One-Dimensional and Three-Dimensional Numerical Optimization and Comparison of Single-Stage Supersonic and Dual-Stage Transonic Radial Inflow Turbines for the ORC

Organic Rankine cycle is one of the most efficient technologies that can utilize low-to-medium grade heat sources and generate useful power. Radial inflow turbine (RIT) is the key component of the ORC and its efficiency has significant effect on the overall cycle performance. Obtaining high cycle thermal efficiency requires large pressure difference (expansion ratio) across the cycle. With the low speed of sound of organic fluids and the high expansion ratios, RIT becomes chocked with supersonic flow regime and shock waves that deteriorate the turbine efficiency and hence reduce the cycle performance. Therefore, developing highly efficient RIT that can both preserve the high expansion ratio requirements of the ORC and maintain the turbine isentropic efficiency is crucial. This paper proposed the complete 1-D and 3-D numerical optimization of two different configurations as single-stage supersonic and dual-stage transonic RITs. Initially, the integrated 1-D modelling of the ORC with RIT coupled with genetic algorithm optimization technique was conducted to maximize the cycle thermal efficiency. The results showed that the dual-stage RIT exhibited considerably higher turbine efficiency in both stages and hence higher cycle efficiency compared to the single-stage supersonic one. Both configurations were further optimized using the 3-D CFD optimization procedure to maximize the turbine efficiency. The CFD results showed that the optimization of each stage individually was successful as the turbine performance increased significantly. The results revealed that the optimizations were more effective for the dual-stage transonic turbine compared to the single-stage supersonic due to the presence of shock waves. Comparison of the optimized single-stage supersonic RIT and complete dual-stage transonic RIT showed that about 15.7%, 10.63kW and 16.08% higher turbine isentropic efficiency, turbine power and cycle thermal efficiency were achieved respectively with the latter configuration.