Real-Gas Effects in ORC Turbine Flow Simulations: Influence of Thermodynamic Models on Flow Fields and Performance Parameters

Abstract

The analysis and design of turbomachinery is usually performed by means of fluid dynamic computations employing ideal gas laws. This can lead to inaccurate redictions for Organic Rankine Cycle (ORC) turbines, which operate partly in the nonideal thermodynamic region. The objective of this work is to evaluate the influence of different equations of state (EoS) on the computed aerodynamic performance and the test case is a 2D nozzle blade of an existing ORC turbine operated with the siloxane MDM as working fluid. Three different fluid models are considered, the simple polytropic ideal gas law, the Peng-Robinson-Stryjek-Vera cubic EoS and the state-of-the-art Span-Wagner EoS. A comparison of the computed Mach number and pressure coefficient distribution along the blade as well as the outlet flow angles, mass flows and loss estimations for both design and off-design operating conditions is presented. The fluid dynamic results are very similar for the computations employing the Span-Wagner and Peng-Robinson- Stryjek-Vera EoS. The calculations performed with the polytropic ideal gas EoS, on the other hand, show large relative differences in almost all parameters, if compared to the accurate Span-Wagner EoS.