Design and numerical analysis of supersonic radial-inflow turbines for transcritical ORC processes

Abstract

Radial inflow turbine design was investigated for ORC systems having transcritical and close to critical point expansion. The analysis was carried out by combining the turbine design and turbine loss analysis with a thermodynamic cycle analysis. Six fluids were investigated, including three hydrocarbons and three fluorocarbons. The objective is to investigate in detail the design and efficiency of ORC turbines designed for transcritical cycles and close to the critical point expansions, where the real gas effects and the density changes along the expansion are significant. The results of the efficiency predictions gained by using the existing enthalpy loss correlations were compared against computational fluid dynamics simulations. It was revealed that using supercritical inlet state leads to more compact turbine wheel dimensions, increased turbine power, as well as increased rotational speed, when compared to subcritical processes. On the other hand, higher turbine efficiencies were reached with subcritical turbine inlet conditions. This is mainly because of the increased stator outlet Mach number, increased expansion ratio, and lower blade height at the rotor inlet with supercritical turbine inlet conditions, resulting into higher losses along the expansion.