In this research, an elastic cylinder that utilized vortex-induced vibration (VIV) was applied to improve convective heat transfer rates by disrupting the thermal boundary layer. Rigid and elastic cylinders were placed across a fluid channel. Vortex shedding around the cylinder led to the periodic vibration of the cylinder. As a result, the flow-structure interaction (FSI) increased the disruption of the thermal boundary layer, and therefore, improved the mixing process at the boundary. This study aims to improve convective heat transfer rate by increasing the perturbation in the fluid flow. A three-dimensional numerical model was constructed to simulate the effects of different flow channel geometries, including a channel with a stationary rigid cylinder, a channel with a elastic cylinder, a channel with two elastic cylinders of the same diameter, and a channel with two elastic cylinders of different diameters. Through the numerical simulations, the channel maximum wall temperature was found to be reduced by approximately 10% with a stationary cylinder and by around 17% when introducing an elastic cylinder in the channel compared with the channel without the cylinder. Channels with two-cylinder conditions were also studied in the current research. The additional cylinder with the same diameter in the fluid channel only reduced the surface wall temperature by 3% compared to the channel without any cylinders because the volume of the second cylinder could occupy some space, and therefore, reduce the effect of the convective heat transfer. By reducing the diameter of the second cylinder by 25% increased the effect of the convection heat transfer and reduced the maximum wall temperature by around 15%. Compared to the channel with no cylinder, the introduction of cylinders into the channel flow was found to increase the average Nusselt number by 55% with the insertion of a stationary rigid cylinder, by 85% with the insertion of an elastic cylinder, by 58% with the insertion of two cylinders of the same diameter, and by approximately 70% with the insertion of two cylinders of different diameters (the second cylinder having the smaller diameter). Furthermore, it was also found that the maximum local Nusselt number could be enhanced by around 200%-400% at the entrance of the fluid channel by using the elastic cylinders compared to the channel without cylinders.

Cette thèse de doctorat s’intéresse aux mécanismes d’amélioration des transferts dans des géométries de dissipateurs thermiques à plaques et ailettes. Une première partie est consacrée à l’étude d’une configuration académique à l’aide de simulations numériques visant à obtenir une amélioration du transfert de chaleur conjugué en modifiant uniquement par des découpes la forme géométrique des ailettes planes conductrices. Une analyse locale approfondie de l’écoulement et des champs thermiques a été effectuée avec notamment le principe de synergie locale, des champs de vitesse et de gradients thermiques, pour comprendre l’effet des modifications géométriques. Ce mémoire présente également le développement de dissipateurs aux performances thermo-aérauliques augmentées pour des applications de refroidissement de coffrets électronique embarqués. L’intensification des transferts thermiques est obtenue par la génération d’écoulements secondaires qui provoquent un brassage de fluide et réduisent la résistance thermique à la paroi en perturbant le développement de la couche limite thermique. Différentes configurations de dissipateurs avec deux types de générateurs d’écoulements secondaires, paires d’ailettes Delta et protrusions, ont été étudiées numériquement, en employant une modélisation de type « RANS ». Les performances thermo-aérauliques des géométries munies de générateurs de vorticité ont été comparées à celle d’un dissipateur thermique de référence « lisse ». Des prototypes ont également été fabriqués et testés sur un banc expérimental spécifiquement développé pour réaliser des mesures des performances globales en termes de puissance thermique et de pertes de charge. Les résultats expérimentaux et numériques ont été confrontés afin de qualifier les simulations réalisées. Par la suite, une étude d’optimisation employant l’analyse factorielle Taguchi a été utilisée afin d’optimiser les paramètres géométriques des dissipateurs retenus. Deux fonctions objectif ont été considérées : la maximisation du facteur de performance thermique à iso puissance de ventilation (PEC) et la réduction de la température moyenne de paroi du dissipateur par rapport au cas de référence. L’analyse des performances thermo-aérauliques globales des géométries étudiées a été complétée par une analyse qualitative locale des champs thermiques et d’écoulement notamment avec le principe de synergie. / This doctoral thesis focuses on mechanisms of heat transfer enhancement in plate and fin heat sink geometries. First part of the thesis is dedicated to study an academic configuration using numerical simulations to achieve an improvement in conjugate heat transfer by modifying only the geometrical shape (through punching) of the conductive plane fins. An in-depth local analysis of the flow and thermal fields was carried out with the local synergy principle, velocity and thermal gradients, to understand the effect of geometric modifications. This thesis also presents the development of heat sinks with increased thermo-hydraulic performance for on-board electronic box cooling applications. The intensification of the heat transfer is obtained by the generation of secondary flows which cause an intensive mixing of fluid and reduces the thermal resistance to the wall by disrupting the development of the thermal boundary layer. Different heat sink geometries with two types of secondary flow generators : delta winglet pair and protrusions were numerically studied using RANS approach. The thermo-hydraulic performances of the geometries equipped with vortex generators were compared with that of a smooth reference heat sink. The prototypes were also manufactured and tested on an experimental bench specifically designed to perform global performance measurements in terms of thermal power and pressure drops. Experimental and numerical results were compared to qualify the simulations performed. Subsequently, an optimization study using Taguchi factorial analysis was used to optimize the geometrical parameters of the chosen dissipaters. Two objective functions were considered : maximization of either iso-pumping power performance criteria (PEC) or average wall temperature of the dissipaters compared to the reference case. The global thermo-hydraulic performance analysis of the studied geometries was completed by a qualitative analysis of local flow and thermal fields, in particular with the local field synergy principle.

Dans le but de réduire la consommation en carburant et les émissions de CO2 des moteurs à combustion interne, un des leviers, qui a intéressé diffèrent acteurs dans le secteur automobile, est la récupération de l’énergie thermique disponible dans les gaz d’échappement. Malgré différents technologie ont été investigués dans le passé; les transferts de chaleur qui apparient dans les gaz d’échappement n’ont pas encore étés suffisamment étudiés. Le fait que les échanges de la chaleur apparent dans des conditions pulsatives, notamment due aux conditions de fonctionnement moteur, rende les connaissances acquis jusqu’à présent limités et ne pas exploitables. A l’état actuel on n’est pas capable de pouvoir prédire le transfert thermique convectif des écoulements pulsé. Les travaux de cette thèse s’instaurent dans la continuité de ce besoin, l’objectif principal est donc l’étude expérimentale du transfert thermique convectif des écoulements turbulent pulsés dans un conduit cylindrique. La première partie de ce travail a été consacrée à le dimensionnement d’un moyen d’essais permettant la création d’un écoulement pulsé type moteur; en suite différents méthodes de mesures ont étés développes afin de connaitre les variations instantanés de vitesse et température de l’écoulement. Plusieurs essais ont été reproduits afin de caractériser l’impact de la pulsation sur le transfert de la chaleur. Les résultats expérimentaux ont été analysés avec deux approches différentes: dans un premier temps une approche analytique 1D a permis de mettre en évidence le mécanisme principal responsable de l’amélioration du transfert thermique convectif,ainsi, il a fourni des éléments supplémentaires pour le futur développement de modèles mathématiques plus adaptés à la prédiction des transferts d’énergie. En suite une approche 2D, supporté d’une phase de modélisation numérique, a permis de caractériser le mécanisme de transport radial d’énergie thermique. / Waste Energy Recovery represents a promising way to go further in fuel saving and greenhouse emissions control for Internal Combustion Engine applications. Although several technologies have been investigated in the past few years, the convective heat transfers, playing an important role in the energy exchanges at the engine exhaust, has not receive enough attention. Heat transfers, in such applications, occur in pulsating conditions because of the engine operating conditions, making thus the actual knowledge of the heat transfer phenomena limited and not exploitable. Nowadays there is not any model capable to predict convective heat transfers for pulsating flows. In this context, the present thesis addresses the purpose to study the convective heat transfer phenomena, by an experimental approach, occurring for turbulent pulsating flows in pipes. In the first part of this work, an experimental apparatus has been designed to reproduce an exhaust type pulsating flow in fully managed conditions, as well as, several measurement techniques have been developed to know the instantaneous profiles of air temperature and velocity. Many experiments have been performed in order to characterize the impact of the flow pulsation on the convective heat transfers. In the second part of this work, the experimental results have been analyzed with two different approaches: firstly, with a 1D assumption the time-average convective heat transfers has been computed, and the major mechanism responsible of the heat transfer enhancement has been pointed out. Furthermore, it has been possible to highlight the mathematical term representative of such mechanism, which should be accounted in future to define a more adapted numerical model for the heat transfer prediction. In a second phase with a 2D assumption, and, with an energy and a fluid-mechanic computational phase, the radial transport of thermal energy has been characterized for a pulsating flow.

Esta pesquisa visa a análise da contribuição de cinco variáveis de entrada e a otimização do desempenho termo-hidráulico de trocadores de calor com venezianas combinados com geradores de vórtices delta-winglets. O desempenho termohidráulico de duas geometrias distintas, aqui nomeadas por GEO1 e GEO2, foram avaliadas. Smoothing Spline ANOVA foi usado para avaliar a contribuição dos parâmetros de entrada na transferência de calor e perda de carga. Considerando aplicação automotiva, foram investigados números de Reynolds iguais a 120 e 240, baseados no diâmetro hidráulico. Os resultados indicaram que o ângulo de venezianas é o maior contribuidor para o aumento do fator de atrito para GEO1 e GEO2, para ambos os números de Reynolds. Para o número de Reynolds menor, o parâmetro mais importante em termos de transferência de calor foi o ângulo das venezianas para ambas as geometrias. Para o número de Reynolds maior, o ângulo de ataque dos geradores de vórtices posicionados na primeira fileira é o maior contribuidor para a tranfesferência de calor, no caso da geometria GEO1, enquanto que o ângulo de ataque dos geradores de vórtices na primeira fileira foi tão importante quanto os ângulos das venezianas para a geometria GEO2. Embora as geometrias analisadas possam ser consideradas como técnicas compostas de intensificação da transferência de calor, não foram observadas interações relevantes entre ângulo de venezianas e parâmetros dos geradores de vórtices. O processo de otimização usa NSGA-II (Non-Dominated Sorting Genetic Algorithm) combinado com redes neurais artificiais. Os resultados mostraram que a adição dos geradores de vórtices em GEO1 aumentaram a transferência de calor em 21% e 23% com aumentos na perda de carga iguais a 24,66% e 36,67% para o menor e maior números de Reynolds, respectivamente. Para GEO2, a transferência de calor aumentou 13% e 15% com aumento na perda de carga de 20,33% e 23,70%, para o menor e maior número de Reynolds, respectivamente. As soluções otimizadas para o fator de Colburn mostraram que a transferência de calor atrás da primeira e da segunda fileiras de geradores de vórtices tem a mesma ordem de magnitude para ambos os números de Reynolds. Os padrões de escoamento e as características de transferência de calor das soluções otimizadas apresentaram comportamentos vi particulares, diferentemente daqueles encontrados quando as duas técnicas de intensificação de transferência de calor são aplicadas separadamente. / This doctoral thesis focuses on screening analysis of five input parameters and heat transfer and pressure drop optimization of flat-tube multi-louvered fin heat exchangers combined with delta-winglet vortex generators. The thermal-hydraulic performance of two distinct geometries, GEO1 and GEO2, were evaluated. Smoothing Spline ANOVA was used to evaluate the contribution of the input parameters such as louver angle, angle of attack of the delta-winglet and streamwise position of the delta-winglet on heat transfer and pressure drop. Taking the automotive application into account, Reynolds numbers of 120 and 240, based on hydraulic diameter, were investigated. The results indicated that the louver angle is the main contributor to increase the Friction factor for GEO1 and GEO2 for both Reynolds numbers. For the lower Reynolds number, the most important heat transfer parameter was the louver angle for both geometries, while at the higher Reynolds number, the angles of attack of the first row of delta-winglets mostly contributed to GEO1, and the angle of attack of the first row of delta-winglets was as important as the louver angle for GEO2. Although those specific geometries can be considered a kind of compound enhancement technique, relevant interactions were not verified between louvers and delta-winglet vortex generators parameters. The surrogatebased optimization procedure uses NSGA-II method (Non-Dominated Sorting Genetic Algorithm) combined with artificial neural networks. The results showed that the addition of DWLs on GEO1 increased the heat transfer of 21.27% and 23.52% with associated pressure loss increasing of 24.66% and 36.67% for the lower and the higher Reynolds numbers, respectively. For GEO2, the heat transfer was increased 13.48% and 15.67% with an increase of the pressure drop of 20.33% and 23.70%, for the lower and the higher Reynolds numbers, respectively. The optimized solutions for the Colburn factor showed that heat transfer behind the second row of deltawinglets has the same order of magnitude of that behind the first row, for both Reynolds numbers. The flow patterns and heat transfer characteristics from optimized solutions presented some particular behavior, differently from the findings when those two heat transfer enhancement techniques are applied separately.

Esta pesquisa visa a análise da contribuição de cinco variáveis de entrada e a otimização do desempenho termo-hidráulico de trocadores de calor com venezianas combinados com geradores de vórtices delta-winglets. O desempenho termohidráulico de duas geometrias distintas, aqui nomeadas por GEO1 e GEO2, foram avaliadas. Smoothing Spline ANOVA foi usado para avaliar a contribuição dos parâmetros de entrada na transferência de calor e perda de carga. Considerando aplicação automotiva, foram investigados números de Reynolds iguais a 120 e 240, baseados no diâmetro hidráulico. Os resultados indicaram que o ângulo de venezianas é o maior contribuidor para o aumento do fator de atrito para GEO1 e GEO2, para ambos os números de Reynolds. Para o número de Reynolds menor, o parâmetro mais importante em termos de transferência de calor foi o ângulo das venezianas para ambas as geometrias. Para o número de Reynolds maior, o ângulo de ataque dos geradores de vórtices posicionados na primeira fileira é o maior contribuidor para a tranfesferência de calor, no caso da geometria GEO1, enquanto que o ângulo de ataque dos geradores de vórtices na primeira fileira foi tão importante quanto os ângulos das venezianas para a geometria GEO2. Embora as geometrias analisadas possam ser consideradas como técnicas compostas de intensificação da transferência de calor, não foram observadas interações relevantes entre ângulo de venezianas e parâmetros dos geradores de vórtices. O processo de otimização usa NSGA-II (Non-Dominated Sorting Genetic Algorithm) combinado com redes neurais artificiais. Os resultados mostraram que a adição dos geradores de vórtices em GEO1 aumentaram a transferência de calor em 21% e 23% com aumentos na perda de carga iguais a 24,66% e 36,67% para o menor e maior números de Reynolds, respectivamente. Para GEO2, a transferência de calor aumentou 13% e 15% com aumento na perda de carga de 20,33% e 23,70%, para o menor e maior número de Reynolds, respectivamente. As soluções otimizadas para o fator de Colburn mostraram que a transferência de calor atrás da primeira e da segunda fileiras de geradores de vórtices tem a mesma ordem de magnitude para ambos os números de Reynolds. Os padrões de escoamento e as características de transferência de calor das soluções otimizadas apresentaram comportamentos vi particulares, diferentemente daqueles encontrados quando as duas técnicas de intensificação de transferência de calor são aplicadas separadamente. / This doctoral thesis focuses on screening analysis of five input parameters and heat transfer and pressure drop optimization of flat-tube multi-louvered fin heat exchangers combined with delta-winglet vortex generators. The thermal-hydraulic performance of two distinct geometries, GEO1 and GEO2, were evaluated. Smoothing Spline ANOVA was used to evaluate the contribution of the input parameters such as louver angle, angle of attack of the delta-winglet and streamwise position of the delta-winglet on heat transfer and pressure drop. Taking the automotive application into account, Reynolds numbers of 120 and 240, based on hydraulic diameter, were investigated. The results indicated that the louver angle is the main contributor to increase the Friction factor for GEO1 and GEO2 for both Reynolds numbers. For the lower Reynolds number, the most important heat transfer parameter was the louver angle for both geometries, while at the higher Reynolds number, the angles of attack of the first row of delta-winglets mostly contributed to GEO1, and the angle of attack of the first row of delta-winglets was as important as the louver angle for GEO2. Although those specific geometries can be considered a kind of compound enhancement technique, relevant interactions were not verified between louvers and delta-winglet vortex generators parameters. The surrogatebased optimization procedure uses NSGA-II method (Non-Dominated Sorting Genetic Algorithm) combined with artificial neural networks. The results showed that the addition of DWLs on GEO1 increased the heat transfer of 21.27% and 23.52% with associated pressure loss increasing of 24.66% and 36.67% for the lower and the higher Reynolds numbers, respectively. For GEO2, the heat transfer was increased 13.48% and 15.67% with an increase of the pressure drop of 20.33% and 23.70%, for the lower and the higher Reynolds numbers, respectively. The optimized solutions for the Colburn factor showed that heat transfer behind the second row of deltawinglets has the same order of magnitude of that behind the first row, for both Reynolds numbers. The flow patterns and heat transfer characteristics from optimized solutions presented some particular behavior, differently from the findings when those two heat transfer enhancement techniques are applied separately.

An experimental and numerical investigation has been conducted to examine steady, internal, nozzle-generated, gas/liquid mist cooling in vertical channels with ultra-thin, evaporating subcooled liquid films. Interest in this research has been motivated by the need for a highly efficient cooling mechanism in high-power lasers for inertial fusion reactor applications. The aim is to quantify the effects of various operating and design parameters, viz. liquid atomization nozzle design (i.e. spray geometry, droplet size distribution, etc.), heat flux, liquid mass fraction, film thickness, carrier gas velocity, temperature, and humidity, injected liquid temperature, gas/liquid combinations, channel geometry, length, and wettability, and flow direction, on mist cooling effectiveness. A fully-instrumented experimental test facility has been designed and constructed. The facility includes three cylindrical and two rectangular electrically-heated test sections with different unheated entry lengths. Water is used as the mist liquid with air, or helium, as the carrier gas. Three types of mist generating nozzles with significantly different spray characteristics are used. Numerous experiments have been conducted; local heat transfer coefficients along the channels are obtained for a wide range of operating conditions. The data indicate that mist cooling can increase the heat transfer coefficient by more than an order of magnitude compared to forced convection using only the carrier gas. The data obtained in this investigation will allow designers of mist-cooled high heat flux engineering systems to predict their performance over a wide range of design and operating parameters. Comparison has been made between the data and predictions of a modified version of the KIVA-3V code, a mechanistic, three-dimensional computer program for internal, transient, dispersed two-phase flow applications. Good agreement has been obtained for downward mist flow at moderate heat fluxes; at high heat fluxes, the code underpredicts the local heat transfer coefficients and does not predict the onset of film rupture. For upward mist flow, the code underpredicts the local heat transfer coefficients and, contrary to experimental observations, predicts early dryout at the test section exit.

Shell and tube heat exchangers (STHEs) are the most common type of heat exchanger in preheat trains (PHT) of oil refineries and in chemical process plants. Most commercial design software tools for STHE assume uniform distribution over all tubes of a tube bundle. This leads to various challenges in the operation of the affected devices. Flow maldistribution reduces heat duty of STHE in many applications and supports fouling buildup in fluids that tend to particle, bio, and crystallization fouling (Verein Deutscher Ingenieure, ed., 2010, Heat Atlas, 2nd ed., VDI-Buch., Springer-Verlag). In this article, a fluid mechanics study about tube side flow distribution of crude oil and related hydrocarbons in two-pass PHT heat exchangers is described. It is shown that the amount of flow maldistribution varies significantly between the different STHE designs. Therefore, a parameter study was conducted to investigate reasons for maldistribution. For instance, the nozzles diameter, type, and orientation were identified as crucial parameters. In consequence, simple design suggestions for reducing tube side flow maldistribution are proposed.