L’étude du transfert de masse turbulent aux interfaces gaz-liquide est d’un grand intérêt dans de nombreuses applications environnementales et industrielles. Bien que ce problème soit étudié depuis de nombreuses années, sa compréhension n’est pas encore suffisante pour la création de modèles de transfert de masse réalistes (de type RANS ou LES sous maille), en particulier en présence d’une phase liquide à rhéologie complexe. Ce travail expérimental a pour but l’étude des aspects fondamentaux du transfert de masse turbulent à une interface plane horizontale entre du dioxyde de carbone gazeux et une phase liquide newtonienne ou non, agitée par une turbulence homogène quasi isotrope. Les milieux liquides non newtoniens étudiés sont des solutions aqueuses d’un polymère dilué à des concentrations variables et aux propriétés viscoélastiques et rhéofluidifiantes. Deux méthodes de mesure optiques permettant l’obtention du champ de vitesse de la phase liquide (SPIV) et de concentration du gaz dissout (I-PLIF) sont couplées tout en maintenant une haute résolution spatiale, afin de déduire les statistiques de vitesse et de concentration couplées dans les premiers millimètres sous la surface. Une nouvelle version de I-PLIF est développée pour les mesures en proche surface. Elle peut également s’appliquer dans différentes études de transfert de masse. La turbulence de fond est générée par un dispositif de grille oscillante. Les mécanismes de production et les caractéristiques de la turbulence sont étudiés. L’importance de la composante oscillante de la turbulence est discutée, et un phénomène d’amplification de l’écoulement moyen est mis en évidence. Les mécanismes du transfert de masse turbulent à l’interface sont finalement observés pour l’eau et une solution de polymère dilué à faible concentration. L’analyse conditionnelle des flux de masse turbulent permet de mettre en évidence les évènements contribuant au transfert de masse et de discuter de leur impact relatif sur le transfert total. / The study of turbulence induced mass transfer at the interface between a gas and a liquid is of great interest in many environmental phenomena and industrial processes. Even though this issue has already been studied for several decades, its understanding is still not good enough to create realistic models (RANS or sub-grid LES), especially when considering a liquid phase with a complex rheology. This experimental work aims at studying fundamental aspects of turbulent mass transfer at a flat interface between carbon dioxide and a Newtonian or non-Newtonian liquid, stirred by homogeneous and quasi isotropic turbulence. Non-Newtonian fluids studied are aqueous solutions of a model polymer, Xanthan gum (XG), at various concentrations, showing viscoelastic and shear-thinning properties. Optical techniques for the acquisition of the liquid phase velocity field (Stereoscopic Particle Image Velocimetry, SPIV) and dissolved gas concentration field (Inhibited Planar Laser Induced Fluorescence, I-PLIF) are for the first time coupled, keeping a high spatial resolution, to access velocity and concentration statistics in the first few millimetres under the interface. A new version of I-PLIF is developed. It is designed to be more efficient for near surface measurements, but its use can be generalized to other single or multiphase mass transfer situations. Bottom shear turbulence in the liquid phase is generated by an oscillating grid apparatus. The mechanisms of turbulence production and the characteristics of oscillating grid turbulence (OGT) are studied. The importance of the oscillatory component of turbulence is discussed. A mean flow enhancement effect upon polymer addition is evidenced. The mechanisms of turbulent mass transfer at a flat interface are finally observed in water and low concentration polymer solutions. A conditional analysis of turbulent mass fluxes allows to distinguish the type of events contributing to mass transfer and discuss their respective impact in water and polymer solutions.

Ces travaux ont pour objectif de modéliser les mécanismes de dispersion dans les aquifères. L’hétérogénéité du champ de vitesse et le transfert de masse entre zones immobiles et mobiles sont deux origines possibles du comportement non-Fickéen, jusqu’alors étudiées de façon séparée. Notre hypothèse de départ est que ces deux mécanismes coexistent. Nos travaux comprennent : 1) des expériences de traçage sur colonnes de billes de verre et carottes de grès de Berea, en mode flow-through et push-pull, et 2) des simulations numériques réalisées à partir d’images en microtomographie RX segmentées en trois phases : solide, vide et microporosité. L’analyse du champ de vitesse (Stokes) montre l’importance de la discrétisation spatiale et de la prise en compte de la microporosité. Les résultats des simulations de transport (en utilisant la méthode time domain random walk) permettent de quantifier l’effet combiné de l’hétérogénéité du champ de vitesse et des transferts diffusifs dans la fraction micro-poreuse de la roche sur la dispersion non-Fickéenne, caractérisée à partir des courbes de restitution (BTC). Ces résultats sont cohérents avec les observations expérimentales. Nous concluons que ces deux effets doivent être pris en compte même si leur identification à partir de la forme des BTCs issues des traçages des milieux naturels (souvent caractérisés par de faible valeurs du nombre de Peclet ) reste difficile. Enfin, un modèle moyen macroscopique 1D est proposé dans le cadre d’une approche de type continuous time random walk dans laquelle des distributions spécifiques du temps de transfert des particules sont construites pour chacun des deux mécanismes de transport. / His work aims at modeling hydrodynamic dispersion mechanisms in aquifers. So far both flow field heterogeneity and mobile-immobile mass transfer have been studied separately for explaining the ubiquitously observed non-Fickian behaviors, but we postulate that both mechanisms contribute simultaneously. Our investigations combine laboratory experiments and pore scale numerical modeling. The experimental rig was designed to enable push-pull and flow through tracer tests on glass bead columns and Berea sandstone cores. Modeling consists in solving Stokes flow and solute transport on 3D X-ray microtomography images segmented into three phases: solid, void and microporosity. Transport is modeled using time domain random walk. Statistical analysis of the flow field emphasizes the importance of the mesh resolution and the inclusion of the microporosity. Results from the simulations show that both the flow field heterogeneity and the diffusive transport in the microporous fraction of the rock contribute to the overall non-Fickian transport behavior observed, for instance, on the breakthrough curves (BTC). These results are supported by our experiments. We conclude that, in general, this dual control must be taken into account, even if these different influences can hardly be distinguished from a qualitative appraisal of the BTC shape, specifically for the low values of the Peclet number that occurs in natural conditions. Finally, a 1D up-scaled model is developed in the framework of the continuous time random walk, where the influences of the flow field heterogeneity and mobile-immobile mass transfer are both taken into account using distinct transition time distributions.

Un nouveau système d’aération de surface pour le traitement des eaux usées a été étudié. Sa spécificité réside dans sa capacité à fonctionner selon deux modes : aération ou simple brassage, en modifiant uniquement le sens de rotation du système. Un pilote a permis de cibler le travail sur l’étude expérimentale du transfert de matière et de l’hydrodynamique. Les champs d'écoulement et les mesures de vitesse à l'intérieur de la cuve agitée ont été réalisés par vélocimétrie laser à effet Doppler (LDV) et par vélocimétrie par images des particules (PIV) pour le mode monophasique (brassage) et pour le mode diphasique (aération). Le transfert d'oxygène se produit à la fois dans la cuve et dans le spray au-dessus de la surface de l'eau. Il a été étudié dans les deux zones. Différentes configurations et conditions opératoires ont été testées afin de comprendre les phénomènes d’interaction : tube de guidage, hélice complémentaire RTP, vitesse de rotation, niveau de submersion des pales de la turbine. La partie expérimentale sur l’hydrodynamique et les champs d'écoulement montre que le mode de fonctionnement en pompage vers le bas (brassage) avec tube de guidage procure les meilleurs résultats en termes de mélange si on se réfère aux champs d'écoulement et à la mesure du temps de mélange. Pour le mode de fonctionnement en pompage vers le haut (aération), les résultats expérimentaux montrent que la configuration du système complet est la plus efficace si on considère le transfert d’oxygène, les vitesses moyennes, l'intensité de l'écoulement turbulent et le temps de mélange. Il est constaté que la meilleure efficacité d'aération standard est atteinte (SAEb = 2.65 kgO2kw-1h-1) lorsque le système complet est utilisé. L'efficacité d'aération standard à 20°C la plus élevée au niveau du spray d'eau est obtenue ((ESP)20 = 51,3%) avec la configuration du système complet. Plusieurs modèles sont proposés pour calculer le transfert d'oxygène dans la cuve et dans le spray, la consommation énergique et le temps de mélange. Ces relations permettent d’évaluer l’influence des différents paramètres géométriques et de fonctionnement dans des systèmes similaires à une échelle industrielle. / A new surface aeration system for water and wastewater treatment has been studied. Its uniqueness lies in its ability to operate in two modes: aeration or simply blending (mixing) by just reversing the direction of rotation. An experimental plant has enabled to focus on mass transfer performance and hydrodynamics. The flow pattern and the velocity field measurements inside the agitated tank were performed by both the Laser Doppler Velocimetry (LDV) and the Particle Image Velocimetry (PIV) techniques for the single phase (Mixing) mode and for the two phases (Aeration) mode. The oxygen mass transfer occurs both in the water bulk and in the spray above water surface and has been independently investigated. Different configurations and operational conditions were tested during the experimental part in order to interpret phenomenon effect of the draft tube and RTP propeller, rotational speed, turbine blades submergence and else on the flow field and the oxygen mass transfer in the agitated system that produced mainly by a cone shape turbine. The experimental part dealing with hydrodynamics and flow field shows that the down-pumping operation mode with the draft tube has the most convenient results in the mixing mode with respect to turbulent flow field and mixing time. Whilst for the up-pumping aeration mode the hydrodynamics experimental results show the whole system configuration is the most convenient with regarded to mean velocities, turbulent flow intensity and mixing time. For the oxygen mass transfer experimental part, it is found that the highest standard liquid bulk aeration efficiency is achieved (SAEb = 2.65 kgO2 kw-1h-1) when the whole system configuration is used. The highest standard aeration efficiency at 20°C for the water spray zone is accomplished ((Esp)20 = 51.3 %) with the whole system configuration. Several correlations models have been derived for the oxygen mass transfer in water bulk and spray zones, power consumption and mixing time, on the basis of experimental results. They can be used as tools to estimate these parameters for geometrical and dynamical similar systems at industrial scales.

<p>Co-axial dripping is one of the many ways to make drops with a core-shell structure for encapsulated materials. However, in systems where the capsule components are not density matched or surfactants are not used, the shell will eventually thin and break if not solidified in time. If the shell fails before solidifying, the core will leak out and result in a non-functional capsule. This study assumes that all capsules will fail once the core has reached 80% eccentricity, meaning a shell region has thinned to 20% of its original thickness (~70 µm). In reality, rupture of the shell depends more on stochastic defects and disturbances, but locally decreasing the shell thickness will increase the probability of capsule rupture. With this assumption, the survival time of a core-shell drop is inversely proportional to the relative velocity of the inner drop, where the greater this relative velocity, the faster the shell phase will thin. Stoke's law is generally used to approximate the speed of a sphere in a fluid. However, this study demonstrates that Stoke's law is insufficient for predicting the inner drop's motion for a compound drop. This is due to internal flows that develop within all fluid drops because of shear forces on the drop’s external face during freefall. For core-shell drops, prior studies report how the inner drop velocity can change in magnitude and direction as a function of its eccentricity, meaning its position within the outer drop. Since previous studies did not analyze this core-shell drop relationship with a 50 vol% core and a high viscosity shell, a model was built in COMSOL Multiphysics to understand how the claims from literature would apply to a previous encapsulation study (Betancourt, 2021). The model was also put through a series of validation tests that confirmed the model’s ability to accurately represent the speed and direction of inner drop motion. The final model configuration was then used to identify the transition point between buoyancy-driven and internal flow-driven failure modes observed during the production of core-shell drops in a previous encapsulation study for phase change materials (Betancourt, 2021). The model results showed how the estimated inner drop velocity was significantly reduced once accounting for the internal flows within the shell phase of a compound drop. While this study does help characterize the motion of an inner drop and could be used to find a material system with a favorable velocity profile, it is still recommended to use an in-air curing system to produce concentric capsules. Achieving a concentric capsule would still require this co-axial dripping setup to be modified significantly. </p> <p>Betancourt-Jimenez, D., Wells, B., Youngblood, J. P., & Martinez, C. J. (2021). Encapsulation of biobased fatty acid amides for phase change material applications. <em>Journal of Renewable and Sustainable Energy</em>, <em>13</em>(6), 064101. https://doi.org/10.1063/5.0072105</p>

<p>  </p> <p>Heat pipes and vapor chambers are passive two-phase heat transport devices that are used for thermal management in electronics. The passive operation of a heat pipe is facilitated by capillary wicking of the working fluid through a porous wick, and thus is subject to an operational limit in terms of the maximum pressure head that the wick can provide. This operational limit, often termed as the capillary limit, is the maximum heat input at which the pressure drop in the wick is balanced by the maximum capillary pressure head; operating a heat pipe or a vapor chamber above the capillary limit at steady-state leads to dryout. It thus becomes important to predict the performance of heat pipes and vapor chambers and explore the parametric design space to provide guidelines for minimized thermal resistance while satisfying this capillary limit. An increasingly critical aspect is to predict the transient thermal response of vapor chambers. Moreover, heat pipes and vapor chambers are extensively being used in electronic systems where the power input is dictated by the end-user activity and is expected to even exceed the capillary limit for brief time intervals. Thus, it is imperative to understand the behavior of heat pipes and vapor chambers when operated at steady and transient heat loads above the capillary limit as dryout occurs. However, review of the literature on heat pipe performance characterization reveals that the regime of dryout operation has been virtually unexplored, and thus this thesis aims to fill this critical gap in understanding.</p> <p>The design for minimized thermal resistance of a vapor chamber or a heat pipe is guided by the relative contribution of thermal resistance due to conduction across the evaporator wick and the saturation temperature gradient in the vapor core. In the limit of very thin form factors, the contribution from the vapor core thermal resistance dominates the overall thermal resistance of the vapor chamber; recent work has focused on working fluid selection to minimize overall thermal resistance in this limit. However, the wick thermal resistance becomes increasingly significant as its thickness increases to support higher heat inputs while avoiding the capillary limit. A thermal resistance network model is thus utilized to investigate the importance of simultaneously considering the contributions of the wick and vapor core thermal resistances. A generalized approach is proposed for vapor chamber design which allows <em>simultaneous</em> selection of the working fluid and wick that provides minimum overall thermal resistance for a given geometry and operating condition. While the thermal resistance network model provides a convenient method for exploring the design space, it cannot be used to predict 3-D temperature fields in the vapor chamber. Moreover, such thermal resistance network models cannot predict transient performance and temperature evolution for a vapor chamber. Therefore, an easy-to-use approach is proposed for mapping of vapor chamber transport to the heat diffusion equation using a set of appropriately defined effective anisotropic thermophysical properties, thus allowing simulation of vapor chamber as a sold conduction block. This effective anisotropic properties approach is validated against a time-stepping analytical model and is shown to have good match for both spatial and transient temperature predictions.</p> <p>Moving the focus from steady-state and transient operation of vapor chambers, a comprehensive characterization of heat pipe operation above capillary limit is performed. Different user needs and device workloads can lead to highly transient heat loads which could exceed the notional capillary limit for brief time intervals. Experiments are performed to characterize the transient thermal response of a heat pipe subjected to heat input pulses of varying duration that exceed the capillary limit. Transient dryout events due to a wick pressure drop exceeding the maximum available capillary pressure can be detected from an analysis of the measured temperature signatures. It is discovered that under such transient heating conditions, a heat pipe can sustain heat loads higher than the steady-state capillary limit for brief periods of time without experiencing dryout. If the heating pulse is sufficiently long as to induce transient dryout, the heat pipe may experience an elevated steady-state temperature even after the heat load is reduced back to a level lower than the capillary limit. The steady-state heat load must then be reduced to a level much below the capillary limit to fully recover the original thermal resistance of the heat pipe. The recovery process of heat pipes is further investigated, and a mechanism is proposed for the thermal hysteresis observed in heat pipe performance after dryout. A model for <em>steady-state</em> heat pipe transport is developed based on the proposed mechanism to predict the parametric trends of thermal resistance following recovery from dryout-induced thermal hysteresis, and the model is mechanistically validated against experiments. The experimental characterization of the recovery process demonstrates the existence of a maximum hysteresis curve, which serves as the worst-case scenario for thermal hysteresis in heat pipe after dryout. Based on the learnings from the experimental characterization, a new procedure is introduced to experimentally characterize the steady-state dryout performance of a heat pipe.</p> <p>To recover the heat pipe performance under steady-state, it has been shown that the heat input needs to be lowered down or <em>throttled</em> significantly below the capillary limit. However, due to the highly transient nature of power dissipation from electronic devices, it becomes imperative to characterize heat pipe recovery from dryout under transient operations. Hence, power-throttling assisted recovery of heat pipe from dryout has been characterized under transient conditions. A minimum throttling time interval, defined as time-to-rewet, is identified to eliminate dryout induced thermal hysteresis using power throttling. Dependence of time-to-rewet on throttling power is explored, and guidelines are presented to advise the throttling need and choice of throttling power under transient conditions. </p> <p>The experimental characterization of heat pipe operation at pulse loads above the capillary limit and power throttling following the pulse load helped define the dryout and recovery performance of a heat pipe. Next, a physics-based model is developed to predict the heat pipe <em>transient</em> thermal response under dryout-inducing pulse load and power throttling assisted recovery. This novel model considers wick as a partially saturated media with spatially and temporally varying liquid saturation, and accounts for the effect of wick partial saturation in heat pipe transport. The model prediction are validated against experiments with commercial heat pipe samples, and it is shown that the model can accurately predict dryout and recovery characteristics, namely time-to-dryout, time-to-rewet, and dryout-induced thermal hysteresis, for heat pipes with a range of wick types, heat pipe lengths and pulse loads above the capillary limit. </p> <p>The work discussed in this thesis opens certain questions that are expected to guide further research in this area. First, the thermal hysteresis mechanism proposed could be further validated with direct visualization of the liquid in a vapor chamber. To achieve this, X-ray microscopy is proposed as a viable option for the imaging <em>in situ</em> wetting dynamics in a vapor chamber. Second, the model developed to predict the dryout and recovery characteristics of the heat pipe can be used to design heat pipe with improved performance under pulse loads and power throttling. Third, novel wick designs can be explored that utilize the understanding developed of governing mechanisms for recovery from dryout, and can eliminate thermal hysteresis at powers closer to capillary limit. Fourth, the modeling approach can be extended to predict dryout and recovery trends in vapor chamber since the heat transfer pathways in a vapor chamber are different than those of a heat pipe. Fifth, and lastly it was observed several times during experiments that some of the heat pipe samples would exhibit complete dryout (sudden catastrophic rise in temperature and thermal resistance at the point of dryout) whereas other samples would exhibit partial dryout (noticeable but small increase in thermal resistance at dryout) at operating powers just above the capillary limit. Exploring and explaining the cause of complete dryout, in particular, would be an extremely valuable contribution to the heat pipe research. </p> <p>The work discussed in this thesis has led to the comprehensive development of a functional and mechanistic understanding of heat pipe operation above the notional capillary limit. The experimental procedures developed in this work are utilized to characterize a heat pipe performance under dryout and recovery. The models based on the mechanistic understanding developed from experimental characterization of dryout and recovery operation of a heat pipe have been experimentally validated and are useful for predicting heat pipe performance under dryout-inducing pulse loads and power-throttling.   </p>

<p>  </p> <p>Cavitation is one of the most common causes of failures in axial piston machines. Due to the detrimental effects that cavitation has on unit performance, it is of important consideration both in the design of new units and in defining the operational limits of existing market products. The work in this thesis aimed to contribute to the current knowledge in both areas, with a focus on design considerations with respect to cavitation scalability, and on operating conditions by measuring cavitation severity under separate and combined inciting parameters. Though the application of unit scaling is common in industry for the design of pump families, there have been no comprehensive attempts to quantify whether cavitation in fluid power units may be adequately accounted for in published scaling laws. In this thesis, the scalability of cavitation phenomena was examined through a CFD scaling study performed using a modified version of the Full Cavitation Model.  Results indicate that linear scaling is consistent in maintaining volumetric efficiency performance within 1% across scaled units up to eight times larger or smaller than the baseline. However, the gas and vapor volume distributions vary significantly between scaled units, due largely to the linear non-scalability of fluid inertia and turbulent factors. Physical exchange between phases within a working fluid was shown to be time-dependent, such that the scaled-down unit exhibits bubble collapse rates up to 30% and 150% greater than the baseline and scaled-up units, respectfully. Considering these effects, the presented work demonstrates a potential for increased cavitation damage area when downscaling a unit and reduced risk in upscaling, despite the scaling law being a reliable indicator for volumetric efficiency. </p> <p>To define a more complete study of cavitation under a variety of operating conditions and inciting parameters, this a new experimental procedure and testing circuit was proposed with focus on repeatability by controlled pressure drops and preliminary quantification of inlet fluid quality. By measuring cavitation conditions under pressure starvation, incomplete filling, and combinations thereof, the direct effect of different inception methods on unit performance was shown to be readily identifiable. Through visualization of the inlet flow, reduction in inlet pressure levels was correlated to fluid cloudiness levels and bubble size, with transparency loss at 0.0 bar_g and transition from bubbly to plug flow at -0.4 bar_g. Incomplete filling-induced cavitation was also shown to be detectable by inlet flow conditions, with a distinct change in bubble coalescence rate when operating under shaft speeds greater than or equal to fill speed for a given inlet pressure. </p>

<p>The present study investigates the capability of computational fluid dynamics (CFD) extensively to predict hydrodynamics and heat transfer characteristics of FC-72 flow boiling in a 2.5-mm ´ 5.0-mm rectangular channel and experimentally explores system instabilities: <em>density wave oscillation</em> (DWO), <em>pressure drop oscillation</em> (PDO) and <em>parallel channel instability</em> (PCI) in a micro-channel heat sink containing 38 parallel channels having a hydraulic diameter of 316-μm. </p> <p>The computational method performs transient analysis to model the entire flow field and bubble behavior for subcooled flow boiling in a rectangular channel heated on two opposite walls at high heat flux conditions of about 40% – 80% of <em>critical heat flux</em> (CHF).  The 3D CFD solver is constructed in ANSYS Fluent in which the <em>volume of fluid</em> (VOF) model is combined with a <em>shear stress transport</em> (SST) <em>k</em>-<em>ω</em> turbulent model, a surface tension model, and interfacial phase change model, along with a model for effects of shear-lift and bubble collision dispersion to overcome a fundamental weakness in modeling multiphase flows.  Detailed information about bubble distribution in the vicinity of the heated surface, thermal conduction inside the heating wall, local heat fluxes passing through the solid-fluid interface, and velocity and temperature profiles, which are not easily observed or measured by experiments, is carefully evaluated.  The simulation results are compared to experimental data to validate the solver’s ability to predict the flow configuration with single/double-side heating.  The added momentum by shear-lift is shown to govern primarily the dynamic behavior of tiny bubbles stuck on the heated bottom wall and therefore has a more significant impact on both heat transfer and heated wall temperature.  By including bubble collision dispersion force, coalescence of densely packed bubbles in the bulk region is significantly inhibited, with more giant bubbles even incurring additional breakup into smaller bubbles and culminating in far less vapor accumulation along the top wall.  Including these momentums is shown to yield better agreement with local interfacial behavior along the channel, overall flow pattern, and heat transfer parameters (wall temperature and heat transfer coefficient) observed and measured in experiments.  The computational approach is also shown to be highly effective at predicting local phenomena (velocity and temperature profiles) not easily determined through experiments.  Different flow regimes predicted along the heated length exhibit a number of dominant mechanisms, including bubble nucleation, bubble growth, coalescence, vapor blankets, interfacial waviness, and residual liquid sub-layer, all of which agree well with the experiment.  Vapor velocity is shown to increase appreciably along the heated length because of increased void fraction, while liquid velocity experiences large fluctuations.  Non-equilibrium effects are accentuated with increasing mass velocity, contributing minor deviations of fluid temperature from simulations compared to those predicted by the analytical method.  Predicted wall temperature is reasonably uniform in the middle of the heated length but increases in the entrance region due to sensible heat transfer in the subcooled liquid and decreases toward the exit, primarily because of flow acceleration resulting from increased void fraction.  When it comes to analyzing heat transfer mechanisms at extremely high heat flux via CFD, predicted flow pattern, bubble behavior, and heat transfer parameters (such as wall temperature excursion and thermal energy concentration) clearly represent phenomena of premature CHF, which take place slightly earlier than actual operating conditions.  But, despite these slight differences, the present computational work does demonstrate the ability to effectively predict the severe degradation in heat transfer performance commonly encountered at heat fluxes nearing CHF.  </p> <p>Much of the published literature addressing flow instabilities in thermal management systems employing micro-channel modules are focused on instability characteristics of the module alone, and far fewer studies have aimed at understanding the relationship between these characteristics and compressive volume in the flow loop external to the module.  From a practical point of view, developers of micro-channel thermal management systems for many modern applications are in pursuit of practical remedies that would significantly mitigate instabilities and their impact on cooling performance.  Experiments are executed using FC-72 as a working fluid with a wide range of mass velocities and a reasonably constant inlet subcooling of ~15°C.  The flow instabilities are reflected in pressure fluctuations detected mainly in the heat sink’s upstream plenum.  Both inlet pressure and pressure drop signals are analyzed in pursuit of amplitude and frequency characteristics for different mass velocities and over a range of heat fluxes.  The current experimental study also examines the effects of compressible volume location in a closed pump-driven flow loop designed to deliver FC-72 to a micro-channel test module having 38 channels with 315-μm hydraulic diameter.  Three accumulator locations are investigated: upstream of the test module, downstream of the test module, and between the condenser and pump.  Both high-frequency temporal parameter data and high-speed video records are analyzed for ranges of mass velocity and heat flux, with inlet subcooling held constant at ~15°C.  PDO is shown to dominate when the accumulator is situated upstream, whereas PCI is dominant for the other two locations.  Appreciable confinement of bubbles in individual channels is shown to promote rapid axial bubble growth.  The study shows significant variations in the amount of vapor generated and dominant flow patterns among channels, a clear manifestation of PCI, especially for low mass velocities and high heat fluxes.  It is also shown effects of the heat sink’s instabilities are felt in other components of the flow loop.  The parametric trends for PCI are investigated with the aid of three different types of stability maps which show different abilities at demarcating stable and unstable operations.  PDO shows severe pressure oscillations across the micro-channel heat sink, with rapid bubble growth and confinement, elongated bubble expansion in both directions, flow stagnation, and flow reversal (including vapor backflow to the inlet plenum) constituting the principal sequence of events characterizing the instability.  Spectral analysis of pressure signals is performed using Fast Fourier Transform, which shows PDO extending the inlet pressure fluctuations with the same dominant frequency to other upstream flow loop components, with higher amplitudes closer to the pump exit.  From a practical system operation point of view, throttling the flow upstream of the heat sink eliminates PDO but renders PCI dominant, and placing the accumulator in the liquid flow segment of the loop between the condenser and pump ensures the most stable operation.</p>

<p dir="ltr">As aircraft engine manufacturers continue to embark on their pursuit of higher-efficiency, lower-emissions gas turbines, a prevailing theme in the industry has been the increase of the engine bypass ratio. As the optimization space for engine bypass ratios trends towards smaller and smaller engine core sizes, the feasibility of centrifugal compressors as the final stage in an axial-centrifugal compressor becomes apparent due to their performance advantages at smaller scales.</p><p dir="ltr">This study performed an investigation into the aerodynamics of a stationary diffusion system intended for use with a final stage aeroengine centrifugal compressor using experimental and numerical techniques. Experimental work was performed at the Purdue Compressor Research Lab at Purdue University’s Maurice J. Zucrow Laboratories. Data were collected from several iterations of rapidly prototyped, additively manufactured diffuser and deswirl parts with internal instrumentation features. Furthermore, computational work on the stage was conducted using the Ansys Turbosystem.</p><p dir="ltr">The goal of this research is to identify trends in stationary diffusion system designs and the geometric features that cause them. Furthermore, the ability of steady computational fluid dynamics methods to predict these changes was evaluated using two turbulence models to produce a simulation of the compressor flow field. When used in conjunction with one another, the efficient use of computational methods to identify an optimal design and rapid prototyping to validate it leads to the determination of the best diffusion system design at a lower cost and time requirement than what is otherwise currently possible.</p><p dir="ltr">The different geometries which were tested identified the negative effects of spanwise vane contouring on the diffuser performance and the ability of endwall divergence to augment the pressure recovery performance of a design at the expense of increased losses. A full understanding of the effect of each design parameter is enabled by iterative inclusion or exclusion of certain design parameters. Furthermore, the use of computational fluid dynamics showed that the BSLEARSM turbulence model performs reasonably well in predicting the build-to-build performance trends. However, neither the BSLEARSM nor the SST turbulence model were able to accurately identify performance trends for the deswirl. For this reason, additional work is warranted to identify an optimal set of parameters to characterize the high axial and meridional turning present in this component.</p>

<p dir="ltr">With increasing power densities in electronic devices, effective thermal management has become an indispensable aspect of electronic systems design. Although phase change materials (PCMs) have been studied as a potential solution, their integration into microelectronic and high-power devices presents a significant challenge due to low thermal conductivity and lack of effective thermal pathways from the heat source to the heat sink. While much work has focused on integrating thermal storage into heat sinks, this dissertation instead investigates integrating PCMs between the heat source and the heat sink in different configurations. By placing the energy storage closer to the heat source, the thermal resistance is reduced, which improves the overall thermal performance of the device. Specifically, this work explores the efficacy of two approaches: (1) direct embedding of a PCM within the die for mobile electronics applications and (2) integration of an auxiliary composite PCM/copper thermal energy storage (TES) component in combination with active liquid cooling for high-power power electronics modules.</p><p><br></p><p dir="ltr">The first study explores die-level thermal management for microelectronics using PCMs. Silicon chips with PCM embedded within the die are modeled using ParaPower, a fast-analysis tool, and a genetic algorithm is used to efficiently optimize the distribution of high-conductivity silicon pathways and high thermal capacitance PCM zones. A thermal test vehicle (TTV) of a realistic microelectronics form factor with an embedded PCM layer is first designed, and a process is developed to fabricate such a TTV. This study is the first to successfully fabricate a TTV with fully encapsulated PCM and validate its thermal response across various operational scenarios. For temperature cycling tests (where the TTV temperature fluctuates between predetermined hot and cold setpoints), the embedded-PCM TTVs extend the operational time by up to 2.8x compared to a baseline all-silicon TTV. For duty cycling tests (with a fixed duration of the periodic heating pulses and off times), the embedded-PCM TTVs suppress the hotspot temperature rise by up to 14% and stabilize quasi-steady state temperature fluctuations by up to 65% through repeated PCM melting and solidification cycles. Thermal performance enhancements are observed even for high heat fluxes of ~65W/cm² . Specifically, a TTV with an embedded square-shaped PCM reservoir reduces temperature instability by an average of 40% across a range of cycle durations.</p><p><br></p><p dir="ltr">The second study investigates the effectiveness of different integration strategies for an auxiliary composite PCM/copper TES block integrated alongside a cold plate, for thermal management of high-power power electronics modules, specifically for electric vehicles. These systems are evaluated for realistic drive cycles of various driving intensities. Computational results indicate that this approach is most effective when the composite TES block is positioned directly above the heat-generating silicon carbide dies. This configuration excels at stabilizing transient temperature fluctuations and absorbing thermal shocks, achieving reductions of up to approximately ~33% compared to current thermal management techniques. This strategy is particularly effective for stop-and-go drive cycles characterized by high rates of acceleration and deceleration, low average driving speeds, and frequent stops, typical of driving schedules for public transport buses and mail delivery vehicles.</p><p><br></p><p dir="ltr">The results from both thermal management approaches demonstrate that the integration of a PCM cooling solution in close proximity to the heat source can significantly enhance its effectiveness by absorbing power bursts and limiting temperature instability via repeated melting and solidification. The contributions of this dissertation include the development of an effective optimization strategy for generating optimized PCM distributions, which reduces the maximum temperature and temperature oscillations in a device with significant computational efficiency. (The same optimization strategy can be applied to other thermal management design challenges.) Notably, TTVs of realistic microelectronics form factors with embedded PCM were designed, modeled, fabricated, and validated. With the PCM thermal buffers, the engineered solution demonstrated superior performance compared to a baseline all-silicon TTV. The second study into the integration of composite PCM/copper TES blocks into high-power power electronics modules established trade-offs between different architectures across various performance metrics, and highlighted its effectiveness for drive cycles with varying intensities. These findings offer an important contribution to the development of embedded thermal management techniques for electronic systems design, which will be critical for the advancement of next-generation microelectronics and high-power devices.</p>

Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2011. / Please refer to full text to view abstract.