Evaluation of Cold Flow Test Conditions and Test Fluid Selection for a 60-kW Radial Inflow Supercritical CO₂ Turbine Available to Purchase

Indian Institute of Science, Bangalore, India, is developing a supercritical CO₂ power block. The power block features a 60-kW single-stage, unshrouded, vaneless radial inflow turbine designed to operate at 55,000 rpm. A cold flow test rig is being built to test the turbine assembly under reduced operating conditions before integrating it into the power block. The cold flow test must mimic the performance of the actual design condition and should be able to operate at design speed. This paper uses similarity method to determine the operating conditions and appropriate test fluid for the cold flow test.

Four candidate test fluids are considered: air, subcritical CO₂, Sulfur hexafluoride, and R134a. A lower turbine inlet pressure is recommended for the cold test to minimize capital costs and ensure safety. However, challenges are encountered to match all similarity parameters at lower turbine inlet pressures. The similarity parameters matching improves as inlet pressure increases. The maximum inlet pressure for R134a and SF₆ is limited by their saturation pressures, and they require speeds below 20,000 rpm. Subcritical CO₂ and air necessitate higher inlet pressures of ∼ 40–60 bar and operating speeds of 35,000 and 49,300 rpm, respectively, to ensure similarity.

Computational fluid dynamic (CFD) simulations are employed to compare the velocity distribution, flow structures, entropy generation, loss distribution, and efficiency between the design and test conditions. The findings highlight air as the best-suited test fluid, offering the best aerodynamic match. The turbine can be tested with air up to full speed without choking. Furthermore, air as test fluid eliminates the need for a discharge storage tank as it can be directly released into the atmosphere.