Design of a supersonic turbine for the organic Rankine cycle system

Abstract

The aerodynamic design of a hundred-kilowatt class supersonic turbine is presented in this study. Its purpose is to improve organic Rankine cycle (ORC) power systems' performance. The ORC systems require a higher pressure ratio across the turbine to generate higher power output. It leads some designs to suffer from a choking condition by the higher pressure ratio. A choked flow leads to saturation of the mass flow rate and power, and a supersonic turbine can be an alternative to this problem. We use the meanline analysis, the method of characteristics, case studies, and validation of the 3D computational fluid dynamics simulations to design the supersonic turbine. 3D CFD is used to simulate the performance of the designed turbine. This approach allows us to understand its aerodynamic characteristics, performance in off-design conditions and also to find the recommended operating conditions. For validation of the numerical model, a comparison was carried out using previous literature that utilized supersonic flow with an organic fluid. The compared results are consistent, validating our numerical model. The designed supersonic turbine has 18 blades for the nozzle and 61 blades for the rotor. The predicted power output using R245fa is about 85.12 kW with a mass flow rate of 3.18 kg/s and a rotational speed of 33120 RPM.