Impact of Inlet Thermodynamic Conditions on sCO2 Compressor Performance

This study investigates the influence of inlet thermodynamic conditions on the performance of a centrifugal compressor operating near the critical point of CO2. In this region, steep property gradients cause strong real gas effects and phase change, which significantly impact aerodynamic performance. To numerically investigate its effect, a CFD framework is established, incorporating a look-up table approach and a Homogeneous Binary Mixture model for phase change. The numerical method is validated against experimental data from a published De Laval nozzle test. Compressor performance– including head coefficient, pressure ratio, and isentropic efficiency–are evaluated for 24 sampled inlet conditions under the design operating condition. As the inlet entropy decreases, the pressure ratio increases by up to 30% while the head coefficient decreases by up to 9%. However, isentropic efficiency varies relatively little across all cases, with deviations less than 0.7%. The observed performance trends are explained by the non-ideal behavior of CO2 near the critical point. The reduction in head is attributed to a decrease in swirl velocity at the impeller exit caused by changes in the isentropic density-pressure derivative, (𝐷𝜌⁄𝐷𝑃)!. The increased pressure ratio is associated with a lower compressibility factor Z. Moreover, the reduced swirl velocity leads to a higher incidence at the diffuser inlet, resulting in additional aerodynamic losses in the diffuser region.