Influence of Geometric Parameters for a 100 kW Inward Flow Radial Supercritical CO2 Turbine

Highly compact and efficient design makes inward flow radial (IFR) turbine a preferred choice for kilowatt scale supercritical CO2 (sCO(2)) power blocks. The influence of geometric design parameters on sCO(2) turbine performance differs from gas turbines because of their small size, high rotational speeds, and lower viscous losses. The paper presents a computational fluid dynamics (CFD) study for a 100 kW IFR turbine to arrive at optimal geometric design parameters-axial length, outlet-to-inlet radius ratio, number of rotor blades, and velocity ratio, and understand their influence on the turbine's performance. The results are compared with well-established gas turbine correlations in the specific speed range of 0.2 to 0.8 to understand the implications on sCO(2) IFR turbines. The analysis shows significant variations in the optimal values of design parameters when compared with gas turbines. It is found that sCO(2) turbines require fewer blades and higher velocity ratios for optimal performance. The maximum turbine efficiency (similar to 82%) is achieved at a lower specific speed of similar to 0.4 compared to a gas turbine with specific speed varying between 0.55 and 0.65. Additionally, higher negative incidence angles in the range of -50 deg to -55 deg are required at high specific speeds to counter the Coriolis effect in the rotor passage. The paper presents the variation of stator, rotor, and exit kinetic energy losses with specific speeds. The cumulative losses are found to be minimum at the specific speed of similar to 0.4.