Thematische Bibliographien / Evaporative flux

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Auswahl der wissenschaftlichen Literatur zum Thema „Evaporative flux“

Autor: Grafiati

Veröffentlicht am 29. März 2025

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Zeitschriftenartikel zum Thema "Evaporative flux"

Gelderblom, Hanneke, Oscar Bloemen und Jacco H. Snoeijer. „Stokes flow near the contact line of an evaporating drop“. Journal of Fluid Mechanics 709 (31.08.2012): 69–84. http://dx.doi.org/10.1017/jfm.2012.321.

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AbstractThe evaporation of sessile drops in quiescent air is usually governed by vapour diffusion. For contact angles below $9{0}^{\ensuremath{\circ} } $, the evaporative flux from the droplet tends to diverge in the vicinity of the contact line. Therefore, the description of the flow inside an evaporating drop has remained a challenge. Here, we focus on the asymptotic behaviour near the pinned contact line, by analytically solving the Stokes equations in a wedge geometry of arbitrary contact angle. The flow field is described by similarity solutions, with exponents that match the singular boundary condition due to evaporation. We demonstrate that there are three contributions to the flow in a wedge: the evaporative flux, the downward motion of the liquid–air interface and the eigenmode solution which fulfils the homogeneous boundary conditions. Below a critical contact angle of $133. {4}^{\ensuremath{\circ} } $, the evaporative flux solution will dominate, while above this angle the eigenmode solution dominates. We demonstrate that for small contact angles, the velocity field is very accurately described by the lubrication approximation. For larger contact angles, the flow separates into regions where the flow is reversing towards the drop centre.

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Anderson, Bruce T., Alex C. Ruane, John O. Roads und Masao Kanamitsu. „Estimating the Influence of Evaporation and Moisture-Flux Convergence upon Seasonal Precipitation Rates. Part II: An Analysis for North America Based upon the NCEP–DOE Reanalysis II Model“. Journal of Hydrometeorology 10, Nr. 4 (01.08.2009): 893–911. http://dx.doi.org/10.1175/2009jhm1063.1.

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Abstract In this paper, a diagnostic metric—termed the local-convergence ratio—is used to analyze the contribution of evaporation and atmospheric moisture-flux convergence to model-based estimates of climatological precipitation over the North American continent. Generally, the fractional evaporative contribution is largest during spring and summer when evaporation is largest and decreases as evaporation decreases. However, there appears to be at least three regions with distinct spatiotemporal seasonal evolutions of this ratio. Over both the northern and western portions of the continent, the fractional evaporative contribution peaks in spring and early summer and decreases during fall and into winter. Over the northern portion, this fall decrease is related to an increase in atmospheric moisture-flux convergence associated with enhanced meridional moisture fluxes into the region; over the western coastal regions, the fall decrease in evaporative contribution is associated with a decrease in evaporation and an increase in total moisture-flux convergence, most likely associated with increased storm activity. In contrast, over the central portions of the continent, the fractional evaporative contribution to precipitation remains relatively low in spring—when enhanced low-level jet activity increases the low-level atmospheric moisture flux convergence into the region—and instead peaks in summer and fall—when the moisture-flux convergence associated with the low-level jet decreases and precipitation is balanced predominantly by local evaporation. Finally, over the southwestern United States and northwestern Mexico, the fractional evaporative contribution to precipitation is found to contain a wintertime minimum as well as a secondary minimum during summer. This latter feature is due to a substantial increase in low-level atmospheric moisture-flux convergence associated with the large-scale monsoon circulation that influences this region during this time.

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Rassam, Daud W., und David J. Williams. „A numerical study of steady state evaporative conditions applied to mine tailings“. Canadian Geotechnical Journal 36, Nr. 4 (22.11.1999): 640–50. http://dx.doi.org/10.1139/t99-030.

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The suction profile of a desiccating soil is dependent on the water table depth, the soil-water retention characteristics, and the climatic conditions. In this paper, an unsaturated flow model, which simulates both liquid and vapour flow, was used to investigate the effects of varying the water table depth and the evaporation rate on the evaporative fluxes from a desiccating tailings deposit under steady-state conditions. Results obtained showed that at a critical evaporation rate, beyond which evaporation is no longer dictated by climatic conditions, the matric suction profiles remain basically unchanged. The critical evaporation rate varies inversely with the water table depth. It is associated with the maximum evaporative flux that might be extracted from a soil at steady-state conditions. The time required to establish steady-state conditions is directly proportional to the water table depth, and it acquires a maximum value at the critical evaporation rate. A detailed investigation of the movement of the drying front demonstrated the significance of attaining a matric suction of about 3000 kPa on the contribution to flow in the vapour phase.Key words: matric suction, mine tailings, potential evaporation, steady state evaporative conditions, surface evaporative flux.

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Harris, Daniel J., Jacinta C. Conrad und Jennifer A. Lewis. „Evaporative lithographic patterning of binary colloidal films“. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 367, Nr. 1909 (28.12.2009): 5157–65. http://dx.doi.org/10.1098/rsta.2009.0157.

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Evaporative lithography offers a promising new route for patterning a broad array of soft materials. In this approach, a mask is placed above a drying film to create regions of free and hindered evaporation, which drive fluid convection and entrained particles to regions of highest evaporative flux. We show that binary colloidal films exhibit remarkable pattern formation when subjected to a periodic evaporative landscape during drying.

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Anderson, Bruce T., Guido Salvucci, Alex C. Ruane, John O. Roads und Masao Kanamitsu. „A New Metric for Estimating the Influence of Evaporation on Seasonal Precipitation Rates“. Journal of Hydrometeorology 9, Nr. 3 (01.06.2008): 576–88. http://dx.doi.org/10.1175/2007jhm968.1.

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Abstract The objective of this paper is to introduce a diagnostic metric—termed the local-convergence ratio—that can be used to quantify the contribution of evaporation (and transpiration) to the atmospheric hydrologic cycle, and precipitation in particular, over a given region. Previous research into regional moisture (or precipitation) recycling has produced numerous methods for estimating the contributions of “local” (i.e., evaporated) moisture to climatological precipitation and its variations. In general, these metrics quantify the evaporative contribution to the mass of precipitable water within an atmospheric column by comparing the vertically integrated atmospheric fluxes of moisture across a region with the fluxes via evaporation. Here a new metric is proposed, based on the atmospheric moisture tendency equation, which quantifies the evaporative contribution to the rate of precipitation by comparing evaporative convergence into the column with large-scale moisture-flux convergence. Using self-consistent, model-derived estimates of the moisture-flux fields and the atmospheric moisture tendency terms, the authors compare estimates of the flux-based moisture-recycling ratio with the newly introduced local-convergence ratio. Differences between the two ratios indicate that they can be considered complementary, but independent, descriptors of the atmospheric hydroclimatology for a given region.

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Suchan, Jared, und Shahid Azam. „Datasets for the Determination of Evaporative Flux from Distilled Water and Saturated Brine Using Bench-Scale Atmospheric Simulators“. Data 7, Nr. 1 (22.12.2021): 1. http://dx.doi.org/10.3390/data7010001.

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Evaporation from fresh water and saline water is critical for the estimation of water budget in the Canadian Prairies. Predictive models using empirical field-based data are subject to significant errors and uncertainty. Therefore, highly controlled test conditions and accurately measured experimental data are required to understand the relationship between atmospheric variables at water surfaces. This paper provides a comprehensive dataset generated for the determination of evaporative flux from distilled water and saturated brine using the bench-scale atmospheric simulator (BAS) and the subsequently improved design (BAS2). Analyses of the weather scenarios from atmospheric parameters and evaporative flux from the experimental data are provided.

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DasGupta, S., I. Y. Kim und P. C. Wayner. „Use of the Kelvin-Clapeyron Equation to Model an Evaporating Curved Microfilm“. Journal of Heat Transfer 116, Nr. 4 (01.11.1994): 1007–15. http://dx.doi.org/10.1115/1.2911436.

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A Kelvin–Clapeyron change-of-phase heat transfer model is used to evaluate experimental data for an evaporating meniscus. The details of the evaporating process near the contact line are obtained. The heat flux and the heat transfer coefficient are a function of the film thickness profile, which is a measure of both the intermolecular stress field in the contact line region and the resistance to conduction. The results indicate that a stationary meniscus with a high evaporative flux is possible. At equilibrium, the augmented Young–Laplace equation accurately predicts the meniscus slope. The interfacial slope is a function of the heat flux.

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Panwar, Annu, Maik Renner und Axel Kleidon. „Imprints of evaporative conditions and vegetation type in diurnal temperature variations“. Hydrology and Earth System Sciences 24, Nr. 10 (20.10.2020): 4923–42. http://dx.doi.org/10.5194/hess-24-4923-2020.

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Abstract. Diurnal temperature variations are strongly shaped by the absorption of solar radiation, but evaporation, or the latent heat flux, also plays an important role. Generally, evaporation cools. Its relation to diurnal temperature variations, however, is unclear. This study investigates the diurnal response of surface and air temperatures to evaporative conditions for different vegetation types. We use the warming rate, defined as the increase in temperature in response to absorbed solar radiation in the morning, and evaluate how it changes with evaporative fraction, which is an indicator of the evaporative conditions. Results for 51 FLUXNET sites show that the warming rate of air temperature carries very weak imprints of evaporative fraction across all vegetation types. However, the warming rate of surface temperature is highly sensitive to evaporative fraction with a value of ∼23×10-3 K (W m-2)-1, indicating stronger evaporative cooling for moister conditions. Contrarily, the warming rates of surface and air temperatures are similar at forest sites and carry literally no imprints of evaporative fraction. We explain these contrasting patterns with an analytical surface energy balance model. The derived expressions reproduce the observed warming rates and their sensitivity to evaporative fraction in all vegetation types. Multiplying the warming rate with daily maximum solar radiation gives an approximation for the diurnal surface temperature range (DTsR). We use our model to compare the individual contributions of solar radiation, evaporative conditions, and vegetation (by its aerodynamic conductance) in shaping DTsR and show that the high aerodynamic conductance of forests reduces DTsR substantially more (−56 %) than evaporative cooling (−22 %). We further show that the strong diurnal variation in aerodynamic conductance (∼2.5 times of the mean across vegetation types) reduces DTsR by ∼35 % in short vegetation and savanna but only by ∼22 % in forests. We conclude that diurnal temperature variations may be useful for predicting evaporation for short vegetation. In forests, however, the diurnal variations in temperatures are mainly governed by their high aerodynamic conductance, resulting in negligible imprints of evaporative conditions.

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Suchan, Jared, und Shahid Azam. „Influence of Desaturation and Shrinkage on Evaporative Flux from Soils“. Geotechnics 2, Nr. 2 (22.05.2022): 412–26. http://dx.doi.org/10.3390/geotechnics2020019.

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An assessment of evaporation losses from soils is critical for sustainable agriculture in semi-arid regions. The purpose of this research was to determine the effect of desaturation and shrinkage on evaporative flux from representative soils. Results indicated that the surface area did not change for silty sand (6% volume reduction) and substantially increased for lean clay (17% volume reduction). The evaporative flux for silty sand decreased from 31 to 25 mg/m2∙s in Stage II, remained constant during Stage III, and decreased to 11 mg/m2∙s in Stage IV. In contrast, the lean clay showed a longer Stage II (34 to 14 mg/m2∙s), a near constant Stage III, albeit a similar Stage IV (13 to 3 mg/m2∙s). The air entry and residual suction values were 1 kPa and 100 kPa for silty sand and 5 kPa and 1400 kPa for lean clay. In both soils, the total suction merged with the matric suction at Stage II–Stage III boundary. Furthermore, the shrinkage curve was J-shaped for silty sand with the only void ratio decrease in Stage II, whereas that for the lean clay showed a significant void ratio decrease in Stage II, marginal decrease in Stage III, and no decrease in Stage IV. Under high demand, the silty sand exhibited Stage III and Stage IV evaporation, whereas the lean clay also showed significant flux during Stage II. For the investigated range of water content, the total water loss under high demand was found to be 7 times that under low demand.

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Suchan, Jared, und Shahid Azam. „Influence of Saline Pore Fluid on Soil Behavior during Evaporation“. Geotechnics 2, Nr. 3 (02.09.2022): 754–64. http://dx.doi.org/10.3390/geotechnics2030036.

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Saline conditions govern soil behavior during evaporation, thereby affecting the water budget in semi-arid regions. This research examined the effects of saline pore fluid on soil behavior during evaporation. The results indicated volumetric reductions of about 5% for silty sand and about 15% for lean clay. The evaporative flux for silty sand decreased from 26 mg/m2∙s to 22 mg/m2∙s in StageII, remained at a constant flux in StageIII, and decreased to 13 mg/m2∙s in StageIV. The air entry and residual suction values were found to be 5 kPa and 100 kPa, respectively, and the total suction of about 5000 kPa merged with matric suction near the Stage II/Stage III boundary. The swell–shrink curve (SSC) was J-shaped with the only void ratio decrease in Stage II. In contrast, the evaporative flux for lean clay decreased from 30 mg/m2∙s to 15 mg/m2∙s in StageII, to 10 mg/m2∙s in StageIII, and then to 5 mg/m2∙s in StageIV. The air entry and residual suction values were 5 kPa and 2000 kPa, respectively, and the total suction during Stage II and Stage III ranged from 1000 kPa to 6000 kPa, with an average value of 3500 kPa. The SSC showed a major void ratio decrease in Stage II, marginal decrease in Stage III, and no decrease in Stage IV. Under high demand, the evaporative flux for silty sand was constant at 180 mg/m2∙s in StageIII and decreased to 50 mg/m2∙s in Stage IV, whereas it decreased for the lean clay from 230 mg/m2∙s to 145 mg/m2∙s in StageII, to 95 mg/m2∙s in StageIII, and then to 25 mg/m2∙s in StageIV. For both soils, the total water loss was found to be six times higher than that under low demand.

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Dissertationen zum Thema "Evaporative flux"

Sircar, Jay D. „Fabrication of a nanoporous membrane device for high heat flux evaporative cooling“. Thesis, Massachusetts Institute of Technology, 2017. http://hdl.handle.net/1721.1/111931.

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Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2017.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 60-63).
We investigated the experimental performance of a nanoporous membrane for ultra-high heat flux dissipation from high performance integrated circuits. The biporous evaporation device utilizes thermally-connected, mechanically-supported, high capillarity membranes that maximize thin film evaporation and high permeability liquid supply channels that allow for lower viscous pressure losses. The 600 nm thick membrane was fabricated on a silicon on insulator (SOI) wafer, fusion-bonded to a separate wafer with larger liquid channels. Spreading effects and overall device performance arising from non-uniform heating and evaporation of methanol was captured experimentally. Heat fluxes up to 412 W/cm2, over an area of O.4x 5 mm, and with a temperature rise of 24.1 K from the heated substrate to ambient vapor, were obtained. These results are in good agreement with a high-fidelity, coupled fluid convection and solid conduction compact model, which was necessitated by computational feasibility, which incorporates non-equilibrium and sub-continuum effects at the liquid-vapor interface. This work provides a proof-of-concept demonstration of our biporous evaporation device. Simulations from the validated model, at optimized operating conditions and with improved working fluids, predict heat dissipation in excess of 1 kW/cm2 with a device temperature rise below 30 K, for this scalable cooling approach.
by Jay D. Sircar.
S.M.

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Dokmak, Akaber. „Investigating the effect of an electric field on the properties and kinetics of a drying pendant droplet : A new experimental approach“. Electronic Thesis or Diss., Ecole nationale des Mines d'Albi-Carmaux, 2025. http://www.theses.fr/2025EMAC0001.

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Le séchage assisté par champ électrique est une technique prometteuse pour la manipulation des produits thermosensibles, mais son étude à l'échelle de la goutte reste limitée en raison du manque de dispositifs expérimentaux adaptés. Cette étude présente un dispositif permettant d'analyser la déformation et le séchage de gouttes pendantes électriquement chargées de CuSO4.5H4O sous un champ électrique (0-800 kV/m) et à différentes concentrations en solides (0-25 % wt/wt). Les résultats montrent une réduction de la tension interfaciale pouvant atteindre 42 % avec l'intensité du champ. L'expansion de surface et la déformation sont également influencées. En séchage diffusif, le champ réduit initialement le taux d'évaporation et le flux évaporatoire des gouttes déformées avant qu'il n'augmente et se stabilise, tandis qu'en séchage convectif, l'expansion de surface réduit temporairement le flux avant stabilisation. Ces résultats apportent de nouvelles perspectives sur les effets électrohydrodynamiques lors du séchage à l'échelle de la goutte
Electrically assisted drying is a promising technique for thermosensitive products, however its study at the droplet scale remains limited due to the lack of suitable experimental setups. This study introduces a novel device to analyze the deformation and drying of pendant electrically charged droplets of CuSO4.5H4O under an electric field (0-800 kV/m) and varying solid concentrations (0 - 25 wt/wt %). Results show up to a 42 % reduction in interfacial tension with increasing field intensity. Surface expansion and deformation are influenced by both field intensity and solid content. In terms of drying, results show that under diffusive drying, the field initially reduces the evaporative flux of highly deformed droplets before it increases and stabilizes, while in convective drying, surface expansion temporarily reduces the flux before stabilization. These findings provide new insights into electrohydrodynamic effects on droplet scale drying dynamics

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Narayanan, Shankar. „Gas assisted thin-film evaporation from confined spaces“. Diss., Georgia Institute of Technology, 2011. http://hdl.handle.net/1853/42780.

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A novel cooling mechanism based on evaporation of thin liquid films is presented for thermal management of confined heat sources, such as microprocessor hotspots. The underlying idea involves utilization of thin nanoporous membranes for maintaining microscopically thin liquid films by capillary action, while providing a pathway for the vapor generated due to evaporation at the liquid-vapor interface. The vapor generated by evaporation is continuously removed by using a dry sweeping gas keeping the membrane outlet dry. This thesis presents a detailed theoretical, computational and experimental investigation of the heat and mass transfer mechanisms that result in dissipating heat. Performance analysis of this cooling mechanism demonstrates heat fluxes over 600W/cm2 for sufficiently thin membrane and film thicknesses (~1-5µm) and by using air jet impingement for advection of vapor from the membrane surface. Based on the results from this performance analysis, a monolithic micro-fluidic device is designed and fabricated incorporating micro and nanoscale features. This MEMS/NEMS device serves multiple functionalities of hotspot simulation, temperature sensing, and evaporative cooling. Subsequent experimental investigations using this microfluidic device demonstrate heat fluxes in excess of 600W/cm2 at 90 C using water as the evaporating coolant. In order to further enhance the device performance, a comprehensive theoretical and computational analysis of heat and mass transfer at micro and nanoscales is carried out. Since the coolant is confined using a nanoporous membrane, a detailed study of evaporation inside a nanoscale cylindrical pore is performed. The continuum analysis of water confined within a cylindrical nanopore determines the effect of electrostatic interaction and Van der Waals forces in addition to capillarity on the interfacial transport characteristics during evaporation. The detailed analysis demonstrates that the effective thermal resistance offered by the interface is negligible in comparison to the thermal resistance due to the thin film and vapor advection. In order to determine the factors limiting the performance of the MEMS device on a micro-scale, a device-level detailed computational analysis of heat and mass transfer is carried out, which is supported by experimental investigation. Identifying the contribution of various simultaneously occurring cooling mechanisms at different operating conditions, this analysis proposes utilization of hydrophilic membranes for maintaining very thin liquid films and further enhancement in vapor advection at the membrane outlet to achieve higher heat fluxes.

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Persson, Tony. „Evaporation and Heat-flux Aggregation in Heterogeneous Boreal Landscapes“. Doctoral thesis, Uppsala University, Department of Earth Sciences, 2004. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-4326.

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The boreal forests represent 8 % of all forested areas on the earth and have a significant role in the control of greenhouse gases and an impact on global climate change. The main objective of this thesis is to increase the understanding of how evaporation and heat-flux processes in the boreal forest zone are affecting the regional and global climate.

A meteorological mesoscale model with an advanced land-surface parameterization has been utilized to study aggregation of fluxes of water vapour and heat. The model has been compared against four other methods for flux estimation in a southern boreal landscape. The results show that the mesoscale model is successfully reproducing 24-hour averages of fractionally weighted mast measurements of sensible and latent heat flux.

The model was also evaluated against in-situ observations of surface fluxes and other meteorological variables. The results reveal that a correct initialization of soil moisture is crucial to simulate a realistic partitioning of the sensible and latent heat fluxes. Significant differences in surface fluxes and friction velocities between two apparently similar forest sites indicate the need for careful assessment of areal representativity when comparing mesoscale model results with in-situ observations.

A parameterization for the absorption of solar radiation of high-latitude sparse forests was implemented and tested in the model that significantly improved the simulation of high wintertime midday sensible heat fluxes. A scheme for heat storage in vegetation was also implemented which improved the results, but the scheme needs further evaluation for high latitude forests.

Two commonly used strategies for the description of land-surface heterogeneity, the effective parameter approach and the mosaic approach, were tested in the mesoscale model against airborne observations of sensible and latent heat fluxes. The results show that the mosaic approach produces better results especially when small lakes are present in model grid-squares.

Norra halvklotets barrskogsbälte representerar 8 % av all skogsbeklädd mark på jorden och har stor betydelse för kontrollen av växthusgaser och påverkan på globala klimatförändringar. Syftet med denna avhandling är att öka förståelsen av hur avdunstning och värmeflöden i den boreala skogszonen påverkar klimatet regionalt och globalt.

En meteorologisk mesoskalemodell med en avancerad landyteparameterisering har använts för att studera aggregering av avdunstning och värmeflöden. Modellen jämfördes med fyra andra metoder för uppskattning av värmeflöden i den boreala skogszonens södra delar. Resultaten visade att mesoskalemodellen reproducerar 24-timmarsmedelvärden av sensibelt och latent värmeflöde från areellt viktade mastmätningar med bra resultat.

Modellen utvärderades även mot markbaserade mätningar av sensibelt och latent värme och andra meteorologiska variabler. Resultaten visar att en korrekt initialisering av markvatteninnehållet är avgörande för att simulera en realistisk uppdelning av de sensibla och latenta värmeflödena. Markanta skillnader i markyteflöden och friktionshastigheter mellan två liknande skogsmätstationer påvisar nödvändigheten av en noggrann bedömning av den areella representativiteten när man jämför resultat från mesoskalemodellen med markbaserade mätningar.

En parameterisering för absorption av solstrålning i glesa skogsbestånd på höga breddgrader infördes och testades i modellen vilket markant förbättrade simuleringen av de höga sensibla värmeflöden som observerats vid middagstid på vintern. Ett uttryck för att beskriva värmelagring i vegetationen infördes också vilket förbättrade resultaten, men uttrycket behöver vidare utvärdering för skogsbestånd på höga breddgrader.

Två ofta använda strategier för att beskriva markytans heterogenitet, effektiva parametermetoden och mosaikmetoden, testades i mesoskalemodellen mot flygburna observationer av sensibla och latenta värmeflöden. Resultaten visar att mosaikmetoden ger bättre resultat särskilt när mindre sjöar förekommer i modellrutorna.

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Janecek, Vladislav. „Evaporation à l'échelle microscopique et à haut flux thermique“. Phd thesis, Université Pierre et Marie Curie - Paris VI, 2012. http://tel.archives-ouvertes.fr/tel-00782517.

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Janeček, Valdislav. „Evaporation à l'échelle microscopique et à haut flux thermique“. Paris 6, 2012. http://www.theses.fr/2012PA066512.

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Cette thèse étudie théoriquement les processus de transport au voisinage de la ligne triple de contact liquide-gaz-solide et leur impact sur l'évaporation macroscopique. Dans la première partie de la thèse, l'hydrodynamique au voisinage de la ligne de contact est étudiée sous les conditions de mouillage partiel. L'évaporation induite par le chauffage du substrat dans l'atmosphère de vapeur du même fluide est considérée. La relaxation de la singularité hydrodynamique de la ligne triple est considérée. La principale conclusion de la thèse est que l'effet Kelvin (dépendance de la température de saturation de la pression) est suffisant en soi, pour faire disparaitre la singularité des variables hydrodynamiques. La microrégion (le voisinage de la ligne de contact) est résolue numériquement et analytiquement pour de faibles pentes de l'interface liquide-vapeur. Les échelles de longueur caractéristiques du problème sont identifiées et la nature multi-échelle du phénomène est prise en compte. Les études paramétriques effectuées révèlent le rôle de la résistance thermique de l'interface vapeur-liquide, de la longueur de glissement, du terme thermocapillaire, du recul de vapeur et ainsi que des forces de surface. Une extension de l'approximation de lubrification pour de pentes élevées de l'interface gaz-liquide à l'évaporation est discutée. Dans la seconde partie de la thèse, le modèle précédemment établi pour la microrégion est couplé à des simulations numériques de la croissance d'une bulle de vapeur. Le départ de la bulle de vapeur de la paroi chauffante pendant l'ébullition a également été étudiée. Il a été proposé dans la thèse, que sous des charges thermiques élevées, l'augmentation de l'angle de contact apparent provoque l'étalement de la bulle de vapeur sur la paroi chauffante. Ce phénomène peut conduire, au séchage de la paroi observé pendant la crise d'ébullition
This thesis theoretically investigates the transport processes in the vicinity of the triple gas-liquid-solid contact line and its impact on macroscopic evaporation. In the first part of the thesis, the hydrodynamics close to the contact line at partial wetting is studied. Specifically, evaporation into the atmosphere of pure vapor driven by heating of the substrate is considered. The question of singularity relaxation is addressed. The main finding of the thesis is that the Kelvin effect (dependence of saturation temperature on pressure) is sufficient by itself to relax the hydrodynamic contact line singularity. The proposed microregion (the contact line vicinity) model for small interface slopes is solved numerically. Asymptotic solutions are found for some specific cases. The governing length scales of the problem are identified and the multiscale nature of the phenomenon is addressed. Parametric studies revealing the role of the thermal resistance of vapor-liquid interface, slip length, thermocapillary term, the vapor recoil and surface forces are also performed. An extension of the lubrication approximation for high slopes of the gas-liquid interface at evaporation is discussed. In the second part of the thesis, the previously established microregion model is coupled to a simplified single vapor bubble growth numerical simulation. The bubble departure from the heater at boiling is also studied. It was proposed in the thesis, that under high heat loads, the increase of the apparent contact angle causes the vapor bubble to spread over the heated substrate. Such a behavior may cause the heater dry-out that occurs during the boiling crisis

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Brashers, Bart A. „Oceanic latent heat flux from satellite data /“. Thesis, Connect to this title online; UW restricted, 1998. http://hdl.handle.net/1773/10045.

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Hanks, Daniel Frank. „Evaporation from nanoporous membranes for high heat flux thermal management“. Thesis, Massachusetts Institute of Technology, 2016. http://hdl.handle.net/1721.1/110889.

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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2016.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 119-123).
Heat dissipation is a critical limitation in a range of electronic devices including microprocessors, solar cells, laser diodes and power amplifiers. The most demanding devices require dissipation of heat fluxes in excess of 1 kW/cm2 with heat transfer coefficients more than 30 W/cm 2K. Advanced thermal management solutions using phase change heat transfer are the most promising approach to address these challenges, yet current solutions are limited due to the combination of heat flux, thermal resistance, size and flow stability. This thesis reports the design, fabrication and experimental characterization for an evaporation device with a nanoporous membrane for high heat flux dissipation. Evaporation in the thin film regime is achieved using nanopores with reduced liquid film thicknesses while liquid pumping is enhanced using the capillary pressure of the 120 nm pores. The membrane is mechanically supported by ridges that form liquid supply channels and also serve as a heat conduction path to the evaporating meniscus at the surface of the membrane. The combination of high capillarity pores with high permeability channels facilitates theoretical critical heat fluxes over 2 kW/cm2 and heat transfer coefficients over 100 W/cm2K. Proof-of-concept devices were fabricated using a two-wafer stack consisting of a bonded silicon-on-insulator (SOI) wafer to a silicon wafer. Pores with diameters 110 - 130 nm were defined with interference lithography and etched in the SOI. Liquid supply microchannels were etched on a silicon wafer and the two wafers were fusion bonded together to form a monolithic evaporator. Once bonded, the membrane was released by etching through the backside of the SOI. Finally, platinum heaters and Resistive Temperature Detectors (RTDs) were deposited by e-beam evaporation and liftoff to heat the sample and measure the device temperature during experiments, respectively. Samples were experimentally characterized in a custom environmental chamber for comparison to the model using R245fa, methanol, pentane, water and isopropyl alcohol as working fluids. A comparison of the results with different working fluids demonstrates that transport at the liquid-vapor interface is the dominant thermal resistance in the system, suggesting a figure of merit: ... The highest heat flux recorded was with pentane at ... and the highest heat transfer coefficient recorded was with ... not including the substrate resistance. However, the samples were observed to clog with soluble, nonvolatile contaminants which limited operation to several minutes. The clogging behavior was captured in a mass diffusion model and a new configuration was suggested which is resistant to clogging. Evaporation from nanopores represents a new paradigm in phase change cooling with a figure of merit that favors high volatility, low surface tension fluids rather than water. The models and experimental results validate the functionality and understanding of the proposed approach and provide recommendations for enhancements in performance and understanding as well as strategies for resistance to clogging. This work demonstrates that nanoporous membranes have the potential for ultra-high heat flux dissipation to address next generation thermal management needs.
by Daniel Frank Hanks.
Ph. D.

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LUZEAU, PIERRE. „Depot et caracterisation de films supraconducteurs bisrcacuo par co-evaporation et modulation des flux“. Paris 11, 1990. http://www.theses.fr/1990PA112204.

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La methode de co-evaporation dans des conditions d'ultravide s'avere la mieux adaptee au depot de couches tres minces. Elles est employee ici pour la fabrication d'oxydes supraconducteurs a haute temperature critique du type (bio)#2sr#2ca#n##1cu#no#2#(#n#+#1#) qui ont des specificites structurales comme par exemple leur caractere lamellaire et la versatilite de leurs structures. Les films minces de ces oxydes ont des applications en electronique et en instrumentation (interconnections, squid, composants hyperfrequence, circuits logiques. . . ). Apres une presentation des oxydes supraconducteurs (en particulier les composes a base de bismuth) et des differentes methodes de depot, le systeme de co-evaporation adapte a l'utilisation d'oxygene en ultravide et les techniques d'analyses sont decrits. Nous montrons l'efficacite d'une source a plasma d'oxygene que nous avons mise au point sur l'oxydation du cuivre. Nous mettons en evidence et nous etudions en detail les problemes d'incorporation du bismuth lors des depots. Nous discutons ensuite les proprietes des films supraconducteurs obtenus. L'utilisation d'oxygene dans le systeme ultravide, les perspectives ouvertes par les evaporations en flux simultanes et en flux modules sont egalement discutees

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Bou-Zeid, Wassim. „Wettability and evaporation of sessile drops of biological fluids“. Thesis, Aix-Marseille, 2014. http://www.theses.fr/2014AIXM4348/document.

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Le processus d'étalement et d'évaporation d'une goutte de suspensions de particules sur une surface solide est très intéressant permettant la formation de motifs. Une étude expérimentale à été effectuée avec du sang total humain et avec des fluides purs dans une chambre sous atmosphère contrôlée en humidité relative. Pour des angles de contact faible, le processus d'étalement/évaporation peut être divisé en deux régimes. Un premier régime rapide gouverné par un équilibre entre les forces visqueuses et les forces capillaires et un deuxième régime plus lent dominé par la cinétique d'évaporation. Nous montrons que les bio-colloïdes jouent un rôle significatif sur la dynamique de la ligne de contact. La vitesse moyenne de la ligne de contact suit la même dynamique d'étalement que le modèle de Tanner, où le temps d'étalement et les paramètres géométriques de la goutte sont fonctions de l'humidité. Dans cette étude, nous montrons que l'humidité relative influence les paramètres géométriques de la goutte et par conséquent le motif a la fin du processus d'évaporation. Un modèle purement diffusif pur a été obtenu dont le diamètre de mouillage et l'angle de contact sont fonction de l'humidité. Pour l'analyse morphologique des motifs de craquelures, une méthode de segmentation manuelle a été utilisée comme une méthode de référence pour la validation de la méthode de segmentation automatique développée dans "iBlood". Par cette méthode, nous montrons que la cinétique d'évaporation influence la distribution structurelle et morphologique des cellules de forme trapézoïdale, et par conséquent, l'espacement des fractures moyenne finale
Spreading/evaporation process of droplets over solid surfaces is a fundamental process and a wide research field because of number of applications in printing, micro-electronics, DNA analysis and even in biomedical. This experimental work aims to investigate the effect of relative humidity on the contact line dynamics, on the evaporation dynamics and on the final pattern of a drop of whole human blood. The spreading of a pure fluid model that has the same physical properties as human blood was studied and compared to the blood. We showed that bio-colloids play significant effect on the dynamics of contact line and the pinning effect of the drop. For low contact angles, we showed that the spreading/evaporation process could be divided into two regimes. A fast first regime determined by a balance between viscous forces and capillary forces and a second slower regime dominated by the evaporation rate. Physical mechanisms that are responsible for the spreading enhancement are proposed and discussed. The average velocity of the contact line was found to follow the same behaviour as Tanner's model, where the spreading dynamics and geometrical parameters of the droplet are function of relative humidity. The experimental measurements are in a good agreement with the purely diffusive model where the equilibrium wetting radius and contact angle are function of relative humidity. For the morphological analysis of crack patterns, a manual segmentation method was used as a reference for the validation of the automatic developed segmentation method. We showed that the evaporation rate influences structural distribution of plaques in the corona region and hence, the mean crack spacing

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Mehr Quellen

Bücher zum Thema "Evaporative flux"

Ltd, Canviro Consultants. An evaluation of flue gas desulphurization wastewater treatment by mechanical evaporation. Kitchener, Ontario: Canviro Consultants Ltd, 1985.

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D, DasGupta, Wayne P. C und United States. National Aeronautics and Space Administration., Hrsg. An augmented Young-Laplace model of an evaporating meniscus in a micro-channel with high heat flux. [Washington, DC: National Aeronautics and Space Administration, 1993.

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1963-, Stephens Craig A., und United States. National Aeronautics and Space Administration. Scientific and Technical Information Program., Hrsg. Predicted thermal response of a cryogenic fuel tank exposed to simulated aerodynamic heating profiles with different cryogens and fill levels. [Washington, D.C.]: National Aeronautics and Space Administration, Office of Management, Scientific and Technical Information Program, 1991.

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Katul, Gabriel. Collection and analysis of evaporative flux data for various climates. 1990.

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Buchteile zum Thema "Evaporative flux"

Nathanson, Gilbert M. „When Liquid Rays Become Gas Rays: Can Evaporation Ever Be Non-Maxwellian?“ In Molecular Beams in Physics and Chemistry, 631–47. Cham: Springer International Publishing, 2021. http://dx.doi.org/10.1007/978-3-030-63963-1_27.

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AbstractA rare mistake by Otto Stern led to a confusion between density and flux in his first measurement of a Maxwellian speed distribution. This error reveals the key role of speed itself in Stern’s development of “the method of molecular rays”. What if the gas-phase speed distributions are not Maxwellian to begin with? The molecular beam technique so beautifully advanced by Stern can also be used to explore the speed distribution of gases evaporating from liquid microjets, a tool developed by Manfred Faubel. We employ liquid water and alkane microjets containing dissolved helium atoms to monitor the speed of evaporating He atoms into vacuum. While most dissolved gases evaporate in Maxwellian speed distributions, the He evaporation flux is super-Maxwellian, with energies up to 70% higher than the flux-weighted average energy of 2 RTliq. The explanation of this high-energy evaporation involves two beautiful concepts in physical chemistry: detailed balancing between He atom evaporation and condensation (starting with gas-surface collisions) and the potential of mean force on the He atom (starting with He atoms just below the surface). We hope that these measurements continue to fulfill Stern’s dream of the “directness and simplicity of the molecular ray method.”

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Abtew, Wossenu, und Assefa Melesse. „Spatially Distributed Surface Energy Flux Modeling“. In Evaporation and Evapotranspiration, 141–59. Dordrecht: Springer Netherlands, 2012. http://dx.doi.org/10.1007/978-94-007-4737-1_10.

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Desjardins, R. L., und J. I. MacPherson. „Water Vapor Flux Measurements from Aircraft“. In Land Surface Evaporation, 245–60. New York, NY: Springer New York, 1991. http://dx.doi.org/10.1007/978-1-4612-3032-8_14.

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Abtew, Wossenu, und Assefa Melesse. „Crop Yield Estimation Using Remote Sensing and Surface Energy Flux Model“. In Evaporation and Evapotranspiration, 161–75. Dordrecht: Springer Netherlands, 2012. http://dx.doi.org/10.1007/978-94-007-4737-1_11.

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Liu, W. Timothy, und Xiaosu Xie. „Ocean-Atmosphere Water Flux and Evaporation“. In Encyclopedia of Remote Sensing, 480–89. New York, NY: Springer New York, 2014. http://dx.doi.org/10.1007/978-0-387-36699-9_122.

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Grossman, Robert L. „Temporal Variation of Heat and Moisture Flux Within the Atmospheric Boundary Layer over a Grassland“. In Land Surface Evaporation, 275–85. New York, NY: Springer New York, 1991. http://dx.doi.org/10.1007/978-1-4612-3032-8_16.

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Gholijani, Alireza, Sebastian Fischer, Tatiana Gambaryan-Roisman und Peter Stephan. „High Resolution Measurements of Heat Transfer During Drop Impingement onto a Heated Wall“. In Fluid Mechanics and Its Applications, 291–310. Cham: Springer International Publishing, 2022. http://dx.doi.org/10.1007/978-3-031-09008-0_15.

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AbstractDrop impact on a hot surface heated above the saturation temperature of the fluid plays an important role in spray cooling. The heat transferred from the wall to the fluid is closely interrelated with drop hydrodynamics. If the surface temperature is below the Leidenfrost temperature, the heat transport strongly depends on the transport phenomena in the vicinity of the three-phase contact line. Due to extremely high local heat flux, a significant fraction of the total heat flow is transported through this region. The local transport processes near the three-phase contact line, and, therefore, the total heat transport, are determined by the wall superheat, contact line velocity, system pressure, fluid composition, surface structure and physical properties on the wall. The effect of the aforementioned influencing parameters on fluid dynamics and heat transport during evaporation of a single meniscus in a capillary slot are studied in a generic experimental setup. The hydrodynamics and evolution of wall heat flux distribution during the impact of a single drop onto a hot wall are also studied experimentally by varying the impact parameters, wall superheat, system pressure, and wall topography. In addition, the fluid dynamics and heat transport behavior during vertical and horizontal coalescence of multiple drops on a heated surface are studied experimentally.

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Heinen, Matthias, Simon Homes, Gabriela Guevara-Carrion und Jadran Vrabec. „Mass Transport Across Droplet Interfaces by Atomistic Simulations“. In Fluid Mechanics and Its Applications, 251–68. Cham: Springer International Publishing, 2022. http://dx.doi.org/10.1007/978-3-031-09008-0_13.

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AbstractDue to availability of powerful computers and efficient algorithms, physical processes occurring at the micrometer scale can nowadays be studied with atomistic simulations. In the framework of the collaborative research center SFB-TRR75 “Droplet dynamics under extreme ambient conditions”, investigations of the mass transport across vapour-liquid interfaces are conducted. Non-equilibrium molecular dynamics simulation is employed to study single- and two-phase shock tube scenarios for a simple noble gas-like fluid. The generated data show an excellent agreement with computational fluid dynamics simulations. Further, particle and energy flux during evaporation are sampled and analysed with respect to their dependence on the interface temperature, employing a newly developed method which ensures a stationary process. In this context, the interface properties between liquid nitrogen and hydrogen under strong gradients of temperature and composition are investigated. Moreover, the Fick diffusion coefficient of strongly diluted species in supercritical CO$$_{2}$$ 2 is predicted by equilibrium molecular dynamics simulation and the Green-Kubo formalism. These results are employed to assess the performance of several predictive equations from the literature.

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Webb, Peter I. „The comparative ecophysiology of water balance in microchiropteran bats“. In Ecology, Evolution and Behaviour of Bats, 203–18. Oxford University PressOxford, 1995. http://dx.doi.org/10.1093/oso/9780198549451.003.0013.

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Abstract Aspects of water balance were studied in the three species of bat commonly found in north-east Scotland: Plecotus auritus, Myotis daubentonii and Pipistrellus pipistrellus. One of these species (M. daubentonii) has a close association with open water in the wild, while one (P. pipistrellus) has a much lower mean body mass (c. 6 g) than the other two (c. 10 g). Faecal water content and maximum urine-concentrating ability were not significantly different between P. auritus and M. daubentonii. However, urine loss was higher (by c. 140 µI/day) and resting evaporative water loss lower (also by c. 140 µI/day) in M. daubentonii than in P. auritus. In free-flying captive colonies of bats, total water intake was highest in M. daubentonii, intermediate in P. auritus and lowest in P. pipistrellus. It seems possible that M. daubentonii demonstrate high water flux outside the day roost and compensate through a low water flux within the day roost. A low rate of water intake in captive P. pipistrellus may simply be a function of small body size.

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Choy, Bruce, und Danny D. Reible. „Constant flux liquid evaporation“. In Diffusion Models of Environmental Transport, 149–52. CRC Press, 2017. http://dx.doi.org/10.1201/9781315141282-10.

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Konferenzberichte zum Thema "Evaporative flux"

Tilton, Donald E., Jay H. Ambrose und Louis C. Chow. „Closed-System, High-Flux Evaporative Spray Cooling“. In Aerospace Technology Conference and Exposition. 400 Commonwealth Drive, Warrendale, PA, United States: SAE International, 1989. http://dx.doi.org/10.4271/892316.

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Nazari, Masoumeh, Matt Gorman und Hadi Ghasemi. „Unprecedented Capillary Evaporative Heat Flux in Nanochannels“. In 2019 18th IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm). IEEE, 2019. http://dx.doi.org/10.1109/itherm.2019.8757337.

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PAIS, M., D. TILTON, L. CHOW und E. MAHEFKEY. „High-heat-flux, low-superheat evaporative spray cooling“. In 27th Aerospace Sciences Meeting. Reston, Virigina: American Institute of Aeronautics and Astronautics, 1989. http://dx.doi.org/10.2514/6.1989-241.

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Lee, Hee Joon, und Shi-Chune Yao. „System Instability of Evaporative Micro-Channels“. In ASME 2009 Heat Transfer Summer Conference collocated with the InterPACK09 and 3rd Energy Sustainability Conferences. ASMEDC, 2009. http://dx.doi.org/10.1115/ht2009-88217.

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During the operation of parallel evaporative micro-channels, system instability may occur in terms of cyclical fluctuations at a long period. This is due to the co-existence of the liquid phase flow at high mass flux and the two-phase flow at a lower mass flux among different parallel channels under the same total pressure drop. For a system at constant flow-rate pumping, with a pressure regulating tank, and a constant heating pre-heater, the system may experience severe alternations between these two states of boiling and non-boiling with a period of minutes. This cyclical system instability has been modeled. In the model, the existence of the liquid phase flow happens at conditions of inlet subcooling and low surface heat flux that the boiling inception is hard to occur. Accordingly, the system instability criteria are established in terms of a System binary states parameter S and a non-dimensional surface heat flux, which is normalized with the boiling incipient heat flux. This model has been validated experimentally.

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Pillutla, Srikrishna, und Wonjae Choi. „Oscillating Bubbles on a Hot Surface“. In ASME 2012 International Mechanical Engineering Congress and Exposition. American Society of Mechanical Engineers, 2012. http://dx.doi.org/10.1115/imece2012-88366.

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This presentation describes a nonlinear dynamics of bubbles forming on a hot surface. In particular, we compare the behavior of bubbles in three regimes — evaporative, nucleate boiling, and film boiling regimes. A bubble appears on a hot surface when the heat flux from the surface is high enough to overcome the Laplace pressure across the surface of the bubble. Once a bubble nucleates on a surface, one might expect that the subsequent evaporation of the liquid phase would make the bubble expand monotonically until it leaves the surface. In real, nucleating bubbles expand monotonically only in evaporative regime. In the other two regimes, the size of the bubbles oscillates with varying frequency and amplitude depending on the heat flux and the convection patterns in the bulk liquid. We suspect that such a behavior suggests that the bubbling dynamics in these three regimes are fundamentally different: while the growth of a bubble in evaporative regime is determined essentially by the evaporation rate on the hot surface, the much more dynamic behavior of a bubble in nucleate and film boiling regime is the result of the interplay between two driving forces — evaporation near the solid surface and condensation near the top of a bubble. Such a competition becomes unsteady when the heat flux is high; therefore, boiling pattern switches from a quasi-static evaporative one to periodic nucleate boiling, and eventually to chaotic film boiling.

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Bhardwaj, Rajneesh, und Daniel Attinger. „A Numerical Model for the Evaporation of a Nanoliter Droplet on a Solid, Heated Substrate“. In ASME 2007 International Mechanical Engineering Congress and Exposition. ASMEDC, 2007. http://dx.doi.org/10.1115/imece2007-43088.

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A finite-element numerical model is developed to study the evaporation of a sessile nanoliter droplet on a flat substrate in the isothermal and non-isothermal case. All equations are expressed in a Lagrangian framework, which provides accurate modeling of the free surface motion and the associated Laplace stresses. The evaporative flux at the free surface is defined as a function of the drop geometry and thermodynamic conditions. The boundary conditions at free surface are the mass and energy jump conditions. Results for fluid dynamics, evaporation time and evaporative flux are described for substrate temperature levels of 27°C (ambient), 82°C and 122°C. Comparisons with experiments are made, in terms of the evolution of volume, wetting angle and wetted radius.

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Burkhart, Collin T., Kara L. Maki und Michael J. Schertzer. „Observation of Contact Line Dynamics in Evaporating Droplets Under the Influence of Electric Fields“. In ASME 2016 14th International Conference on Nanochannels, Microchannels, and Minichannels collocated with the ASME 2016 Heat Transfer Summer Conference and the ASME 2016 Fluids Engineering Division Summer Meeting. American Society of Mechanical Engineers, 2016. http://dx.doi.org/10.1115/icnmm2016-7988.

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Deposition of colloidal material in evaporating droplets is important in many applications including DNA sequencing and medical diagnostic testing. When colloidal droplets evaporate, the majority of material is often deposited at the periphery of the resultant deposition in a coffee-ring pattern. Formation of this pattern is the result of contact line pinning and the interplay between evaporative and surface tension effects in the droplet. When the contact line is pinned and the evaporative flux in the droplet is highest at the periphery, a radially outward flow is generated to conserve mass that deposits particles in the fluid at the contact line. Evaporation at the contact line can also create a temperature gradient across the droplet. This gradient gives rise to a surface tension driven flow that can resuspend particles in the droplet. When the evaporative flow dominates, particles are deposited at the contact line in a coffee-ring pattern. The presence of the coffee-ring pattern is undesirable in many printing and medical diagnostic processes. Suppression of the coffee-ring effect has been achieved by addition of surfactant, enhancement of surface tension flow, surface modification, alteration of particle shape, and application of an electric field. Manipulation of the coffee-ring effect has been achieved through the application of both AC and DC electric fields. One result of the presence of this field is the electrowetting force at the contact line which acts to reduce the contact angle and increase contact area. Since this force acts at the contact line, it may disrupt typical contact line dynamics, including evaporative dynamics, which are responsible for the formation of the coffee-ring effect. This work will experimentally examine contact line dynamics of evaporating droplets in the presence of DC electric fields. Droplets of water will be desiccated on a device with a photolithographically patterned electrode covered with a thin layer of SU8-3005. Experimental cases with applied DC fields will be compared with unactuated control cases to examine changes in transient interface shape and contact diameter.

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„Development and evaluation of global land surface evaporative flux records“. In 21st International Congress on Modelling and Simulation (MODSIM2015). Modelling and Simulation Society of Australia and New Zealand, 2015. http://dx.doi.org/10.36334/modsim.2015.l9.mccabe.

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Wayner, Peter C. „Effects of Interfacial Phenomena and Conduction on an Evaporating Meniscus“. In ASME 2007 5th International Conference on Nanochannels, Microchannels, and Minichannels. ASMEDC, 2007. http://dx.doi.org/10.1115/icnmm2007-30021.

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An overview of some of the theoretical models describing the effects of chemical potential, excess free energy, free energy gradient, film thickness profile, temperature profile, superheat, thermal conduction, concentration gradient, velocity profile, slip velocity, apparent contact angle, and kinetic theory on the phase change heat transfer processes in an evaporating meniscus are presented. The relative importance of the parameters is demonstrated. Experimental techniques and confirming experimental data are also presented. In essence, the microscopic thickness profile of the evaporating meniscus is measured optically to obtain the details of the liquid pressure field and modeled to give the fluid flow rate and the evaporative heat flux. The macroscopic temperature field of the substrate is measured and numerically modeled to give the microscopic temperature field and a complementary calculation of the evaporative heat flux. For closure, the values of the slip velocity and concentration change on evaporation need to be correctly assumed. The interfacial transport processes are very sensitive to small interfacial temperature and concentration changes, which are difficult, if not impossible, to measure directly. However, the liquid pressure gradients can be directly measured. The effects of the interacting phenomena on the phase change processes are demonstrated using these complementary experimental-modeling procedures. The processes are found to be very complex and simple modeling/experiments can only confirm the general phenomena and give insight.

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Jeong, Siyoung, Eunsang Cho und Hark-koo Kim. „Evaporative Heat Transfer and Pressure Drop of CO2 in a Microchannel Tube“. In ASME 3rd International Conference on Microchannels and Minichannels. ASMEDC, 2005. http://dx.doi.org/10.1115/icmm2005-75180.

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Evaporation heat transfer and pressure drop characteristics of carbon dioxide were investigated in a multi-channel micro tube. The aluminum tube has 3 square channels with a hydraulic diameter of 2mm, a wall thickness of 1.5mm, and a length of 5m. The tube was heated directly by electric current. Experiments were conducted at heat fluxes ranging 4–16 kW/m2, mass fluxes from 150 to 750 kg/m2s, evaporative temperature from 0 to 10°C, and qualities from 0 to superheated state. The heat transfer coefficient measured was in the range of 6–15kW/m2K, and the pressure drop was 3–23kPa/m. For the qualities lower than 0.5, the heat transfer coefficient was found to increase with the quality, which is assumed to be the effect of convective boiling. For the qualities higher than 0.6, sudden drop in heat transfer coefficients was sometimes observed due to local dry-out. It was found that dry-out occurred at lower quality if mass flux was smaller. The average heat transfer coefficient was found to increase with increasing heat flux, mass flux, and evaporation temperature, of which the effect of heat flux was the greatest. At given experimental conditions the pressure drop increased almost linearly with increasing quality. The total pressure drop was found to increase with increasing heat flux, mass flux, and evaporation temperature, of which the effect of mass flux was the greatest. From the experimental results simple correlations for heat transfer coefficients and pressure drop were developed.

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Berichte der Organisationen zum Thema "Evaporative flux"

Wang, Evelyn N. Nanoengineered Surfaces for High Flux Thin Film Evaporation. Fort Belvoir, VA: Defense Technical Information Center, Juli 2013. http://dx.doi.org/10.21236/ada583176.

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Seginer, Ido, Daniel H. Willits, Michael Raviv und Mary M. Peet. Transpirational Cooling of Greenhouse Crops. United States Department of Agriculture, März 2000. http://dx.doi.org/10.32747/2000.7573072.bard.

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Background Transplanting vegetable seedlings to final spacing in the greenhouse is common practice. At the time of transplanting, the transpiring leaf area is a small fraction of the ground area and its cooling effect is rather limited. A preliminary modeling study suggested that if water supply from root to canopy is not limiting, a sparse crop could maintain about the same canopy temperature as a mature crop, at the expense of a considerably higher transpiration flux per leaf (and root) area. The objectives of this project were (1) to test the predictions of the model, (2) to select suitable cooling methods, and (3) to compare the drought resistance of differently prepared seedlings. Procedure Plants were grown in several configurations in high heat load environments, which were moderated by various environmental control methods. The difference between the three experimental locations was mainly in terms of scale, age of plants, and environmental control. Young potted plants were tested for a few days in small growth chambers at Technion and Newe Ya'ar. At NCSU, tomato plants of different ages and planting densities were compared over a whole growing season under conditions similar to commercial greenhouses. Results Effect of spacing: Densely spaced plants transpired less per plant and more per unit ground area than sparsely spaced plants. The canopy temperature of the densely spaced plants was lower. Air temperature was lower and humidity higher in the compartments with the densely spaced plants. The difference between species is mainly in the canopy-to-air Bowen ratio, which is positive for pepper and negative for tomato. Effect of cooling methods: Ventilation and evaporative pad cooling were found to be effective and synergitic. Air mixing turned out to be very ineffective, indicating that the canopy-to-air transfer coefficient is not the limiting factor in the ventilation process. Shading and misting, both affecting the leaf temperature directly, proved to be very effective canopy cooling methods. However, in view of their side effects, they should only be considered as emergency measures. On-line measures of stress: Chlorophyll fluorescence was shown to accurately predict photosynthesis. This is potentially useful as a rapid, non-contact way of assessing canopy heat stress. Normalized canopy temperature and transpiration rate were shown to correlate with water stress. Drought resistance of seedlings: Comparison between normal seedlings and partially defoliated ones, all subjected to prolonged drought, indicated that removing about half of the lowermost leaves prior to transplanting, may facilitate adjustment to the more stressful conditions in the greenhouse. Implications The results of this experimental study may lead to: (1) An improved model for a sparse canopy in a greenhouse. (2) A better ventilation design procedure utilizing improved estimates of the evaporation coefficient for different species and plant configurations. (3) A test for the stress resistance of transplants.

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Cohen, Shabtai, Melvin Tyree, Amos Naor, Alan N. Lakso, Terence L. Robinson und Yehezkiel Cohen. Influence of hydraulic properties of rootstocks and the rootstock-scion graft on water use and productivity of apple trees. United States Department of Agriculture, 2001. http://dx.doi.org/10.32747/2001.7587219.bard.

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This one year exploratory project investigated hydraulic architecture of apple dwarfing rootstocks. The hypothesis was that hydraulic conductance is correlated with rootstock vigor. A previous study of trees on three rootstocks in Israel showed that dwarfed trees used less water than un-dwarfed trees. Analysis showed that if the tree maintains leaf water potentials above minimum values, then this implies that the dwarfed trees have lower leaf conductance, which may also be the cause of dwarfing. The current project studied small 2-year old unworked rootstock trees, and full sized trees bearing commercial yields. In both cases hydraulic conductance was determined with two methods - the non-destructive evaporative flux (EF)-leaf water potential (L WP) method, and a destructive method in which water was forced through the plant at known pressure using the "high pressure flow meter" (HPFM). Detailed work allowed measurement of conductance of the rootstock-scion union. This was achieved both with the HPFM and with the EF-LWP methods, the former in the US and the latter in Israel. Direct measurements of leaf conductance were made, and carbon isotope ratios ( d ¹³ C) were determined for leaves sampled at the end of the season. The latter can indicate sustained differences in leaf conductance behavior. HPFM and EF-LWP methods did not give the same results. In the small plants results were similar in magnitude, but not significantly correlated. In large trees, EF- L WP measurements were a fraction of those obtained with the HPFM. The latter indicates that some of the xylem is not normally functional but transports water when pressurized. Additional experimental work targeted this result. Xylem was stained before and after perfusion with water at high pressure. This showed that at least for one rootstock a significant amount of xylem was blocked before perfusion. The "air method" for determining xylem vessel properties was improved and employed. Length, radius and density of xylem vessels of different rootstocks were found to be similar, and significant differences found were not clearly related to rootstock vigor. Measurements in the commercial orchard in Israel showed that the graft union in a dwarfing rootstock was a large obstacle for water transport (i.e. had a high resistance). This apparently led to low leaf conductance to water vapor, as indicated by lower d ¹³ C, which implies low internal CO ₂ concentrations. In the US orchard, d ¹³ C in 2001 was correlated with rootstock vigor, and significant differences were found in leaf conductance. However, the d ¹³ C differences were not observed in 2002, were opposite to those found in the Israeli orchard, and measurements of the graft union with the HPFM did not find large resistances. We speculate that the graft union is not necessarily a large impediment to water transport unless the scion starts to separate from the rootstock. It was concluded that significant differences in hydraulic conductance exist between different dwarfing rootstocks. These differences may be caused by differences in xylem properties and in the degree of cavitation, as well as resistance in the graft union. However, no general relationship to rootstock vigor was found. Therefore, hydraulic conductance alone cannot explain dwarfing, but may be one of two or more factors that lead to dwarfing. Future work should integrate more factors with hydraulic relations, e.g. nutrient and solute transport and production of hormones.

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