An experimental study was performed on a low frequency transcritical thermoacoustic engine developed at Maurice J. Zucrow Laboratories. The goal of the experiment was to characterize the effects of engine geometry on the thermoacoustic production of the working fluid and to use insights gained to design a power extraction device for the transcritical thermoacoustic engine. The effects of geometry were investigated by parametrically varying the length of the resonator and the diameter of the resonator and measuring the pressure amplitude and frequency of thermoacoustic instabilities developed at varying ∆T and one bulk pressure of P Pcr = 1.1. It was found that increasing resonator length increases pres?sure amplitude, decreases frequency, and increases acoustic power developed. Increasing resonator diameter decreases pressure amplitude, increases frequency, and increases acoustic power developed. It was also experimentally proven that coiled tube sections in the res?onator attenuate the thermoacoustic pressure wave. After testing, the knowledge gained was applied to the design of a bidirectional impulse turbine for eventual integration into a scaled-up version of the current thermoacoustic engine to be used to extract power from the thermoacoustic instabilities developed in the rig.

River dynamic behaviour is affected by variations both in the water phase and in the transported sediment phase. A change in the water regime may lead to significant morphological changes in the bed profile, which in turn may strongly influence the flow conditions. Transcritical flows over mobile beds are particularly challenging to model due to the rapid variation in space and time of the solid transport, and to the specific treatment required for boundary conditions. The one-dimensional numerical model presented in this dissertation divides the flow in two fully coupled layers: a water layer and a water-sediment transport layer. This model was initially designed to depict dam-break flows, which does not require a specific treatment of boundary conditions. An extension of the two-layer approach is proposed in order to properly take into account boundary conditions. The treatment of boundary conditions commonly relies on characteristics. Within a two-layer model, which embodies five governing equations, an appropriate eigenstructure analysis is developed based on numerical estimations. This novel approach results in a new characterization of the critical stage by defining a specific two-layer Froude number. The model is compared to the classical Saint-Venant – Exner approach and favourably applied to several typical situations: uniform flow, which allows a straightforward calibration of the model parameters; regressive erosion around a mild-to-steep slope transition; evolution of a mobile bed under a hydraulic jump; and scour hole formation downstream of a fixed bed.

When released into the atmosphere, traditional refrigerants contribute to climate change several orders of magnitude more than a corresponding amount of carbon dioxide. For that reason, an increasing amount of interest has been paid to transcritical vapor compression systems in recent years, which use carbon dioxide as a refrigerant. Vapor compression systems also impact the environment through their consumption of energy. This can be greatly increased by faulty operation. Automated techniques for detecting and diagnosing faults have been widely tested for subcritical systems, but have not been applied to transcritical systems. These methods can involve either dynamic analysis of the vapor compression cycle or a variety of algorithms based on steady state behavior. In this thesis, the viability of dynamic fault detection is tested in relation to that of static fault detection for a transcritical refrigeration system. Step tests are used to determine that transient behavior does not give additional useful information. The same tests are performed on a subcritical air-conditioner showing little value in dynamic fault detection. A static component based method of fault detection which has been applied to subcritical systems is also tested for all pairings of four faults: over/undercharge, evaporator fouling, gas cooler fouling, and compressor valve leakage. This technique allows for low cost measurement and independent detection of individual faults even when multiple faults are present. Results of this method are promising and allow distinction between faulty and fault-free behavior.

Dans les moteurs cryotechniques, les ergols sont refroidis pour être stockés sous forme liquide et limiter ainsi la taille des réservoirs. Ils sont ensuite mis sous pression, grâce à une turbopompe, avant d’être injectés dans la chambre de combustion. Pour augmenter les rendements des moteurs, la pression de chambre est augmentée et peut dépasser les pressions critiques des ergols. Le régime de combustion supercritique est alors appelé transcritique lorsque l’oxygène est injecté à une température inférieure à sa température critique avec une densité équivalente à celle d’un liquide. Ce régime possède certaines propriétés des injections diphasiques avec un dard dense mais sans présenter de phénomène d’atomisation ce qui le rapproche des injections gaz-gaz. L’étude de la transition dense-dilué de l’oxygène a été le dénominateur commun de cette thèse. En régime supercritique, l’épaisseur de cette transition diminue avec la pression jusqu’à devenir infiniment fine à la pression critique. Le manque de discrétisation des zones à forts gradients conduit à des instabilités numériques. Cette situation est analogue numériquement au cas d’une interface liquide vapeur subcritique. C’est pourquoi nous avons étendu dans cette thèse des méthodes d’interface diffuse au régime supercritique. La méthode dite de second gradient introduite par van der Waals a permis de simuler des flammes étirées subcritiques et supercritiques. Tandis que l’approche multifluide a été utilisée pour réaliser des simulations aux grandes échelles du banc d'essai MASCOTTE en régime supercritique avec le code CEDRE. / In cryogenic engines, the propellants are refrigerated and stored liquid in order to limit the dimension of the tanks. They are pressurized by turbopumps before their injection in the combustion chamber. To increase the efficiency of the engines, the chamber pressure is increased and can be above the critical pressure of the propellants. This combustion regime is called transcritical. It exhibits some properties of diphasic injection with a high density core jet but does not have a phenomenon of atomization. The study of the oxygen transition from dense to light has been the main objectives of this thesis. In supercritical regime, the width of this transition decreases with the pressure till it reaches the critical pressure where it becomes infinitely thin. The lack of discretization in the zone of strong gradients leads to numerical instabilities. This situation is analog from a numerical point of view to a liquid-vapor interface. Thus, in this thesis we have extented the diffuse interface methods to the supercritical regime. The second gradient method introduced by van der Waals has allowed the simulation of subcritical and supercritical flames. The multifluid approach has been implemented in the solver CEDRE for the computation of a large eddy simulation of the experimental bench MASCOTTE in supercritcal conditions.

An experimental study of a low-frequency transcritical thermoacoustic device has been conducted at Purdue University's Maurice J. Zucrow Laboratories. The purpose of this study was to characterize the thermoacoustic response of transcritical R-218 and asses it's feasibility for energy extraction and waste heat removal. This rig operated as a standing-wave configuration and achieved pressure amplitudes as high as 690 KPa (100 psi) at a temperature difference of 150 K and a bulk pressure of 1.3 P/P_cr(3.43 MPa). To the author's knowledge, this is the highest ever thermoacoustic pressure amplitude achieved in a non-reacting flow. The thermoacoustic response was characterized by varying temperature difference and bulk pressure parametrically. The effect of resonator length was characterized in a set of tests where resonator length and bulk pressure was varied parametrically at a single temperature difference. Finally, the feasibility for energy extraction was assessed in a set of tests which characterized the ability of the working fluid to pump itself through a recirculation line with check valves. This set of tests showed that the working fluid was able to create self-sustained circulation by inducing a pressure differential across the check valves with the thermoacoustic response. This circulation was induced while still maintaining a significant pressure amplitude, demonstrating promising results as a feasible method for energy extraction and waste heat removal.

A compreensão, monitoramento e simulação da dinâmica da rede de drenagem de um centro urbano, inclusive os rios, podem auxiliar a análise de risco e o desenvolvimento e correção de projetos de rede de drenagem urbana. Entretanto, a simulação de rios urbanos é um grande desafio, devido a rápida variação das condições de escoamento (nível d\'água e velocidade) durante a passagem de onda de cheia, trazendo dificuldades tanto no monitoramento como na modelagem. Por se tratar de escoamentos não permanentes com significativo efeito de histerese da relação nível-vazão, torna-se necessária a utilização de modelos hidrodinâmicos. Além disso, a possível ocorrência de escoamento transcrítico aumenta a complexidade de representação do problema. Neste trabalho, um modelo hidrodinâmico capaz de resolver escoamentos transcríticos foi desenvolvido com o objetivo de simular eventos de passagem de onda de cheia em rios urbanos. O modelo, implementado na linguagem Fortran, foi baseado nas equações unidimensionais completas de Saint-Venant e utiliza o esquema de resolução de Preissmann adaptado por um redutor dos termos de inércia. Isso permitiu a resolução de escoamentos transcríticos, conforme comprovado em problemas teóricos padrões. Na aplicação prática do modelo, eventos de onda de cheia foram monitorados em um trecho do canal do córrego do Gregório na cidade de São Carlos, São Paulo, e simulações foram realizadas considerando apenas dados de níveis. Os resultados obtidos foram satisfatórios (Eficiência de Nash-Sutcliffe acima de 0,7) e permitiram a utilização das vazões estimadas para a proposição de uma curva-chave para o córrego. A metodologia aplicada neste trabalho se mostrou promissora para a definição de curva-chave de rios urbanos que apresentam dificuldades de medição de vazão. / The recognition, monitoring and simulation of the drainage system\'s dynamics of an urban center, including rivers, can assist in the risk assessment and the development and correction of urban drainage network projects. However, the simulation of urban rivers is a great challenge, due to the rapid variation of the flow conditions (water level and velocity) during the passing of flood waves, and poses difficulties for their monitoring and modeling. The use of a hydrodynamic model becomes necessary, as the flow is unsteady and produces a hysteresis effect of stage-discharge relation. Furthermore, the possible occurrence of transcritical flow increases the complexity of the problem representation even more. This manuscript addresses the development of a hydrodynamic model that solves transcritical flows and simulates flood wave events in urban rivers. The model, implemented in the Fortran programming language, was based on the one-dimensional complete equations of Saint Venant and uses the Preissmann scheme of resolution adapted by a reduction factor of the inertial terms. Such features enabled the resolution of transcritical flows, as evidenced in standard theoretical problems. In a practical application of the model, flood wave events were monitored in the Gregório creek in São Carlos, São Paulo, and simulations were performed only with level data. The results were satisfactory (Nash-Sutcliffe efficiency above 0.7) and enabled the use of the flows estimated for the proposal of a rating curve for the creek. The methodology applied has shown promising for the definition of a rating curve for urban rivers of difficult flow measurements.

Cette étude consiste en l’expérimentation de mélanges de fluides frigorigènes à base de CO2 dans les applications de pompes à chaleur domestiques. L’objectif est l’obtention de performances supérieures à une pompe à chaleur au CO2 double-service (eau chaude sanitaire et chauffage). L’ajout d’autres composés au CO2 déplace le point critique et de façon plus générale modifie les lignes d’équilibre de phases. On peut alors s’attendre à la réduction des pressions de fonctionnement et à une augmentation du rendement énergétique du cycle thermodynamique. Une pompe à chaleur mono-étagée au CO2 est utilisée comme référence, et les conditions de température externes à la boucle thermodynamique sont invariantes. Deux scénarii sont considérés : La production d’eau chaude sanitaire (ECS), où l’eau est chauffée de 10 °C à 65 °C ; la production d’eau de chauffage (CH) où l’au est chauffée de 30 °C à 35 °C. Dans les deux cas, l’eau glycolée en l’entrée de l’évaporateur est régulée à 7 °C. Afin de pouvoir analyser le comportement des cycles thermodynamiques avec mélanges, il est essentiel de connaitre la composition du fluide frigorigène en circulation. Pour ce faire, nous avons mis au point une technique de mesure statistique et non-intrusive de la composition: Des cellules optiques installées sur les tubes de la boucle permettent de recueillir les spectres d’absorption du fluide en circulation, grâce à un spectromètre proche infrarouge à transformée de Fourier. Un étalonnage méticuleux est effectué via l’acquisition de nombreux spectres d’échantillons ayant des compositions connues. Un modèle statistique est alors créé pour lier les concentrations aux données spectrales. Les compositions peuvent ensuite être estimées à partir de nouveau spectres, dont l’acquisition rapide permet l’analyse de la composition du fluide même en fonctionnement dynamique de la pompe à chaleur. Des mélanges de CO2 & propane, et CO2 & R-1234yf ont été testés, montrant des potentiels d’amélioration des performances de la pompe à chaleur pour les applications de chauffage des locaux. / The aim of this work is to experiment CO2 based mixtures as working fluids for heat pump applications in buildings, in order to enhance their performances compared to pure CO2 dual services heat pumps. Since adding other chemicals to CO2 moves the critical point and generally equilibrium lines, it is expected that lower operating pressures as well as higher global efficiencies can be reached. A simple stage pure CO2 cycle is used as reference, with fixed external conditions. Two scenarios are considered: water is heated from 10 °C to 65 °C for Domestic Hot Water (DHW) scenario and from 30 °C to 35 °C for Central Heating (CH) scenario. In both cases, water at the evaporator inlet is set at 7 °C to account for such outdoor temperature conditions. In order to understand the dynamic behaviour of thermodynamic cycles with mixtures, it is essential to measure the fluid circulating composition. To this end, we have developed a non intrusive method. Online optical flow cells allow the recording of infrared spectra by means of a Fourier Transform Infra Red spectrometer. A careful calibration is performed by measuring a statistically significant number of spectra for samples of known composition. Then, a statistical model is constructed to relate spectra to compositions. After calibration, compositions are obtained by recording the spectrum in few seconds, thus allowing for a dynamic analysis. Mixtures of CO2 & propane and CO2 & R-1234yf have been tested and showed great potential on enhancing performances of the heat pump for central heating applications.

A compreensão, monitoramento e simulação da dinâmica da rede de drenagem de um centro urbano, inclusive os rios, podem auxiliar a análise de risco e o desenvolvimento e correção de projetos de rede de drenagem urbana. Entretanto, a simulação de rios urbanos é um grande desafio, devido a rápida variação das condições de escoamento (nível d\'água e velocidade) durante a passagem de onda de cheia, trazendo dificuldades tanto no monitoramento como na modelagem. Por se tratar de escoamentos não permanentes com significativo efeito de histerese da relação nível-vazão, torna-se necessária a utilização de modelos hidrodinâmicos. Além disso, a possível ocorrência de escoamento transcrítico aumenta a complexidade de representação do problema. Neste trabalho, um modelo hidrodinâmico capaz de resolver escoamentos transcríticos foi desenvolvido com o objetivo de simular eventos de passagem de onda de cheia em rios urbanos. O modelo, implementado na linguagem Fortran, foi baseado nas equações unidimensionais completas de Saint-Venant e utiliza o esquema de resolução de Preissmann adaptado por um redutor dos termos de inércia. Isso permitiu a resolução de escoamentos transcríticos, conforme comprovado em problemas teóricos padrões. Na aplicação prática do modelo, eventos de onda de cheia foram monitorados em um trecho do canal do córrego do Gregório na cidade de São Carlos, São Paulo, e simulações foram realizadas considerando apenas dados de níveis. Os resultados obtidos foram satisfatórios (Eficiência de Nash-Sutcliffe acima de 0,7) e permitiram a utilização das vazões estimadas para a proposição de uma curva-chave para o córrego. A metodologia aplicada neste trabalho se mostrou promissora para a definição de curva-chave de rios urbanos que apresentam dificuldades de medição de vazão. / The recognition, monitoring and simulation of the drainage system\'s dynamics of an urban center, including rivers, can assist in the risk assessment and the development and correction of urban drainage network projects. However, the simulation of urban rivers is a great challenge, due to the rapid variation of the flow conditions (water level and velocity) during the passing of flood waves, and poses difficulties for their monitoring and modeling. The use of a hydrodynamic model becomes necessary, as the flow is unsteady and produces a hysteresis effect of stage-discharge relation. Furthermore, the possible occurrence of transcritical flow increases the complexity of the problem representation even more. This manuscript addresses the development of a hydrodynamic model that solves transcritical flows and simulates flood wave events in urban rivers. The model, implemented in the Fortran programming language, was based on the one-dimensional complete equations of Saint Venant and uses the Preissmann scheme of resolution adapted by a reduction factor of the inertial terms. Such features enabled the resolution of transcritical flows, as evidenced in standard theoretical problems. In a practical application of the model, flood wave events were monitored in the Gregório creek in São Carlos, São Paulo, and simulations were performed only with level data. The results were satisfactory (Nash-Sutcliffe efficiency above 0.7) and enabled the use of the flows estimated for the proposal of a rating curve for the creek. The methodology applied has shown promising for the definition of a rating curve for urban rivers of difficult flow measurements.

Pour satisfaire aux dernières réglementations en matière d'émissions, des progrès importants sont encore attendus des moteurs à combustion interne. De plus, améliorer l'efficacité du moteur pour réduire les émissions et la consommation de carburant est devenu plus essentiel qu'auparavant. Mais, de nombreux phénomènes complexes restent mal compris dans ce domaine, tels que le processus d'injection de carburant. Nombreux logiciels pour la dynamique des fluides numérique (CFD) prenant en compte le changement de phase (comme la cavitation) et la modélisation de l’injection ont été développés et utilisés avec succès dans le processus d’injection. Néanmoins, il existe peu de codes CFD capables de simuler avec précision des conditions d’injection transcritiques, à partir d'une condition de température de carburant sous-critique vers un mélange supercritique dans la chambre de combustion. En effet, la plupart des modèles existants peuvent simuler des écoulements à phase unique, éventuellement dans des conditions supercritiques, ou des écoulements diphasiques dans des conditions sous-critiques. Par conséquent, il manque un modèle complet capable de traiter les conditions transcritiques, y compris la transition de phase possible entre les régimes souscritiques et supercritiques, ou entre les écoulements monophasiques et diphasiques, de manière dynamique. Cette thèse a pour objectif de relever ce défi.Pour cela, des modèles d'écoulement diphasique compressible de fluide réel basés sur une approche eulérienne-eulérienne avec prise en compte de l'équilibre de phase ont été développés et discutés dans le présent travail. Plus précisément, un modèle à 6-équation entièrement compressibles incluant les équations de bilan des phases liquide et gazeuse résolues séparément ; et un modèle à 4-équation qui résout les équations des bilans liquide et gazeux en équilibre mécanique et thermique sont proposés dans ce manuscrit. L’équation d’état Peng-Robinson EoS est sélectionné pour fermer les deux systèmes et pour faire face aux éventuels changements de phase et à la transition ou à la séparation des phases. En particulier, un solveur d'équilibre de phase a été développé et validé. Ensuite, une série de tests académiques 1D portant sur les phénomènes d'évaporation et de condensation effectués dans des conditions sous-critiques et supercritiques a été simulée et comparée aux données de la littérature et aux résultats académiques disponibles. Ensuite, les modèles d'écoulement en deux phases entièrement compressibles (systèmes à 6-équation et à 4- équation) ont été utilisés pour simuler les phénomènes de cavitation dans une buse 3D de taille réelle afin d'étudier l'effet de l’azote dissous sur la création et le développement de la cavitation. Le bon accord avec les données expérimentales prouve que le solveur proposé est capable de gérer le comportement complexe du changement de phase dans des conditions sous-critiques. Enfin, la capacité du solveur à traiter l’injection transcritique à des pressions et températures élevées a été validée par la modélisation réussie de l’injecteur Spray A du réseau de combustion moteur (ECN). / To satisfy latest stringent emission regulations, important progress is still be expected from internal combustion engines. In addition, improving engine efficiency to reduce the emission and fuel consumption has become more essential than before. But many complex phenomena remain poorly understood in this field, such as the fuel injection process. Numerous software programs for computational fluid dynamics (CFD) considering phase change (such as cavitation) and injection modelling, have been developed and used successfully in the injection process. Nevertheless, there are few CFD codes able to simulate correctly transcritical conditions starting from a subcritical fuel temperature condition towards a supercritical mixture in the combustion chamber. Indeed, most of the existing models can simulate either single-phase flows possibly in supercritical condition or two-phase flows in subcritical condition; lacking therefore, a comprehensive model which can deal with transcritical condition including possible phase transition from subcritical to supercritical regimes, or from single-phase to two-phase flows, dynamically. This thesis aims at dealing with this challenge. For that, real fluid compressible two-phase flow models based on Eulerian-Eulerian approach with the consideration of phase equilibrium have been developed and discussed in the present work. More precisely, a fully compressible 6-equation model including liquid and gas phases balance equations solved separately; and a 4-equation model which solves the liquid and gas balance equations in mechanical and thermal equilibrium, are proposed in this manuscript. The Peng-Robinson equation of state (EoS) is selected to close both systems and to deal with the eventual phase change or phase transition. Particularly, a phase equilibrium solver has been developed and validated. Then, a series of 1D academic tests involving the evaporation and condensation phenomena performed under subcritical and supercritical conditions have been simulated and compared with available literature data and analytical results. Then the fully compressible two-phase flow models (6-Equation and 4-Equation systems) have been employed to simulate the cavitation phenomena in a real size 3D nozzle to investigate the effect of dissolved N2 on the inception and developing of cavitation. The good agreement with experimental data proves the solver can handle the complex phase change behavior in subcritical condition. Finally, the capability of the solver in dealing with the transcritical injection at high pressure and temperature conditions has been further validated through the successful modelling of the engine combustion network (ECN) Spray A injector.

Ces cinquantes dernières années, la majorité des paramètres de conception des moteurs cryotechniques ont été ajustés en l'absence d'une compréhension détaillée des phénomènes de combustion, en raison des limites des diagnostiques expérimentaux et des capacités de calcul. L'objectif de cette thèse est de réaliser des simulations numériques instationnaires d'écoulements réactifs transcritiques de haute fidélité, pour permettre une meilleure compréhension de la dynamique de flamme dans les moteurs cryotechniques et finalement guider leur amélioration. Dans un premier temps, la thermodynamique gaz-réel et son impact sur les schémas numériques sont présentés. Comme la Simulation aux Grandes Echelles (SGE) comporte des équations filtrées, les effets de filtrages induits par la thermodynamique gaz-réel sont ensuite mis en évidence dans une configuration transcritique type et un opérateur de diffusion artificiel, spécifique au gaz réel, est proposé pour lisser les gradients transcritiques en SGE. Dans un deuxième temps, une étude fondamentale du mélange turbulent et de la combustion dans la zone proche-injecteur des moteurs cryotechniques est menée grâce à la Simulation Numérique Directe (SND). Dans le cas non-réactif, les lâchers tourbillonnaires dans le sillage de la lèvre de l’injecteur jouent un rôle majeur dans le mélange turbulent et provoquent la formation de structures en peigne déjà observées expérimentalement dans des conditions similaires. Dans le cas réactif, la flamme reste attachée à la lèvre de l'injecteur, sans extinction locale, et les structures en peigne disparaissent. La structure de flamme est analysée et différents modes de combustion sont identifiés. Enfin, une étude de flamme-jet transcritique H2/O2, accrochée à un injecteur coaxial avec et sans retrait interne, est menée. Les résultats numériques sont d'abord validés par des données expérimentales pour l'injecteur sans retrait. Ensuite, la configuration avec retrait est comparée à la solution de référence sans retrait et à des données experimentales pour observer les effets de ce paramètre de conception sur l'efficacité de combustion. / In the past fifty years, most design parameters of the combustion chamber of Liquid Rocket Engines (LREs) have been adjusted without a detailed understanding of combustion phenomena, because of both limited experimental diagnostics and numerical capabilities. The objective of the present thesis work is to conduct high-fidelity unsteady numerical simulations of transcritical reacting flows, in order to improve the understanding of flame dynamics in LRE, and eventually provide guidelines for their improvement. First real-gas thermodynamics and its impact on numerical schemes are presented. As Large-Eddy Simulation (LES) involves filtered equations, the filtering effects induced by real-gas thermodynamics are then highlighted in a typical 1D transcritical configuration and a specific real-gas artificial dissipation is proposed to smooth transcritical density gradients in LES. Then, a Direct Numerical Simulation (DNS) study of turbulent mixing and combustion in the near-injector region of LREs is conducted. In the non-reacting case, vortex shedding in the wake of the lip of the injector is shown to play a major role in turbulent mixing, and induces the formation of finger-like structures as observed experimentally in similar operating conditions. In the reacting case, the flame is attached to the injector rim without local extinction and the finger-like structures disappear. The flame structure is analyzed and various combustion modes are identified. Finally, a LES study of a transcritical H2/O2 jet flame, issuing from a coaxial injector with and without inner recess, is conducted. Numerical results are first validated against experimental data for the injector without recess. Then, the recessed configuration is compared to the reference solution and to experimental results, to scrutinize the effects of this design parameter on combustion efficiency.