Flow Coefficient as a Normalising Operating Parameter for Comparison of Drop-in Fluid Replacement in Centrifugal Compressors

The phase-out of high-GWP refrigerants means working fluids are increasingly replaced in compressors already designed and sized for the original fluid. Assessing the fluid replacement in a fixed geometry is complicated by the choice of operating parameter: at a matched mass flow rate, fluids with different densities will exhibit different volumetric flow. This study uses the flow coefficient as the main flow-side operating parameter, with the mass flow rate being calculated individually for each fluid and rotational speed so that all fluids are compared at the matched non-dimensional operating point. Eight working fluids—air, nitrogen, argon, carbon dioxide, R22, R134a, R1234yf and R1234ze(E)—were simulated in a single fixed-geometry centrifugal impeller geometry using ANSYS2024 R2 CFX across six rotational speeds from 10,000 to 20,000 RPM and six flow coefficients. The efficiency optima of all eight fluids cluster to a common flow coefficient between 0.0636 and 0.0697. However, the width of the characteristic and the onset of choke remain fluid-dependent. While the low-density fluids operate over the full range of flow coefficients, the refrigerants exhibit choke at the higher end of the flow coefficient range, a feature which increases with the rotational velocity.