Preliminary Design of Small-Scale Supercritical CO₂ Radial Inflow Turbines

In recent years, supercritical CO₂ (sCO₂) Brayton cycles have drawn the attention of researchers due to their high cycle efficiencies, compact turbomachinery, and environmental friendliness. For small-scale cycles, radial inflow turbines (RIT) are the prevailing choice and one of the key components. A mean line design procedure for sCO₂ RIT is developed and design space exploration conducted for a 100 kW-class turbine for a low-temperature waste-heat utilization sCO₂ Brayton cycle. By varying the two design parameters, specific speed and velocity ratio, different turbine configurations are setup and compared numerically by means of computational fluid dynamics (CFD) simulations. Results are analyzed to conclude on optimum design parameters with regard to turbine efficiency and expansion ratio. Specific speeds between 0.2 and 0.5 are recommended for sCO₂ RIT with small though flow (3 kg/s). The higher the velocity ratio, the bigger the turbine expansion ratio. Pairs of optimum design parameters that effectuate maximum efficiency are identified, with smaller velocity ratios prevailing for smaller specific speeds. The turbine simulation results for sCO₂ are compared to well-established recommendations for the design of RIT from literature, such as the Balje diagram. It is concluded that for the design of sCO₂ RITs, the same principles can be used as for those for air turbines. By achieving total-to-static stage and rotor efficiencies of 84% and 86%, respectively, the developed mean line design procedure has proven to be an effective and easily applicable tool for the preliminary design of small-scale sCO₂ RIT.