Specific Speed Considerations for Inward Flow Radial Supercritical CO2 Turbines

Abstract

Inward flow radial supercritical CO2 turbines require higher operating speeds than steam or gas turbines making only low specific speed designs practically realizable. A theoretical model is presented to estimate the low specific speed design regime. The model incorporates boundary conditions across the turbine and engineering limitations such as blade height, Mach number, and flow angle at the rotor inlet to predict the minimum allowable rotational speed. A CFD study is subsequently performed to review the applicability of gas turbine design principles on sCO2 turbines for low specific speed designs. The effect of specific speed and velocity ratio on turbine efficiency and flow physics is studied for power output ranging from 100 kW to 5 MW. The results show a significant deviation in the optimal specific speed, velocity ratio, and incidence angle values than traditional gas turbine designs. It is found that viscous losses dominate low specific speeds, while Coriolis effects dominate the high specific speed designs. High specific speed designs require higher negative incidence angles ∼ −50° to −55° to overcome the flow stagnation at blade pressure surface arising from the Coriolis effect. Maximum turbine efficiencies (∼ 83%) are achieved at lower specific speeds of ∼ 0.4 than gas turbines’ optimal specific speed of ∼ 0.55 to 0.65. Variations of stator, rotor, and exit kinetic energy losses with specific speeds are also presented. Finally, the results are superposed on Balje’s Ns-Ds diagram and compared with gas turbine designs.