Limiting Inlet Conditions for Phase Change Avoidance in Supercritical CO2 Compressors

Abstract

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.

Effect detection of using a modified Redlich-Kwong-Aungier equation of state on the calculation of carbon dioxide flow in a centrifugal compressor

“…To reduce the difference between the isotherms, the scaling factor n_liquid (3) is introduced into the coefficient A(T) of equation (1). Its value can be determined for temperatures from 216.59 K to 300 K using the empirical function (4). Comparison of the dependencies of the original coefficient A(T) and the modified coefficient A(T ) liquid on temperature shows a similar trend in Fig.…”

“…Fluid acceleration near the compressor inlet may lead to potential condensation or cavitation at the inlet. Despite possible mitigation effects or evidence in the literature, the possibility of operating in the liquid phase is a high-risk condition that cannot be recommended for the high-reliability system design [4]. For compressor operation in the supercritical mode, close to the critical point, there is a hypothesis of the working fluid transitioning to the subcritical mode.…”

Thermo-economic analysis of a supercritical CO2-based waste heat recovery system

“…As a matter of fact, lower fluid temperature at compressor inlet really reduces the compression work due to the higher fluid density near to the critical point, where the temperature is around 31°C. However, the potential for phase change in the inlet flow passages of the sCO 2 compressor should be taken into account [15][16][17]. Indeed, phase change could occur at the compressor inlet because of local flow acceleration and the related reduction in static pressure and temperature.…”

Effects of Near-Critical Condensation and Cavitation on the Performance of S-CO2 Compressor

“…It is still difficult to measure the internal flow through experiments due to the small size of the impeller. Although some studies state that the phase change has little effect on a compressor's performance, most studies consider a potential two-phase operation as a high risk and do not recommend it for a high-reliability system [22]. Therefore, it is necessary to investigate the quantitative effect of the near-critical nonequilibrium phase change on the performance of a S-CO2 centrifugal compressor and provide theoretical guidance for the design and operation strategy for S-CO2 compressors.…”