passively drive additional working fluid towards the heated surface; thereby increasing the dryout limitations of the thermal device. Very little data is available in literature regarding the surface tension of these fluids due to the complexity involved in fluid handling, heating, and experimentation. Careful experiments were performed to investigate the surface tension of n-Pentanol + water. The concentration and temperature range investigated were from 0.25%wt. to1.8%wt and 25°C to 85°C, respectively.

APA, Harvard, Vancouver, ISO, and other styles

25

Rebhann, Marco Frank. "Bewirtschaftung von Niedermooren in der gemäßigten Klimazone: Treibhausgasemissionen und Rentabilität." Doctoral thesis, Humboldt-Universität zu Berlin, 2018. http://dx.doi.org/10.18452/19334.

Full text

Abstract:

Niedermoore sind durch Grundwasser beeinflusste Standorte, in denen Torf gebildet wird oder oberflächig ansteht. Sie emittieren durch mikrobiologische Prozesse Treibhausgase (THG). Wird der Grundwasserstand gesenkt, steigen Kohlendioxid und Lachgas Emissionen, während bei einer Anhebung vermehrt Methan emittiert wird. Weltweit wird davon ausgegangen, dass Moore 5 % der anthropogen verursachten Emissionen verursachen. Um THG-Emissionen entgegenzuwirken, wird die Anhebung des Grundwasserstandes diskutiert. Dies führt durch eingeschränkte Befahrbarkeit und geringeres Kulturartenspektrum zu einer kaum gegebenen Wirtschaftlichkeit. Es fehlt ein Vergleich der THG-Emissionen und der Rentabilität der Bewirtschaftungssysteme verschiedener Landnutzungssysteme für Niedermoorstandorte der gemäßigten Klimazone. Die bodenbürtigen THG-Emissionen wurden mit einer Metaanalyse zu Veröffentlichungen von THG-Messungen in Niedermooren der gemäßigten Klimazone ermittelt. Ebenfalls wurden die bewirtschaftungsabhängigen Emissionen ermittelt und zu Gesamtemissionen zusammengefasst. Für den Vergleich der Rentabilität der Bewirtschaftungssysteme wurden Kosten und Erlöse berechnet. Abschließend wurden Treibhausgasvermeidungskosten abgeschätzt Bei zunehmender Landnutzungsintensität und größeren Grundwasserflurabstand nahmen die Gesamt-THG-Emissionen zu. Durch die Bewirtschaftung verursachte Emissionen spielten im Vergleich zu den bodenbürtigen Emissionen kaum eine Rolle. Zur THG-Einsparung und der Torferhaltung sollte die Bewirtschaftung extensiviert werden. Im Rentabilitätsvergleich erwiesen sich intensive, im Vergleich zu extensive Bewirtschaftungssysteme ohne Förderung als wirtschaftlich. Subventionen und Förderungen um torf- und klimaschonende Praktiken zu unterstützen, sind unverzichtbar. Mit dieser Arbeit wurde erstmalig für Niedermoore ein systematischer Überblick über die Gesamt-THG-Emissionen und die Rentabilität gängiger Bewirtschaftungssysteme verschiedener Landnutzungssysteme gegeben.
Fens are sites influenced by the groundwater where peat is formed in or located in the upper soil layer. They emit greenhouse gases (GHG) through microbiological processes in which the main influencing factor is the groundwater level. The lowering of the groundwater level increases carbon dioxide and dinitrogen monoxide emissions, while higher groundwater levels lead to methane emissions. It is assumed that fens cause 5% of global anthropogenic GHG emissions. In order to save peat and reduce GHG emissions, rewetting fens has been discussed and applied. This leads to low profitable land use due to limited trafficability and cultivation options. There is a lack of comparison of management systems in different land use systems regarding GHG emissions and profitability on fenlands in the temperate climate zone. The soil-borne GHG emissions for the land use systems were identified with a meta-analysis of published measurements of GHG emissions on fenlands in the temperate climate zone. The GHG emissions due to cultivation and possible land use changes were identified and summarized as the overall emissions. Costs and revenues were calculated for comparability purposes of profitability of management systems. GHG abatement costs were estimated too. Results indicated that overall GHG emissions increase with higher land use intensity and deeper groundwater level. Management induced GHG emissions are negligible compared to the soil-borne emissions. Fenlands should be extensified to save peats and reduce GHG emissions. The comparison of profitability showed that management systems of intensive land use systems, in contrast to extensive land use systems, with the low groundwater level are broadly profitable without subsidies. Subsidies and grants are indispensable to promote peat-saving and climate-friendly practices. This thesis provides a systematic overview of overall GHG emissions and profitability of common management systems of fenlands.

APA, Harvard, Vancouver, ISO, and other styles

26

Gu, Sen. "Release of dissolved and colloidal phosphorus from riparian wetlands : a field and laboratory assessment of the mechanisms and controlling factors." Thesis, Rennes 1, 2017. http://www.theses.fr/2017REN1S063/document.

Full text

Abstract:

Le phosphore (P) est un nutriment essentiel dans le contrôle de l'eutrophisation des eaux de surface. La majorité du P causant cette eutrophisation dans les pays occidentaux est aujourd'hui issu des sols agricoles, ce qui explique pourquoi les recherches actuelles sur l'eutrophisation se focalisent sur la compréhension des mécanismes par lequel le P est relargué de ces sols. Dans cette thèse, nous étudions ces mécanismes en nous focalisant sur la fraction dissoute (DP) du P, fraction la plus menaçante du point de vue de l'eutrophisation. Une double approche a été utilisée, combinant le suivi de la composition d'eaux du sol et d'eaux de ruisseau dans un petit bassin versant (BV) agricole représentatif (BV de Kervidy-Naizin, France) et des simulations expérimentales au laboratoire. Les suivis de terrain ont révélé que les zones humides ripariennes (RW) étaient les principales zones de relargage de DP dans le BV étudié, via deux mécanismes essentiellement déclenchée par les fluctuations des hauteurs de nappe, i) la réhumectation des sols (DRW) et ii) la dissolution réductrice des oxydes de fer du sol dans de périodes d'anoxie. Ces mêmes suivis ont révélé la présence de fortes variations spatiales de la nature chimique (inorganique et organique/colloïdale) du DP relargué. Les variations saisonnières et interannuelles de l'hydroclimat, combinées aux variations locales de topographie ont été démontrées être les deux facteurs principaux contrôlant i) la fréquence des épisodes DRW, et ii) la durée des périodes anoxiques, entraînant au final de fortes variations saisonnières et interannuelles de la dynamique de relargage du DP. Comme indiqué dans un modèle conceptuel général, la topographie est sans doute le facteur clé de contrôle des variations observées, en raison de son rôle sur i) le transfert de P à partir des parcelles agricoles amont, ii) le taux de minéralisation du P organique du sol P et iii), le déclenchement des deux mécanismes de relargage précités. Les expériences en laboratoire ont confirmé le rôle des événements DRW comme processus clé causant le relargage de DP dans les RWs. Les résultats ont démontré que le DP relargué consistait non seulement de "vrai" DP inorganique et organique, mais aussi de P colloïdal, le P colloïdal et le DP organique étant les plus réactifs aux événements DRW. Les données ont aussi révélées que ces différentes formes de P provenaient de différentes sources dans le sol (méso et macroporosité pour P colloïdal et le DP organique; microporosité pour DP inorganique), et que la quantité de P colloïdal relargué était positivement corrélée avec la teneur en matière organique et la taille de biomasse microbienne du sol. Ces mêmes expériences ont confirmé le rôle des conditions anoxiques comme conditions favorisant la libération de DP dans les RWs. La dissolution réductive de sol Fe-oxyhydroxydes n'est cependant pas le seul processus impliqué, un autre processus étant la hausse du pH causée par des réactions de réduction. Les résultats obtenus démontrent que l'augmentation de pH contrôle la libération de DP dans les sols riches en matière organique, alors que ce relargage est contrôlé principalement par la réduction des oxydes de fer dans les sols pauvres en matière organique. Les données expérimentales démontrent également que l'apport de sédiments issus des sols agricoles amont accroit le relargage de DP dans les RW, probablement en raison de la dissolution des oxydes de fer de ces mêmes sédiments par les bactéries ferroréductrices des Rws. Au final, cette thèse permet de mieux contraindre les mécanismes et facteurs responsables du relargage de DP dans les bassins versants agricoles. Une conséquence très pratique de ce travail est que la conception de stratégies pour limiter les fuites de DP dans ces bassins ne peut se faire sans une prise en compte des rôles de l'hydroclimat, de la topographie locale et des propriétés du sol sur ce relargage
Phosphorus (P) is a key nutrient in controlling surface water eutrophication. Because of the decrease of urban and industrial P emissions, most of the P nowadays causing surface water eutrophication in western countries consists of P transferred from agricultural soils, explaining why current eutrophication research focused on understanding the mechanisms by which P is released from soils. In this thesis, we studied these release mechanisms for dissolved P (DP) – i.e. the most bioavailable P component for algae - using an approach combining field monitoring of soil and stream water compositions in a small, headwater catchment typical of western countries agricultural catchments (the Kervidy-Naizn catchment, France), and laboratory experimental simulations. Field monitoring data revealed that riparian wetlands (RW) are the main zones of DP release and DP production in the studied catchment, through essentially two mechanisms triggered by groundwater table fluctuations, namely i) rewetting of dry soils (DRW), and ii) reductive dissolution of soil Fe (hydr)oxides during anaerobic periods. Field monitoring data also revealed the presence of strong spatial variations in the chemical nature (inorganic vs. organic/colloidal) of the released DP, which was in relation to differences in soil properties and local topography. Seasonal and inter-annual hydroclimate variations, combined with variations in local topography were found to control the frequency of soil DRW events and duration of anaerobic periods, resulting in strong seasonal and inter-annual variations of DP release dynamics. As shown in a conceptual model, topography is likely to be the key driver of the observed spatial and temporal variations, because of its combined control on i) the transfer of P from upland fields to RW zones, ii) the mineralization rates of soil organic P and iii) the triggering of the above two release mechanisms. Laboratory leaching experiments on the same soils confirmed the role of DRW events as a major process causing DP release pulses in RWs. The data demonstrated that the released DP consisted not only of true dissolved inorganic and organic P but also of colloidal P, the latter phase being the most reactive to DRW events. The data also revealed that the different P forms came from different P sources in the soil (soil macro/mesopores for colloidal P and organic DP; soil micropores for inorganic DP) and that the amount of released colloidal P correlated positively with the organic matter contents and soil microbial biomass size of the soil. Anaerobic incubation experiments, on their hand, confirmed the role of anoxic conditions as conditions favoring the release of DP in RW. Reductive dissolution of soil Fe-oxyhydroxide was, however, not the sole process involved in that release, another process being the rise in pH caused by reduction reactions. Experimental data showed that the pH rise controlled the DP release in organic-rich soils, this release being on the contrary mainly controlled by soil Fe-oxyhydroxides reductive dissolution in organic-poor soils. Experimental data also showed that the input of soil sediments from upland fields enhanced the release of DP in RW, most likely due to the enhanced dissolution of sediment Fe-oxyhydroxides by RW Fe-reducing bacteria. Overall, this thesis allowed new constraints to be placed on the release mechanisms of DP in headwater agricultural catchments. One very practical output is that great care should be taken of hydroclimate variability, local topography, and soil property when designing and implementing management options to reduce DP release and transfer in agricultural catchments

APA, Harvard, Vancouver, ISO, and other styles

27

Zaaroura, Ibrahim. "Amélioration du transfert de chaleur et de masse à l'aide de nanofluides auto-réhumidifiants." Thesis, Valenciennes, Université Polytechnique Hauts-de-France, 2020. http://www.theses.fr/2020UPHF0024.

Full text

Abstract:

La gestion thermique est actuellement un enjeu majeur dans de nombreux secteurs industriels. Les dispositifs de transfert de chaleur (comme les caloducs) sont des systèmes intégrés qui utilisent des modes de transfert par convection, évaporation et condensation. L'efficacité de l'échange thermique peut être améliorée en utilisant une nouvelle méthode de nanotechnologie connue sous le nom de nanofluides. Les nanofluides sont des suspensions de particules nanométriques (de 2 à 100 nm) dans les fluides. Cette amélioration est due à l'augmentation significative de la conductivité effective dans la plage de 10 à 50% ainsi qu'à leurs coefficients de transfert de chaleur par convection. Cependant, des limitations liées aux phénomènes de nanoparticules, de sédimentation et d'agrégation apparaissent au cours de différentes phases. Dans ce projet, afin d'optimiser les performances et la stabilité des nanofluides, deux méthodes différentes ont été utilisées: une optique couplée à une méthode de thermographie infrarouge et une méthode acoustique à haute fréquence. La sélection des nanoparticules a été basée sur leurs performances thermiques lors de l'évaporation de gouttelettes sessiles à l'aide d'un système Kruss, dans les mêmes conditions et à des concentrations différentes. Ainsi, des ondes acoustiques haute fréquence, en surface, ont permis d'analyser la stabilité de nanoparticules lors de l’évaporation où ces particules sont en suspension avec des fluides de base tels que l'eau ionisée, des mélanges binaires auto-réhumidifiant (Pour améliorer la circulation des fluides à l'intérieur caloducs grâce à leurs propriétés physiques comme les phénomènes Marangoni thermique, Marangoni de concentration, capillaires ...). Les nanofluides sélectionnés seront utilisés par la suite pour analyser expérimentalement leurs performances sur les dispositifs de transfert de chaleur à boucle de fluide capillaire biphasé et dans les échangeurs de chaleur à micro-canaux en trouvant et en calculant la résistance thermique du système, le coefficient de transfert de chaleur par convection et la température de paroi
Thermal management is currently a major issue in many sectors of industry. Heat transfer devices (like Heat Pipes, Heat exchangers…) are integrated systems that use transfer modes by convection, evaporation and condensation. The efficiency of heat exchange can be improved by using a new nanotechnology method known as nanofluids. Nanofluids are suspensions of nano-size particles (from 2 to 100nm) in the fluids. This improvement is due to the significant increase in the effective conductivity in the range 10-50% as well as their convective heat transfer coefficients. However, limitations related to the phenomena of nanoparticle, sedimentation and aggregation appear during different phases. In this project, in order to optimize the performance and stability of nanofluids, two different methods were used: an optical one coupled to an infrared thermography method and a high frequency acoustic method. The selection of nanoparticles was based on their thermal performance during evaporation of sessile droplets using a Kruss system, under same conditions and at different concentrations. Thus, high frequency acoustic waves, at the surface, make it possible to analyze the stabilities of nanoparticles during its evaporation where these particles are in suspension with base fluids such as ionized water, self-rewetting binary mixture (To improve the circulation of fluids inside heat pipes thanks to their physical properties like thermal Marangoni, concentration Marangoni, capillary...). The selected nanofluids will be used later to analyze experimentally their performance on two-phase capillary fluid loop heat transfer devices and in micro-channel heat exchangers by finding and calculating the thermal resistance of the system, convection heat transfer coefficient and wall temperature

APA, Harvard, Vancouver, ISO, and other styles

28

Kheir, Maya. "Fonctionnement des sols méditerranéens sous changement climatique : effet de la préexposition au stress, de la mixité des litières et de l'identité de l'espèce végétale." Thesis, Aix-Marseille, 2020. http://theses.univ-amu.fr.lama.univ-amu.fr/200710_KHEIR_495pzb687n410mxugdf923oaivtm_TH.pdf.

Full text

Abstract:

Ce travail se situe dans un contexte du changement climatique. L’objectif est d’explorer le fonctionnement des communautés microbiennes de litières végétales dépendant des contrastes climatiques (sub-humide vs semi-aride) Nord/Sud de la Méditerranée mais aussi selon le contexte côtier/continental,et d’évaluer leur réponse au stress d'aridification en considérant les effets i) du type de litière (identité de l’espèce végétale et mixité) et ii) de la préexposition au stress intrinsèques des milieux côtiers. Le stress d’aridité a été appliqué en laboratoire (cycles de séchage/réhumectation) et in natura via des transferts de «litter bags» de France vers l’Algérie. Les résultats montrent que la biomasse microbienne et la respiration basale dépendent des traits chimiques de la litière (C/N et lignine/N): lorsque ces ratios augmentent la biomasse microbienne diminue, alors que l’effet sur la respiration dépend des contrastes climatiques. L’effet de la mixité binaire des litières sur les microbes dépend du contexte climatique et de la composition du mélange: certains mélanges semblent limiter le relâchement du carbone par respiration hétérotrophique en contexte aride Algérien. En outre, un fonctionnement microbien particulier des environnements côtiers a été mis en évidence: i) la préexposition aux contraintes côtières a limité l’effet du stress sur les structures cataboliques microbiennes en laboratoire ii) l’expérience in natura a montré que les réponses microbiennes au stress dépendent du contexte (côtier/continental) mais aussi du type de la litière, mettant en valeur une influence de la litière qui dépasse l’échelle locale pour s’imposer même à une large échelle spatiale
This work takes place in the context of climate change. We aimed to explore plant litter microbial communities' functioning depending on climate contrasts Northern/Southern Mediterranean but also according to inland/coastal context, and to assess their response to aridification stresses, considering the effect i) of the type of litter (plant species identity and mixture) and ii) of the preexposure to stress intrinsic of coastal environments. Aridity stress was applied in the laboratory (drying /rewetting cycles) and in natura via "litter bags" transfer from France to Algeria. Results show that microbial biomass and basal respiration depend on litter chemical traits (C/N and lignin/N): when these ratios increase, microbial biomass decreases, while the effect on respiration depends on the climate contrasts. The effect of binary litter mixtures on microbes depends on the climate context and the mixture composition: some mixtures seem to limit carbon release through heterotrophic respiration in Algerian arid context. In addition, a particular microbial functioning of coastal environments was highlighted: i) preexposure to coastal constraints limited the effect of stress applied in the laboratory on microbial catabolic structures ii) the in natura experiment showed that microbial responses to stress depend on the context (coastal/inland) but also on litter type, highlighting litter influence even at large spatial scales

APA, Harvard, Vancouver, ISO, and other styles

29

Nguyen, Ngoc diep. "Characterization and contribution of colloidal fraction in the mobility and bioavailability of sedimentary phosphorus." Thesis, Limoges, 2020. http://aurore.unilim.fr/theses/nxfile/default/9f8fcb76-5d28-468c-8289-23bd366f1bb7/blobholder:0/2020LIMO0078.pdf.

Full text

Abstract:

Le relargage de phosphore sédimentaire constitue une source interne de nutriments dans les eaux conduisant à l’eutrophisation des milieux aquatiques, en particulier des retenues. Les connaissances actuelles concernant le relargage du P sédimentaire ne tiennent pas compte de la fraction colloïdale, qui est pourtant considérée comme une importante phase porteuse de P, très mobile et potentiellement biodisponible. Cette thèse souligne le rôle majeur des colloïdes sédimentaires vis-à-vis de leur teneur en phosphore ainsi que leur fort potentiel à être mobilisé dans l’eau. Il a été démontré que les colloïdes sont une composante intrinsèque essentielle des sédiments au sein du barrage de Champsanglard (Creuse, France) et qu’ils peuvent être remobilisés à hauteur de 2.3 % de la masse totale du sédiment lors de la resuspension de ce dernier. Le phosphore colloïdal représentait jusqu’à 6 % du phosphore sédimentaire total et 80 % du phosphore total mobilisable. L’étude des différents protocoles d'extraction, de séparation et de conservation des sédiments a permis de proposer une méthodologie opérationnelle permettant la sélection du protocole le plus approprié. Il est ainsi recommandé de réaliser les extractions de colloïdes sur sédiment humides, par une méthode douce (agitation) et de réaliser la séparation des colloïdes extraits par filtration avec une étape de pré-séparation. Les centrifugations successives sont déconseillées car elles conduisent à une estimation biaisée de la quantité et de la qualité des colloïdes extraits. Au travers de l’étude de la retenue de Champsanglard, il a été mis en évidence une forte hétérogénéité de la répartition spatiale de la quantité et de la qualité des colloïdes sédimentaires et du P associé. La granulométrie des sédiments, l'hydrodynamique des écoulements, la présence d'affluents et le marnage apparaissent comme des facteurs clés. La plus importante quantité de colloïdes mobilisables et en particulier les colloïdes de plus grande taille (0.2 – 1 μm) concerne les sédiments lacustres des zones profondes, en amont de la confluence avec les tributaires. Les sédiments de berge régulièrement impactés par le marnage de l’eau relarguent du phosphore sous une forme biodisponible (forme dissoute et nano/petits colloïdes). En condition anoxique, les sédiments ont la capacité de libérer du phosphore colloïdal, dans des proportions largement supérieures au phosphore dissous. Jusqu’à 40 % du phosphore sédimentaire total a ainsi été remobilisé sous forme colloïdale. Lorsque les conditions redeviennent oxiques, une majeure partie du phosphore mobilisé persiste en solution et notamment sous la forme de colloïdes de tailles petites et intermédiaires (300 KDa - 0.45 μm). En conditions d’oxydo-réduction variables, les processus biogéochimiques impliquant le fer et la matière organique seraient les paramètres clés contrôlant la mobilité du P colloïdale. Ce travail met en évidence la nécessité de considérer la contribution des colloïdes pour l’évaluation de la mobilité et de la biodisponibilité du P, en particulier en contexte de retenue de barrage
The mobilization of phosphorus (P) from sediment is a persistent internal nutrient source sustaining eutrophication, especially in reservoir. Current knowledge about sedimentary P release does not consider colloidal form, although it is well-known for its efficiency as P-carrier, its high mobility and potentially bioavailability. This thesis provides new constraints to place sedimentary colloids and sedimentary colloidal P into light concerning their important stock and release potential in the reservoir. Colloids are shown to be an intrinsic component of reservoir sediment, and their recovery under sediment resuspension indicated the contribution of water mobilizable colloids up to 2.3 % of the sediment mass in Champsanglard dam reservoir (Creuse, France). The amount of P associated with colloids was up to 6 % of total sedimentary P and 80 % of water-mobilizable P. After the application of different protocols for colloid extraction, separation and for sediment storage, a framework for selecting appropriated method for colloid recovering from a sediment is proposed. The recommendation for colloid recovery is to work with wet sediment, avoid using high-power extraction protocol (i.e. sonication), using filtration with pre-separation step instead of successive centrifugation to avoid under or overestimation in quantity and quality of recovered colloids. The quantity and quality of sedimentary colloids and associated P varied spatially in the reservoir according to sediment size grain distribution and the influence of flow hydrodynamics, the presence of tributaries and fluctuations in the water-level. The highest content of water mobilizable colloids and especially large colloids (0.2 - 1 μm) were found in lacustrine bottom sediments, downstream of the tributary confluence. Bank sediments impacted by water-level fluctuations released P in bioavailable forms (dissolved P and small/nano P-colloids). Under anoxic condition, the potential of sediment to mobilize colloidal P was significantly higher than truly dissolved P and represented up to 40 % of total sedimentary P. When condition became oxic subsequently, major portion of released P remained in suspension under small/intermediate size (300 kDa - 0.45 μm). In redox changing conditions, the closely linked biogeochemical cycles of iron and organic matter could be the key parameters involved in mobility of colloidal P. This thesis highlights the need to consider colloids and their contribution in the P mobility at the boundary of sediment-water and associated bioavailability to aquatic plankton, particularly in dam reservoir context

APA, Harvard, Vancouver, ISO, and other styles

30

Scheiff, Valentin. "Etude expérimentale et modélisation du transfert de chaleur de l'ébullition transitoire." Thesis, Toulouse, INPT, 2018. http://www.theses.fr/2018INPT0145/document.

Full text

Abstract:

L’étude de l’ébullition transitoire est un enjeu important pour la sureté nucléaire. Un tel phénomène peut se produire lors d’un accident de type RIA (Reactivity Initiated Accident)dans un réacteur nucléaire où le pic de puissance au niveau d’un crayon de combustible peut déclencher une ébullition transitoire conduisant à une forte augmentation de la température de la gaine et à un risque de rupture. Plusieurs études en conditions réacteurs ont permis d’obtenir des courbes d’ébullition transitoires mais la modélisation qui en découle manque encore de fiabilité. Dans le cadre d’une collaboration avec l’Institut de Radioprotection et de Sûreté Nucléaire (IRSN), une expérience modèle a été construite à l’Institut de Mécanique des Fluides de Toulouse (IMFT). Elle génère un écoulement de réfrigérant HFE7000 dans un canal de section semi-annulaire, simulant l’écoulement autour d’un crayon de combustible, dont la partie intérieure, composée d’une feuille de métal, est chauffée rapidement par effet Joule, simulant l’échauffement de la gaine du crayon. La thermographie infra-rouge permet de mesurer la température de la paroi externe du métal. L’application d’une peinture noire sur le métal augmente son émissivité mais aussi la résistance thermique de la paroi. La précision de la mesure de la température d’intérêt a été optimisée en fonction de l’épaisseur de peinture et une correction sur le bilan d’énergie prend en compte ce paramètre. Ces mesures sont couplées avec une caméra rapide qui permet de visualiser les régimes d’ébullition et d’obtenir des tailles de bulles à l’aide de la mise en place d’algorithmes de traitement d’image. On représente sur un diagramme flux-température les transferts thermiques lors des différents régimes en stationnaire et en transitoire. Chaque régime d’ébullition, en conditions stationnaire ou transitoire, est alors passé en revue : la convection, le déclenchement de l’ébullition, l’ébullition nucléée, la crise d’ébullition, l’ébullition en film et le remouillage. Les régimes stationnaires sont correctement modélisés par des corrélations usuelles. La convection transitoire est caractérisée sur toute la paroi et son évolution se rapproche de la solution quasistationnaire. Il est montré que les transferts thermiques lors du passage vers l’ébullition nucléée sont dépendants de la formation d’une importante poche de vapeur qui se propage sur la paroi. Une étude locale de cette propagation est alors nécessaire. Afin de simuler des transitoires de température durant l’ébullition nucléée, un système d’asservissement de type P.I.D. permet d’imposer des créneaux ou des rampes de températures (de 5 à 500 K.s 1 ). Les résultats en ébullition nucléée sont conformes avec ceux de la littérature, tant en conditions stationnaire que transitoire. L’expérience permet d’étudier le transfert de chaleur lorsqu’un film de vapeur se forme et isole la paroi. Ce régime d’ébullition en film, pendant la chauffe ou le refroidissement de la paroi peut ainsi être stabilisée pendant plusieurs secondes avec ce système. On caractérise ainsi les conditions de déclenchement de l’ébullition en film, la dynamique de sa propagation et les transferts une fois établi. Enfin, l’implémentation des caractéristiques physiques de notre expérience dans le code SCANAIR de l’IRSN, permet de commencer à calculer et comparer nos résultats expérimentaux avec les simulations numériques. Des calculs de conduction instationnaire sont notamment considérés en imposant la température mesurée pour analyser nos résultats lors du régime de convection et après le déclenchement de l’ébullition
The study of rapid transient boiling is an important issue in the nuclear safety. Such a phenomenon may occur in the case of a RIA (Reactivity Initiated Accident) in the core of a nuclear reactor powerplant, where a power excursion can trigger the formation of a vapour film around the fuel rod, leading to an important rise of the rod temperature and a risk of failure. Some studies in reactor conditions provided transient boiling curves but the modeling lacks of reliability. In collaboration with the IRSN (Institut de Radioprotection et de Sûreté Nucléaire), an experiment model was built at the Institute of Fluid Mechanics of Toulouse. It generates the flow of a refrigerant, HFE7000, in a semi-annular section channel, whose inner wall is made of a metal foil rapidly heated by Joule effect, simulating the heating of a fuel rod. Infrared thermography is used to measure the temperature of the metal foil, painted with a black paint to increase its emissivity, causing also an increase of the wall thermal resistance. The measurement accuracy of the interest temperature has been optimized according to the paint thickness and a correction on the energy balance takes account this parameter. These measurements are coupled with a high-speed camera that allows visualizing the boiling regimes and get bubble sizes using image processing algorithms. On a flux-temperature diagram, the heat transfers are represented both for steady and transient regimes. Each boiling regime is then reviewed : convection, onset of nucleate boiling, nucleate boiling, boiling crisis, film boiling and rewetting. Steady regimes are correctly modeled by usual correlations. Transient convection is characterized over the whole wall and its evolution is closed to the quasi-steady solution. It is shown that heat transfer during the transition to nucleate boiling are strongly related to the formation of a large vapor phase that spreads on the wall. A local study of this propagation is then necessary. In order to simulate and control transient temperature during nucleate boiling, a P.I.D. is implemented to impose a steady or ramps temperature (from 5 to 500 K.s 1 ). The results in nucleate boiling make it possible to recover the results of the literature in both steady and transient conditions. The experiment allows to study the heat transfer when a vapor film is formed and insulates the wall. The film boiling regime during heating or the cooling of the wall can thus be stabilized for several seconds with this system. The conditions for triggering of film boiling are thus characterized, as its spread dynamic and its transfers once established. Finally, the implementation of the physical characteristics of our experience in IRSN’s SCANAIR code allows us to begin to calculate and compare our experimental results with numerical simulations. Unsteady conduction calculations are applied to the measured temperature to analyze our results during the convection regime and after the onset of boiling

APA, Harvard, Vancouver, ISO, and other styles

31

Meyer, Ann-Carolin. "Physiologische Untersuchungen am Stamm und im Kronenraum eines Fichtenaltbestandes nach experimenteller Manipulation des Wasser- und Ionenhaushaltes (Dachprojekt Solling)." Doctoral thesis, [S.l. : s.n.], 2001. http://webdoc.sub.gwdg.de/diss/2002/meyer/meyer.pdf.

Full text

APA, Harvard, Vancouver, ISO, and other styles

32

Shatadal, Pankaj. "Thin-layer rewetting rates of canola (Brassica campestris L.)." 2009. http://hdl.handle.net/1993/3603.

Full text

Abstract:

Thin-layer rewetting tests were conducted for seeds of canola (Brassica campestris L.) in the temperature range of 7.5*C to 30.0*C at two relative humidities (80 and 90%) using a thin-layer wetting unit. Separate tests were done at 5.0, 7.0 and 10.0% initial moisture contents of seeds and 0.10, 0.25, and 0.43 m/s air velocities while the air was maintained at 30*C and 90% relative humidity. In the tests, a thin-layer (one to two kernels thick) of canola was held in the vertical plane and the conditioned air was passed through the layer, thus fully exposing the thin-layer of canola to the air. The gain in the mass of the thin-layer of canola with time was recorded using a micro-computer-based data acquisition system. The liquid diffusion equation for an isotropic and homogeneous sphere did not describe the rewetting rate of canola satisfactorily. The thin-layer rewetting rate data agreed well with Page's equation. The parameter n in Page's equation was assumed as a product-dependent constant which made it easy to compare the effects of independent variables on the rewetting rate without causing considerable error in predicting the rewetting rate for canola. A linear relationship was found between the parameter k, temperature and relative humidity. The initial moisture content, in the range from 5 to 10% wet mass basis, had no significant effect (p > 0.05) on the rewetting rate. The rewetting rate did not change significantly with air velocity in the range from 0.25 to 0.43 m/s (p > 0.05). The rewetting rate, however, was slower at 0.10 m/s air velocity. The thin-layer wetting model developed in this study will be of importance in predicting seed moisture changes during near-ambient air drying of bulks of canola.

APA, Harvard, Vancouver, ISO, and other styles

33

Lin, Wei-Jhih, and 林威志. "The Application of Self-Rewetting Fluids on Loop Heat Pipe." Thesis, 2013. http://ndltd.ncl.edu.tw/handle/95945667975312908438.

Full text

Abstract:

碩士
國立臺灣大學
機械工程學研究所
101
The objective of this study is the application of self-rewetting fluid as the working fluid on loop heat pipe (LHP), with sintered copper as the chosen capillary structure material; this study also investigates the effect of using different components and concentrations of self-rewetting fluid as well as compares their heat transfer　performances with that of water. Previous studies have show that using self-rewetting fluid as working fluid can enhance the heat transfer mechanisms of pool boiling,traditional heat pipe, and wickless heat pipe. Compared with using pure substance as working fluid, where the surface tension decreases linearly with increasing temperature, self-rewetting fluid’s surface tension has a non-linear relationship with temperature changes; therefore, at a certain temperature, the self-rewetting fluid’s surface tension increases with increasing temperature, resulting in the Marangoni effect, and the condensed liquid can be transported to the heating surface, delaying the occurrence of dryout and thus increasing the critical heat load. Concerning the effect of varying the concentration of butanol aqueous solution on heat transfer performance of LHP, concentrations ranging from 2% to 8% is investigated. Experimental results show that 6% butanol aqueous solution results in the the best heat transfer performance of LHP; compared with that of water, the critical heat load is increased by 130% and the total thermal resistance is decreased on average by 50%. Concerning the effect of changing the components of self-rewetting working fluid, the fluids considered are butanol, pentanol, hexanol, and heptanol, with the concentration of each as the maximum solubility concentration in water under standard conditions. Experimental results show that, compared with using water as working fluid, using self-rewetting fluid can allow the total thermal resistance of LHP system to decrease, increasing the critical heat load. Concerning the heat transfer performance of different self-rewetting fluids, under operating temperature of 90°C or lower, hexanol aqueous solution achieves the largest heat load of 250W and lowest total thermal resistance of 0.33°C/W; at operating temperatures higher than 90°C, hexanol aqueous solution has already reach the critical heat load, causing the system to be unstable, but butanol aqueous solution achieves the best results, with maximum critical heat load of 350W and minimum total thermal resistance of 0.32°C/W. Therefore, after analysis of the heat transfer performance of various self-rewetting fluids, butanol water solution has the largest operating temperature range, highest critical heat load, and lowest total thermal resistance, indicating that butanol water solution is most effective on heat transfer performance of LHP.

APA, Harvard, Vancouver, ISO, and other styles

34

Xie, Stella Xiao Min. "Roughening due to inkjet rewetting: Effect of paper treatment and composition." 2006. http://link.library.utoronto.ca/eir/EIRdetail.cfm?Resources__ID=442004&T=F.

Full text

APA, Harvard, Vancouver, ISO, and other styles

35

Chu, Mao-Long, and 朱茂榕. "Heat Transfer Enhancement of Loop Heat Pipe with Self-Rewetting fluid." Thesis, 2014. http://ndltd.ncl.edu.tw/handle/62422980327924299494.

Full text

Abstract:

碩士
國立臺灣大學
機械工程學研究所
102
The objective of this study is the application of self-rewetting fluid as the working fluid on loop heat pipe (LHP), with sintered copper as the chosen capillary structure material; this study also investigates the effect of using different contents and concentrations of self-rewetting fluid on heat transfer performances of LHP as well as compares the results with those from using water as working fluid. Previous studies have shown that using self-rewetting fluid as working fluid can enhance the heat transfer mechanisms of pool boiling, traditional heat pipes, and wickless heat pipes. Compared to using pure substance as working fluid, where the surface tension decreases linearly with increasing temperature, self-rewetting fluid’s surface tension has a non-linear relationship with temperature changes; therefore, at a certain temperature, the self-rewetting fluid’s surface tension increases with increasing temperature, resulting in the Marangoni effect, and the condensed liquid can be transported to the heating surface, delaying the occurrence of dry out and thus increasing the critical heat load. Concerning the effect of varying the concentration of butanol and petanol aqueous solutions on heat transfer performance of LHP, butanol concentrations ranging from 2% to 8% is investigated, and pentanol concentrations ranging from 1% to 3% is investigated. Experimental results show that 6% butanol aqueous solution results in the best heat transfer performance of LHP; compared with that of water, the critical heat load is increased by 100% and the total thermal resistance is decreased on average by 30%. Concerning the effect of changing the components of self-rewetting working fluid, the fluids considered are butanol, pentanol, hexanol, with the concentration of each being the maximum solubility concentration in water under standard conditions. Experimental results show that, compared with those from using water as working fluid, using self-rewetting fluid can allow the total thermal resistance of LHP system to decrease, increasing the critical heat load. Concerning the heat transfer performance of different self-rewetting fluids, under operating temperature of 90°C or lower, hexanol aqueous solution achieves the largest heat load of 200W and lowest total thermal resistance of 0.33°C/W; at operating temperatures higher than 90°C, hexanol aqueous solution has already reach the critical heat load, causing the system to be unstable, but butanol aqueous solution achieves the best results, with maximum critical heat load of 500W and minimum total thermal resistance of 0.26°C/W. Therefore, after analysis of the heat transfer performance of various self-rewetting fluids, butanol water solution has the largest operating temperature range, highest critical heat load, and lowest total thermal resistance, indicating that butanol aqueous solution is the most effective in enhancing the heat transfer performance of LHP.

APA, Harvard, Vancouver, ISO, and other styles

36

Lin, Wei-Jhih, and 林煒智. "Study of Self-Rewetting Fluid Applied to PTFE Wick of Loop Heat Pipe." Thesis, 2015. http://ndltd.ncl.edu.tw/handle/23665625930080743052.

Full text

Abstract:

碩士
中華科技大學
航空機械系飛機系統工程碩士班
103
This study applied butanol aqueous solution to form self-rewetting fluid to LHP with a PTFE wick. Porous PTFE structure advantages including low sintering temperature and resistant to oxidation; however, due to PTFE’s hydrophobic nature, with water as working fluid, for example, it has been observed that PTFE cannot effectively pump the working fluid by capillary action. In this paper, self-rewetting fluid (butanol aqueous solution) was used to address the hydrophobic nature of PTFE, allowing the working fluid to successfully flow through the PTFE wick by capillary forces. 0 ~ 7wt% butanol aqueous solutions and 1~2.75mm wick thickness were tested for effectiveness and ease in allowing the working fluid to flow through the wick and LHP heat transfer performance test. Experimental results showed that, with 6wt% butanol aqueous solution and 2mm wick thickness, heat transfer performance is 550W, the minimal total thermal resistance is approximately 0.273℃/W, and the heat flux is 28W/cm2. Dimensionless performance figure of loop heat pipe, and can be used as reference for future wick designs.

APA, Harvard, Vancouver, ISO, and other styles

37

Butterly, Clayton Robert. "Drying/rewetting cycles in southern Australian agricultural soils: effects on turnover of soil phosphorus, carbon and the microbial biomass." 2008. http://hdl.handle.net/2440/48167.

Full text

Abstract:

Phosphorus (P) limitations to agricultural productivity commonly occur in Australian soils and have largely been overcome by the use of inorganic fertilisers. However, studies have shown that most of the P taken up by plants is from native P pools. The turnover of P and native soil organic matter may be strongly affected by drying and rewetting (DRW). Rewetting dry soil results in a pulse of respiration activity and available nutrients. In Mediterranean-type climates surface soils naturally undergo recurrent DRW cycles. In southern Australia, soils experience DRW due to erratic rainfall within the growing season, and short, high intensity thunderstorms also during summer periods. The principal objective of this thesis was to determine the significance of dry-rewet events, for altering P availability and cycling in agricultural soils in Australia. Soils representing a wide range of soil types and climatic zones of southern Australia, showed large flushes in carbon (C) mineralisation after a single DRW event. For some soils these were comparable with reported values, however large variability in flush size between soils was observed. Soils that commonly experience DRW did not appear to be more resilient to DRW than soils from areas with fewer DRW events. Even when soils had relatively small respiration flushes, as a result of low soil organic matter, a high proportion of the soil C was mineralised after rewetting. Soil physiochemical properties (total C, total N, organic C, humus, microbial biomass P, organic P, sand and silt) were correlated to the size of the flush, hence nutrient availability and soil texture appear to primarily determine flush size. Therefore, the influence of climate on DRW may relate to determining the quantity of organic matter and microbial biomass that is available for turnover. Different size and composition of the microbial biomass within the same soil matrix were achieved by adding three different C substrates (glucose, starch and cellulose at 2.5 g kg-¹) at 5 times over 25 weeks. The treatments showed disparate responses to DRW, due to greater biomass (larger flushes) and effects of community composition, highlighting the central role of the soil microbes in DRW processes. When subjected to multiple DRW events these soils showed smaller rewetting respiration flushes with subsequent rewetting events. In contrast, the amount of P released after rewetting was the same. This study showed that increases in P after rewetting were transient and rapid immobilisation of P by microbes occurred, which may limit the availability to plants. The composition of the microbial community was changed by DRW with a reduction in fungi and gram negative bacteria, showing that certain species are more susceptible to DRW than others. Closer investigation at 2 hourly intervals after rewetting confirmed the transient nature of P flushes. The response in microbial respiration after rewetting was immediate, with the highest activity occurring within the first 2 h. Phosphorus availability was increased by DRW but remained stable over the following 48 h incubation period. The study highlights the rapid nature of changes in available nutrients after rewetting. Furthermore, while potentially only a small component of the P flush that occurred, the DRW soil had higher levels of P than most incubated soil at 48 h, this would be potentially available for plant uptake or movement with the soil solution. Long-term water regimes (continuously moist or air-dry, or DRW occurring at different times during incubation) that were imposed on two soils from different climatic regions over a 14 wk period, did not alter available nutrient (P and C) pools or the size of the microbial biomass. However, these long-term water regimes determined the respiration response of the soils to experimental DRW. The largest flushes occurred in the treatment with the longest dry period, and confirm findings of reported studies that the response of a soil at rewetting is determined by the length of the period that it is dried. Microbial biomass was little affected by experimental DRW, but showed large changes in C:P ratio. Thus, changes in physiological state or community composition may be more affected by DRW than the size of the microbial biomass. Microbial communities were altered by DRW irrespective of climatic history (warm wet summer and temperate Mediterranean), however these changes were not related to specific groups of organisms. In addition, the disparate respiration responses and inhibition of phosphatase by DRW, indicate that functional changes may be induced by DRW but can not be sufficiently explained by quantifying available nutrient pools or the microbial biomass. The use of wheat seedlings bio-indicators of P availability after the long-term water regimes, confirmed that plant available P was altered by DRW, indicated by differences in growth, although the large variability in seedling growth made it difficult to quantify these differences. However, the distribution of labile P, available at planting, in soil and plant pools at harvest, showed that long-term water regimes increased P allocation in plant tissue in one soil and decreased it in another. Furthermore, only a small fraction of the labile P present at planting was taken up by plants, which confirms the superior ability of soil microbes to immobilise P that is released by DRW. Nevertheless, since the long-term water regimes increased P availability, this may be transported via surface water or leaching. DRW is important for C and P turnover in soils of southern Australia. However, P flushes occur rapidly after rewetting and are transient. Therefore, DRW appears to have only minor consequences for P availability to plants.
http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1321018
Thesis (Ph.D.) -- University of Adelaide, School of Earth and Environmental Sciences, 2008

APA, Harvard, Vancouver, ISO, and other styles

38

Asota, Celestine Ngozi. "Effects of repeated rewetting and drying on breakage and moisture desorption characteristics of soybeans." 1987. http://catalog.hathitrust.org/api/volumes/oclc/17860661.html.

Full text

Abstract:

Thesis (Ph. D.)--University of Wisconsin--Madison, 1987.
Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 220-245).

APA, Harvard, Vancouver, ISO, and other styles

39

Chia-Hsuan, Lin, and 林家璿. "On the performance analysis of using self-rewetting fluid in intermittent spray cooling system." Thesis, 2018. http://ndltd.ncl.edu.tw/handle/kygu89.

Full text

Abstract:

碩士
國立臺灣大學
機械工程學研究所
106
In this study, we utilize the self-rewetting fluid in an intermittent spray system and study the effects of pulsed duration and duty cycle on the heat transfer mechanism and the cooling capacity. Compared to water, using the self-rewetting fluid can successfully induced inverse Marangoni convection and proven to prolong the cooling time within a cycle. This leads to a lower fluid consumption because comparable heat transfer rate can be delivered by spraying the self-rewetting fluid at much lower frequency. For a given duty cycle, increasing the pulsed duration leads to a higher heat transfer rate per period. The working fluid left on the heated surface is so abundant that the inverse Marangoni convection is sustained. However, longer spray pulse decreases the spray frequency and is unfavorable when a given time span is consider. Although a shorter pulsed duration only shows moderate cooling capacity per cycle, the time-averaged heat transfer rate is improved by the increase in spray frequency. On the other hand, reducing the duty cycle also augments the heat transfer rate per period when the pulsed duration is fixed. As the spray frequency decreases, the surface temperature increases, so that a stronger inverse Marangoni convection is induced. Nevertheless, dry-out may occur if the duty cycle becomes too small. As a result, we find that a pulsed duration of 1.5 s with a 7.5% duty cycle produces the best cooling effect and with the highest spray cooling efficiency and the lowest fluid consumption.

APA, Harvard, Vancouver, ISO, and other styles

40

Hour, Bor Kae, and 侯博凱. "A Study on the Thermal-Fluid Phenomenoa of Rewetting along a F uel Rod." Thesis, 1995. http://ndltd.ncl.edu.tw/handle/31438893204012959644.

Full text

APA, Harvard, Vancouver, ISO, and other styles

41

Chou, Ta-Li, and 周大立. "Heat Transfer Enhancement of Loop Heat Pipe with Metal and Polymer Capillary Structure using Self-Rewetting Fluid." Thesis, 2015. http://ndltd.ncl.edu.tw/handle/23095570107961268676.

Full text

Abstract:

碩士
國立臺灣大學
機械工程學研究所
104
The objective of this study is the application of self-rewetting fluid as the working fluid on loop heat pipe (LHP), with nickel and teflon(PTFE) as the capillary structure materials respectively. Self-rewetting fluids, generally the dilute aqueous solutions of high carbon alcohols(number of carbon atoms ≥ 4), shows not only better wettability compared with pure water but also a special thermophysical effect that the surface tension increases with increasing temperature. Since the Marangoni effect induces a flow from liquid with lower surface tension to the liquid with higher surface tension, which results a liquid flow to hotter interface, it is advantageous to use self-rewetting fluid as working fluid on heat-transfer devices. In this study, we measure the surface tension of different self-rewetting fluids at different concentration in the temperature range 25~70 oC, and the working fluids of 1-butanol, 1-pentanol, 1-hexanol and 2-butanol(isomer of 1-butanol) are choosed. The result shows that self-rewetting fluids possess higher positive surface tension gradient("σ" _"T" ) at saturation. Furthermore, the 1-butanol (6%) and 2-butanol (20%) show higher "σ" _"T" than others, which are 0.27 mN/moC and 0.30 mN/moC respectively. These two self-rewetting fluids are then applied to the LHP with nickel capillary structure. The maximum heat load are 500W and 600W relatively when the working fluid is 1-butanol (6%) and 2-butanol (20%) , which was increased by 50 % and 70 % compared with using water as working fluid, and the total thermal resistence decreased from 0.31 oC/W to 0.22 oC/W. Due to the hydrophobicity of PTFE, water can’t be used on PTFE capillary structure. This study applied the self-rewetting fluids, which possess better wettability than water, on LHP with PTFE capillary structure. The results show that when the working fluid is 2-butanol (20%), the LHP system successfully work and the maximun heat load can reach 250W with total thermal resistence 0.54 oC/W, and the maximum heat flux is 12.9 W/cm2. Compared with results in other literatures, the maximum heat flux in this study is higher by 3 to 5 times other reference.

APA, Harvard, Vancouver, ISO, and other styles

42

YEN, SHIH-HSUAN, and 顏士軒. "Study of Self-Rewetting Fluid Applied to PTFE Wick structure of Loop Heat Pipe with flat evaporator." Thesis, 2016. http://ndltd.ncl.edu.tw/handle/00634478782462936917.

Full text

Abstract:

碩士
中華科技大學
飛機系統工程研究所
104
This study investigated the application of self-rewetting ﬂuid to Nickel and PTFE wick structure of Loop Heat Pipe with flat evaporators (FLHP) and its effect on the heat transfer performance of FLHP. PTFE wick structure can prevent FLHP heat leakage problem, however, PTFE material with strong hydrophobic nature, cannot use the water as the working fluid. Therefore, this study attempts to use Self-rewetting fluid's good wettability to overcome the PTFE wick's hydrophobicity. At a certain temperature, a self-rewetting ﬂuid, contrary to common newtonian ﬂuids, has the ability to reverse its surface tension's trend, inducing colder ﬂuid to ﬂow to the heated surface and delaying the occurrence of dry-out in FLHP during operation. Thus, this study focused on a few self-rewetting ﬂuids; the surface tensions of these ﬂuids were measured, and then it was applied to FLHP as working ﬂuid to investigate its effect on the heat transfer performance. Heat transfer performance test results shows that DI water and 1-Butanol;1-Heptanol aqueous solution in nickel wick’s FLHP, the maximum heat load can be up to 280W,Compared to DI water's 140W can improve heat transfer performance about 70 to 100% . DI water and 1-Butanol;1-Heptanol aqueous solution in PTFE wick's FLHP, the maximum heat load can be up to 240W,Compared to DI water's 40W can improve heat transfer performance about 300 to 500% .

APA, Harvard, Vancouver, ISO, and other styles

43

Hung, Chia-Yin, and 洪嘉吟. "Exploring the cooling capability of self-rewetting fluid deposited on the cold end of a copper strip with horizontal temperature gradient." Thesis, 2019. http://ndltd.ncl.edu.tw/handle/9xmnz8.

Full text

Abstract:

碩士
國立臺灣大學
機械工程學研究所
107
This study aims at applying the inverse Marangoni convection to cooling augmentation of a super-hydrophilic copper strip with a horizontal temperature gradient. The self-rewetting fluid used herein is the 0.2%wt 1-heptanol dilute aqueous solution and there are two different supplied quantity: 1 ml and 1.5 ml. The horizontal temperature gradient is varied by adjusting the inlet temperature of coolant and the initial temperature of the hot end. We find that the inverse Marangoni convection is able to cool the hot end effectively and result in higher accumulated cooling amount comparing to water. For a given initial temperature of the copper strip in the hot end, warmer coolant at the results in lower inlet temperature gradient and therefore weaker inverse Marangoni convection. When temperature gradient exceeds 3°C cm-1 on the other hand, applying less working fluid produces a higher temperature gradient, therefore, the stronger inverse Marangoni convection and the higher accumulated cooling amount. However, a temperature gradient less than 3°C cm-1 leads to very weak inverse Marangoni convection, and the low temperature of copper strip in the hot end makes the change in sensible heat of fluid dominate the heat transfer mechanism. As the result, 1.5 ml of working fluid produces a higher accumulated cooling amount comparing to that of 1 ml. In addition, dryout can be effectively delayed by strong inverse Marangoni convection when 1 ml of the self-rewetting fluid is applied with a large temperature gradient in horizontal direction.

APA, Harvard, Vancouver, ISO, and other styles

44

Ketcheson, Scott James. "An evaluation of the water balance and moisture dynamics within Sphagnum mosses following the restoration (rewetting) of an abandoned block-cut bog." Thesis, 2011. http://hdl.handle.net/10012/6328.

Full text

Abstract:

Artificial drainage networks established throughout peatlands during the peat extraction process often remain active following abandonment, maintaining a water table relatively far from the surface of the peat and hindering the survival and reestablishment of Sphagnum mosses. Since cutover peatlands are characterized by low (negative) soil water pressures, sufficient internal water storage and balanced water fluxes are critical for the physiological function of spontaneously regenerated Sphagnum mosses. The relative importance of water exchanges between spontaneously regenerated Sphagnum moss cushions and their surroundings are addressed through investigation of the sensitivity of moss moisture dynamics to a range of environmental variables. Precipitation waters are poorly retained within the cushions, which indicated that rain event water can only be relied upon by the mosses for a short period of time. An imbalance between water inputs and losses from moss cushions identified that additional (small) sources of water, such as dewfall and distillation, are potentially important for physiological processes under dry conditions, common in disturbed peatland ecosystems. As an initial restoration effort, rewetting of the peatland by blocking drainage ditches consequently reduced the runoff efficiency and caused the site-average water table to rise by 32 cm. Higher water tables and a blocked drainage network created conditions more favourable for Sphagnum survival through increasing the moisture content and soil-water pressures within the remnant peat deposit. The hydrologic connectivity between moss cushions and the remnant peat was strong when conditions were wet and the water table was within 30 cm of the surface of the cutover peat but weakened as conditions became drier, as reflected by weakened upward hydraulic gradients in the unsaturated zone below the moss cushions. Runoff variability increased following rewetting, and displayed a greater dependence upon antecedent conditions (capacity to retain additional water on-site) and event-based precipitation dynamics. Evapotranspiration rates were 25% higher following rewetting (3.6 mm day-1) compared to pre-restoration ET rates of 2.7 mm day-1. Total storage changes were restricted following rewetting, as a factor of the reduced runoff losses limiting water table drawdown, thereby constraining peat compression and preventing undue drying of the unsaturated zone. Changes to the system hydrology following rewetting of the peatland by blocking drainage ditches created conditions more favourable for Sphagnum survival through increasing the moisture content and soil-water pressures within the remnant peat deposit; although restoration efforts should aim to constrain water table fluctuations to within the upper 30 cm.

APA, Harvard, Vancouver, ISO, and other styles

45

Malloy, Shannon. "Fen restoration on a bog cut down to sedge peat: A hydrological assessment of rewetting and the impact of a subsurface gyttja layer." Thesis, 2013. http://hdl.handle.net/10012/7355.

Full text

Abstract:

Vacuum harvested peatlands do not easily regain their hydrological function after peat production therefore restoration is required to create hydrological conditions appropriate for the growth of peat producing fen plant species. Peat extraction at Bic-Saint-Fabien peatland by the block-cut method began in 1946, converted to vacuum harvesting in the early 1970’s and ceased in 2000. After exploitation, about 0.04-1 m of residual minerotrophic sedge peat remained; therefore Bic-Saint-Fabien was restored as a fen. Research occurred in 2008-2011 and restoration began fall 2009 so we have two pre-restoration and two post-restoration study years. Rewetting consisted of blocking active drainage ditches, contouring the peat surface into level terraces to even out elevation differences, and building peat ridges (bunds) to retain runoff. Vegetation was introduced to some recontoured parts of the site by the surface layer transfer method. A straw mulch treatment was applied to minimize evapotranspiration. A water budget was created for the harvested cutover area and an adjacent undisturbed section of Bic-Saint- Fabien for all study years. Data collection for the water budget occurred June 2-August 7 (day of year 153-219). Water table, volumetric soil moisture content and soil-water pressure were also examined to better understand the impact of reconfiguration on the hydrology of the system. A 1-1.5 m thick layer of gyttja (a low permeability, high porosity lake-bed sediment) underlies the residual peat; it was parameterized and assessed to see if it could potentially compress and supply water to the overlying peat when the system is stressed. In 2011, the volumetric moisture content and thickness of gyttja were monitored in the field to estimate the strain placed on the gyttja by seasonal water table variability. Gyttja samples were collected and brought back to the laboratory for parameterization and compressibility tests. A water budget was completed annually for the cutover and undisturbed areas of the peatland. There were no distinguishable differences between study years except that 2008 and 2009 were climatologically wetter. During the water budget period surface and groundwater run-on into the cutover area were negligible making precipitation the principle water input. The dominant water loss from the cutover area was evapotranspiration since runoff was only 2 and 9 mm in 2010 and 2011, respectively. Rewetting did not result in a uniform wetness across the cutover site chiefly due to local differences in peat surface elevation. An interior section of Bic- Saint-Fabien remained saturated for nearly all of 2011; it had mean seasonal water table of +2.8 cm, and volumetric soil moisture content and soil-water pressure, 5 cm below the peat surface were 86% and +4 mbar. At a peripheral section (~100 m away) the values were -14.4 cm, 67% and -13 mbar, respectively. While the interior was generally wetter than the peripheral regions, there were some exceptions, notably near where dams were installed on peripheral drainage ditches. The markedly different spatial patterns of wetness suggests that a uniform prescription regarding vegetation re-establishment in the rewetted section may not be warranted. The bulk density, particle density and porosity of gyttja averaged 0.12 g cm-3 1.57g cm-3 and 92%, in the top 40 cm of the layer. The organic matter content of gyttja decreased with depth from about 70% at a 5 cm depth to 45% at a 45 cm depth. Laboratory compression tests showed 9 and 72% strain at effective stresses of 3.5 and 200 kPa, respectively, demonstrating the potential for releasing water upon compression, which in the field is caused by water table lowering. From day of year 192 to 202 in 2011, when a ~8 cm water level change occurred (effective stress range ~0.8 kPa), the volumetric moisture content and thickness of the top 30 cm of gyttja decreased by 0.4%, and 0.5 cm, respectively, representing 0.1 and 1.7% strain, respectively, as determined from these two different approaches. The compression of gyttja after Bic-Saint-Fabien was rewetted (2011) was small but might have been significant under drier conditions with greater water table variability, such as during the early stages of site drainage when it was being prepared for peat extraction or after peat production ceased. The release of water to the peat layer from the compression of gyttja after peat production finished and before rewetting occurred, might have been an important self-preservation mechanism, eventually making it easier to rewet. Water table drawdown in 2011 produced very small strain rates suggesting gyttja compression in this year had no important role; hence rewetting success was more reliant on other rewetting techniques implemented at this site.

APA, Harvard, Vancouver, ISO, and other styles

46

Kozub, Łukasz. "Wpływ restytucji torfowiska niskiego metodą usuwania warstwy murszu na warunki siedliskowe, produktywność i bilans gazów cieplarnianych." Doctoral thesis, 2016. https://depotuw.ceon.pl/handle/item/1819.

Full text

Abstract:

Streszczenie Naturalne, niezaburzone antropogenicznie torfowiska pełnią niezwykle istotne funkcje. Są to, jedne z największych globalnych magazynów węgla, a w skali zlewni stanowią ważne miejsce retencji pierwiastków biogennych. Typowe dla torfowisk stałe wysokie uwodnienie oraz wycofywanie z obiegu pierwiastków biogennych wywołują w zbiorowisku roślinnym stresy anoksji oraz niskiej żyzności. Czynniki te ograniczają produkcję pierwotną i zwiększają szanse przetrwana na torfowiskach gatunków roślin słabych konkurencyjnie, często zagrożonych. Równocześnie torfowiska należą dziś do ekosystemów najszybciej degradujących się za sprawą działalności człowieka. W wyniku osuszenia powierzchniowe warstwy torfu ulegają mineralizacji, co powoduje emisję węgla do atmosfery oraz eutrofizację wód powierzchniowych. Ponadto na skutek zaniku czynników stresowych silnie konkurencyjne gatunki wypierają słabsze gatunki wyspecjalizowane. Aby ograniczyć negatywne skutki degradacji torfowisk rozpoczęto realizację projektów restytucji przyrodniczej tych ekosystemów. Początkowo metodą stosowaną w tych zabiegach było podniesienie poziomu wody poprzez zablokowanie rowów odwadniających. Metodę tą krytykowano jednak za jej negatywny wpływ na efekt cieplarniany (okresowe znaczne zwiększanie emisji metanu) oraz z powodu wzrostu żyzności (następującego równocześnie ze zwiększeniem uwilgotnienia), który uniemożliwia odtworzenie populacji rzadkich gatunków torfowiskowych. Drugą rzadziej stosowaną w restytucji torfowisk niskich metodą jest usuwanie wierzchniej, zdegradowanej warstwy torfu. Skuteczność tej metody w odtwarzaniu zbiorowisk roślinnych typowych dla torfowisk niskich została dobrze udokumentowana. Jednak do dziś nie wiadomo, jak metoda ta wpływa na relatywną dostępność poszczególnych pierwiastków biogennych, co może mieć duże znaczenie dla rozwoju fitocenoz. Co więcej, wiedza o wpływie tej metody na odtworzenie usług ekosystemowych torfowisk, takich jak wpływ na bilans gazów cieplarnianych czy obieg pierwiastków w zlewni, jest skąpa i oparta na pośrednim wnioskowaniu. Celem niniejszej rozprawy doktorskiej była ocena skuteczności restytucji torfowiska niskiego metodą usuwania zdegradowanej warstwy gleby w zakresie jej wpływu na: (1) warunki wzrostu roślin; (2) przywrócenie usług ekosystemowych związanych z obiegiem pierwiastków; (3) efekt cieplarniany, oraz porównanie tej metody z metodą powtórnego nawodnienia zdegradowanego torfowiska. Terenem badań było torfowisko Bagno Całowanie, na którym realizowany był projekt restytucji przyrodniczej, obejmujący m. in. usunięcie warstwy zdegradowanego torfu z powierzchni ok. 2 ha. Badania prowadziłem na tym obszarze oraz na zlokalizowanych wokół niego powierzchniach kontrolnych (zdegradowane, przesuszone partie torfowiska), referencyjnych (refugia roślinności nawiązującej do mechowisk zlokalizowane w obszarach dawnego wydobycia torfu) oraz poletkach eksperymentalnych, na których imitowano restytucję metodą powtórnego nawodnienia. Uzyskane przeze mnie wyniki wskazują, że metoda usuwania warstwy zdegradowanego torfu, w odróżnieniu od metody powtórnego nawodnienia zdegradowanego torfowiska, pozwala ograniczyć stężenia pierwiastków biogennych w roztworze glebowym oraz nie wywołuje zwiększenia emisji metanu. Z drugiej strony okazało się, że zastosowanie tej metody nie doprowadziło do odtworzenia warunków siedliskowych charakteryzujących się zasobnością w kationy metali zbliżoną do ekosystemów referencyjnych. Ponadto, względna dostępność fosforu w stosunku do azotu nie została ograniczona w stopniu wystarczającym, by odtworzyć warunki, w których produkcja pierwotna byłaby limitowana niedoborem tego pierwiastka (co sprzyjałoby zagrożonym gatunkom roślin). Ponadto, gdy w analizie wpływu na efekt cieplarniany uwzględniłem także efekt mineralizacji usuniętej zdegradowanej warstwy torfu, metoda usuwania warstwy zdegradowanej okazała się mniej korzystna dla klimatu (wskutek emisji dwutlenku węgla do atmosfery) niż metoda powtórnego nawodnienia. Pomimo wykazanych w pracy wad usuwania warstwy zdegradowanego torfu, ta metoda może zostać uznana za bardziej efektywną pod względem odtwarzania usług ekosystemowych oraz warunków występowania słabych konkurencyjnie gatunków roślin od powtórnego nawodnienia w ogólnym porównaniu obydwu metod. Ograniczenie stężeń azotanów i fosforanów sprzyja odtworzeniu niskoproduktywnych ekosystemów torfowiskowych. Ponadto w następstwie restytucji ograniczony zostaje ładunek azotu trafiający do zlewni, w której znajduje się torfowisko, a ilość dopływającego do niej fosforu pozostaje bez zmian. Problem wpływu dwutlenku węgla z mineralizującego się, usuniętego zdegradowanego torfu, na efekt cieplarniany może zaś zostać rozwiązany, jeśli torf ten zostanie wykorzystany w rolnictwie, leśnictwie lub ogrodnictwie jako substytut torfu wydobytego z niezaburzonych hydrologicznie torfowisk. W ten sposób emisje gazów cieplarnianych wywołane przez działania restytucji zostaną skompensowane przez ograniczenie emisji na innym obszarze.

APA, Harvard, Vancouver, ISO, and other styles

47

Knorr, Klaus-Holger [Verfasser]. "Dynamics of soil processes under extreme meteorological boundary conditions : response of below-ground carbon, sulfur, and iron cycling in fen soils ; effects of experimental drought and subsequent rewetting on internal carbon, sulfur, iron, and arsenic turnover in a soil from a northern temperate fen / vorgelegt von Klaus-Holger Knorr." 2009. http://d-nb.info/992484421/34.

Full text

APA, Harvard, Vancouver, ISO, and other styles

Dissertations / Theses on the topic 'Rewetting'

Create a spot-on reference in APA, MLA, Chicago, Harvard, and other styles