Potential of the range extension of compressors with a variable inlet prewhirl for automotive turbocharged engines with an ultra-high-power density

Abstract

Downsizing is an effective way to improve the fuel economy of an engine and to reduce the emissions. For turbocharged engines, further downsizing or increasing the power density requires air to be compressed to a higher pressure. However, conventional compressors have a narrow stable operating range at high pressure ratios and cannot provide such highly pressurized air for ultra-high-power-density engines. Thus, a variable inlet prewhirl is employed to extend the stable operating range of the compressor. This paper investigates the performance of a centrifugal compressor with different prewhirls and speeds, quantitatively estimates the potential of range extension with a variable inlet prewhirl and discusses the mechanisms thought to be responsible for the range extension. The approach combines steady three-dimensional Reynolds-averaged Navier–Stokes simulations with theoretical analysis. In order to make the study universal to various applications with a inlet prewhirl, the inlet prewhirl was generated by giving a velocity direction at the inlet boundary of the numerical method. A variable inlet prewhirl from 0° to 60° improves the stable operating range from 23.5% to 49.0% at a pressure ratio of 4.8. The extended range is based on the shifts in the choke line and the surge line with a positive inlet prewhirl. The decreased choking flow with a positive inlet prewhirl is mainly caused by the drop in the work input from the impeller. The dominant factor in reducing the surge flow of compressor is the highly increased stability of the impeller. It is also found that the impeller tends to choke and the diffuser tends to surge with increasing prewhirl. By extending the stable operating range of the compressor with a variable inlet prewhirl from 0° to 60°, the improvement in the maximum pressure for turbocharged engine is about 64% and the increase in the brake torque is estimated to be up to 42%.