Effects of inlet conditions on centrifugal diffuser performance

Abstract

This report examines the influence of inlet flow conditions, including Mach number, flow angle, blockage, and axial flow non-uniformity, on the performance and operating range of a straight channel centrifugal compressor diffuser. The research was carried out in a unique facility specifically developed to provide the diffuser with a controlled inlet flow. The tests were carried out for inlet Mach number up to values greater than unity and for a range of inlet flow angles up to the onset of rotating stall. It was found that expressing the overall diffuser pressure recovery coefficient, defined using availability or mass averaged inlet total pressure, as a function of momentum averaged diffuser inlet flow angle yields a relationship which is essentially independent of diffuser inlet flow distortion, blockage, or Mach number. Further, the operating range of the diffuser was limited by the onset of rotating stall at a momentum averaged diffuser inlet flow angle ([alpha]crit = 70.5 ±0.5), which was also independent of the inlet flow field axial distortion and Mach number. The straight channel diffuser was designed to be comparable to a previously tested discrete passage diffuser and the performance of the two was compared; the overall pressure recovery of the former was found to be roughly 10% higher than that of the latter. Both diffuser types, straight channel and discrete passage diffuser showed similar behavior regarding the insensitivity of the performance and operating range to inlet flow axial non-uniformities and Mach number. The report also presents information on recent developments in the area of centrifugal compressor diffusers together with a detailed review of the open literature.