A Flow Physics Approach to Determining Critical Angle of Inlet Total Pressure Distortion for a Centrifugal Compressor

Abstract Characterizing the critical circumferential angle of total pressure inlet distortion, where compressor performance and stability are degraded, is of paramount importance for gas turbine inlet integration. This effort utilizes steady and transient full-annulus simulations to characterize the critical angle of total pressure inlet distortion of the single stage centrifugal compressor (SSCC) at Purdue University. This is a high-specific speed centrifugal compressor with a vaned diffuser. A method of tracking distortion through the impeller utilizing rothalpy is presented. A comparison of reduced frequency methods to determine the critical distortion extent and the physical mechanism behind this has been conducted. Both integrated and approximate methods of calculating the reduced frequency of inlet distortion are presented. The critical angle of distortion for this compressor correlates with the acoustic propagation time through the rotor. At the critical distortion extent, a stationary stall cell was developed in the diffuser leading to a reduction in stage efficiency and total pressure ratio (TPR). The stalled diffuser vane correlated to the acoustic propagation location of the ingested distortion. The critical acoustic reduced frequency for this centrifugal compressor was unity. This allowed for use of a single passage steady model compared to the full-annulus transient to assess the potential for a simplified computational fluid dynamics (CFD) model to estimate the critical angle with less computational cost. The critical angle of distortion calculated from the steady single passage model was within 0.2 deg to that of the full-annulus transient simulations.