The technical feasibility of hypersonic flight (i.e., re-entry, hypersonic flight vehicles, cruise missiles, etc.) hinges on our ability to understand, predict, and control the transport of turbulence in the presence of non-equilibrium effects. A theoretical analysis of the governing equations suggests a mechanism by which fluctuations in internal energy are coupled to the transport of turbulence. Numerical studies of these flows have been conducted, but limited computational power results in reduced fidelity. Experimental studies are exceedingly rare and, consequently, experimental data available to build and evaluate turbulence models is nearly non-existent. The Decaying Mesh Turbulence (DMT) facility was designed and constructed to generate a fundamental decaying mesh turbulent flow field with passive grids. Vibrational non-equilibrium was achieved via a capacitively-coupled radio-frequency (RF) plasma discharge which required an operating pressure of 30 Torr. The flow velocity was 30 m/s. Data was recorded with each grid at multiple plasma powers (Off, 150 W, and 300 W). Over two terabytes of highly resolved (3,450 image pairs) two-dimensional particle image velocimetry (PIV) was acquired and archived. Temperature measurements were carried out using coherent anti-Stokes Raman spectroscopy (CARS). The primary objective of this study was to answer the fundamental scientific question: "Does thermal non-equilibrium alter the decay rate of turbulence?" The results of this study show that the answer is "Yes." The results demonstrate a clear coupling between thermal non-equilibrium and turbulence transport. The trends observed agree with those expected based on an analysis of the Reynolds stress transport equations, which provides confidence in transport equation-based modeling. A non-trivial reduction (~30%) in the decay rate downstream of the 300 W plasma discharge was observed. The data also show that the decay of TKE downstream of the plasma discharge was delayed (~20% downstream shift). In addition, the thermal non-equilbrium was observed to have no effect on the transverse stress. This suggests that, for this flow, the energy dilatation terms are small and unaffected by the plasma discharge, which simplifies modeling.

Thermo-mechanical failure of components in planar-type solid oxide fuel cells (SOFCs) is a major obstacle on the path to bringing this technology to commercial viability. The probability of material degradation and failure in SOFCs depends strongly on the local temperature gradients at the interfaces of different materials. Therefore, it is of paramount importance to accurately predict and manage the temperature fields within the stack, especially near the interfaces. In this work we consider three effects in detail. First, we analyze radiative heat transfer effects within the semi-transparent solid electrolyte and compared them to thermal conduction. We also, present the modeling approach for calculation of surface-to-surface exchange within the flow channels and from the stack to the environment. The simplifying assumptions are identified and their carefully justified range of applicability to the problem at hand is established. This allows thermal radiation effects to be properly included in overall thermal modeling efforts with the minimum computational expense requirement. Second, we developed a series of reduced-order models for the transient heating and cooling of a cell, leading to a framework for optimization of these processes. The optimal design is one that minimizes heating time while maintaining thermal gradients below an allowable threshold. To this end, we formulated reduced order models (validated by rigorous CFD simulations) that yield simple algebraic design rules for predicting maximum thermal gradients and heating time requirements. Several governing dimensionless parameters and time scales were identified that shed light on the essential physics of the process. Finally, an analysis was performed to assess the degree of local thermal non-equilibrium (LTNE) within porous SOFC electrodes, and through a simple scaling analysis we discovered the parameter that gives an estimate of the magnitude of LTNE effects. We conclude that because of efficient heat transfer between the solid and gas in the microscale pores of the electrodes, the temperature difference between gas and solid is often negligible. However, if local variations in current density are significant, the LTNE effects may become significant and should be considered.

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Un modèle constitutif complètement couplé est présenté pour l'analyse rigoureuse de la déformation, de l'écoulement de fluides et de transfert de chaleur dans les milieux poreux saturés à double porosité soumis à des chargements thermo-hydro-mécaniques, y compris ceux induisant un non-équilibre thermique local. La phase solide contient deux cavités distinctes: le bloc poreux et le réseau des fissures. Les équations de champs sont obtenues à partir des équations de conservation de la masse, du mouvement et de l'énergie et sont résolues par une approche par élément finis. Le modèle est utilisé pour deux types d'applications: la stabilité d'un puits de forage stimulée thermiquement pour la récupération de pétrole et l'extraction de chaleur dans un réservoir géothermique fracturé. Les différences substantielles, particulièrement de la contrainte effective, soulignent l'influence majeure de la double porosité et du non-équilibre thermique pour prédire le comportement des milieux fracturés. / A fully coupled constitutive model is presented for a rigorous analysis of deformation, hydraulic and heat flows in saturated dual porosity media subject to thermo-hydro-mechanical loadings including those able to cause local thermal non-equilibrium. The solid phase is assumed to contain two distinct cavities: the porous blocks and the fissure network. The governing equations are derived based on the equations of conservation of mass, momentum and energy. Solution to the governing equations is obtained numerically using the finite element approach. The capabilities of the model address two energy applications: the stability of a borehole in a thermally enhanced oil recovery context and the heat extraction of enhanced geothermal systems. Substantial differences, particularly in the effective stress response, highlight the major influence of the dual porosity model and the importance of the local thermal non-equilibrium assumption to predict the behaviour of fractured media.

The numerical modelling of the VKI-Longshot facility remains a challeng-ing task as it requires multi-physical numerical methods in order to simulate all the components. In the current dissertation, numerical tools were developed in order to study each component of the facility separately and a deep investigations of each stage of the shot were performed. This helped to better understand the different processes involved in the flow development inside this hypersonic wind tunnel. However the numerical computation of different regions of the facility treated as independent from each others remains an approximation at best.The accuracy of the rebuilding code for determining the free stream conditions and the total enthalpy in the VKI-Longshot facility was investigated by using a series of unsteady numerical computations of axisymmetric hypersonic flow over a heat flux probe. Good agreement was obtained between the numerical results and the measured data for both the stagnation pressure and the heat flux dur- ing the useful test time.The driver-driven part of the Longshot facility was modelled using the quasi one-dimensional Lagrangian solver L1d2. The three main conditions used for the experiments —low, medium and high Reynolds number —were considered.The chambrage effect due to the junction between the driver and the driven tubes in the VKI-Longshot facility was investigated. The computation showed great ben- efit of the chambrage in increasing the speed of the piston and thus the final compression ratio of the test gas.Two dimensional simulations of the flow in the driver and the driven tube were performed using Arbitrary Lagrangian Eulerian (ALE) solver in COOLFLuiD. A parallel multi-domain strategy was developed in order to integrate the moving piston within the computational domain.The computed pressure in the reservoir is compared to the one provided by the experiment and good agreement was obtained for both con- editions.Finally, an attempt was made to compute the starting process of the flow in the contoured nozzle. The transient computation of the flow showed how the primary shock initiates the flow in the nozzle before reaching the exit plan at time of 1.5 [ms] after the diaphragm rupture. The complex interactions of the reflected shocks in the throat raise the temperature above 9500 [K] which was not expected. Chemical dissociation of Nitrogen was not taken into account during this transient investigation which may play a key role considering the range of temperature reached near the throat. / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

Ce travail concerne l'utilisation de la technique de Fracturation Hydraulique (FH) pour exploiter l'énergie géothermique des réservoirs profonds de roches sèches chaudes (HDR). La fracturation hydraulique est réalisée par injection de fluides géothermiques dans des réservoirs partiellement fracturés de faible perméabilité. Les fluides à haute pression sont destinés à faire évoluer les fissures et leur connectivité. Les valeurs de débit/pression auxquelles les fluides géothermiques doivent être pompés, ainsi que le calendrier de pompage pour initier la fracturation hydraulique, dépendent principalement des conditions géostatiques existantes (contraintes géostatiques, pression fluide et température initiales de l'HDR) ainsi que des propriétés des fissures de l'HDR (longueur, épaisseur, densité et distribution directionnelle initiales moyennes de fissures). Tous ces éléments, en sus de leurs effets sur la stabilité des forages, sont analysés dans cette recherche. Des modèles de fracturation, qui sont capables de suivre l'évolution des fissures dans toutes les orientations spatiales possibles, sont utilisés pour obtenir le tenseur anisotrope de perméabilité. Ces modèles sont intégrés dans un code domestique d'éléments finis qui est développé pour résoudre des problèmes aux limites thermo-poroélastiques. Pour supprimer/diminuer les oscillations qui accompagnent les solutions paraboliques et/ou hyperboliques lors de la convection forcée, plusieurs techniques de stabilisation ont dû être implémentées. / The application of the Hydraulic Fracturing (HF) technology to exploit geothermal energy from Hot Dry Rocks (HDR) reservoirs is addressed. HF is achieved by extensively pumping geothermal fluids to already existing fractured HDR reservoirs of low permeability. High fluid pressures are expected to drive cracks to evolve and connect. The newly created burgeoning hydraulic conduits should supposedly enhance the permeability of the existing HDR reservoirs. The flow rate/pressure values at which geothermal fluids should be pumped, as well as the pumping schedule to initiate HF, depend primarily on the existing geostatic conditions (geostatic stresses, initial HDR pressure and temperature) as well as on HDR fracture properties (initial mean fracture length, mean fracture aperture, density and orientational distribution of fractures). While these components, in addition to their effects on borehole stability, are scrutinized in this research, focus is on the evolution during circulation processes of the fracture properties. A fracturing model that is capable of tracking fracture evolution in all possible spatial orientations is used to obtain the time course of the anisotropic permeability tensor. This evolving property is integrated into a domestic finite element code which is developed to solve thermo–poroelastic BVPs: emphasis is laid on the efficiency of the doublet flow technique where a fluid gains thermal energy by circulating through the HDR reservoir from the injection well to the production well. The spurious oscillations in the hyperbolic solutions of the approximated finite element approach that are commensal with the phenomenon of forced heat convection are healed/mitigated through several stabilization approaches.

The hypersonic laminar to turbulent transition problem above Mach 10 is addressed experimentally in the short duration VKI Longshot gun tunnel. Reentry conditions are partially duplicated in terms of Mach and Reynolds numbers. Pure nitrogen is used as a test gas with flow enthalpies sufficiently low to avoid its dissociation, thus approaching a perfect gas behavior. The stabilizing effects of Mach number and nosetip bluntness on the development of natural boundary layer disturbances are evaluated over a 7 degrees half-angle conical geometry without angle of attack. <p><p>Emphasis is initially placed on the flow characterization of the Longshot wind tunnel where these experiments are performed. Free-stream static pressure diagnostics are implemented in order to complete existing stagnation point pressure and heat flux measurements on a hemispherical probe. An alternative method used to determine accurate free-stream flow conditions is then derived following a rigorous theoretical approach coupled to the VKI Mutation thermo-chemical library. Resulting sensitivities of free-stream quantities to the experimental inputs are determined and the corresponding uncertainties are quantified and discussed. The benefits of this different approach are underlined, revealing the severe weaknesses of traditional methods based on the measurement of reservoir conditions and the following assumptions of an isentropic and adiabatic flow through the nozzle. The operational map of the Longshot wind tunnel is redefined accordingly. The practical limits associated with the onset of nitrogen flow condensation under non-equilibrium conditions are also accounted for. <p><p>Boundary layer transition experiments are then performed in this environment with free-stream Mach numbers ranging between 10-12. Instrumentation along the 800mm long conical model includes flush-mounted thermocouples and fast-response pressure sensors. Transition locations on sharp cones compare favorably with engineering correlations. A strong stabilizing effect of nosetip bluntness is reported and no transition reversal regime is observed for Re_RN<120000. Wavelet analysis of wall pressure traces denote the presence of inviscid instabilities belonging to Mack's second mode. An excellent agreement with Linear Stability Theory results is obtained from which the N-factor of the Longshot wind tunnel in these conditions is inferred. A novel Schlieren technique using a short duration laser light source is developed, allowing for high-quality flow visualization of the boundary layer disturbances. Comparisons of these measurement techniques between each other are finally reported, providing a detailed view of the transition process above Mach 10. / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished