Simulation des grandes échelles d'écoulements de gaz dense à travers des grilles d'aube

Abstract : Organic Rankine Cycles (ORC) are a promising technology used for energy extraction from low-temperature heat sources. Unlike classical Rankine cycles, they use a low-boiling, dense organic fluid instead of water, leading to more compact and efficient expanders. The molecular complexity of organic working fluids induces, for thermodynamic conditions close to the liquid/vapor coexistence curve and temperatures and pressures of the order of magnitude of the critical point, considerable real gas effects, which need to be modelled by means of advances equations of state and transport-property laws. For medium to high power ORC, the expander is generally a turbine, caracterized by a small number of highly loaded stages working in the supersonic or transonic flow regimes. In order to improve ORC turbine design, it is essential to understand and predict loss mechanisms due to the formation of shock waves and to their interaction with the transitional or turbulent surrounding boundary layers. In this work we carry out large eddy simulations (LES) of transonic and supersonic dense gas flows through turbine cascades. For that purpose, we first set-up a suitable numerical strategy, with focus on efficient time interation schemes for flows dominated by the advective time step. The proposed methodology is validated for test cases of increasing difficulty, including the LES of the VKI LS-89 high-pressure turbine cascade. In the past, such a configuration has been extensively investigated both experimentally and numerically, using a perfect gas as the working fluid. Afterwards, LES of the VKI LS-89 configuration and of a supersonic turbine guide vane specifically design for ORC applications are carried out at various operating conditions by using working fluids leading to strong non-ideal effects, namely, the heavy fluorocarbon PP11 and the refrigerant r245fa. The results show up the influence of dense gas effects on shock wave formation and laminar-to-turbulent transition. Comparisons with simulations based on Reynolds-Averaged Navier-Stokes (RANS) equations supplemented by a turbulence model, the workhorse of ORC turbine design, show significant discrepancies due to the transitional nature of turbine flows, pointing out the importance of using advanced models in turbine design.
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Jean-Christophe Hoarau. Simulation des grandes échelles d'écoulements de gaz dense à travers des grilles d'aube. Autre [cond-mat.other]. Ecole nationale supérieure d'arts et métiers - ENSAM, 2019. Français. ⟨NNT : 2019ENAM0044⟩. ⟨tel-02478837v1⟩

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