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Communication Dans Un Congrès Année : 2013

Numerical investigation of dense gas flows through transcritical multistage axial Organic Rankine Cycle turbines

Luca Sciacovelli
P. Cinnella

Résumé

Many recent studies suggest that supercritical Organic Rankine Cycles (ORCs), i.e., ORCs in which heat is supplied at a pressure greater than the liquid/vaport critical point pressure, have a great potential for low-temperature heat recovery applications, since they allow better recovery efficiency for a simplified cycle architecture. In this work we investigate supercritical flows of dense gases through axial, multi-stage, ORC turbines, using a numerical code including advanced equations of state and a high-order discretization scheme. At this stage, we focus on inviscid flow effects due to the peculiar thermodynamic behavior of the working fluids. Bidimensional numerical simulations are carried out, initially for a single stage and then for the complete turbine, for three working fluids: the refrigerants R134a and R245fa and carbon dioxide (CO2). For the last one, the turbine is simulated for both inlet and outlet supercritical conditions. For R134a and R245fa, both supercritical and subcritical inlet turbine conditions are considered, the outlet pressure being always subcritical. Numerical simulations are used to evaluate the turbine adiabatic efficiencies found in the different cases, and an in-detail investigation of the flow field across the turbine is carried out to understand the main loss mechanisms. Shock-wave formation is found to have a crucial impact on the overall performance: carbon dioxide is shown to provide an optimal behavior since, because of the high values of the speed of sound in this fluid, the flow field is completely subsonic and no shock-waves are created. However, carbon dioxide requires to work at pressures of the order of 50 bars, which leads to higher installation costs. The use of R134a ensures satisfactory adiabatic efficiencies, despite the presence of weak shocks at the suction sides of rotor blades, whereas R245fa develops, for the turbine configuration considered in this work, strong shocks leading to significant losses. For both fluids, using supercritical inlet conditions tends to increase turbine isentropic efficiency for a given pressure ratio since, at high pressures, their thermodynamic behavior significantly deviates from that of a perfect gas, slowing down the increase of the Mach number during turbine expansion, and leading to weaker shocks.
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Dates et versions

hal-03440450 , version 1 (22-11-2021)

Identifiants

  • HAL Id : hal-03440450 , version 1

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Luca Sciacovelli, P. Cinnella. Numerical investigation of dense gas flows through transcritical multistage axial Organic Rankine Cycle turbines. CFM 2013 - 21ème Congrès Français de Mécanique, Aug 2013, Bordeaux, France. ⟨hal-03440450⟩

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