Computation of performance maps for centrifugal compressors in supercritical CO2 applications: Influence of two-phase flow on choking mass flow rate

This paper is dedicated to the performance map computation of centrifugal compressors operated with carbon dioxide at supercritical states (sCO2) and inlet conditions in the vicinity of the critical point. Three-dimensional computational fluid dynamics (3D-CFD) simulations are utilised. First, different approaches to model two-phase flows are reviewed. Second, the impact of two-phase flow on the speed of sound and the choking limit is further investigated. The flow through Laval nozzles is analysed to simplify the problem to its fundamental aspects. The 3D-CFD calculations match well with the ones conducted with simple one-dimensional CFD programs and the theory of equilibrium phase change. Third, the throttle curve of the NASA low-speed centrifugal compressor (LSCC) and of an industrial-scale centrifugal compressor is computed and compared to the one for air. Total inlet conditions are supercritical in the so-called liquid-like region. A considerable shift of the choking line towards lower flow coefficients is found (reduction by a factor of up to 2.0 depending on the inlet conditions). The reason for this shift is a drop in the speed of sound when bubbles are formed in the liquid, and a two-phase flow is established while the flow is accelerated around the compressor’s leading edge. Finally, a log(p)-h diagram is provided, enabling a quick assessment of the risk of two-phase flow in centrifugal compressors and serving as a practical engineering tool.