Numerical Investigation of Loss Mechanisms in a Partially Loaded Supersonic ORC Axial Turbine Stage

Industrial processes, small-scale power plants or internal combustion engines produce a high amount of waste heat as a by-product. The Organic Rankine Cycle (ORC) technology allows to recover that heat more efficiently compared to steam/water in conventional Clausius-Rankine cycles. For a comparably efficient and economical operation over a wide range of operating conditions, partial admission control appears to be a well-suited design option for ORC turbines. However, accurate numerical performance prediction of a partial admitted turbine stage is challenging and requires full annulus CFD computations of the partial admitted turbine stage. In the present study, a comprehensive analysis of the internal flow and aerodynamic loss mechanisms in a supersonic, axial single stage impulse 18.3 kW ORC turbine operating with an ethanol/water gas-mixture as working fluid at a partial admission ratio of 40 % based on steady-state CFD computations is presented. A comparison of numerical and experimental results for a partial admission ratio of 20 % and 40 % shows, that for a partial admission ratio of 40 % efficiency predictions based on steady-state simulations are within the measurement uncertainty. To extract and quantify the magnitude of the occurring loss mechanisms, the entropy generation rate is analysed. The results show an entropy generation between the rotor blades and the closed stator passages, which has a significant influence on the turbine performance and leads to a reduction of efficiency of about 2 to 4.5 ppt. This was found to be related to a strong jet induced in the narrow gaps between the rotor leading edges and the trailing edges of the closed stator passages, which mixes with the stagnant flow in the following nozzle sections.