Inlet Distortion Studies on a Centrifugal Compressor

Abstract

Abstract Centrifugal compressors can be subjected to non-uniformity in inlet flows due to complicated inlet pipe geometries arising from space constraints and the wakes of protruding instrumentation, impacting the compressor’s performance. Inlet distortion can cause premature stall initiation and surge, effectively reducing the operating range of the device. This motivates a detailed characterization of the performance for different types of inlet distortion. While the impact of distortion in axial compressors is studied extensively, there are relatively fewer studies for centrifugal compressors. The present study investigates the effects of inlet total pressure distortion on the performance of a centrifugal compressor. The rig employing a centrifugal compressor of a turbocharger is developed and built for the study to analyze full-speed compressor aerodynamics. The experimental setup comprises of a turbocharger, whose turbine is driven through flow-metered compressed air, enabling independent control of the shaft speed by adjusting the flow rate and pressure. The compressor section line is instrumented with pressure sensors, thermocouples, shaft RPM sensor, to enable the measurements required to generate the compressor characteristics. A typical operating point of the compressor with clean inflow at 130,000 RPM is a pressure ratio of 1.4 with a mass flow rate of 0.02 kg/s. Provision is added to include distortion screens to generate total pressure distortion at the inlet duct of the compressor. A PC-based data acquisition program is developed to acquire and monitor the compressor map data as well as monitor other parameters for the purposes of safety and maintenance. The distortion screens are fabricated and characterized as per the SAE ARP 1420 distortion descriptors to generate circumferential and radial total pressure distortion for different cases. The compressor performance with clean inflow is compared with different distortion screens in terms of the static pressure ratio, isentropic efficiency, for a given normalized mass flow rate and normalized RPM. In the presence of inflow distortion, the compressor performance is expectedly found to deteriorate, particularly with a lower pressure rise at the same RPM and flow rate. For a given RPM, the effect is substantial near choke compared to the near-surge region. The decrease in isentropic efficiency is more significant at higher RPMs and near choke. A detailed account of the impact of different types of distortion, in terms of their circumferential and radial distortion intensities, on the compressor global performance, along with insights gained from the local measurements, are presented.