Orientador: Elaine Maria Cardoso / Resumo: A intensificação da transferência de calor por meio de alterações na morfologia da superfície aquecida vem sendo estudada no meio científico, a fim de suprir a crescente demanda de resfriamento de dispositivos com alta capacidade de processamento e dimensões cada vez menores. O presente trabalho apresenta o estudo experimental do efeito de superfícies nanoestruturadas e do espaçamento do canal de confinamento durante a ebulição em piscina da água deionizada, à temperatura de saturação na pressão atmosférica, sobre o coeficiente de transferência de calor, HTC, e fluxo crítico de calor, CHF. As superfícies nanoestruturadas foram obtidas pelo processo de ebulição do nanofluido de Al2O3-água deionizada em duas diferentes concentrações más-sicas: 0,03 g/l (“baixa” concentração, LC) e 0,3 g/l (“alta” concentração, HC). Foram realizados testes livres, com espaçamento, entre a superfície aquecida e a superfície adiabática, de 30 mm (correspondendo a Bo = 12), e testes sob condições de confinamento, com espaçamento de 1,0 mm (Bo = 0,4). As superfícies de teste foram caracterizadas por meio de medição da rugosidade média (Ra), do ângulo de contato estático (molhabilidade), e imagens MEV. Foi observado um aumento médio de 45% no HTC do teste com superfície lisa nanoestruturada em baixa concentração de nanofluido, em relação à superfície lisa sem deposição. Esse ganho está relacionado com o aumento do número de sítios ativos de nucleação causado pela deposição das nanopartículas sobre a ... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: The intensification of heat transfer through changes in the heated surface morphology has been studied in the scientific community to meet the increase demand for cooling of devices with high processing power and smaller dimensions. This work presents the experimental study of the effect of nanocoated surfaces and gap size during nucleated boiling of deionized water, in saturation temperature at atmospheric pressure, about heat transfer coefficient, HTC, and critical heat flux, CHF. The pool boiling process of Al2O3-water based nanofluid at two different mass concentrations: 0.03 g/l (“low” concentration, LC) and 0.3 g/l (“high” concentration, HC), produced nanostructured surfaces. Unconfined tests were analyzed, with gap size between the heated surface and the adiabatic surface of 30 mm (corresponding to Bo = 12), and tests under confinement conditions, with gap size of 1.0 mm (Bo = 0.4). The tested surfaces were characterized by means of surface roughness (Ra) measurement, static contact angle (wettability), and SEM images. An average increase of 45% in HTC of the test with nanocoated smooth surface in low nanofluid concentration was observed in relation to smooth surface without deposition. This enhancement is related to the increase in the number of active nucleation sites caused by the nanoparticle’s deposition on the smooth surface. For all tests with rough nanocoated surfaces and nanocoated smooth one with high nanofluid concentration, there was degradation of the HTC ... (Complete abstract click electronic access below) / Mestre

Submitted by Jessica Martha Nunes (je.nunes25@gmail.com) on 2018-10-04T01:31:03Z No. of bitstreams: 1 Dissertação_NunesJM.pdf: 7307066 bytes, checksum: 4cd6a3e84b8e10900f14961caf7df4e5 (MD5) / Approved for entry into archive by Cristina Alexandra de Godoy null (cristina@adm.feis.unesp.br) on 2018-10-08T14:23:35Z (GMT) No. of bitstreams: 1 nunes_jm_me_ilha.pdf: 7307066 bytes, checksum: 4cd6a3e84b8e10900f14961caf7df4e5 (MD5) / Made available in DSpace on 2018-10-08T14:23:35Z (GMT). No. of bitstreams: 1 nunes_jm_me_ilha.pdf: 7307066 bytes, checksum: 4cd6a3e84b8e10900f14961caf7df4e5 (MD5) Previous issue date: 2018-08-10 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / A intensificação da transferência de calor por meio de alterações na morfologia da superfície aquecida vem sendo estudada no meio científico, a fim de suprir a crescente demanda de resfriamento de dispositivos com alta capacidade de processamento e dimensões cada vez menores. O presente trabalho apresenta o estudo experimental do efeito de superfícies nanoestruturadas e do espaçamento do canal de confinamento durante a ebulição em piscina da água deionizada, à temperatura de saturação na pressão atmosférica, sobre o coeficiente de transferência de calor, HTC, e fluxo crítico de calor, CHF. As superfícies nanoestruturadas foram obtidas pelo processo de ebulição do nanofluido de Al2O3-água deionizada em duas diferentes concentrações más-sicas: 0,03 g/l (“baixa” concentração, LC) e 0,3 g/l (“alta” concentração, HC). Foram realizados testes livres, com espaçamento, entre a superfície aquecida e a superfície adiabática, de 30 mm (correspondendo a Bo = 12), e testes sob condições de confinamento, com espaçamento de 1,0 mm (Bo = 0,4). As superfícies de teste foram caracterizadas por meio de medição da rugosidade média (Ra), do ângulo de contato estático (molhabilidade), e imagens MEV. Foi observado um aumento médio de 45% no HTC do teste com superfície lisa nanoestruturada em baixa concentração de nanofluido, em relação à superfície lisa sem deposição. Esse ganho está relacionado com o aumento do número de sítios ativos de nucleação causado pela deposição das nanopartículas sobre a superfície lisa. Para todos os testes com superfícies rugosas nanoestruturadas e lisa nanoestruturada com alta concentração, houve degradação do HTC, devido ao efeito de preenchimento das cavidades e formação de uma resistência térmica adicional. Para baixos fluxos de calor, houve um aumento no HTC para os casos confinados em comparação aos livres, como consequência da evaporação do filme líquido presente entre a superfície aquecida e a bolha de vapor. Porém com o aumento do fluxo de calor, o fenômeno do dryout é antecipado em relação aos testes livres, o que compromete o desempenho de componentes sob essas condições. Nos testes sob confinamento foram observados ganhos no fluxo de calor de início do dryout para todas as superfícies nanoestruturadas testadas, chegando a 52% para a superfície lisa nanoestruturada em alta concentração, em comparação à superfície lisa sem nanoestrutura. Isso mostra que a nanoestruturação, apesar de não promover ganho no HTC, auxilia no ganho do fluxo de calor de início do dryout, que é o limite operacional de sistemas que trabalham sob confinamento. / The intensification of heat transfer through changes in the heated surface morphology has been studied in the scientific community to meet the increase demand for cooling of devices with high processing power and smaller dimensions. This work presents the experimental study of the effect of nanocoated surfaces and gap size during nucleated boiling of deionized water, in saturation temperature at atmospheric pressure, about heat transfer coefficient, HTC, and critical heat flux, CHF. The pool boiling process of Al2O3-water based nanofluid at two different mass concentrations: 0.03 g/l (“low” concentration, LC) and 0.3 g/l (“high” concentration, HC), produced nanostructured surfaces. Unconfined tests were analyzed, with gap size between the heated surface and the adiabatic surface of 30 mm (corresponding to Bo = 12), and tests under confinement conditions, with gap size of 1.0 mm (Bo = 0.4). The tested surfaces were characterized by means of surface roughness (Ra) measurement, static contact angle (wettability), and SEM images. An average increase of 45% in HTC of the test with nanocoated smooth surface in low nanofluid concentration was observed in relation to smooth surface without deposition. This enhancement is related to the increase in the number of active nucleation sites caused by the nanoparticle’s deposition on the smooth surface. For all tests with rough nanocoated surfaces and nanocoated smooth one with high nanofluid concentration, there was degradation of the HTC due to the filling effect of the cavities and the formation of an additional thermal resistance. For low heat fluxes, the HTC increased for confined cases compare to unconfined ones, as consequence of the liquid film evaporation present between the heated surface and the vapor bubble. However, with heat flux increase, the dryout phenomenon incipience is precipitated in relation to unconfined tests, which compromises the performance of components under these conditions. In the confined tests, enhancement in dryout incipience heat flux were observed for all nanocoated surfaces tested, reaching 52% for the nanocoated smooth surface in high concentration, compared to the smooth surface without nanostructure. This shows that nanostructure, while not promoting HTC enhancement, helps to delay the dryout incipience heat flux, which is the operational limit of systems that work under confinement.

Suite à un défaut de contrôle de la réaction nucléaire, un accident d’insertion de réactivité (RIA) peut survenir dans une centrale. Un pic de puissance se produit alors dans certains crayons de combustible, suffisamment important pour entraîner l’ébullition en film du réfrigérant qui les entoure. Ceci provoque la chute du refroidissement des crayons et donc une rapide et importante augmentation de la température de la gaine qui les entoure. L’évaluation du risque de rupture de la gaine est un sujet d’étude de l’Institut de Radioprotection et de Sûreté Nucléaire. Ces échanges de chaleur transitoires ne sont toujours pas compris et modélisés. Pour comprendre ces phénomènes, une boucle expérimentale a été construite à l’Institut de Mécanique des Fluides de Toulouse. Du HFE7000 circule de bas en haut dans une section d’essais verticale de géométrie semi-annulaire. Le demi-cylindre intérieur est une feuille de métal chauffée par effet Joule. Sa température est mesurée par une caméra infrarouge, couplée avec une caméra rapide pour la visualisation de l’écoulement. La courbe d’ébullition entière est étudiée en régimes stationnaire et transitoire : convection, déclenchement de l’ébullition, ébullition nucléée, passage en film, ébullition en film et remouillage. Les régimes stationnaires sont bien modélisés par des corrélations de la littérature. Différents modèles sont proposés pour représenter les transferts de chaleur transitoires : l’évolution de la convection et de l’ébullition nucléée se font de manière auto similaire pendant un palier de puissance. Ce constat permet de modéliser des évolutions plus compliquées telles des rampes de température. Le modèle de Hsu instationnaire prédit bien le déclenchement de l’ébullition. Pour des créneaux de puissance, le passage en film se fait à une température constante et le flux critique augmente avec la puissance, tandis que pour des rampes de puissance la température augmente mais le flux critique diminue avec l’augmentation de la puissance. Quand la paroi est chauffée, les flux de chaleur en ébullition en film sont beaucoup plus importants qu’en stationnaire mais ce régime est encore mal compris. Le refroidissement en ébullition en film et le remouillage sont bien caractérisés par un modèle à deux fluides. / A failure in the control system of the power of a nuclear reactor can lead to a Reactivity Initiated Accident in a nuclear power plant. Then, a power peak occurs in some fuel rods, high enough to lead to the coolant film boiling. It leads to an important increase of the temperature of the rod. The possible risk of the clad’s failure is a matter of interest for the Institut de Radioprotection et de Sûreté Nucléaire. The transient boiling heat transfer is not yet understood and modelled. An experimental set-up has been built at the Institut de Mécanique des Fluides de Toulouse (IMFT). Subcooled HFE-7000 flows vertically upward in a semi annulus test section. The inner half cylinder simulates the clad and is made of a stainless steel foil, heated by Joule effect. Its temperature is measured by an infrared camera, coupled with a high speed camera for the visualization of the flow topology. The whole boiling curve is studied in steady state and transient regimes: convection, onset of boiling, nucleate boiling, criticial heat flux, film boiling and rewetting. The steady state heat transfers are well modelled by literature correlations. Models are suggested for the transient heat flux: the convection and nucleate boiling evolutions are self-similar during a power step. This observation allows to model more complex evolutions, as temperature ramps. The transient Hsu model well represents the onset of nucleate boiling. When the intensity of the power step increases, the film boiling begins at the same temperature but with an increasing heat flux. For power ramps, the critical heat flux decreases while the corresponding temperature increases with the heating rate. When the wall is heated, the film boiling heat transfer is higher than in steady state but it is not understood. A two-fluid model well simulates the cooling film boiling and the rewetting.

Interest in the use of heat pipe technology for heat recovery and energy saving in a vast range of engineering applications has been on the rise in recent years. Heat pipes are playing a more important role in many industrial applications, especially in increasing energy savings in commercial applications and improving the thermal performance of heat exchangers. Computational techniques play an important role in solving complex flow problems for a large number of engineering applications due to their universality, flexibility, accuracy and efficiency. However, up to now, computational studies on heat pipes are still at an early stage due to the complexity of multiphase flow characteristics and heat and mass transfer phase changes. Therefore, the main objective of this study is to develop a CFD modelling that includes the complex physical phenomena of both the heat transfer processes of evaporation and condensation and the mass transfer process of phase change during the pool boiling and film condensation. In this thesis, two novel numerical models were developed in ANSYS FLUENT. In the first, a two-dimensional CFD model was developed to visualise the two-phase flow and the evaporation, condensation and heat transfer phenomena during the operation of a wickless heat pipe, that otherwise could not be visualised by empirical or experimental work. An in-house code was developed using user-defined functions (UDFs) to enhance the ability of FLUENT to simulate the phase change occurring inside the heat pipe. Three different fluids, water, R134a and R404a, were selected as the working fluids of the investigated wickless heat pipe. The cooling system of the condenser section was simulated separately as a three-dimensional CFD model of a parallel-flow double pipe heat exchanger to model the heat transfer across the condenser section's heat exchanger and predict the heat transfer coefficients. The overall effective thermal resistance along with the temperature profile along the wickless heat pipe have been investigated. An experimental apparatus was built to carry out a thermal performance investigation on a typical wickless heat pipe for the purpose of validating the CFD simulation. A theoretical model based on empirical correlations was developed to predict the heat transfer thermal resistances in the evaporator and the condenser section. The second model was developed to combine the two-dimensional CFD simulation of the wickless heat pipe and the three-dimensional CFD simulation of the condenser section's heat exchanger to simulate the two-phase flow phenomena of boiling and condensation and the cooling system of the condenser section through a comprehensive three-dimensional CFD model of a wickless heat pipe. Two fluids, water and R134a, were selected as the working fluids of the investigated wickless heat pipe. This model was validated using a transparent glass wickless heat pipe to visualise the phenomena of pool boiling and comparing the results with the three-dimensional CFD flow visualisation. This study demonstrated that the proposed CFD models of a wickless heat pipe can successfully reproduce the complex physical phenomena of both the heat transfer process of evaporation and condensation and the mass transfer process of phase change during the pool boiling that takes place in the evaporator section and the filmwise condensation that takes place in the condenser section. The CFD simulation was successful in modelling and visualising the multiphase flow characteristics for water, R134a and R404a, emphasising the difference in pool boiling behaviour between these working fluids. The CFD simulation results were compared with experimental measurements, with good agreement obtained between predicted temperature profiles and experimental temperature data.

The use of nanofluids as heat transfer fluids has received a lot of attention from the heat transfer research community. Due to the increased thermal conductivity of nanofluids over their base fluids, the number of nanofluids scientific publications increased significantly in the past decade. The effects of the heated surface roughness, nanoparticles and surfactant concentrations on pool boiling of nanofluids have been thoroughly investigated. However, contradicting findings have been observed under what appeared to similar test conditions. In this experimental investigation, two boiling surfaces have been prepared with an average surface roughness of 6 and 60 nm using high precision machining. Alumina Oxide-Water based nanofluids have been used in this investigation. The initial nanoparticle size reported by the manufacturer is 10 nm. The nanoparticles concentration has been kept at 0.05 wt. %. A Sodium Dodecylbenzenesulfonate (SDBS) surfactant has been added to the nanofluids in order to improve its stability. Results showed that the nanofluids boiling performance depended on the boiling surface roughness. The heat transfer coefficient (HTC) obtained in the case of the smooth, mirror finished surface showed an enhancement of 205% with respect to pure water. This trend was reversed in the case of the rough surface which is believed to be due to significant nanoparticles deposition. The HTC obtained with the rough surface was 12% lower than that of pure water. The effect of the surfactant concentration on nanoparticles deposition has been investigated by changing the surfactant concentration from 0.1 to 1.0 wt. %. In the case of the rough surface, the increase of surfactant concentration was found to reduce the formation of the nanoparticles deposition layer. The HTC obtained with the higher surfactant concentration was increased by 46 %. The effect of nanoparticles concentration on the smooth surface shows an unexpected trend of 20 % reduction of the transfer rate of the nanofluids coupled with the increase of the nanoparticle concentration from 0.05 to 0.1 wt. %. However all concentrations showed heat transfer enhancement with respect to pure water. The minimum heat transfer coefficient ratio enhancement was 11 % using 0.1 wt. % nanofluids with respect to pure water. Since nanoparticles deposition has been observed and attributed to micro-layer evaporation, an investigation has been carried out to examine the nucleation process during the pure water and nanofluids pool boiling. The bubble growth rate in both cases was analyzed at different wall degrees of superheat ranging from 104.3 to 105.9 ºC. In addition, the bubble departure diameter and frequency have been measured and compared for both cases. The nanofluid bubble size was about 80 % smaller than that of pure water. The nanofluid bubble departure had almost constant frequency of 500 Hz over the range of wall superheats whereas the maximum bubble frequency in the case of pure water was 22.72 Hz. / Thesis / Master of Applied Science (MASc)

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

A novel surface structure comprising dendritically ordered nano-particles of copper was developed during the duration of this thesis research project. A high current density electrodeposition process, where hydrogen bubbles functioned as a dynamic mask for the materials deposition, was used as a basic fabrication method. A post processing annealing treatment was further developed to stabilize and enhance the mechanical stability of the structure. The structure was studied quite extensively in various pool boiling experiments in refrigerants; R134a and FC-72. Different parameters were investigated, such as; thickness of the porous layer, presence of vapor escape channels, annealed or non-annealed structure. Some of the tests were filmed with a high speed camera, from which visual observation were made as well as quantitative bubble data extracted. The overall heat transfer coefficient in R134a was enhanced by about an order of magnitude compared to a plain reference surface and bubble image data suggests that both single- and two-phase heat transfer mechanisms were important to the enhancement. A quantitative and semi-empirical boiling model was presented where the main two-phase heat transfer mechanism inside the porous structure was assumed to be; micro-layer evaporation formed by an oscillating vapor-liquid meniscus front with low resistance vapor transport through escape channels. Laminar liquid motion induced by the oscillating vapor front was suggested as the primary single-phase heat transfer mechanism. The structure was applied to a standard plate heat exchanger evaporator with varying hydraulic diameter in the refrigerant channel. Again, a 10 times improved heat transfer coefficient in the refrigerant channel was recorded, resulting in an improvement of the overall heat transfer coefficient with over 100%. A superposition model was used to evaluate the results and it was found that for the enhanced boiling structure, variations of the hydraulic diameter caused a change in the nucleate boiling mechanism, which accounted for the largest effect on the heat transfer performance. For the standard heat exchanger, it was mostly the convective boiling mechanism that was affected by the change in hydraulic diameter. The structure was also applied to the evaporator surface in a two-phase thermosyphon with R134a as working fluid. The nucleate boiling mechanism was found to be enhanced with about 4 times and high speed videos of the enhanced evaporator reveal an isolated bubble flow regime, similar to that of smooth channels with larger hydraulic diameters. The number and frequency of the produced bubbles were significantly higher for the enhanced surface compared to that of the plain evaporator. This enhanced turbulence and continuous boiling on the porous structure resulted in decreased oscillations in the thermosyphon for the entire range of heat fluxes. / QC 20111111

<p dir="ltr">Boiling processes have long had an important role in power generation and air conditioning applications. The efficient and reliable heat dissipation afforded through the phase change process in the boiling has led to their generation of a substantial body of work in this field over several decades. Despite decades of efforts, the heat transfer performance prediction in boiling has been highly empirical with models working only for a narrow range of surface/fluids or other operating conditions. The limitation in these models is a result of a lack of mechanistic understanding of the underlying heat and mass transfer process. Surface dryout or boiling crisis is a process wherein there is a spontaneous formation of vapor film on top of the surface causing a catastrophic increase in surface temperature. The heat flux at which this formation of vapor film occurs is called critical heat flux (CHF). The CHF demarcates the upper limit to the regime of stable nucleating bubbles called nucleate boiling. The mechanism causing dryout is under debate for over half a century and several conflicting theories that cause dryout have been suggested since the 1950s including hydrodynamic, irreversible dryspot expansion, macrolayer dryout/liftoff, critical bubble distributions, vapor-recoil based theories and more. The lack of consensus is due to limitation in the information collected on the dynamic multiscale and chaotic bubble interactions. Recent advances in high-fidelity spatiotemporal phase, temperature, and heat flux measurements now enable diagnostic tools that can be leveraged to understand the complex heat transfer processes emerging from bubble-surface interaction on the boiling surface. In this work, we develop such techniques to understand various transport mechanisms underlying boiling and its crisis.</p><p dir="ltr">In this work, an experimental technique for collecting synchronized through-substrate visual and infrared (IR) measurements of a boiling surface is developed. An IR and visually transparent sapphire substrate with an IR-opaque indium-tin-oxide (ITO) heater layer is used to measure the phase (liquid and vapor areas) and temperature of the ITO layer. The visual camera collects the light reflected off the substrate from a red LED and the images collected show a contrast between liquid and vapor areas that is used to generate binarized phase maps. The temperature from the IR camera is used as boundary condition to solve a conduction problem for heat fluxes going into the fluid. Four distinct heat flux signatures corresponding to liquid, contact line, vapor and rewetting regions are observed. A post-processing methodology utilizing synchronous phase measurements to identify and partition these regions is introduced. The high-fidelity phase measurements allow for detection of fine features that are not discernable using heat flux maps alone. Analysis of the heat flux and temperature maps of partitioned regions for HFE-7100 fluid on the ITO surface show qualitative agreement with the trends in mechanisms underlying those areas. The experiment and post-processing methodology introduced in this work is the first to provide partitioning of underlying heat transfer mechanisms for multi-bubbles throughout the entire range of the boiling curve during both steady and transient scenarios.</p><p dir="ltr">The technique developed is used to probe the mechanisms underlying the boiling crisis. Theories suggested in the literature for boiling crisis are carefully evaluated and evidence against hydrodynamic instability, macrolayer dryout, vapor recoil, irreversible expansion of dryspots, macrolayer liftoff model, and bifurcations from critical distributions is observed. The signature in the peak of the spatially averaged fluid heat flux is observed to precede any other signs of dryout. Beyond the peak heat flux an increase in superheat leads to reduced heat dissipated by boiling and further increases the temperature causing a thermal runaway in the substrate that eventually leads to dryout. Hence, the boiling crisis is found to be a consequence of a peak in the nucleate boiling curve. The cause for the peak in the boiling heat flux for the surface-fluid combination tested was due to degradation of heat transfer caused by the replacement of high-heat-transfer contact line region with lower-heat-transfer vapor covered regions, among the multiple competing mechanisms. Hence, we propose that mechanistically modeling the boiling crisis rests on prediction of the peak in the upper portion of the nucleate boiling curve by considering all underlying heat transfer mechanisms. A modeling framework based on heat flux partitioning, where the overall heat transferred during boiling is calculated as the sum of the heat transferred by individual mechanisms is demonstrated as potential pathway to predict the upper portion of the nucleate boiling curve and thereby critical heat flux. Based on the terms involved in summation for individual mechanisms, we propose that the boiling curve for any given surface be interpreted as a path on a multidimensional surface (boiling manifold). Estimation of such a boiling manifold allows for prediction of the boiling curve for any surface, given development of the relations between these parameters and surface-fluid properties, and can further be used to backtrack relevant thermophysical or nucleation properties for enhanced boiling performance.</p><p dir="ltr">Enhancement of pool boiling heat transfer performance using surface modifications is of major interest to applications and this work further delves into characterizing the boiling performance using traditional surface averaged measurements of microstructured surfaces using HFE-7100. We find that microlayer evaporation from the imbibed liquid layer underneath the growing vapor bubbles is the key mechanism of boiling heat transfer enhancement in microstructures. Further, this implies that characterization of microstructured surfaces for evaporative performance can serve as an important proxy to enable heat transfer coefficient enhancement prediction during pool boiling. Hence, we also developed an easily calculated Figure of Merit (FOM) that characterizes the efficacy of evaporation from microstructured surfaces.</p><p dir="ltr">To summarize, in this work we developed an experimental technique using synchronous through-substrate high-speed visual and IR imaging methods. New post-processing techniques for partitioning of different heat transfer mechanisms are proposed and used to analyze boiling on an ITO-coated sapphire substrate with HFE-7100 as the working fluid. We reveal thermal runaway in the substrate caused due to a negative-sloping boiling curve as the mechanism of dryout. Mechanistic modeling of the critical heat flux thus involves calculating the peak in the nucleate boiling curve. A framework to predict the nucleate boiling curve and subsequently critical heat flux is proposed based on the partitioning analysis. The experimental method developed lays the groundwork for measuring heat flux and superheats associated with various mechanisms, and hence, enables validation of future partitioning-based boiling heat transfer models that intrinsically enable prediction of the peak.</p>

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Doctor of Philosophy / Department of Mechanical and Nuclear Engineering / Steven J. Eckels / The current research presents the experimental investigation of the effect of tube pitch on enhanced tube bundles’ performance. The typical application of this research is flooded refrigerant evaporators. Boosting evaporator’s performance through optimizing tube spacing reduces cost and energy consumption. R-134a with the enhanced tube Turbo BII-HP and R-123 with Turbo BII-LP were used in this study. Three tube pitches were tested P/D 1.167, P/D 1.33, and P/D 1.5. Each tube bundle includes 20 tubes (19.05 mm outer diameter and 1 m long each) constructed in four passes. The test facility’s design allows controlling three variables, heat flux, mass flux, and inlet quality. The type of analysis used is local to one location in the bundle. This was accomplished by measuring the water temperature drop in the four passes. The water-side pressure drop is included in the data analysis. A new method called the EBHT (Enthalpy Based Heat Transfer) was introduced, which uses the water-side pressure drop in performing the heat transfer analysis. The input variables ranges are: 15-55 kg/m².s for mass flux, 5-60 kW/m² for heat flux, and 10-70% for inlet quality. The effect of local heat flux, local quality, and mass flux on the local heat transfer coefficient was investigated. The comparison between the bundle performance and single tube performance was included in the results of each tube bundle. The smallest tube pitch has the lowest performance in both refrigerants, with a significantly lower performance in the case of R-134a. However, the two bigger tube pitches have very similar performance at low heat flux. Moreover, the largest tube pitch performance approaches that of the single tube at medium and high heat fluxes. For the R-123 study, the smallest tube bundle experienced quick decease in performance at high qualities, exhibiting tube enhancement dry-out at certain flow rates and high qualities. The flow pattern effect was demonstrated by the dry-out phenomena. At medium and high heat fluxes, as the tube pitch increases, the performance approaches that of the single tube. All tube bundles experience quick decrease in performance at high qualities. Evidently, P/D 1.33 is the optimum tube pitch for the studied refrigerants and enhanced tubes combinations.