Tesi sul tema "Critical flux"
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1
Yang, Jun. "Effect of non-uniform axial heat-flux distribution on critical heat flux". Thesis, University of Ottawa (Canada), 2004. http://hdl.handle.net/10393/26816.
Testo completoAbstract (sommario):
An experimental study of the effect of axial flux distribution (AFD) on critical heat flux (CHF) was conducted in directly heated tubes at the Freon-equivalent CANDU reactor conditions of interest. CHF measurements were obtained on test sections with four nonuniform AFD profiles as well as a uniform AFD profile using HFC-134a as a test fluid. Each of the non-uniform AFD test sections had a stepped cosine heat flux profile with approximately 16 heat flux steps. The test conditions covered a pressure range of 1662 to 2389 kPa, a mass flux range of 2827 to 4648 kg m-2 s -1 and an inlet quality range of -0.909 to -0.002.
The results showed that the AFD has a strong effect on CHF at high dryout qualities. (Abstract shortened by UMI.)
2
Manning, Jonathan Paul. "Critical heat flux in non-circular channels". Thesis, Imperial College London, 2018. http://hdl.handle.net/10044/1/61534.
Testo completoAbstract (sommario):
In the design of nuclear reactors adequate cooling must be demonstrated for all operational states as well as during and after design basis accidents. A key aspect of this design activity is the prediction of the Critical Heat Flux (CHF). The focus of the work in this thesis was the prediction of CHF in non-circular channels. The Look Up Table was used to analyse several burnout studies for non-circular channels in the literature and was found to be a poor predictive tool for these geometries. A conventional phenomenological model developed for round tubes was also shown to give poor predictions, with a mean error of 25% and root mean square error of 31%. Phenomenological modelling requires correlations for the mass transfer processes in annular flow. Deposition rates for annular flow in rectangular channels have been determined by an analysis of upstream burnout data. This showed good agreement with the rates in round tubes and validated this aspect of the phenomenological approach. The conventional one-dimensional phenomenological model was extended to include a variation in film thickness around the periphery. This model was fitted to experimental data from the literature for burnout in asymmetrically heated tubes. The low mean and root mean square errors, 0.8% and 3.0% respectively, confirmed the principle of the model. A flow visualisation rig has been designed and successfully operated to produce a flow-regime map for a rectangular channel of 25 mm by 2.5 mm. This map showed that the gas momentum flux required to cause annular flow was higher than that in round tubes. A wide range of annular flow conditions were observed and shown to be generally consistent with the phenomenological modelling approach. However it was seen that there were novel flow features that will need to be accounted for when predicting CHF in these geometries.
3
Thompson, Jordan Lee. "Direct Measurement of Boiling Water Heat Flux for Predicting and Controlling Near Critical Heat Flux". Thesis, Virginia Tech, 2013. http://hdl.handle.net/10919/23091.
Testo completoAbstract (sommario):
A novel method for measuring heat flux of boiling water is designed and built to study critical heat flux (CHF) and observe the response of a heat flux sensor when CHF occurs. A high temperature heat flux sensor is embedded in the wall of a pipe to get a direct measurement of the surface heat flux and sensor temperature. By submerging the pipe in water and applying a controlled heat flux to the inside diameter over the area where the sensor is located, boiling is created on the outer surface while measuring the heat flux. The heat flux is gradually increased up to CHF and the heat flux response is observed to determine if the heat flux sensor could sense CHF when it occurred. The heat flux sensor is able to consistently measure the value for CHF, which is approximately 510 kW/m" for this system. It is also observed during the experiments that the heat flux response undergoes an inflection of the heat transfer coefficient at a consistent temperature just before reaching CHF. This observed inflection caused the heat flux response to deviate from its cubic relationship with the temperature and drastically increase for a very small change in temperature. This inflection response can be used as an indication for approaching CHF and can also be used to approximate its value without prior knowledge of when it occurs.Master of Science
4
Joober, Khaled. "The effect of flow geometry on critical heat flux". Thesis, University of Ottawa (Canada), 1993. http://hdl.handle.net/10393/6544.
Testo completoAbstract (sommario):
An extensive and systematic literature review on the effect of flow geometry on the Critical Heat Flux (CHF) has been performed. This review covers most of the flow geometries such as tubes, concentric and eccentric annuli, rectangular channels, irregular-shaped channels and bundles. The following geometric parameters have been found to strongly influence the CHF: (i) hydraulic-equivalent diameter, (ii) heated-equivalent diameter, (iii) gap size, (iv) unheated adjacent surface, (v) heated adjacent surface, (vi) curvature, (vii) eccentricity (including bowing), and (viii) channel shape. It is found that some of the geometric effects on CHF depend on the flow conditions and the CHF type. For each geometry the parametric trends have been described, whenever sufficient experimental results are available. A review and assessment of the available prediction methods is conducted. The following trends have been identified in this study: (i) in general the CHF in annuli (concentric and eccentric) is lower than that in tubes, especially for high quality and narrow gaps; (ii) for rectangular channels and irregular-shaped channels, corners can cause a large CHF reduction; (iii) the CHF for concave surface is significantly higher than the CHF for a convex surface; (iv) the effect of gap size in concentric annuli is different for a departure from nucleate boiling (DNB) type CHF and CHF type for the annular flow regime. For the first CHF type reduction in gap size results in a CHF decrease, while for the second CHF type it results in CHF enhancement; and (v) heating the adjacent surface results in a CHF increase. Based on the observed trends, CHF correction factors have been derived for each geometry. Finally, an interim CHF prediction method for subchannels and flow conditions of interest to CANDU$\sp*$ reactors has been proposed. ftn$\sp*$CANDU--CANada Deuterium Uranium, a registered trademark.
5
Le, Clech Pierre. "Process configurations and fouling in membrane bioreactors". Thesis, Cranfield University, 2002. http://dspace.lib.cranfield.ac.uk/handle/1826/11336.
Testo completoAbstract (sommario):
MBR process consists of a suspended growth biological reactor combined with a membrane unit. The widespread of this system for waste water treatment is contained by membrane fouling, which is strongly influenced by three factors: biomass characteristics, operating conditions and membrane characteristics. Fouling control techniques mainly include low-flux operation (sub-critical flux operation) and/or high-shear slug flow aeration in submerged. configuration. Based on the concept of the critical flux (Jo), the flux-step method has been developed to more fully characterise transmembrane pressure (TMP) behaviour during constant-fluxoperation. A zero rate of TMP increase was never attained during the trial, such that no critical flux, in its strictest definition, could be defined in this study for a submerged MBRs challenged with real and simulant sewage. Under similar operating conditions, Jc was obtained around 18 and 10 L.m-2.h-1 for a submerged MBR fed by real and synthetic sewage respectively. Three TMP-based parameters have been defined, all indicating the same flux value at which fouling starts to be more significant (the weak form of Jo). Results from factorial experimental designs revealed the relative effect of MLSS levels, aeration rate and membrane pore size on J, The MLSS effect on Jc was generally around double that of the aeration effect. The calculation of mean sub-critical values for the different TMP-based parameters suggest lower short-term fouling resistance for large pore sized membranes. A direct comparison between the two MBR configurations revealed a greater J, for the submerged compared to the SS MBR (22 and 11 L.m-2.h-1 respectively) under similar hydraulic conditions. The fluid hydrodynamics has been studied for both configurations, leading to an accurate calculation of shear at the membrane surface in SS MBR and to the determination of the minimum gas velocity required for Taylor bubble formation in submerged MBR (around 0.1 m.s-1). Finally, the effect of operating conditions such as process configuration, feed nature, and aeration type on biomass characteristics has been assessed and link to membrane fouling. Key words: Fouling, MBR, critical flux, process configuration, biomass characterisation.
6
Truong, Bao H. (Bao Hoai). "Determination of pool boiling Critical Heat Flux enhancement in nanofluids". Thesis, Massachusetts Institute of Technology, 2007. http://hdl.handle.net/1721.1/41689.
Testo completoAbstract (sommario):
Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, June 2007."May 2007."
Includes bibliographical references (leaves 51-53).
Nanofluids are engineered colloids composed of nano-size particles dispersed in common fluids such as water or refrigerants. Using an electrically controlled wire heater, pool boiling Critical Heat Flux (CHF) of Alumina and Silica water-based nanofluids of concentration less than or equal to 0.1 percent by volume were measured. Silica nanofluids showed CHF enhancement up to 68% and there seems to be a monotonic relationship between nanoparticle concentration and magnitude of enhancement. Alumina nanofluids had CHF enhancement up to 56% but the peak occurred at the intermediate concentration. The boiling curves in nanofluid were found to shift to the left of that of water and correspond to higher nucleate boiling heat transfer coefficients in the two-phase flow regime. SEM images show a porous coating layer of nanoparticles on wires subjected to nanofluid CHF tests. These coating layers change the morphology of the heater's surface, and are responsible for the CHF enhancement. The thickness of the coating was estimated using SEM and was found ranging from 3.0 to 6.0 micrometers for Alumina, and 3.0 to 15.0 micrometers for Silica. Inductively Coupled Plasma Spectroscopy (ICP-OES) analyses were also attempted to quantify the mass of the particle deposition but the results were inconsistent with the estimates from the SEM measurement.
by Bao H. Truong.
S.B.
7
Chen, Geng. "Analytical and experimental studies of critical heat flux in complex geometry". Thesis, National Library of Canada = Bibliothèque nationale du Canada, 2001. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp05/NQ66137.pdf.
Testo completo
8
Lalonde, Richard. "Flux line interactions in conventional and high critical transition temperature superconductors". Thesis, University of Ottawa (Canada), 1990. http://hdl.handle.net/10393/6031.
Testo completoAbstract (sommario):
We have developed a novel experimental approach for the study of the interaction of sheets of non parallel flux lines in hysteric type II superconductors. We continuously monitor the evolution of the components of the magnetic flux density $\Vert$ and $\perp$ to $H\sb{a}$ (i.e. $\langle B\sb{z}\rangle$ and $\langle B\sb{y}\rangle$) as $H\sb{a}$ is raised to various intensities, and then reduced to zero. In our investigation of a high $T\sb{c}(YBa\sb{2}Cu\sb{3}O\sb{7-x}$) ceramic, $H\sb{a}$ exceeded $H\sb{*}$, the first full penetration field. We applied the phenomenological Clem general critical state model to the analysis of our extensive observations. A computer program was developed to solve the four coupled differential equations of this theory with appropriate physical constraints for the situations prevailing in our experiment. This analysis provides detailed insight into the evolution of the intricate configurations of the magnetic flux density $\vec B$(x), the critical current density, $\vec J$(x), and electric field $\vec E$(x) patterns as the injected and trapped flux lines are made to interact, unpin, migrate and undergo flux cutting processes. The model is seen to generate the variety of complicated measured curves of $\langle B\sb{y}\rangle$ and $\langle B\sb{z}\rangle$ vs $H\sb{a}$ very satisfactorily. (Abstract shortened by UMI.)
9
Tanase, Aurelian. "Improved methodology for deriving the critical heat flux look-up table". Thesis, University of Ottawa (Canada), 2007. http://hdl.handle.net/10393/27923.
Testo completoAbstract (sommario):
A literature review on critical heat flux (CHF) prediction methods confirmed that the CHF look-up table (LUT) has many advantages over the other prediction methods: it covers the widest range of flow conditions, it is the most accurate CHF prediction method and it is computationally very efficient. The LUT has been included in the major thermalhydraulics and safety analysis computer codes.
The LUT accuracy has increased over the years, although several areas have been identified where further improvements are desirable. These areas include (i) the screening of the experimental data, (ii) effect of the heated channel diameter and length on the CHF, and (iii) difficulties in predicting the CHF in the limiting quality region in LUT, at low flow/low pressure conditions and in the very high dryout quality range.
This thesis describes the various improvements that have been made to the LUT derivation. In addition to the improvements in the LUT derivation methodology, a new visual analysis technique that allows simultaneous LUT trend visualization and comparison in all parametric directions has been developed. Based on the findings and improvements in the LUT derivation methodology, a new version of the LUT has been developed.
The error analysis revealed that refined data screening and removal of outliers is an effective method for improving the CHF LUT accuracy. Because the majority of the experimental data were obtained for diameters close to the standard 8 mm ID, a better correction of diameter effect on the CHF does not significantly affect the overall LUT accuracy, although it appears to be very important at specific conditions such as low flow or extreme diameters.
10
Roach, Gregory M. Jr. "Onset of flow instability and critical heat flux in uniformly-heated microchannels". Thesis, Georgia Institute of Technology, 1998. http://hdl.handle.net/1853/19048.
Testo completo
11
Cui, Xingdong. "Prediction of critical heat flux in bundles using tube look-up table". Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk2/tape17/PQDD_0002/MQ28415.pdf.
Testo completo
12
Kuan, Wai Keat. "Experimental study of flow boiling heat transfer and critical heat flux in microchannels /". Link to online version, 2006. https://ritdml.rit.edu/dspace/handle/1850/1887.
Testo completo
13
Youravong, Wirote. "Critical flux and fouling during ultrafiltration of milk components : hydrodynamic and physicochemical effects". Thesis, University of Reading, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.252239.
Testo completo
14
VALENTE, BELMIRO RUFINI. "ANALYSIS OF CRITICAL HEAT FLUX IN PWR NUCLEAR REACTORS USING ARTIFICIAL NEURAL NETWORKS". PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 1996. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=19433@1.
Testo completoAbstract (sommario):
CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOA ocorrência de fluxo crítico de calor – FCC – é o principal fator termo-hidráulico limitante à produção de energia em reatores nucleares do tipo PWR (Reator a Água Pressurizada). O método usual de determinação de FCC é baseado em simulação numérica, utilizando programas como os COBRA, desenvolvidos a partir da análise dos subcanais do núcleo do reator. Esses programas implementam uma correlação, ou função empírica, que interpola os resultados obtidos por simulação experimental, realizada nas Seções de testes – ST-, de forma a obter o FCC numa ampla faixa operacional do reator. Esta dissertação propõe e investiga um método alternativo de determinação de FCC empregando, como correlação, redes neuronais artificiais – RNA. Neste método, as RNA são obtidas a partir de treinamento, utilizando o paradigma de backpropapagation, realizado com o mesmo conjunto de dados experimentais oriundos das STs.
Critical Heat Flux – CHF – occurence is the main thermo-hydraulical factor that restrains the energy produced in Pressurized Water Reactor – PWR – nuclear plants. The usual method of determining CFCH is based upon numerical simulation performed by computer programs such as COBRA, which were developed considering the reactor core sub-channel analysis. These programs implement a correlation, or empirical function, wich interpolates the results obtained through experimental simulation, acocomplished on test sections – TSs – for the sake of obtaining CHF in a wide core operational range. This work investigate and analyze an alternate method of detrmining CHF using, as a correlation, artificial neural networks – ANNs. In this method, the ANNs are obtained through trainning, making use of backpropagation paradigm, against the same experimental data set that came from the TSs.
15
Truong, Bao H. (Bao Hoai). "Critical heat flux enhancement via surface modification using colloidal dispersions of nanoparticles (Nanofluids)". Thesis, Massachusetts Institute of Technology, 2008. http://hdl.handle.net/1721.1/44775.
Testo completoAbstract (sommario):
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2008.Includes bibliographical references (leaves 97-103).
Nanofluids are engineered colloidal dispersions of nanoparticles (1-100nm) in common fluids (water, refrigerants, or ethanol...). Materials used for nanoparticles include chemically stable metals (e.g., gold, silver, copper), metal oxides (e.g., alumina, zirconia, silica, titania) and carbon in various forms (e.g., diamond, graphite, carbon nanotubes). The attractive properties of nanofluids include higher thermal conductivity, heat transfer coefficients (HTC) and boiling critical heat flux (CHF) than that of the respective base fluid. Nanofluids have been found to exhibit a very significant enhancement up to 200% of the boiling CHF at low nanoparticle concentrations. In this study, nanofluids were investigated as an agent to modify a heater surface to enhance Critical Heat Flux (CHF). First, the CHF of diamond, Zinc Oxide and Alumina water-based nanofluids at low volume concentration (<1 vol%) were measured to determine if nanofluid enhances CHF as seen in literature. Subsequently, the heaters are coated with nanoparticles via nucleate boiling of nanofluids. The CHF of water was measured using these nanoparticle precoated heaters to determine the magnitude of the CHF enhancement. Characterization of the heaters after CHF experiments using SEM, confocal, and contact angle were conducted to explain possible mechanisms for the observed enhancement. The coating thickness of the nanoparticle deposition on a wire heater as a function of boiling time was also investigated. Finally, theoretical analyses of the maximum CHF and HTC enhancement in term of wettability were performed and compared with the experimental data. The CHF of nanofluids was as much as 85% higher than that of water, while the nanoparticle pre-coated surfaces yielded up to 35% CHF enhancement compared to bare heaters.
(cont.) Surface characterization of the heaters after CHF experiments showed a change in morphology due to the nanoparticles deposition. The coating thickness of nanoparticle was found to deposit rather quickly on the wire surface. Within five minutes of boiling, the coating thickness of more than 1 pm was achieved. Existing CHF correlations overestimated the experimental data.
by Bao H. Truong.
S.M.
16
Skirpan, Zachary. "Multiphase CFD benchmark of experimental critical heat flux data at PWR operating conditions". Thesis, Massachusetts Institute of Technology, 2020. https://hdl.handle.net/1721.1/127300.
Testo completoAbstract (sommario):
Thesis: S.M., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, May, 2020Cataloged from the official PDF of thesis.
Includes bibliographical references (pages 85-89).
Critical Heat Flux (CHF) in flow boiling represents the fundamental upper limit for thermal hydraulic performance of PWR fuel. Currently, the nuclear industry relies on expensive, prototypical experiments using electrically heated, full-height rod assemblies to determine the limit of the boiling crisis. The development of next-generation Multiphase Computational Fluid Dynamics (M-CFD) approaches for the prediction of CHF seeks to represent the detailed physics of the boiling process up to its critical condition, rather than estimating it from ad-hoc thresholds. In this work we evaluate the advancement in M-CFD boiling attained by the Consortium for Advanced Simulation of Light water reactors (CASL). The CASL approach builds off an industry-lab-university collaboration with individual validation of interfacial momentum closures and wall boiling models.
The M-CFD simulations were implemented in the commercially available STAR-CCM+ software, and benchmarked against experimental observations of CHF collected at the University of Wisconsin by Duarte. In this work, 15 M-CFD simulations were completed. Boiling curves were generated for each test case. The modelled boiling characteristics were then compared to expected physical parameters to determine model accuracy. Structural spacers are the main driver of vapor accumulations leading to the breakdown of boiling heat transfer at CHF. Interestingly, M-CFD solutions indicate that the Departure from Nucleate Boiling (DNB) first occurs in areas not measured by the experimental thermal couples used to detect CHF, possibly resulting in a late experimental detection. Additionally, sensitivity studies are conducted for relevant model terms to understand their impact on CHF.
Leveraging the results from this sensitivity study, it is suggested that improved predictions could 1) increase the turbulent dispersion at the wall to account for turbulence under-prediction and 2) increase the minimum bubble size limiter to reflect physically observed coalesced bubble sizes after departure. For future work, higher spatial resolution measurements for detecting DNB are suggested for better experimental CHF predictions. A fully mechanistic approach for modeling the heat flux partitioning and subsequent wall boiling in M-CFD is also needed to more effectively simulate the proper heat transfer mechanisms and boiling physics before CHF. It is the hope that through this work and further M-CFD heat transfer investigations that similar methods may be validated for CHF detection and streamline the fuel design process in the nuclear industry.
by Zachary Skirpan.
S.M.
S.M. Massachusetts Institute of Technology, Department of Nuclear Science and Engineering
17
Ahmad, Masroor. "Critical heat flux and associated phenomena in forced convective boiling in nuclear systems". Thesis, Imperial College London, 2012. http://hdl.handle.net/10044/1/9181.
Testo completoAbstract (sommario):
In evaporation of a liquid flowing in a tube or nuclear fuel element, there exists a transition (known as "dryout", "burnout", "boiling crisis" or "critical heat flux", CHF) from a high heat transfer coefficient regime to one of greatly reduced heat transfer coefficient. The conditions leading to dryout or CHF and the behaviour of wall temperatures in the ("post dryout or post CHF") region beyond it are of immense importance in nuclear reactor safety. In a nuclear reactor, the clad temperature excursion in the post-dryout region may be unacceptably high and the prediction of the location of dryout and the magnitude of the temperature excursion into the post-dryout region is of great importance. Moreover, the dryout transition and its effects are important not only in nuclear plant but also in many other types of heat transfer equipment. The main focus of work described in this thesis was the improvement and validation of phenomenological models for the prediction of CHF and of heat transfer beyond CHF ("post CHF" or "post dryout" heat transfer). The main focus has been on the process of annular film dryout. In phenomenological modelling of this process the dryout location prediction is sensitive to the boundary value of entrained fraction at churn annular transition, especially at high flow rates. The model was extended to churn flow so that integration of entrainment, deposition and evaporation processes could be started from onset of churn flow. A new correlation for the prediction of entrainment rate in churn flow was presented. The application of the new methodology to experimental data leads to improved predictions of CHF. Another long-standing problem, i.e. effect of heat flux on droplet entrainment, is addressed by analysing the contradictory results of previous experiments by using the annular film dryout model. The capability of phenomenological models to cover the whole range of CHF scenarios, i.e. from subcooled or very low quality to very high quality CHF, was demonstrated by using a possible transition criterion from bubble crowding model (an improved version of the Weisman Pie model) to annular film dryout model. These improved phenomenological models captured trends of CHF data very well (including the Look Up Table data of Groeneveld et al. 2007) and produced improved results over a wide range of system parameters such as pressure, mass flux and critical quality. The implementation of the phenomenological models was pursued by modifying and developing an Imperial College computer code GRAMP. In addition to its application in modelling CHF, the GRAMP code was extended to the post dryout region and predictions for this region compared to a range of data and the results were found to be satisfactory.
18
Wong, Wai-Chuen. "Effect of tube diameter on critical heat flux in vertical steam-water flow". Thesis, University of Ottawa (Canada), 1997. http://hdl.handle.net/10393/4295.
Testo completoAbstract (sommario):
The effect of tube diameter on critical heat flux (CHF) has been examined by (1) analyzing existing experimental data, (2) reviewing existing correlations and semi-analytical models, and (3) obtaining CHF measurements in two vertical tubes of different diameters. Based on constant local flow conditions, the parametric trend of available CHF data shows generally a decreasing CHF with increasing tube diameter. Due to the large data scatter (inherent uncertainty within different sets of data), a definite conclusion cannot be drawn. The observations indicated that the tube-diameter effect on CHF is a complex phenomenon and is affected by pressure, mass flux and thermodynamic quality as well as tube diameter itself. An improved correction factor has been derived to account for the effect of tube diameter on CHF. It is also presented in terms of the tube diameter to a power "n", consistent with existing recommendations. Rather than a constant value, the coefficient is expressed as a function of mass flux and thermodynamic quality to include other compounded effects. (Abstract shortened by UMI.)
19
Bhatia, Mohit. "MgB2 superconductors: processing, characterization and enhancement of critical fields". Columbus, Ohio : Ohio State University, 2007. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1190055943.
Testo completo
20
Stoddard, Ryan Manse. "Onset of flow instability and critical heat flux in horizontal, thin, uniformly-heated annuli". Thesis, Georgia Institute of Technology, 2000. http://hdl.handle.net/1853/17135.
Testo completo
21
Carson, Robert J. "Critical heat flux for a heated surface impacted by a stream of liquid droplets". Thesis, Georgia Institute of Technology, 1991. http://hdl.handle.net/1853/19579.
Testo completo
22
Neal, Peter Ross Chemical Sciences & Engineering Faculty of Engineering UNSW. "An examination of the nature of critical flux and membrane fouling by direct observation". Awarded by:University of New South Wales. School of Chemical Sciences and Engineering, 2006. http://handle.unsw.edu.au/1959.4/30584.
Testo completoAbstract (sommario):
Securing water in the right quantities at the right quality for the right price is a major issue around the world. Membranes are making an increasingly important contribution to meeting this need; however their performance is limited by fouling. This thesis reports on an investigation into the fouling of systems related to water treatment using the Direct Observation Through the Membrane (DOTM). The investigation focused on the measurement of critical flux and observation of particle behaviour under a variety of conditions and for a number of different particles. The range of meanings attributed to critical flux in the literature was analysed and several proposals made for the improved use of the concept. In particular, critical flux determination techniques were classified by whether they measure resistance changes or particle deposition; leading to the definition of Critical Resistance and Critical Deposition Fluxes. In this thesis the deposition definition is used exclusively. The effect of Reynolds number and spacer orientation on critical flux was correlated for spacer-filled channels. The heterogeneous deposition patterns observed with regions of heavy deposition next to areas of little or no deposition. This pattern was related to the local hydrodynamics of spacer cells (a few mm2 in size). The correlations developed for critical flux in spacer-filled channels were adjusted for submicron particle size and incorporated into a SpiralWound Module (SWM) leaf model and then used to simulate the fouling of SWM leaves under a range of operating conditions and operating policies. The Mass Balance technique of critical flux determination was also briefly assessed. The applicability of critical flux criteria to SWM arrays was discussed. Fouling, particle behaviour and critical flux were also investigated in air-sparged systems. The post-cleaning water flux was found to be enhanced when the membrane is fouled in the presence of bubbles. The rate of flux decline was reduced by bubbles. Critical flux increased with air flowrate, and decreased with increased liquid flowrate and concentration. Bubbles caused particles to periodically deposit on the membrane. Particles were observed to stream past the membrane under the influence of back-diffusive forces. Video clips of particulate fouling are provided.
23
Andersson, Patrik. "Predicting the deflection of electric heater rods in a critical heat flux test loop". Thesis, KTH, Fysik, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-147354.
Testo completo
24
Phillips, Bren Andrew. "Nano-engineering the boiling surface for optimal heat transfer rate and critical heat flux". Thesis, Massachusetts Institute of Technology, 2011. http://hdl.handle.net/1721.1/76536.
Testo completoAbstract (sommario):
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2011.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 130-133).
The effects on pool boiling characteristics such as critical heat flux and the heat transfer coefficient of different surface characteristics such as surface wettability, roughness, morphology, and porosity are not well understood. Layer-by-layer nanoparticle coatings were used to modify the surface of a sapphire heater to control the surface roughness, the layer thickness, and the surface chemistry. The surface was then tested in a water boiling test at atmospheric pressure while imaging the surface with high speed infrared thermography yielding a 2D time dependent temperature profile. The critical heat flux and heat transfer coefficient were enhanced by over 100% by optimizing the surface parameters. It was found that particle size of the nanoparticles in coating, the coating thickness, and the wettability of the surface have a large impact on CHF and the heat transfer coefficient. Surfaces were also patterned with hydrophobic "islands" within a hydrophilic "sea" by coupling the Layer-by-layer nanoparticle coatings with an ultraviolet ozone technique that patterned the wettability of the surface. The patterning was an attempt to increase the nucleation site density with hydrophobic dots while still maintaining a large hydrophilic region to allow for rewetting of the surface during the ebullition cycle and thus maintaining a high critical heat flux. The patterned surfaces exhibited similar critical heat fluxes and heat transfer coefficients to the surfaces that were only modified with layer-by-layer nanoparticle coatings. However, the patterned surfaces also exhibited highly preferential nucleation from the hydrophobic regions demonstrating an ability to control the nucleation site layout of a surface and opening an avenue for further study.
by Bren Andrew Phillips.
S.M.
25
Ahmed, Amer Naji. "Rejection and critical flux of calcium sulphate in a ceramic titanium dioxide nanofiltration membrane". Thesis, University of Manchester, 2013. https://www.research.manchester.ac.uk/portal/en/theses/rejection-and-critical-flux-of-calcium-sulphate-in-a-ceramic-titanium-dioxide-nanofiltration-membrane(19fb66ec-3ba5-4b13-b65f-690fbb80ddc3).html.
Testo completoAbstract (sommario):
This thesis describes the rejection efficiency and the fouling behaviour of calcium sulphate solutes in a 1 nm tubular ceramic titanium dioxide nanofiltration membrane. Calcium sulphate is considered as one of the greatest scaling potential inorganic salts that responsible for membrane fouling which represents a main challenge in the expansion of membrane processes for desalination of brackish and saline water. The surface charge type and magnitude for the composite amphoteric TiO_2 membrane were characterised using streaming potential measurements. Electrokinetic membrane experiments were conducted in a background electrolyte comprising 0.01 M (NaCl). The zeta potential was estimated from the measured streaming potential using the Helmoholtz-Smoluchowski equation and the surface charge density was subsequently calculated using the Gouy-Chapman and Graham equations. The experimental results showed that the membrane was negatively charged at neutral pH and its iso-electrical point (i.e.p) was at pH of 4.0. The rejection behaviour of calcium sulphate at three different initial concentrations (0.001, 0.005 and 0.01 M) were investigated compared to other naturally occurring minerals (NaCl, Na_2 SO_4, CaCl_2) in single salt solutions. The rejection experiments were conducted at five different applied trans-membrane pressures ranged from 1.0 to 5.0 bars. Salt retention measurements showed that the rejection sequence was R (CaSO_4) > R (Na_2 SO_4) > R (CaCl_2) > R (NaCl). This rejection sequence behaviour showed an inverse relationship with the diffusion coefficients of the four salts. The salt with the lowest diffusion coefficient (CaSO_4) showed the highest rejection (43.3%), whereas that with the highest diffusion coefficient showed the lowest rejection. The rejection of calcium sulphate solution at saturation concentration was also conducted after a suspension solution of 0.015 M (CaSO_4) was prepared and filtered. The ionic analysis for calcium sulphate permeates indicated that, for the negatively charged TiO_2 membrane, the rejection for bivalent anion (SO_4^(2-) ) was higher than that of the bivalent cation (Ca^(2+) ).The critical flux (CF) experiments were carried out at six trans-membrane pressure ranged from 1.0 to 6.0 bars to identify the form and the onset of calcium sulphate fouling (as gypsum) using different concentrations below saturation concentration (0.001, 0.005, 0.01 M) and at saturation concentration. Two different flux-pressure techniques have been applied and compared to determine the critical flux values; these are: step by step technique and standard stepping technique. The obtained critical flux results from both measuring techniques (for all the four sessions) confirmed that the critical flux was reached and exceeded. The present work indicated that the resulting critical flux values from both measuring procedures were decreased as the ionic strengths of the calcium sulphate solutes were increased. A mathematical model has been proposed to identify the key parameters that affect the transport performance inside the TiO_2 nanofiltration membrane. The original Donnan steric pore model (DSPM) was used to simulate the rejection of 0.01 M sodium chloride as a reference solution. The membrane effective pore radius was estimated using two different transport models, both of these models depend on the permeation test of uncharged solute (glucose). The Donnan potential was determined based on the membrane effective fixed charge density which was determined by supposing that the membrane surface charge was uniformly distributed in the void volume of cylindrical pores. The theoretical rejection of NaCl solute for the present DSPM model was found to be in agreement with the experimental data.
26
Kiameh, Bassam Philip. "Prediction of critical heat flux (CHF) for non-aqueous fluids in forced convective boiling". Thesis, University of Ottawa (Canada), 1986. http://hdl.handle.net/10393/21731.
Testo completo
27
Zhou, Sihai. "Processing and characterisation of MgB₂ superconductors". Access electronically, 2004. http://www.library.uow.edu.au/adt-NWU/public/adt-NWU20041111.135706/index.html.
Testo completo
28
Bockwoldt, Todd S. "Induced convective enhancement of the critical heat flux for partially heated surfaces in pool boiling". Thesis, Georgia Institute of Technology, 1990. http://hdl.handle.net/1853/13094.
Testo completo
29
Gandolfini, Germain. "Comparison of flux line cutting behaviour in high critical temperature and conventional Type II superconductors". Thesis, University of Ottawa (Canada), 1990. http://hdl.handle.net/10393/5757.
Testo completoAbstract (sommario):
The magnetic behaviour (magnetization curves, Meissner effect, hysteresis losses, remanent flux and flux line cutting) for high $T\sb{c}$ samples of Nd and $YBa\sb{2}Cu\sb{3}O\sb{7-x}$ at 77 K, semi-reversible PbIn and hysteretic VTi at 4.2 K have been investigated and compared. The magnetic behaviour of the high $T\sb{c}$ samples and of the PbIn are remarkably similar but contrasts dramatically with the phenomena observed in the VTi. A hump structure appears in the low field region of the initial magnetization of the high $T\sb{c}$ samples. This phenomenon occurs because the sintered samples consist of a compact agglomeration of small irregularly shaped grains which are electrically coupled before the hump structure but are isolated after this feature. We show that the appropriate calibration should be based on the diamagnetic response of the uncoupled grains. A large ratio of $j\sb{c\Vert}$ to $j\sb{c\perp}$ (the critical current densities $\Vert$ and $\perp$ to the flux line density) accounts for the observations on the VTi in the flux cutting regime whereas $j\sb{c\Vert} \approx j\sb{c\perp}$ is indicated by the behaviour of the high $T\sb{c}$ and PbIn samples.
30
Tetreault-Friend, Melanie. "Systematic investigation of the effects of hydrophilic porosity on boiling heat transfer and critical heat flux". Thesis, Massachusetts Institute of Technology, 2014. http://hdl.handle.net/1721.1/95571.
Testo completoAbstract (sommario):
Thesis: S.M., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2014.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 97-99).
Predicting the conditions of critical heat flux (CHF) is of considerable importance for safety and economic reasons in heat transfer units, such as in nuclear power plants. It is greatly advantageous to increase this thermal limit and much effort has been devoted to studying the effects of surface characteristics on it. In particular, recent work carried out by O'Hanley demonstrated the separate effects of surface wettability, porosity, and roughness on CHF, and found that porous hydrophilic surface coatings provided the largest CHF increase, with a 50-60% enhancement over the base case. In the present study, a systematic investigation of the effects that the physical characteristics of the hydrophilic layers have on heat transfer was conducted. Parameters experimentally explored include porous layer thickness, pore size, and void fraction (pore volume fraction). The surface characteristics are created by depositing layer-by-layer (LbL) thin compact coatings made of hydrophilic SiO₂ nanoparticles of various sizes. A new coating was developed to reduce the void fraction by using polymers to partially fill the voids in the porous layers. All test surfaces are prepared on indium tin oxide - sapphire heaters and tested in a pool boiling facility at atmospheric pressure in MIT's Thermal-Hydraulics Laboratory. Results indicate that CHF follows a trend with respect to each parameter studied and clear CHF maxima reaching up to 114% enhancement are observed for specific thickness and pore size values. ZnO₂ nanofluid-generated coatings are also prepared and their boiling performance is compared to the boiling performance of the engineered LbL coatings. The results highlight the dependence of CHF on capillary wicking and are expected to allow further optimization of the nanoengineered surfaces.
by Melanie Tetreault-Friend.
S.M.
31
DeWitt, Gregory L. "Investigation of downward facing critical heat flux with water-based nanofluids for In-Vessel Retention applications". Thesis, Massachusetts Institute of Technology, 2011. http://hdl.handle.net/1721.1/76495.
Testo completoAbstract (sommario):
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2011.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 359-368).
In-Vessel Retention ("IVR") is a severe accident management strategy that is power limiting to the Westinghouse AP1000 due to critical heat flux ("CHF") at the outer surface of the reactor vessel. Increasing the CHF level by altering the cooling fluid would increase the safety margin of current design power or allow for higher power. The modification to current licensed design to implement a new cooling fluid would not require significant changes to the containment and associated systems. Previous research at MIT and other institutions has demonstrated that CHF of water on a heated metal surface can be increased from 30% to 200% with the introduction of nanoparticles. Alumina has shown the best CHF enhancement of the nanoparticles tested to date at MIT. Alumina nanoparticles and water based nanofluids have also shown long term stability in solution, which is important for the long time frame (hours to days) of IVR. To measure the CHF of geometry and conditions relevant to IVR for the AP1000, a two-phase flow loop has been designed and built. The test section designed to have hydrodynamic similarity to the AP 1000 and allows for all angles that represent the bottom surface of the reactor vessel. Research completed herein measured CHF for varied conditions of orientation angle, pressure, mass flux, fluid type, and surface material. Results for stainless steel with water based alumina 0.001% by volume nanofluid indicate an average 70% CHF enhancement with a range of 17% to 108% for geometry and conditions expected for IVR. Experiments also indicate that only about thirty minutes of boiling time is needed to obtain CHF enhancement. Implementation could involve storage tanks of high concentration nanofluids installed in containment. Once the IVR strategy is initiated with flooding of the vessel cavity with water from the In-containment Refueling Water Storage Tank ("IRWST"), the nanofluids would be released to mix as the natural circulation flow sets up along the gap between the vessel and the insulation mounted to the concrete wall in the vessel cavity. Boiling then plates nanoparticles onto the surface enhancing CHF.
by Gregory Lee DeWitt.
Ph.D.
32
Kim, Sung Joong Ph D. Massachusetts Institute of Technology. "Subcooled flow boiling heat transfer and critical heat flux in water-based nanofluids at low pressure". Thesis, Massachusetts Institute of Technology, 2009. http://hdl.handle.net/1721.1/53274.
Testo completoAbstract (sommario):
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2009.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 285-290).
A nanofluid is a colloidal suspension of nano-scale particles in water, or other base fluids. Previous pool boiling studies have shown that nanofluids can improve the critical heat flux (CHF) by as much as 200%. In this study, subcooled flow boiling heat transfer and CHF experiments were performed with low concentrations of alumina, zinc oxide, and diamond nanoparticles in water (< 0.1 % by volume) at atmospheric pressure. It was found that for comparable test conditions the values of the nanofluid and water heat transfer coefficient (HTC) are similar (within ±20%). The HTC increased with mass flux and heat flux for water and nanofluids alike, as expected in flow boiling. The CHF tests were conducted at 0.1 MPa and at three different mass fluxes (1500, 2000, 2500 kg/m2s) under subcooled conditions. The maximum CHF enhancement was 53%, 53% and 38% for alumina, zinc oxide and diamond, respectively, always obtained at the highest mass flux. The measurement uncertainty of the CHF was less than 6.2%. A post-mortem analysis of the boiling surface reveals that its morphology is altered by deposition of the particles during nanofluids boiling. A confocal-microscopy-based examination of the test section revealed nanoparticles deposition not only changes the number of micro-cavities on the surface, but also the surface wettability. A simple model was used to estimate the ensuing nucleation site density changes, but no definitive correlation between the nucleation site density and the heat transfer coefficient data could be found.
(cont.) Wettability of the surface was substantially increased for heater coupons boiled in alumina and zinc oxide nanofluids, and such wettability increase seems to correlate reasonably well with the observed marked CHF enhancement for the respective nanofluids. Interpretation of the experimental data was conducted in light of the governing surface parameters and existing models. It was found that no single parameter could explain the observed HTC or CHF phenomena. The existing models were limited in studying the surface effects, suggesting that more accurate models incorporating surface effects need to be developed. Finally, the research activities performed in this thesis help identify the research gaps and indicate future research directions.
by Sung Joon Kim.
Ph.D.
33
Soltanian, Saeid. "Development of superconducting magnesium diboride conductors". Institute for Superconducting & Electronic Materials - Faculty of Engineering, 2004. http://ro.uow.edu.au/theses/381.
Testo completoAbstract (sommario):
The work in this thesis concentrates on the fabrication and characterization of MgB2 superconducting bulk wire and tape. An overview of the research on MgB2 superconductor during the last three years is also provided. High transport and magnetic critical current density values above 105 A/cm2 have been obtained for metal-clad wires and tapes. Fe-clad MgB2 tapes were fabricated using a powder-in-tube technique. The tape shows a sharp transition with a transition width ΔTc of 0.2 K and a Tc0 of 37.5 K. An high transport critical current value of 1.7 × 104 A/cm2 for both 29.5 K in 1 Tesla and 33 K in zero applied field has been achieved. The effects of sintering time and temperature on the formation and critical current densities of Feclad MgB2 wires is also investigated. MgB2 wires were sintered for different periods of time at predetermined temperatures. In contrast to the common practice of sintering for several hours, results show that there is no need for prolonged heat treatment in the fabrication of Fe/MgB2 wires. A total time in the furnace of several minutes is enough to form nearly pure MgB2. Jc of 4.5×105 A/cm2 in zero field and above 105 A/cm2 in 2 T at 15 K has been achieved for Fe/MgB2 wires sintered for a short time. These findings substantially simplify the fabrication process, making it possible to have a continuous process for fabrication and reducing the costs for large-scale production of MgB2 wires. Ag and Cu clad MgB2 wires were also fabricated using an in-situ reaction method. The effects of a shorter than usual sintering on the critical current densities of Ag and Cu clad MgB2 wires were studied. For Ag clad wire Jc is improved by more than two times after the short period sintering process. Jc values of 1.2×105 A/cm2 in zero field and above 104 A/cm2 in 2 T at 20 K have been achieved for Ag clad MgB2 wire which is only sintered for a few minutes at 800 oC. However, a remarkable degree of reaction has been found between the superconducting cores and the sheath materials, leading to the formation of Cu2Mg and Ag3Mg for copper and silver clad wires, respectively. The results show that the short sintering causes less reaction between the magnesium and the sheath materials and markedly improves the critical current density. Our results also show that iron is still the best sheath material for MgB2 superconductor wire and tape. Sixteen-filament stainless steel/Fe/MgB2 wires were fabricated by the powder-in-tube method followed by groove rolling. Magnetic critical current densities of 3.4×105 A/cm2 in 0.5 T and about 1.9×105 A/cm2 in 1 T at 5 K were achieved. Results on transport Jc of solenoid coils up to 100 turns fabricated with Cu-sheathed MgB2 wires using a windreaction in-situ technique are reported. Despite the low density of the single core and some reaction between the Mg and the Cu-sheath, our results demonstrate that the decrease in transport Jc with increasing length of MgB2 wires is insignificant. Solenoid coils with diameters as small as 10 mm can be readily fabricated using a wind-reaction in-situ technique. The Jc of coils is essentially the same as for straight wires. Jc values of 133,000 A/cm2 and 125,000 A/cm2 at 4 K and self field have been achieved for small coil wound using Cu-sheathed tape and Cu-sheathed wire respectively. The results indicate that the MgB2 wires have potential for large scale applications. The effect of chemical doping on the superconductivity and critical current density of MgB2 superconductor is investigated. Enhancements in the Jc field performance as well as the irreversibility field were obtained due to chemical doping with both C and SiC nano-particles. Doping MgB2-x(SiC)x/2 with x = 0, 0.2 and 0.3 and a 10 wt% nano-SiC doped MgB2 sample, led to slight decrease in Tc and significantly enhanced Hc2, Hirr and Jc at high magnetic fields. Compared to the non-doped sample, Jc for the 10 wt% doped sample increased by a factor of 32 at 5 K and 8 T, 42 at 20 K and 5 T, and 14 at 30 K and 2 T. At 20 K, which is considered to be a benchmark operating temperature for MgB2, the best Jc for the doped sample was 2.4×105 A/cm2 at 2 T, which is comparable to Jc of the best Ag/Bi-2223 tapes. At 20 K and 4 T, Jc was 36,000 A/cm2, which is an order of magnitude higher than for the Fe/MgB2 tape. Our results show that there are two distinguishable but closely related mechanisms: increase of Hc2 and improvement of flux pinning that control the performance of Jc(H) in the samples. SiC-doping introduced many nano-scale precipitates and disorders at B and Mg sites, provoking a high resistivity of ρ (40K) = 300 μΩ-cm (RRR = 1.75) for the SiC-doped sample, leading to significant enhancement of both Hc2 and Hirr with only minor effects on Tc. EELS and TEM analysis revealed impurity phases: Mg2Si, MgO, MgB4, BOx, SixByOz, and BC at a scale below 10 m and an extensive domain structure of 2-4nm domains in the doped sample which serve as strong pinning centers. The effect of nano-SiC doping on the critical current density and flux pinning of Fe/MgB2 wires is also investigated. The depression of Tc with increasing SiC doping level remained rather small. High level SiC doping resulted in a substantial enhancement in the Jc(H) performance. The transport Jc for all the wires is comparable to the magnetic Jc at higher fields despite the low density of the samples. The transport Ic for the 10 wt% doped Fe/MgB2 wire reached 675 A at 24 K and 1 T (Jc = 140,000 A/cm2) and 500 A at 20 K and 2T (Jc = 103,000A/cm2). The transport Jc for the 10wt% SiC doped MgB2 wire is 30 times higher than for the undoped wire. SiC doped MgB2 polycrystalline samples were fabricated using different grain sizes (20 nm, 100 nm, and 37 μm) of SiC and different doping levels (0, 8, 10, 12, 15 wt %) in order to investigate the effect of the particle size of the starting SiC powder on the properties of samples. Results show that grain sizes of the starting precursors of SiC have a strong effect on the critical current density and its field dependence. The smaller the SiC grains are, the better the Jc field performance is. Significant enhancement of Jc and the irreversibility field Hirr were revealed for all the SiC doped MgB2 with additions up to 15 wt%. A Jc as high as 20,000 A/cm2 in 8 T at 5 K was achieved for the sample doped with 10 wt% SiC with a grain size of 20 nm. Results indicate that the nano-inclusions and substitution inside MgB2 are responsible for the enhancement of flux pinning. Polycrystalline MgB2-xCx samples with x=0.05, 0.1, 0.2, 0.3, 0.4 nano-particle carbon powder were prepared using an in-situ reaction method under well-controlled conditions to limit the extent of C substitution. It was found that both the a-axis lattice parameter and the Tc decreased monotonically with increasing doping level. However, for the sample doped with the highest nominal composition of x=0.4 the Tc dropped only 2.7 K. The nano-C doped samples showed an improved field dependence of the Jc compared with the undoped sample over a wide temperature range. The nhancement by C-doping is not as strong as for nano-SiC doped MgB2. X-ray diffraction results indicate that C reacted with Mg to form nano-size Mg2C3 and MgB2C2 particles. A study of ac susceptibility, magnetic shielding and the sample size effect is presented in Chapter 6. Systematic ac susceptibility measurements were performed on MgB2 bulk samples. It is shown that the flux creep activation energy is a nonlinear function of the current density U , indicating a nonlogarithmic relaxation of the current density in this material. The dependence of the activation energy on the magnetic field is determined to be a power law (J ) ∝ J −0.2 U(B) ∝ B−1.33 , showing a steep decline in the activation energy with magnetic field, which accounts for the steep drop in the critical current density with magnetic field that is observed in MgB2. Magnetic shielding was investigated by means of transport critical current measurements for Fe-sheathed MgB2 round wires. Strong magnetic shielding by the iron sheath was observed, resulting in a decrease in Ic by only 15% in a field of 0.6 T at 32 K. In addition to shielding, interaction between the iron sheath and the superconductor resulted in a constant Ic between 0.2 and 0.6 T. This was well beyond the maximum field for effective shielding of 0.2 T. This effect can be used to substantially improve the field performance of MgB2/Fe wires at fields at least 3 times higher than the range allowed by mere magnetic shielding by the iron sheath. The dependence of Ic on the angle between the field and the current showed that the transport current does not flow straight across the wire, but meanders between the grains. The effect of sample size on the critical current density and the flux pinning of pure and SiC doped MgB2 bulk samples has been investigated. At high fields a systematic degradation of magnetic Jc and Hirr was observed as the sample size decreased. However, Jc remarkably increased on decreasing the sample volume at low magnetic fields below 1 T. The SiC doped samples show less sample size effect than the pure samples, indicating a larger n-factor and therefore a stronger pinning effect due to SiC doping.
34
Yao, Qiwen. "Study of synthesis, structures and superconducting properties of magnesium diboride MgB₂ material". Access electonically, 2004. http://ro.uow.edu.au/theses/404.
Testo completo
35
Aritake, T., T. Noda, H. Shimizu, Y. Yokomizu, T. Matsumura e N. Murayama. "Relation between critical current density and flux flow resistivity in Bi2223 bulk element for fault current limiter". IEEE, 2003. http://hdl.handle.net/2237/6788.
Testo completo
36
Nyhus, Jørgen. "Resonant Ultrasonic System Design, and Measurements of Critical Behaviour and Flux-Line Elasticity in High Temperature Superconductors". Doctoral thesis, Norwegian University of Science and Technology, Department of Physics, 2001. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-2117.
Testo completo
37
Vaitekunas, David A. "An investigation of the effect of flow obstructions on critical heat flux, pressure drop and heat transfer". Thesis, National Library of Canada = Bibliothèque nationale du Canada, 2000. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape4/PQDD_0020/NQ57073.pdf.
Testo completo
38
Kang, Yong Tae. "Experimental investigation of critical heat flux in transient boiling systems with vertical thin rectangular parallel plate channels /". The Ohio State University, 1994. http://rave.ohiolink.edu/etdc/view?acc_num=osu1244826053.
Testo completo
39
Shimizu, H., Y. Yokomizu, M. Goto, T. Matsumura e N. Murayama. "A study on required volume of superconducting element for flux flow resistance type fault current limiter". IEEE, 2003. http://hdl.handle.net/2237/6787.
Testo completo
40
Gong, Shengjie. "An Experimental Study on Micro-Hydrodynamics of Evaporating/Boiling Liquid Film". Doctoral thesis, KTH, Kärnkraftsäkerhet, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-50216.
Testo completoAbstract (sommario):
Study of liquid film dynamics is of significant importance to the understanding and control of various industrial processes that involve spray cooling (condensation), heating (boiling), coating, cleaning and lubrication. For instance, the critical heat flux (CHF) of boiling heat transfer is one of the key parameters ensuring the efficiency and safety of nuclear power plants under both operational and accident conditions, which occurs as the liquid layers (microlayer and macrolayer) near the heater wall lose their integrity. However, an experimental quantification of thin liquid film dynamics is not straightforward, since the measurement at micro-scale is a challenge, and further complicated by the chaotic nature of boiling process. The object of present study is to develop experimental methods for the diagnosis of liquid film dynamics, and to obtain data for the film instability under various conditions. A dedicated test facility was designed and constructed where micro conductive probes and confocal optical sensors were used to measure the thickness and dynamic characteristics of a thin liquid film on various heater surfaces, while a high speed camera was used to get visual observation. Extensive tests were performed to calibrate and verify the two thickness measuring systems. The micro conductive measuring system was proven to have a high reliability and repeatability with maximum system error less than 5µm, while the optical measuring system is capable of recording the film dynamics with spatial resolution of less than 1 mm. The simultaneous measurement on the same liquid film shows that the two techniques are in a good agreement with respect to accuracy, but the optical sensors have a much higher acquisition rate up to 30 kHz, which are more suitable for rapid process. The confocal optical sensors were therefore employed to measure the dynamic thickness of liquid films (ethanol, hexane and water) evaporating on various horizontal heater surfaces (aluminum, copper, silicon, stainless steel and titanium) to investigate the influences of heat flux, the surface and liquid properties on the film instability and the critical thickness. The critical thickness of water film evaporating on various surfaces was measured in the range of 60-150 mm, increasing with the increased contact angle or increased heat flux (evaporating rate) and also with the decreased thermal conductivity of the heater material. The data suggest the conjugate heat transfer nature of the evaporating liquid film dynamics at higher heat fluxes of interest to boiling and burnout. In the case of hexane on the aged titanium surface with contact angle of ~3o, the liquid film is found resilient to rupture, with film oscillations at relatively large amplitude ensuing as the averaged film thickness decreases below 15 µm. To interpret our experimental findings on liquid film evolution and its critical thickness at rupture, a theoretical analysis is also performed to analyze the dynamics of liquid films evaporating on heater surfaces. While the influences of liquid properties, heat flux, and thermal conductivity of heater surface are captured by the simulation of the lubrication theory, influence of the wettability is considered via a minimum free energy criterion. The thinning processes of the liquid films are generally captured by the simulation of the lubrication theory. For the case with ideally uniform heat flux over the heater surface, the instability of the liquid film occurs at the thickness level of tens micro meters, while for the case of non-uniform heating, the critical thicknesses for the film rupture are closer to the experimental data but still underestimated by the lubrication theory simulation. By introducing the minimum free energy criterion to considering the influence of surface wettability, the obtained critical thicknesses have a good agreement with the experimental ones for both titanium and copper surfaces, with a maximum deviation less than ±10%. The simulations also explain why the critical thickness on a copper surface is thinner than that on a titanium surface. It is because the good thermal conductivity of copper surface leads to uniform temperature distribution on the heat surface, which is responsible for the resilience of the liquid film to rupture. A silicon wafer with an artificial cavity fabricated by Micro Electronic Mechanical System (MEMS) technology was used as a heater to investigate the dynamics of a single bubble in both a thick and thin liquid layer under low heat flux (<60 kW/m2). The maximum departure diameter of an isolated bubble in a thick liquid film was measured to be 3.2 mm which is well predicted by the Fritz equation. However, in a thin liquid layer with its thickness less than the bubble departure diameter, the bubble was stuck on the heater surface with a dry spot beneath. A threshold thickness of the liquid film which enables the dry spot rewettable was obtained, and its value linearly increases with increasing heat flux. In addition, another test section was designed to achieve a constant liquid film flow on a titanium nano-heater surface which helps to successfully carry boiling in the liquid film from low heat flux until CHF. Again, the confocal optical sensor was employed to measure the dynamics of the liquid film on the heater surface under varied heat flux conditions. A statistical analysis of the measured thickness signals that emerge in a certain period indicates three distinct liquid film thickness ranges: 0~50 µm as microlayer, 50~500 µm as macrolayer, 500~2500 µm as bulk layer. With increasing heat flux, the bulk layer disappears, and then the macrolayer gradually decreases to ~105 µm, beyond which instability of the liquid film may lose its integrity and CHF occurs. In addition, the high-speed camera was applied to directly visualize and record the bubbles dynamics and liquid film evolution. Dry spots were observed under some bubbles occasionally from 313 kW/m2 until CHF with the maximum occupation fraction within 5%. A dry spot was rewetted either by liquid receding after the rupture of a bubble or by the liquid spreading from bubbles’ growth in the vicinity. This implies that the bubbles’ behavior (growth and rupture) and their interactions in particular are of paramount importance to the integrity of liquid film under nucleate boiling regime.QC 20111205
VR-2005-5729, MSWI
41
Stromberger, Jöerg H. "Effects of forced wall vibration on the onset of flow instability and critical heat flux in uniformly-heated microchannels". Diss., Georgia Institute of Technology, 2003. http://hdl.handle.net/1853/17062.
Testo completo
42
Edwards, Bronwyn K. "Effect of combined nanoparticle and polymeric dispersions on critical heat flux, nucleate boiling heat transfer coefficient, and coating adhesion". Thesis, Massachusetts Institute of Technology, 2009. http://hdl.handle.net/1721.1/53288.
Testo completoAbstract (sommario):
Thesis (S.M. and S.B.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2009.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 123-126).
An experimental study was performed to determine thermal performance and adhesion effects of a combined nanoparticle and polymeric dispersion coating. The critical heat flux (CHF) values and nucleate boiling heat transfer coefficients (HTC) of nickel wires pre-coated using 1.0% alumina, 0.1% alumina, 500ppm polyallylamine hydrochloride (PAH), and 0.1% alumina combined with 500ppm PAH dispersions were determined using the pool-boiling method. The adhesion of 0.1% alumina and combined 0.1% alumina and 500ppm PAH coatings was evaluated using the tape and modified bend test methods. Results of the pool boiling experiments showed that the wire heaters pre-coated with combined 0.1% alumina and 500ppm PAH dispersion increase the CHF in water by -40% compared to bare wire heaters, compared to an enhancement of -37% with a 0.1% alumina coating. The combined 0.1% alumina and 500ppm PAH dispersion degrades the wire HTC by less than 1%, compared to a degradation of over 26% with a 0.1% alumina coating. Results from the tape test indicate qualitatively that the combined 0.1% alumina and 500ppm PAH dispersion coating adheres better than the 0.1% alumina nanoparticle coating. Results from the modified bend test showed that the combined 0.1% alumina and 500ppm PAH dispersion coating did not fail at the failure strain of the 0.1% alumina nanoparticle coating (8.108x 10-4). The addition of PAH to alumina nanofluid for creating a nanoparticle coating through boiling deposition was found to improve both coating thermal performance and adhesion over the pure alumina nanofluid.
by Bronwyn K. Edwards.
S.M.and S.B.
43
Milliken, Damion Alexander. "Uranium doping of silver sheathed bismuth-strontium-calcium-copper-oxide superconducting tapes for increased critical current density through enhanced flux pinning". Access electronically, 2004. http://www.library.uow.edu.au/adt-NWU/public/adt-NWU20040810.154223/index.html.
Testo completo
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Chan, Robert Chemical Engineering & Industrial Chemistry UNSW. "Fouling mechanisms in the membrane filtration of single and binary protein solutions". Awarded by:University of New South Wales. Chemical Engineering and Industrial Chemistry, 2002. http://handle.unsw.edu.au/1959.4/18832.
Testo completoAbstract (sommario):
In this study the fouling mechanisms of various microfiltration and ultrafiltration membranes were studied when subjected to crossflow filtration using various protein solutions. Experimentation was carried out using controlled flux experiments and fouling phenomena was investigated via the monitoring of the transmembrane pressure (TMP) and rejection. Electron microscopy was employed to study fouling on microfiltration membranes when single protein solutions were filtered while a novel method involving Matrix-Assisted Laser Desorption Ionisation Mass Spectrometry (MALDI-MS) was developed to qualitatively and quantitatively analyse mixed proteins fouled on ultrafiltration membranes. An apparent critical flux was identified whereby fouling was shown to occur at fluxes where there was no increase in TMP. TMP increase is one of the common indicators of fouling in controlled flux operation. Microfiltration experiments showed that the imposition of the apparent critical flux is accompanied by rapid increases in hydraulic resistance and the membrane wall concentration. Pore blockage and narrowing occurred at somewhat higher fluxes as indicated by increases in the observed rejection. Fouling was not influenced greatly by the addition of electrolytes for microfiltration membranes but observed transmissions were found to be greater than 100% when ultrafiltration membranes were employed. For all membranes used, the actual value of the apparent critical flux was shown to be independent of the salt concentration but dependent on pH. Sub apparent-critical constant flux microfiltration showed the existence of an aggregation/deposition time lag after which the membrane experiences a rapid increase in resistance due to protein aggregates blocking a majority of pores. This phenomenon was shown to be dependent on pH and ionic strength and was concluded to be the product of a balance between electrostatic, solubility and hydrophobic interactions.
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Wong, Pit-Leong. "Performance Improvements of Multi-Channel Interleaving Voltage Regulator Modules with Integrated Coupling Inductors". Diss., Virginia Tech, 2001. http://hdl.handle.net/10919/27148.
Testo completoAbstract (sommario):
The emergence of the Intel Pentium TM processor necessitates that a dedicated converter, the voltage regulator module (VRM), be physically located very close to the processor in computer power systems. The efficiency and transient response specifications of the VRM place contradictory requirements on the inductance. This dissertation discusses possible VRM inductor designs to improve efficiency without compromising transient responses.
The multi-channel interleaving buck converter is the most popular topology for present VRMs. Analysis in this work shows that the small-signal model of an n-channel interleaving buck can be simplified as a single buck converter. The equivalent inductance is 1/n of the inductance in the interleaving channel. The equivalent switching frequency is n times the switching frequency in each channel.
Through the transient response analysis, the critical inductance of the VRM is identified. The critical inductance is a tradeoff point between transient response and efficiency. The inductances smaller than the critical inductance have equal transient responses. For the inductances larger than the critical inductance, the VRM transient voltage spikes increase with the inductance. The critical inductance is the largest inductance that gives the fastest transient responses. The critical inductance is a function of the control bandwidth and the load transient steps.
Although multi-channel interleaving reduces the current ripple stress on the output capacitors, it cannot reduce the current ripples in each channel. The large current ripples reduce the efficiency of the VRM. With the proposed concept of integrated coupling inductors between channels, the converters have larger equivalent inductances in steady-state operation and smaller equivalent inductances in transient response. The steady-state current ripples can be reduced without compromising the transient response. The overall efficiency of the converter is improved.
In order to evaluate the application of the coupling inductor concept in multi-channels, an appropriate magnetic model is required. This dissertation proposes a flux reluctance model for the core and winding structures. With this reluctance model and mathematical transformations, the coupled inductors can be decoupled in the electric circuit simulation model. This reduces the complexity of the model when a large number of inductors are coupled. The model can be easily scaled to model the structures that involve more inductors. Examples are presented to show the application of this proposed model.Ph. D.
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Gourbil, Ange. "Etude expérimentale de l'ébullition convective en milieu poreux : assèchement et flux critique". Phd thesis, Toulouse, INPT, 2017. http://oatao.univ-toulouse.fr/18597/1/GOURBIL_Ange.pdf.
Testo completoAbstract (sommario):
Cette thèse est motivée par le besoin de compléter les connaissances actuelles des phénomènes ayant lieu lors d’un renvoi d’eau dans un lit de débris radioactifs, opération appelée « renoyage » et qui intervient dans une séquence d’accident grave où un cœur de réacteur nucléaire est dégradé suite à une perte prolongée de refroidissement primaire. Notre étude, de nature expérimentale, vise à mieux caractériser la crise d’ébullition en convection forcée, dans un milieu poreux chauffant. Le cœur du dispositif expérimental est un milieu poreux modèle quasibidimensionnel, composé de 276 cylindres disposés entre deux plaques de céramique distantes de 3 mm, dont l’une, transparente, permet de visualiser les écoulements. Les cylindres, de 2 mm de diamètre, sont des sondes thermo-résistives qui ont une double fonction : elles sont utilisées comme éléments chauffants et comme capteurs de température. Une boucle fluide permet de contrôler le débit d’injection de liquide dans la section test, la température d’injection ainsi que la pression. La section test est placée verticalement, le liquide est injecté par le bas à une température proche de la saturation. Dans une première série d’expériences, la puissance thermique dissipée globalement par un ensemble de cylindres chauffants est augmentée de façon progressive jusqu’à atteindre l’assèchement d’une zone du milieu poreux. Les résultats montrent deux types de phénoménologies dans le déclenchement de la crise d’ébullition. Pour des débits d’injection faibles (densités de flux massique de l’ordre de 4 kg.m^-2.s^-1 maximum), l’atteinte de la puissance d’assèchement se traduit par un lent recul du front diphasique jusqu’à sa stabilisation en haut de la zone chauffée ; en aval de la zone chauffée, l’écoulement est monophasique vapeur. Pour des débits d’injection plus élevés, la crise d’ébullition apparaît autour d’un des éléments chauffants, conduisant à une ébullition en film localisée, tandis qu’un écoulement diphasique liquide-vapeur continue de parcourir l’aval de la section test. Les visualisations de ces expériences permettent d’identifier qualitativement la structure des écoulements. D’autres expériences consistent à mesurer le flux critique local autour d’un cylindre choisi, pour différentes configurations d’écoulements. Le débit d’injection est fixé. Une puissance de chauffe est imposée à une ligne horizontale de cylindres en amont du cylindre choisi. Les résultats montrent que le flux critique sur ce cylindre diminue en fonction de la puissance délivrée à la ligne chauffée. La distance du cylindre étudié à la ligne chauffée semble avoir peu d’influence sur le flux critique. Des visualisations expérimentales sont utilisées pour caractériser l’écoulement diphasique en aval de la ligne chauffée, dans le but de mettre en relation le flux critique local avec des paramètres hydrodynamiques (saturations, vitesses des phases). Les images obtenues sont difficiles à exploiter. Afin de calibrer les paramètres des algorithmes de traitement d’images, nous avons reproduit une cellule d’essai de géométrie identique à l’originale, mais où l’on injecte du gaz par une ligne de cylindres en amont de la section test dans une configuration d’écoulement diphasique isotherme. Dans ce dispositif, le débit d’injection de gaz est contrôlé et mesuré. Les visualisations obtenues servent alors de références auxquelles sont comparées les visualisations d’ébullition convective.
47
Hayes, Benjamin Zed. "Experimental study of the effect of channel orientation and flow oscillations on nucleate boiling heat transfer and the critical heat flux". Diss., Restricted to subscribing institutions, 2007. http://proquest.umi.com/pqdweb?did=1383482001&sid=1&Fmt=2&clientId=1564&RQT=309&VName=PQD.
Testo completo
48
Forrest, Eric Christopher. "Nanoscale modification of key surface parameters to augment pool boiling heat transfer and critical heat flux in water and dielectric fluids". Thesis, Massachusetts Institute of Technology, 2009. http://hdl.handle.net/1721.1/52799.
Testo completoAbstract (sommario):
Thesis (S.M. and S.B.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2009.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (p. 123-130).
Surface effects on pool boiling heat transfer and the critical heat flux are well documented but poorly understood. This study investigates the pool boiling characteristics of various fluids, and demonstrates that surface effects can drastically alter the nucleate boiling heat transfer coefficient as well as the critical heat flux. Changes in surface morphology and surface chemistry are suspected to be the primary factors influencing pool boiling heat transfer. The relative impact of surface properties is shown to depend strongly upon the working fluid. To evaluate the effects of chemical constituency and surface texture on the pool boiling of water, nanoparticle thin-film coatings are applied to nickel and stainless steel substrates using the layer-by-layer assembly method. This study shows that such coatings, with thicknesses on the order of one micron or less, are capable of enhancing the critical heat flux of water up to 100%, and enhancing the nucleate boiling heat transfer coefficient over 100%. Through the use of thin-film coatings, the importance of nanoscale surface texture, porosity, and chemical constituency on boiling mechanisms is revealed. Low surface tension dielectric fluids, including a recently developed fluorinated ketone with a low global warming potential, are tested to determine their pool boiling heat transfer capabilities. The potential for nanoparticle-based pool boiling enhancement in well-wetting dielectric fluids is investigated. The role of surface wettability and adhesion tension on the incipience of boiling, nucleate boiling, and critical heat flux are considered.
(cont.) Results indicate that the low global warming potential fluorinated ketone may be a viable alternative in the cooling of electronic devices. Additionally, results demonstrate that enhancement of boiling heat transfer is possible for well-wetting dielectric fluids, with 40% enhancement in the critical heat flux using dilute suspensions of aluminum or silica nanoparticles in the fluorinated ketone.
by Eric Christopher Forrest.
S.M.and S.B.
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O'Hanley, Harrison Fagan. "Separate effects of surface roughness, wettability and porosity on boiling heat transfer and critical heat flux and optimization of boiling surfaces". Thesis, Massachusetts Institute of Technology, 2012. http://hdl.handle.net/1721.1/78208.
Testo completoAbstract (sommario):
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering; and, (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2012.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 157-161).
The separate effects of surface wettability, porosity, and roughness on critical heat flux (CHF) and heat transfer coefficient (HTC) were examined using carefully-engineered surfaces. All test surfaces were prepared on nanosmooth indium tin oxide - sapphire heaters and tested in a pool boiling facility in MIT's Reactor Thermal Hydraulics Laboratory. Roughness was controlled through fabrication of micro-posts of diameter 20[mu]m and height 15[mu]m; intrinsic wettability was controlled through deposition of thin compact coatings made of hydrophilic SiO₂ (typically, 20nm thick) and hydrophobic fluorosilane (monolayer thickness); porosity and pore size were controlled through deposition of layer-by-layer coatings made of SiO₂ nanoparticles. The ranges explored were: 0 - 15[mu] for roughness (Rz), 0 - 135 degrees for intrinsic wettability, and 0 - 50% and 50nm for porosity and pore size, respectively. During testing, the active heaters were imaged with an infrared camera to map the surface temperature profile and locate distinct nucleation sites. It was determined that wettability can play a large role on a porous surface, but has a limited effect on a smooth non-porous surface. Porosity had very pronounced effects on CHF. When coupled with hydrophilicity, a porous structure enhanced CHF by approximately 50% - 60%. However, when combined with a hydrophobic surface, porosity resulted in a reduction of CHF by 97% with respect to the reference surface. Surface roughness did not have an appreciable effect, regardless of the other surface parameters present. Hydrophilic porous surfaces realized a slight HTC enhancement, while the HTC of hydrophobic porous surfaces was greatly reduced. Roughness had little effect on HTC. A second investigation used spot patterning aimed at creating a surface with optimal characteristics for both CHF and HTC. Hydrophobic spots (meant to be preferential nucleation sites) were patterned on a porous hydrophilic surface. The spots indeed were activated as nucleation sites, as recognized via the IR signal. However, CHF and HTC were not enhanced by the spots. In some instances, CHF was actually decreased by the spots, when compared to a homogenous porous hydrophilic surface.
by Harrison Fagan O'Hanley.
S.B.
S.M.
50
Sayee, Mohan Kaushik. "Pool Boiling of FC 770 on Graphene Oxide Coatings: A Study of Critical Heat Flux and Boiling Heat Transfer Enhancement Mechanisms". Thesis, Virginia Tech, 2016. http://hdl.handle.net/10919/71873.
Testo completoAbstract (sommario):
This thesis investigates pool boiling heat transfer from bare and graphene-coated NiCr wires in a saturated liquid of FC 770, a fluorocarbon fluid. Of particular interest was the effect of graphene-oxide platelets, dip-coated onto the heater surface, in enhancing the nucleate boiling heat transfer (BHT) rates and the critical heat flux (CHF) value. In the course of the pool boiling experiment, the primary focus was on the reduction mechanism of graphene oxide. The transition from hydrophilic to hydrophobic behavior of the graphene oxide-coated surface was captured, and the attendant effects on surface wettability, porosity and thermal activity were observed. A parametric sensitivity analysis of these surface factors was performed to understand the CHF and BHT enhancement mechanisms.
In the presence of graphene-oxide coating, the data indicated an increase of 50% in CHF. As the experiment continued, a partial reduction of graphene oxide occurred, accompanied by (a) further enhancement in the CHF to 77% larger compared to the bare wire. It was shown that the reduction of graphene oxide progressively altered the porosity and thermal conductivity of the coating layer without changing the wettability of FC 770. Further enhancement in CHF was explained in terms of improved porosity and thermal activity that resulted from the partial reduction of graphene-oxide. An implication of these results is that a graphene-oxide coating is potentially a viable option for thermal management of high-power electronics by immersion cooling technology.Master of Science