Pressure Characteristic Rollover of a Transonic Centrifugal Impeller

This paper presents a detailed numerical investigation of a transonic centrifugal compressor to understand the mechanism causing its pressure rise characteristic rollover, which fundamentally impacts compressor stability. Distinct characteristic rollover behaviors at different compressor speeds are predicted and studied. It is found that the impeller characteristic rollover occurs at high blade tip Mach number (>1) conditions. It is the result of a combination of the inducer and exducer performance. The inducer is found to stall early, while the exducer is mostly a stable part maintaining the overall impeller stability. The overall impeller characteristic rolls over when the exducer’s performance deteriorates significantly, which happens at higher flow conditions toward high speed. This is due to the flow compressibility effect (density change). It shows that the flow density across the impeller increases with the blade tip Mach number. The increased density leads to a reduced exducer exit flow coefficient with higher workload and aerodynamic losses. Detailed analysis is carried out to understand the 1D and 3D flow mechanisms governing the inducer and exducer, hence the impeller characteristic.