Laminar, two-dimensional, constant-property numerical simulations of flat tube heat exchanger devices operating in flow regimes in which self-sustained oscillations occur were performed. The unsteady flow regimes were transition flow regimes characterized by cyclic variations of flow parameters such as stream-wise or cross-stream velocity. A computer code was developed to perform the numerical simulations. Spatial discretization was based upon a Control Volume Finite Element Method (CVFEM). Temporal discretization was based upon a semi-implicit Runge-Kutta method. Double Cyclic conditions were used to limit the numerical domains to one repeating geometric module. Nine geometric domains representing flat tube heat exchanger devices were tested over a range of Reynolds numbers. A maximum Reynolds number (Re) of 2000 was established to keep the study within the transition range. For each domain, a critical Reynolds number (Re_crit) was found such that for Re < Re_crit the flow was steady, laminar flow and for Re > Re_crit the flow exhibited cyclic oscillations. For the cases tested, the variation in longitudinal pitch had little impact on the Re_crit value for a fixed transverse pitch. However, for a fixed longitudinal pitch, the Re_crit was increased for decreasing transverse pitch. The results demonstrate the importance of using unsteady simulation methods for these cases. Nusselt numbers predicted by the unsteady method were on the order of 65% higher than predicted by steady methods for the same Reynolds numbers. Data for required pumping power versus resultant Nusselt number were collected which showed four distinct operating regions for these devices spanning the low Reynolds number, steady flow region through the self-sustained oscillating flow region. Based on the data, the recommended operating region is the region of self-sustained oscillations as this region is characterized by the highest increase in Nusselt number per increase in required pumping power.

Thermo-acoustic instabilities in combustion chambers are generated by the interactions between a flame and the combustor acoustics, leading to a resonant coupling. These self-sustained oscillations may be observed in many practical systems such as domestic boilers, industrial furnaces, gas turbines or rocket engines. Although this phenomenon has already been the topic of many investigations, there is yet no generalized robust framework to predict the onset of these self-sustained oscillations and to determine the evolution of the flow variables within the combustor during unstable operation. This work builds on previous models and experiments to improve the description of the response of laminar conical flames to flow perturbations and the prediction of thermoacoustic instability in burners operating with conical flames. In the first part of the manuscript, an extensive review of conical flame dynamics modeling is undertaken and a general framework for the modeling of their Flame Transfer Function (FTF) is presented. The experimental setup and the diagnostics used to characterize their response to flow disturbances are then described. They are used to measure the FTF when the flames are submitted to harmonic flow perturbations. A novel experimental technique is also proposed to control the flow perturbation level at the burner outlet. It enables to modulate the flow with random white noise perturbations and to measure the FTF with a better frequency resolution. Results with this alternative technique compare well with results from the classical method using harmonic signals for small disturbances. Limits of this technique are also highlighted when the perturbation level increases. Different analytical expressions for the FTF of conical flames are derived in the second part of the thesis by progressively introducing more physics into the models. Models based on convected flow disturbances are extended by taking into account the incompressible nature of the perturbed velocity field. It is shown that the prediction of the FTF phase lag of a conical flame is greatly improved and collapses well with measurements. Then, a thorough investigation of the flame base dynamics interacting with the anchoring device is conducted by considering unsteady heat loss from the flame to the burner. This mechanism is shown to drive the motion of the flame base and the flame dynamics at high frequencies. It is also shown that this contribution to the FTF rules the high frequency behavior of the FTF as well as the nonlinear evolution of the FTF when the perturbation level increases. Finally, an analysis is conducted on the dynamics of a single conical flame placed into cylindrical flame tubes featuring different diameters. It is shown that confinement effects need to be taken into account when the burnt gases cannot fully expand. Large differences are observed between FTF measured for different confinement tube diameters. A new dimensionless number is derived to take these effects into account and make all the FTF collapse on a single curve. These different models are then used to model the response of a collection of small conical flames stabilized on a perforated plate. It is shown that by sorting out the different contributing mechanisms to the FTF, the expressions proposed in this work may be combined to capture the main behavior and correct phase lag evolution of these flames in the frequency range of interest for thermo-acoustic instability prediction.

In this study, forced convection cooling of package of 2-D parallel boards with heat generating chips is investigated. The main objective of this study is to determine the optimal board-to-board spacing to maintain the temperature of the components below the allowable temperature limit and maximize the rate of heat transfer from parallel heat generating boards cooled by forced convection under constant pressure drop across the package. Constant heat flux and constant wall temperature boundary conditions on the chips are applied for laminar and turbulent flows. Finite elements method is used to solve the governing continuity, momentum and energy equations. Ansys-Flotran computational fluid dynamics solver is utilized to obtain the numerical results. The solution approach and results are compared with the experimental, numerical and theoretical results in the literature. The results are presented for both the laminar and turbulent flows. Laminar flow results improve existing relations in the literature. It introduces the effect of chip spacing on the optimum board spacing and corresponding maximum heat transfer. Turbulent flow results are original in the sense that a complete solution of turbulent flow through the boards with discrete heat sources with constant temperature and constant heat flux boundary conditions are obtained for the first time. Moreover, optimization of board-to-board spacing and maximum heat transfer rate is introduced, including the effects of chip spacing.

Les instabilités thermo-acoustiques présentes dans les chambres de combustion sont générées par des interactions entre une flamme et l’acoustique du foyer. Ces oscillations auto-entretenues peuvent être observées dans de nombreux systèmes industriels tels que des chaudières domestiques, des fours industriels, des turbines à gaz ou des moteurs fusée. Bien que ce phénomène ait fait l’objet de nombreux travaux, il n’existe toujours pas de cadre d’étude assez général et robuste pour prédire le déclenchement de ces oscillations auto-entretenues et pour déterminer l’évolution des variables de l’écoulement à l’intérieur de la chambre de combustion. Ce travail s’appuie à la fois sur des modèles et des expériences. L’objectif est d’améliorer la description de la réponse de flammes coniques laminaires prémélangées à des perturbations de l’écoulement et les prédictions d’instabilités thermo-acoustiques dans des foyers alimentés par des flammes coniques. Dans la première partie du manuscrit, une revue des modèles décrivant la dynamique de flammes coniques est entreprise et un cadre général d’étude pour la modélisation de la Fonction de Transfert de Flamme (FTF) est présenté. Le dispositif expérimental ainsi que les diagnostics utilisés sont ensuite décrits. Ces systèmes sont utilisés pour mesurer la FTF de flammes coniques laminaires prémélangées soumises à des perturbations harmoniques de l’écoulement. Une nouvelle technique expérimentale est proposée pour contrôler les perturbations de l’écoulement à la sortie du brûleur. Elle est utilisée pour moduler l’écoulement avec un bruit blanc aléatoire et déterminer la FTF avec une résolution fréquentielle bien meilleure. Pour de faibles niveaux d’excitation, les résultats obtenus avec cette technique sont en accord avec ceux obtenus par la méthode classique utilisant des perturbations harmoniques. Les limites de cette technique sont décrites lorsque le niveau de perturbation augmente. Plusieurs expressions analytiques de la FTF de flammes coniques sont établies dans la seconde partie de cette thèse en introduisant progressivement plus de phénomènes physiques dans le modèle. Les modèles basés sur des perturbations convectées par l’écoulement sont étendus en tenant compte de la nature incompressible du champ de perturbation de vitesse. La prévision de la phase de la FTF de flamme conique est améliorée et présente un bon accord avec les mesures. Ensuite, une étude détaillée des interactions de la base de la flamme avec le bord du brûleur est conduite en tenant compte des pertes thermiques instationnaires de la flamme vers le brûleur. Ce mécanisme contrôle le mouvement de la base de la flamme et la dynamique de flamme à haute fréquence. Cette contribution à la FTF détermine le comportement haute fréquence de la FTF ainsi que l’évolution non-linéaire de la FTF lorsque le niveau de perturbation augmente. Enfin, une analyse de la dynamique des flammes coniques est entreprise pour des flammes placées dans des tubes de différents diamètres. Il est montré que les effets de confinement doivent être pris en compte lorsque les gaz brûlés ne peuvent se dilater complètement. Des différences importantes sont observées entre des FTF mesurées pour des tubes de confinement de diamètres différents. Un nouveau nombre sans dimension est établi pour prendre en compte ces effets. Ces différents modèles sont ensuite utilisés pour modéliser la réponse d’une collection de petites flammes coniques stabilisées sur une plaque perforée. Il est montré qu’une combinaison de ces modèles permet de capturer le comportement de ces flammes ainsi que l’évolution de la phase de la FTF couvrant le spectre fréquentiel pertinent pour la prédiction d’instabilités thermo-acoustiques. / Thermo-acoustic instabilities in combustion chambers are generated by the interactions between a flame and the combustor acoustics, leading to a resonant coupling. These self-sustained oscillations may be observed in many practical systems such as domestic boilers, industrial furnaces, gas turbines or rocket engines. Although this phenomenon has already been the topic of many investigations, there is yet no generalized robust framework to predict the onset of these self-sustained oscillations and to determine the evolution of the flow variables within the combustor during unstable operation. This work builds on previous models and experiments to improve the description of the response of laminar conical flames to flow perturbations and the prediction of thermoacoustic instability in burners operating with conical flames. In the first part of the manuscript, an extensive review of conical flame dynamics modeling is undertaken and a general framework for the modeling of their Flame Transfer Function (FTF) is presented. The experimental setup and the diagnostics used to characterize their response to flow disturbances are then described. They are used to measure the FTF when the flames are submitted to harmonic flow perturbations. A novel experimental technique is also proposed to control the flow perturbation level at the burner outlet. It enables to modulate the flow with random white noise perturbations and to measure the FTF with a better frequency resolution. Results with this alternative technique compare well with results from the classical method using harmonic signals for small disturbances. Limits of this technique are also highlighted when the perturbation level increases. Different analytical expressions for the FTF of conical flames are derived in the second part of the thesis by progressively introducing more physics into the models. Models based on convected flow disturbances are extended by taking into account the incompressible nature of the perturbed velocity field. It is shown that the prediction of the FTF phase lag of a conical flame is greatly improved and collapses well with measurements. Then, a thorough investigation of the flame base dynamics interacting with the anchoring device is conducted by considering unsteady heat loss from the flame to the burner. This mechanism is shown to drive the motion of the flame base and the flame dynamics at high frequencies. It is also shown that this contribution to the FTF rules the high frequency behavior of the FTF as well as the nonlinear evolution of the FTF when the perturbation level increases. Finally, an analysis is conducted on the dynamics of a single conical flame placed into cylindrical flame tubes featuring different diameters. It is shown that confinement effects need to be taken into account when the burnt gases cannot fully expand. Large differences are observed between FTF measured for different confinement tube diameters. A new dimensionless number is derived to take these effects into account and make all the FTF collapse on a single curve. These different models are then used to model the response of a collection of small conical flames stabilized on a perforated plate. It is shown that by sorting out the different contributing mechanisms to the FTF, the expressions proposed in this work may be combined to capture the main behavior and correct phase lag evolution of these flames in the frequency range of interest for thermo-acoustic instability prediction.

Aeroacoustic whistles are a significant source of customer complaints to automotive manufacturers. Whistles can occur on many such components, but the relative position and configuration of rearview mirrors means they are a more problematic source of tonal noise on vehicles. The low subsonic complex turbulent flow, combined with small cavity scales, determines the possible whistle mechanisms. The one considered to be most problematic, fluid-dynamic cavity resonance, is the topic of this research thesis. The research scope is limited to the automotive environment of external rearview mirrors and the fluid-dynamic resonance mechanism: low subsonic Mach number, M = 0.05 - 0.13; laminar boundary layers; and two-dimensional, acoustically compact cavities. The low unit-cost of rearview mirrors and the desire to have simple identification and prediction schemes, that could be used by production engineers, determined an empirical approach. A search of the existing literature revealed that there were some data on cavities of the above scale in low Mach number flow, but quoted errors in empirical descriptions were large and there was very little research on the effects of flow yaw angle on the chosen resonance mechanism. The research therefore aims to determine whether existing empirical descriptions of fluid-dynamic cavity resonance are suitable for the prediction of the resonance characteristics, with sufficient accuracy to enable unambiguous identification of the presence of the resonance and its mechanism. A second aim is to investigate the effects of a feature of the automotive flow environment, flow yaw angle, on the resonance. Flow yaw angle is determined by those components of the flow in the same plane as the surface in which the cavity is situated. An experimental program was undertaken using a purpose-built aeroacoustic wind tunnel and a simple cavity model. Testing with two types of cavity configurations, as well as flow visualisation, investigated the main features of the resonance in time-averaged yawed flow. Within the scope of this thesis, it is shown that fluid-dynamic cavity resonance characteristics can be accurately identified by a simple empirical model, even in yawed flow. Various descriptors allow identification of the resonance threshold, stage, frequency and relative amplitude in non-yawed flow, while the frequency and stage can also be identified in yawed flow. The relative decrease in resonance amplitude in yawed flow, although identified for these experiments, would depend on the degree of spanwise variation in the boundary layer characteristics for a given cavity configuration. The results also identify significant issues with testing in a free jet tunnel, due to the nature of fluid-dynamic cavity resonance and the fluctuation energy content in free shear layers. Despite this, the thesis aims are achieved, and appropriate design guidelines are produced for automotive designers.

New methodologies and concepts for developing compact and energy efficient automotive exhaust systems have been studied. This originates in the growing concern for global warming, to which road transportation is a major contributor. The focus has been on commercial vehicles—most often powered by diesel engines—for which the emission legislation has been dramatically increased over the last decade. The emissions of particulates and nitrogen oxides have been successfully reduced by the introduction of filters and catalytic converters, but the fuel consumption, which basically determines the emissions of carbon dioxides, has not been improved accordingly. The potential reduction of fuel consumption by optimising the exhaust after-treatment system (assuming fixed after-treatment components) of a typical heavy-duty commercial vehicle is ~4%, which would have a significant impact on both the environment and the overall economy of the vehicle. First, methodologies to efficiently model complex flow duct networks such as exhaust systems are investigated. The well-established linear multiport approach is extended to include flow-acoustic interaction effects. This introduces an effective way of quantifying amplification and attenuation of incident sound, and, perhaps more importantly, the possibility of predicting nonlinear phenomena such as self-sustained oscillations—whistling—using linear models. The methodology is demonstrated on T-junctions, which is a configuration well known to be prone to self-sustained oscillations for grazing flow past the side branch orifice. It is shown, and validated experimentally, that the existence and frequency of self-sustained oscillations can be predicted using linear theory. Further, the aeroacoustics of T-junctions are studied. A test rig for the full determination of the scattering matrix defining the linear three-port representing the T-junction is developed, allowing for any combination of grazing-bias flow. It is shown that the constructive flow-acoustic coupling not only varies with the flow configuration but also with the incidence of the acoustic disturbance. Configurations where flow from the side branch joins the grazing flow are still prone to whistling, while flow bleeding off from the main branch effectively cancels any constructive flow-acoustic coupling. Two silencer concepts are evaluated: first the classic Herschel-Quincke tube and second a novel modified flow reversal silencer. The Herschel-Quincke tube is capable of providing effective attenuation with very low pressure loss penalty. The attenuation conditions are derived and their sensitivity to mean flow explained. Two implementations have been modelled using the multiport methodology and then validated experimentally. The first configuration, where the nodal points are composed of T-junctions, proves to be an example where internal reflections in the system can provide sufficient feedback for self-sustained oscillation. Again, this is predicted accurately by the linear theory. The second implementation, with nodal points made from Y-junctions, was designed to allow for equal flow distribution between the two parallel ducts, thus allowing for the demonstration of the passive properties of the system. Experimental results presented for these two configurations correlate well with the derived theory. The second silencer concept studied consists of a flow reversal chamber that is converted to a resonator by acoustically short-circuiting the inlet and outlet ducts. The eigenfrequency of the resonator is easily shifted by varying the geometry of the short circuit, thus making the proposed concept ideal for implementation as a semi-active device. Again the concept is modelled using the multiport approach and validated experimentally. It is shown to provide significant attenuation over a wide frequency range with a very compact design, while adding little or no pressure loss to the system. / QC 20110208

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