The Impeller Exit Flow Coefficient As a Performance Map Variable for Predicting Centrifugal Compressor Off-Design Operation Applied to a Supercritical CO2 Working Fluid

A multi-stage centrifugal compressor model is presented with emphasis on analyzing use of an exit flow coefficient vs. an inlet flow coefficient performance parameter to predict off-design conditions in the critical region of a supercritical carbon dioxide (CO₂) power cycle. A description of the performance parameters is given along with their implementation in a design model (number of stages, basic sizing, etc.) and a dynamic model (for use in transient studies). A design case is shown for two compressors, a bypass compressor and a main compressor, as defined in a process simulation of a 10 megawatt (MW) supercritical CO₂ recompression Brayton cycle. Simulation results are presented for a simple open cycle and closed cycle process with changes to the inlet temperature of the main compressor which operates near the CO₂ critical point. Results showed some difference in results using the exit vs. inlet flow coefficient correction, however, it was not significant for the range of conditions examined. This paper also serves as a reference for future works, including a full process simulation of the 10 MW recompression Brayton cycle.