Limiting Inlet Conditions for Phase Change Avoidance in Supercritical CO2 Compressors

In many supercritical CO2 cycle implementations, compressor or pump inlet conditions are relatively near the two-phase region. Fluid acceleration near the compressor inlet can result in the potential for condensation or cavitation at the inlet. Despite potential mitigating effects or evidence in the literature, potential two-phase operation is a high-risk condition and may not be recommended for high-reliability system design. This paper presents a summary of the existing literature documenting inlet phase change in sCO2, and presents an analysis of required conditions to avoid phase change as a function of inlet pressure, temperature, and Mach number. Static conditions at the inlet are calculated based on the real gas approach documented in ASME PTC-10, Appendix G. In addition, various total-to-static iteration challenges are discussed and avoided through solution of the inverse problem to convert limiting static conditions at saturation to the full range of limiting total conditions for various Mach numbers up to 1.0. The results show that a threshold total temperature exists above which phase change cannot occur, ranging from 31.1 to 66.95 °C and increasing with Mach number. Lower temperatures below this threshold may also avoid phase change depending on the total pressure. The documented results are useful as a reference for use by cycle designers to impose design limits that minimize risks associated with two-phase flow in the compressor.