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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.
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Altalidi, Sulaiman Saleh. "Two-Phase Spray Cooling with HFC-134a and HFO-1234yf for Thermal Management of Automotive Power Electronics using Practical Enhanced Surfaces." Thesis, University of North Texas, 2017. https://digital.library.unt.edu/ark:/67531/metadc1011876/.
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The objective of this research was to investigate the performance of two-phase spray cooling with HFC-134a and HFO-1234yf refrigerants using practical enhanced heat transfer surfaces. Results of the study were expected to provide a quantitative spray cooling performance comparison with working fluids representing the current and next-generation mobile air conditioning refrigerants, and demonstrate the feasibility of this approach as an alternative active cooling technology for the thermal management of high heat flux power electronics (i.e., IGBTs) in electric-drive vehicles. Potential benefits of two-phase spray cooling include achieving more efficient and reliable operation, as well as compact and lightweight system design that would lead to cost reduction. The experimental work involved testing of four different enhanced boiling surfaces in comparison to a plain reference surface, using a commercial pressure-atomizing spray nozzle at a range of liquid flow rates for each refrigerant to determine the spray cooling performance with respect to heat transfer coefficient (HTC) and critical heat flux (CHF). The heater surfaces were prepared using dual-stage electroplating, brush coating, sanding, and particle blasting, all featuring "practical" room temperature processes that do not require specialized equipment. Based on the obtained results, HFC-134a provided a better heat transfer performance through higher HTC and CHF values compared to HFO-1234yf at all tested surfaces and flow rates. While majority of the tested surfaces provided comparable HTC and modestly higher CHF values compared to the reference surface, one of the enhanced surfaces offered significant heat transfer enhancement.
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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.
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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.
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Mlakar, Genesis. "Effects of Surface Engineering on HFE-7100 Pool Boiling Heat Transfer." Case Western Reserve University School of Graduate Studies / OhioLINK, 2021. http://rave.ohiolink.edu/etdc/view?acc_num=case1619036502968687.
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Wong, Y. L. "Generalized CHF prediction for horizontal tubes with uniform heat flux." Thesis, University of Ottawa (Canada), 1988. http://hdl.handle.net/10393/5471.
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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.
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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.)
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Manning, Jonathan Paul. "Critical heat flux in non-circular channels." Thesis, Imperial College London, 2018. http://hdl.handle.net/10044/1/61534.
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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.
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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.
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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
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Joober, Khaled. "The effect of flow geometry on critical heat flux." Thesis, University of Ottawa (Canada), 1993. http://hdl.handle.net/10393/6544.
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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.
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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.
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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.
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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.
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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.
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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.
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Scheiff, Valentin. "Etude expérimentale et modélisation du transfert de chaleur de l'ébullition transitoire." Thesis, Toulouse, INPT, 2018. http://www.theses.fr/2018INPT0145/document.
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L’étude de l’ébullition transitoire est un enjeu important pour la sureté nucléaire. Un tel phénomène peut se produire lors d’un accident de type RIA (Reactivity Initiated Accident)dans un réacteur nucléaire où le pic de puissance au niveau d’un crayon de combustible peut déclencher une ébullition transitoire conduisant à une forte augmentation de la température de la gaine et à un risque de rupture. Plusieurs études en conditions réacteurs ont permis d’obtenir des courbes d’ébullition transitoires mais la modélisation qui en découle manque encore de fiabilité. Dans le cadre d’une collaboration avec l’Institut de Radioprotection et de Sûreté Nucléaire (IRSN), une expérience modèle a été construite à l’Institut de Mécanique des Fluides de Toulouse (IMFT). Elle génère un écoulement de réfrigérant HFE7000 dans un canal de section semi-annulaire, simulant l’écoulement autour d’un crayon de combustible, dont la partie intérieure, composée d’une feuille de métal, est chauffée rapidement par effet Joule, simulant l’échauffement de la gaine du crayon. La thermographie infra-rouge permet de mesurer la température de la paroi externe du métal. L’application d’une peinture noire sur le métal augmente son émissivité mais aussi la résistance thermique de la paroi. La précision de la mesure de la température d’intérêt a été optimisée en fonction de l’épaisseur de peinture et une correction sur le bilan d’énergie prend en compte ce paramètre. Ces mesures sont couplées avec une caméra rapide qui permet de visualiser les régimes d’ébullition et d’obtenir des tailles de bulles à l’aide de la mise en place d’algorithmes de traitement d’image. On représente sur un diagramme flux-température les transferts thermiques lors des différents régimes en stationnaire et en transitoire. Chaque régime d’ébullition, en conditions stationnaire ou transitoire, est alors passé en revue : la convection, le déclenchement de l’ébullition, l’ébullition nucléée, la crise d’ébullition, l’ébullition en film et le remouillage. Les régimes stationnaires sont correctement modélisés par des corrélations usuelles. La convection transitoire est caractérisée sur toute la paroi et son évolution se rapproche de la solution quasistationnaire. Il est montré que les transferts thermiques lors du passage vers l’ébullition nucléée sont dépendants de la formation d’une importante poche de vapeur qui se propage sur la paroi. Une étude locale de cette propagation est alors nécessaire. Afin de simuler des transitoires de température durant l’ébullition nucléée, un système d’asservissement de type P.I.D. permet d’imposer des créneaux ou des rampes de températures (de 5 à 500 K.s 1 ). Les résultats en ébullition nucléée sont conformes avec ceux de la littérature, tant en conditions stationnaire que transitoire. L’expérience permet d’étudier le transfert de chaleur lorsqu’un film de vapeur se forme et isole la paroi. Ce régime d’ébullition en film, pendant la chauffe ou le refroidissement de la paroi peut ainsi être stabilisée pendant plusieurs secondes avec ce système. On caractérise ainsi les conditions de déclenchement de l’ébullition en film, la dynamique de sa propagation et les transferts une fois établi. Enfin, l’implémentation des caractéristiques physiques de notre expérience dans le code SCANAIR de l’IRSN, permet de commencer à calculer et comparer nos résultats expérimentaux avec les simulations numériques. Des calculs de conduction instationnaire sont notamment considérés en imposant la température mesurée pour analyser nos résultats lors du régime de convection et après le déclenchement de l’ébullitionThe study of rapid transient boiling is an important issue in the nuclear safety. Such a phenomenon may occur in the case of a RIA (Reactivity Initiated Accident) in the core of a nuclear reactor powerplant, where a power excursion can trigger the formation of a vapour film around the fuel rod, leading to an important rise of the rod temperature and a risk of failure. Some studies in reactor conditions provided transient boiling curves but the modeling lacks of reliability. In collaboration with the IRSN (Institut de Radioprotection et de Sûreté Nucléaire), an experiment model was built at the Institute of Fluid Mechanics of Toulouse. It generates the flow of a refrigerant, HFE7000, in a semi-annular section channel, whose inner wall is made of a metal foil rapidly heated by Joule effect, simulating the heating of a fuel rod. Infrared thermography is used to measure the temperature of the metal foil, painted with a black paint to increase its emissivity, causing also an increase of the wall thermal resistance. The measurement accuracy of the interest temperature has been optimized according to the paint thickness and a correction on the energy balance takes account this parameter. These measurements are coupled with a high-speed camera that allows visualizing the boiling regimes and get bubble sizes using image processing algorithms. On a flux-temperature diagram, the heat transfers are represented both for steady and transient regimes. Each boiling regime is then reviewed : convection, onset of nucleate boiling, nucleate boiling, boiling crisis, film boiling and rewetting. Steady regimes are correctly modeled by usual correlations. Transient convection is characterized over the whole wall and its evolution is closed to the quasi-steady solution. It is shown that heat transfer during the transition to nucleate boiling are strongly related to the formation of a large vapor phase that spreads on the wall. A local study of this propagation is then necessary. In order to simulate and control transient temperature during nucleate boiling, a P.I.D. is implemented to impose a steady or ramps temperature (from 5 to 500 K.s 1 ). The results in nucleate boiling make it possible to recover the results of the literature in both steady and transient conditions. The experiment allows to study the heat transfer when a vapor film is formed and insulates the wall. The film boiling regime during heating or the cooling of the wall can thus be stabilized for several seconds with this system. The conditions for triggering of film boiling are thus characterized, as its spread dynamic and its transfers once established. Finally, the implementation of the physical characteristics of our experience in IRSN’s SCANAIR code allows us to begin to calculate and compare our experimental results with numerical simulations. Unsteady conduction calculations are applied to the measured temperature to analyze our results during the convection regime and after the onset of boiling
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
Full textAbstract:
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
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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.
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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.
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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.
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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.)
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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.
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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.
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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.
Full textAbstract:
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.
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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.
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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.
Full textAbstract:
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.
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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.
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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.
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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
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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.
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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.
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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.
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Martinez, Christian David. "Heat transfer enhancement of spray cooling with nanofluids." [Tampa, Fla] : University of South Florida, 2009. http://purl.fcla.edu/usf/dc/et/SFE0003237.
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Potier, Luc. "Large Eddy Simulation of the combustion and heat transfer in sub-critical rocket engines." Thesis, Toulouse, INPT, 2018. http://www.theses.fr/2018INPT0043/document.
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La combustion cryogénique dans les moteurs de fusée dits à propulsion liquide utilise généralement un couple d'ergols, le plus couramment composé d'hydrogène/oxygène (H2/O2). Privilégiée pour le fort pouvoir calorifique du dihydrogène, cette combustion à haute pression, induit des températures de fonctionnement très élevées et nécessite l'intégration d'un système de refroidissement. La prédiction des flux thermiques aux parois est donc un élément essentiel de la conception d'une chambre de combustion de moteur fusée. Ces flux sont le résultat d'écoulements fortement turbulents, compressibles, avec une cinétique chimique violente induisant de forts gradients d'espèces et de température. La simulation de ces phénomènes nécessite des approches spécifiques telles que la Simulation aux Grandes Echelles (SGE) qui réalise un très bon compromis entre précision et coût de calcul. Cette thèse a ainsi pour objectif la simulation par SGE des transferts de chaleur aux parois dans les chambres de combustion de moteurs fusée opérant en régime sous-critique. Le régime sous-critique implique un état liquide pour un des ergols, dont il faut traiter l'injection et l'atomisation. Dans un premier temps ce travail s'intéresse à plusieurs éléments de modélisation nécessaire pour réaliser les simulations visées. Le comportement des flammes H2/O2 est décrit par un schéma cinétique réduit et validé sur des configurations académiques. La prédictivité de ce schéma est évaluée sur une large gamme de fonctionnement dans des conditions représentatives des moteurs fusée. La simulation de l'injection de l'oxygène liquide (LOx) est un autre point critique qui nécessite de décrire l'atomisation et la phase dispersée ainsi que son couplage avec la phase gazeuse. La déstabilisation et l'atomisation primaire du jet liquide, trop complexe à simuler en SGE 3D, sont omises ici pour injecter directement un spray paramétré grâce à des corrélations empiriques. Enfin, la prédiction des flux thermiques utilise un modèle de loi de paroi spécifiquement dédiée aux écoulements à fort gradient de température. Cette loi de paroi est validée sur des configurations de canaux turbulents par comparaison avec des simulations avec résolution directe de la couche limite. La méthodologie basée sur les modèles développés est ensuite employée pour la simulation d'une chambre de combustion représentative du fonctionnement des moteurs cryogéniques. Il s'agit de la configuration CONFORTH testée sur le banc MASCOTTE (ONERA) et pour laquelle des mesures de température de paroi et de flux thermiques sont disponibles. Les résultats des SGE montrent un bon accord avec l'expérience et démontrent la capacité de la SGE à prédire les flux thermiques dans une chambre de combustion de moteur fusée. Enfin, dans un dernier chapitre ce travail s'intéresse à une méthode d'augmentation des transferts thermiques via une expérience de JAXA utilisant des parois rainurées dans la direction axiale. Par comparaison avec une chambre à parois lisses, les résultats démontrent la bonne prédiction par la SGE de l'augmentation du flux de chaleur grâce aux rainures et confirment la validité de la méthode développée pour des géométries de paroi complexesCombustion in cryogenic engines is a complex phenomenon, involving either liquid or supercritical fluids at high pressure, strong and fast oxidation chemistry, and high turbulence intensity. Due to extreme operating conditions, a particularly critical issue in rocket engine is wall heat transfer which requires efficient cooling of the combustor walls. The concern goes beyond material resistance: heat fluxes extracted through the chamber walls may be reused to reduce ergol mass or increase the power of the engine. In expander-type engine cycle, this is even more important since the heat extracted by the cooling system is used to drive the turbo-pumps that feed the chamber in fuel and oxidizer. The design of rocket combustors requires therefore an accurate prediction of wall heat flux. To understand and control the physics at play in such combustor, the Large Eddy Simulation (LES) approach is an efficient and reliable numerical tool. In this thesis work, the objective is to predict wall fluxes in a subcritical rocket engine configuration by means of LES. In such condition, ergols may be in their liquid state and it is necessary to model liquid jet atomization, dispersion and evaporation.The physics that have to be treated in such engine are: highly turbulent reactive flow, liquid jet atomization, fast and strong kinetic chemistry and finally important wall heat fluxes. This work first focuses on several modeling aspects that are needed to perform the target simulations. H2/O2 flames are driven by a very fast chemistry, modeled with a reduced mechanism validated on academic configurations for a large range of operating conditions in laminar pre- mixed and non-premixed flames. To form the spray issued from the atomization of liquid oxygen (LOx) an injection model is proposed based on empirical correlations. Finally, a wall law is employed to recover the wall fluxes without resolving directly the boundary layer. It has been specifically developed for important temperature gradients at the wall and validated on turbulent channel configurations by comparison with wall resolved LES. The above models are then applied first to the simulation of the CONFORTH sub-scale thrust chamber. This configuration studied on the MASCOTTE test facility (ONERA) has been measured in terms of wall temperature and heat flux. The LES shows a good agreement compared to experiment, which demonstrates the capability of LES to predict heat fluxes in rocket combustion chambers. Finally, the JAXA experiment conducted at JAXA/Kakuda space center to observe heat transfer enhancement brought by longitudinal ribs along the chamber inner walls is also simulated with the same methodology. Temperature and wall fluxes measured with smooth walls and ribbed walls are well recovered by LES. This confirms that the LES methodology proposed in this work is able to handle wall fluxes in complex geometries for rocket operating conditions
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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.
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Bock, Bradley D. "Surface influences on falling film boiling and pool boiling of saturated refrigerants : influences of nanostructures, roughness and material on heat transfer, dryout and critical heat flux of tubes." Thesis, University of Pretoria, 2020. http://hdl.handle.net/2263/78711.
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Falling film evaporators that operate in the nucleate boiling regime in the refrigeration industry offer a number of advantages over their flooded counterparts such as lower refrigerant charge and at times improved heat transfer. Existing literature has not characterised the influence of surface characteristics on the falling film boiling process, and they are poorly understood for the pool boiling process. The purpose of this study was therefore to experimentally measure the influence of roughness, material and nanostructures on the heat transfer of falling film boiling and pool boiling of saturated refrigerants on the outside of horizontal tubes. The critical heat flux point was measured if it occurred, and the falling film heat transfer enhancement ratio, critical dryout threshold and general dryout characteristics were investigated in the study.
The tubes tested consisted of plain copper, stainless steel and mild steel tubes that were polished and roughened with various grades of sandpaper. Furthermore, three types of nanostructured surfaces were applied to polished copper tubes, namely a layer-by-layer (LbL) coating of silica nanoparticles, a copper oxide (CuO) nanostructure coating and a commercial nanocoating process termed nanoFLUX.
The nanoFLUX tube had the highest heat transfer coefficients of tubes tested under both pool boiling and falling film conditions, with between 40 and 200% higher heat transfer coefficients than those of a polished copper tube. The nanoFLUX surface outperformed the other surfaces due to a combination of rougher microstructure and a unique heat transfer mechanism, possibly linked to capillary wicking of liquid inside the nanochannels of the porous coating.
The falling film heat transfer enhancement ratio was found to increases as surface roughness was increased on plain tubes, suggested to be as a result of enhanced microlayer evaporation from the trapped sliding bubbles in the thin flowing film.
The nanoFLUX and CuO surfaces experienced lower critical heat flux as a result of departure from nucleate boiling under pool boiling and falling film boiling conditions compared with plain surfaces.. However, the nanoFLUX and CuO tubes performed well in terms of critical dryout at lower heat fluxes. The wicking capabilities of the nanoFLUX and CuO surfaces were thought to be the cause of their improved dryout capabilities at lower heat fluxes, but increased heat fluxes possibly led to dryout of the nanostructures resulting in operation in the Cassie-Baxter state and subsequent reduced wettability.Thesis (PhD)--University of Pretoria, 2020.
Mechanical and Aeronautical Engineering
PhD (Mechanical Engineering)
Unrestricted
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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.
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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
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Venkataraman, Manoj. "THE EFFECT OF COLLOIDAL STABILITY ON THE HEAT TRANSFER CHARACTERISTICS OF NANOSILICA DISPERSED FLUIDS." Master's thesis, University of Central Florida, 2005. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/3656.
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Addition of nano particles to cooling fluids has shown marked improvement in the heat transfer capabilities. Nanofluids, liquids that contain dispersed nanoparticles, are an emerging class of fluids that have great potential in many applications. There is a need to understand the fundamental behavior of nano dispersed particles with respect to their agglomeration characteristics and how it relates to the heat transfer capability. Such an understanding is important for the development and commercialization of nanofluids. In this work, the stability of nano particles was studied by measuring the zeta potential of colloidal particles, particle concentration and size. Two different sizes of silica nano particles, 10 nm and 20 nm are used in this investigation at 0.2 vol. % and 0.5 vol. % concentrations. The measurements were made in deionized (DI) water, buffer solutions at various pH, DI water plus HCl acid solution (acidic pH) and DI water plus NaOH solution (basic pH). The stability or instability of silica dispersions in these solutions was related to the zeta potential of colloidal particles and confirmed by particle sizing measurements and independently by TEM observations. Low zeta potentials resulted in agglomeration as expected and the measured particle size was greater. The heat transfer characteristics of stable or unstable silica dispersions using the above solutions were experimentally determined by measuring heat flux as a function of temperature differential between a nichrome wire and the surrounding fluid. These experiments allowed the determination of the critical heat flux (CHF), which was then related to the dispersion characteristics of the nanosilica in various fluids described above. The thickness of the diffuse layer on nano particles was computed and experimentally confirmed in selected conditions for which there was no agglomeration. As the thickness of the diffuse layer decreased due to the increase in salt content or the ionic content, the electrostatic force of repulsion cease to exist and Van der Waal's force of agglomeration prevailed causing the particles to agglomerate affecting the CHF. The 10nm size silica particle dispersions showed better heat transfer characteristics compared to 20nm dispersion. It was also observed that at low zeta potential values, where agglomeration prevailed in the dispersion, the silica nano particles had a tendency to deposit on the nickel chromium wire used in CHF experiments. The thickness of the deposition was measured and the results show that with a very high deposition, CHF is enhanced due to the porosity on the wire. The 10nm size silica particles show higher CHF compared to 20nm silica particles. In addition, for both 10nm and 20nm silica particles, 0.5 vol. % concentration yielded higher heat transfer compared to 0.2 vol. % concentration. It is believed that although CHF is significantly increased with nano silica containing fluids compared to pure fluids, formation of particle clusters in unstable slurries will lead to detrimental long time performance, compared to that with stable silica dispersions.M.S.M.S.E.
Department of Mechanical, Materials and Aerospace Engineering;
Engineering and Computer Science
Materials Science and Engineering
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Haas, Christoph [Verfasser]. "Critical Heat Flux for Flow Boiling of Water at Low Pressure on Smooth and Micro-Structured Zircaloy Tube Surfaces (KIT Scientific Reports ; 7627) / Christoph Haas." Karlsruhe : KIT Scientific Publishing, 2012. http://www.ksp.kit.edu.
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Martin, Callizo Claudi. "Flow Boiling Heat Transfer in Single Vertical Channels of Small Diameter." Doctoral thesis, KTH, Tillämpad termodynamik och kylteknik, 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-25797.
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Microchannel heat exchangers present many advantages, such as reduced size, high thermal efficiency and low fluid inventory; and are increasingly being used for heat transfer in a wide variety of applications including heat pumps, automotive air conditioners and for cooling of electronics.However, the fundamentals of fluid flow and heat transfer in microscalegeometries are not yet fully understood. The aim of this thesis is to contribute to a better understanding of the underlying physical phenomena in single-phase and specially flow boiling heat transfer of refrigerants in small channels. For this purpose, well-characterized heat transfer experiments have been performed in uniformly heated, single, circular, vertical channels ranging from 0.64 to 1.70 mm in diameter and using R-134a, R-22 and R-245fa as working fluids. Furthermore, flow visualization tests have been carried out to clarify the relation between the two-phase flow behavior and the boiling heat transfer characteristics. Single-phase flow experiments with subcooled liquid refrigerant have confirmed that conventional macroscale theory on single-phase flow and heat transfer is valid for circular channels as small as 640μm in diameter. Through high-speed flow boiling visualization of R-134a under non adiabatic conditions seven flow patterns have been observed: isolated bubbly flow, confined bubbly flow, slug flow, churn flow, slug-annular flow, annular flow, and mist flow. Two-phase flow pattern observations are presented in the form of flow pattern maps. Annular-type flow patterns are dominant for vapor qualities above 0.2. Onset of nucleate boiling and subcooled flow boiling heat transfer of R-134a has been investigated. The wall superheat needed to initiate boiling was found as large as 18 ºC. The experimental heat transfer coefficients have been compared to predictions from subcooled flow boiling correlationsav ailable in the literature showing poor agreement. Saturated flow boiling heat transfer experiments have been performed with the 640 μm diameter test section. The heat transfer coefficient has been found to increase with heat flux and system pressure and not to change with vapor quality or mass flux when the quality is less than ∼0.5. For vapor qualities above this value, the heat transfer coefficient decreases with vapor quality. This deterioration of the heat transfer coefficient is believed to be caused by the occurrence of intermittent dryout in this vapor quality range. The experimental database, consisting of 1027 data points, has been compared against predictions from correlations available in the literature. The best results are obtained with the correlations by Liu and Winterton (1991) and by Bertsch et al. (2009). However, better design tools to correctly predict the flow boiling heat transfer coefficient in small geometries need to be developed. Dryout incipience and critical heat flux (CHF) have been investigated in detail. CHF data is compared to existing macro and microscale correlations. The comparison shows best agreement with the classical Katto and Ohno (1984) correlation, developed for conventional large tubes.QC 20101101
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Santos, Wilton Fogaça da Silva. "Uma nova técnica para contenção de acidentes em reatores nucleares de água pressurizada." Universidade de São Paulo, 2018. http://www.teses.usp.br/teses/disponiveis/3/3139/tde-09042018-144934/.
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Durante um acidente em uma usina nuclear, a integridade do vaso pressurizado deve ser assegurada. Em resposta a um possível derretimento do combustível nuclear, a atual geração de usinas possui um sistema para a injeção de água potável na cavidade do vaso pressurizado com intuito de resfriar sua parede, prevenindo danos a sua estrutura e evitando o vazamento de material radioativo. Esse estudo considerou o uso de água marinha como refrigerante para inundar a cavidade do vaso pressurizado combinado com a fixação de um estrutura porosa em forma de grade em sua parede externa como meio de aprimorar a margem de segurança durante a contenção de acidentes. Experimentos de longa duração para a ebulição em piscina de água marinha artificial foram conduzidos em uma superfície circular de cobre plana com 30 mm de diâmetro. Foi encontrado um fluxo de calor crítico de 1; 6 MW/m2 sob pressão atmosférica. Esse valor é significantemente maior que aquele obtido (1; 0 MW/m2) nas mesmas condições experimentais. Foi verificado que os depósitos de sais marinhos podem aumentar a molhabilidade e a capilaridade da superfície de teste, aprimorando assim o fluxo crítico. Combinando a água marinha e a fixação da estrutura porosa sobre a superfície de teste, verificou-se um melhora no coeficiente de transmissão de calor e no fluxo de calor crítico de até 110 % (2; 1 MW/m2), quando comparado a água destilada na superfície limpa, sem a instalação da estrutura. Após os experimentos, foi identificado que muitos dos poros presentes nas superfícies da estrutura porosa encontravam-se bloqueados devido ao aglutinamento de sais marinhos. Isso levou a conclusão que o aumento no valor do fluxo crítico observado para a água marinha artificial ocorreu devido, principalmente, a separação das fases líquida e gasosa do fluido na região próxima a superfície de teste, efeito proporcionado pela forma de grade da estrutura porosa, e ao aumento da molhabilidade e capilaridade da superfície devido a formação dos depósitos marinhos.During a severe nuclear power plant accident, the integrity of the reactor pressure vessel must be assured. In response to a possible fuel meltdown, operators of the current generation of nuclear power plants are likely to inject water into the reactor pressure vessel to cool down the reactor vessel wall, preserving its integrity and avoiding leakage of radioactive material. This study considers the use of seawater to flood a reactor pressure vessel combined with the attachment of a honeycomb porous plate (HPP) on the vessel outer wall as a way to improve the safety margins for in-vessel retention of fuel. In long-duration experiments, saturated pool boiling of artificial seawater was performed with an upward-facing plain copper heated surface 30 mm in diameter. The resulting value for critical heat flux (CHF) was 1; 6 MW/m2 at atmospheric pressure, a value significantly higher than the CHF obtained when the working fluid was distilled water (1; 0 MW/m2). It was verified that sea-salt deposits could greatly improve surface wettability and capillarity, enhancing the CHF. The combination of artificial seawater and an HPP attached to the heated surface improved the boiling heat transfer coefficient and increased the CHF up to 110% (2; 1 MW/m2) as compared to distilled water on a bare surface. After the artificial seawater experiments, most of the wall micropores of the HPP were clogged because of sea-salt aggregation on the HPP top and bottom surfaces. Thus, the CHF enhancement observed in this case was attributed mainly to the separation of liquid and vapor phases provided by the HPP channel structure and improvement of surface wettability and capillarity by sea-salt deposition.
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Suk, Ladislav. "Konstrukční návrh části zařízení pro studijní účely krize varu." Master's thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2012. http://www.nusl.cz/ntk/nusl-230091.
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Graduation these deals with investigation of critical heat flux in pressurized water nuclear reactors. Theoretical part covers fundamental terms from area hydrodynamics of two-phase flow and critical heat flux. Here are also mentioned the individual approaches to description of physical process of heat transfer crisis. Practical part is devoted to systems design of measuring stand for critical heat flux in vertical canal allowing visualization of two-phase flow.
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Milanova, Denitsa. "Next generation heat transfer fluids : experimental study of nano-oxide and carbon nanotube suspensions in water." Honors in the Major Thesis, University of Central Florida, 2008. http://digital.library.ucf.edu/cdm/ref/collection/ETH/id/1117.
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This item is only available in print in the UCF Libraries. If this is your Honors Thesis, you can help us make it available online for use by researchers around the world by following the instructions on the distribution consent form at http://library.ucf.edu/Systems/DigitalInitiatives/DigitalCollections/InternetDistributionConsentAgreementForm.pdf You may also contact the project coordinator, Kerri Bottorff, at kerri.bottorff@ucf.edu for more information.Bachelors
Engineering and Computer Science
Mechanical Engineering
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LIMA, RITA de C. F. de. "Comportamento termoidraulico de vareta aquecida eletricamente durante transitorio de fluxo critico de calor." reponame:Repositório Institucional do IPEN, 1997. http://repositorio.ipen.br:8080/xmlui/handle/123456789/10640.
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Made available in DSpace on 2014-10-09T12:42:40Z (GMT). No. of bitstreams: 0Made available in DSpace on 2014-10-09T13:57:16Z (GMT). No. of bitstreams: 1 05031.pdf: 4962096 bytes, checksum: 39c12c06c0063abb20c1c82005ecef33 (MD5)
Tese (Doutoramento)
IPEN/T
Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP
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Tibiriçá, Cristiano Bigonha. "Estudo teórico-experimental da transferência de calor e do fluxo crítico durante a ebulição convectiva no interior de microcanais." Universidade de São Paulo, 2011. http://www.teses.usp.br/teses/disponiveis/18/18147/tde-22092011-161901/.
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A pesquisa realizada tratou do estudo da transferência de calor e do fluxo crítico durante a ebulição convectiva no interior de canais de diâmetro reduzidos a partir de dados levantados em bancadas experimentais construídas para esta finalidade. Extensa pesquisa bibliográfica foi efetuada e os principais métodos disponíveis para previsão de coeficiente de transferência de calor, fluxo crítico e mapas de escoamento foram levantados. Os resultados obtidos foram parametricamente analisados e comparados com os métodos da literatura. Pela primeira vez para microcanais, resultados experimentais foram levantados por um mesmo autor em laboratórios distintos buscando verificar a tendência e comportamentos. Tal comparação tem sua importância destacada em face das elevadas discrepâncias observadas na literatura quando resultados de autores distintos, obtidos em condições similares, são comparados. Os resultados levantados foram utilizados na elaboração de modelos que consideram os padrões de escoamento observados em microcanais. A incorporação dos padrões permitiu o desenvolvimento de modelos mecanísticos para coeficiente de transferência de calor, fluxo crítico e critérios para a caracterização da transição entre macro e microcanais baseados na formação do padrão de escoamento estratificado e na simetria do filme líquido no escoamento anular.This research comprises an experimental and theoretical study on flow boiling heat transfer and critical heat flux inside small diameter tubes based on data obtained in experimental facilities specially designed for this purpose. A broad literature review was carried out and the main methods to predict the heat transfer coefficient, critical heat flux and flow patterns were pointed out. The experimental results were parametrically analyzed and compared against the predictive methods from literature. For the first time, microchannels experimental results obtained by an unique researcher in distinct laboratories were compared and a reasonable agreement was observed. The importance of such a comparison is high-lighted for flow boiling inside microchannels due to the high discrepancies ob-served when results from independent laboratories obtained under similar experimental conditions are compared. Moreover, the experimental results obtained in the present study were used to develop correlations and models for the heat transfer coefficient and heat flux that takes into account the flow patterns observed in microchannels. The heat transfer coefficient and critical heat flux models were developed based on mechanistic approach. In addition, criteria to characterize macro to microchannel transition were proposed based in the occurrence of the stratified flow pattern and the liquid film symmetry under annular flow conditions.
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Terng, Nilton. "Desenvolvimento e validação de uma rede neural para análise de fluxo crítico de calor em reatores nucleares do tipo PWR." reponame:Repositório Institucional da UFABC, 2015.
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Orientador: Prof. Dr. Pedro CarajilescovDissertação (mestrado) - Universidade Federal do ABC. Programa de Pós-Graduação em Energia, 2015.
Fluxo Crítico de Calor (FCC) consiste no principal limite termohidráulico de reatores do tipo PWR, que representa a opção nuclear brasileira. Trata-se, ainda, de um fenômeno de entendimento limitado. Em projetos, a estimativa de seu valor é realizada apenas por correlações empíricas, resultando valores aproximados, com elevadas incertezas. O presente projeto consiste no desenvolvimento de um método computacional para o cálculo do FCC, através de conceitos de Redes Neurais Artificiais, programado na linguagem Fortran, utilizando para treinamento e teste os dados das chamadas "Look up Tables" (LUT). Considerou-se a faixa de variação dos dados das tabelas, com a pressão variando de 1 e 21 MPa, fluxo de massa, na faixa de 50 a 8000 kg m-2 s-1 e título do escoamento entre - 0,5 a 0,9. Comparando os resultados da RN com a LUT, a média da razão dos valores resultou em 0,993, com o erro médio quadrático de 13,3%. Com a rede neural foi realizado o estudo paramétrico do FCC, para observar a influência dos parâmetros operacionais tais como pressão, fluxo de massa e título termodinâmico. Observa-se o aumento do FCC com o aumento do fluxo de massa e a atenuação do FCC com o aumento da pressão e título, como esperado. Porém algumas tendências imprevistas ocorreram, as quais podem ser atribuídas à incerteza dos dados, ou por fatos desconhecidos do fenômeno. A aplicação da rede neural em geometrias de feixe de varetas com arranjo quadrado apresentou bons resultados pelo método de balanço de energia (HBM) e a correção de PEI com erro médio quadrático de até 20,08%. Pelo método da substituição direta (DSM), foram elaborados diversos métodos de correção para adaptar os valores da rede neural à geometria de feixe de varetas. Os resultados não foram satisfatórios, pois apresentaram erro médio quadrático elevado, sendo o menor erro médio quadrático alcançado de 19,92%, utilizando uma rede neural com o espectro de parâmetros de entradas restritos e fator de correção multivariável. A correlação de EPRI com a correção de PEI apresentou resultado de erro médio quadrático de 18,73%, sendo menor que todos os métodos desenvolvidos nesse projeto. Portanto, o método de rede neural, desenvolvido nesse trabalho, não se revelou satisfatório para aplicação em feixe de varetas.
The critical heat flux (CHF) is one of the principal thermal hydraulic limits of PWR type nuclear reactors. To date, the CHF phenomenon is not well understood. So, for design purpose, the CHF is usually estimated by empirical correlation, resulting in approximate values, with high uncertainties. As an alternative to traditional methods, the present work consists in the development of an artificial neural network (ANN) to estimate the CHF, based on Look Up Table CHF data, published by Groeneveld (2006). Three parameters were considered in the development of the ANN: the pressure in the range of 1 to 21 MPa, the mass flux in the range of 50 to 8000 kg m-2 s-1 and the thermodynamic quality in the range of - 0,5 to 0,9. Comparing the ANN predictions with the data of the Look Up Table, it was observed an average of the ratio of 0.993 and a root mean square error (rms) of 13.3%. With the developed ANN, a parametric study of CHF was performed to observe the influence of each parameter in the FCC. It was possible to note that the CHF decreases with the increase of pressure and thermodynamic quality, while CHF increases with the mass flow rate, as expected. However, some erratic trends were also observed which can be attributed to either unknown aspect of the FCC phenomenon or uncertainties in the data. The ANN application in square array of rods bundle demonstrated nice result for the heat balance method (HBM) with the PEI correction resulting in rms of 20,08%. A few methods of correction were developed for the direct substitution method (DSM) to adapt the ANN in rod bundle geometry. The results wasn¿t satisfactory, because the best rms reached was 19.92%, using the ANN with restricted input range and multivariable correction factor. EPRI correlation with PEI correction results in rms of 18.73%, being better than all of developed methods in this project. Therefore, the ANN method, developed in this work, does not seem to be satisfactory for the application in rod bundle.