Performance impact of impeller blade trimming on centrifugal compressors

Abstract

Centrifugal impeller blade trimming can be used to modify an existing impeller design to meet a new flow or pressure ratio design point. In flow trimming, the passage area is reduced from inlet to outlet along the entire meridional length of the impeller to reduce the flow coefficient of the impeller while maintaining the pressure ratio of the untrimmed impeller. Axial trimming is a method of reducing the blade height at the impeller exit while maintaining the shroud profile of the original impeller in order to reduce the head coefficient of the impeller while maintaining the original flow range. In this work, computational fluid dynamics was employed to numerically model four impellers of varying geometries, speeds, and performance characteristics to study the performance effects and limits of modifying the impeller geometry by either flow or axial trimming. Flow trimming was found to be capable of reducing the flow coefficient by between 20 and 50% while maintaining the pressure ratio and efficiency of the baseline impeller. The suitability of an impeller for flow trimming was found to correlate strongly with flow incidence angle. Reducing the exit blade height of a centrifugal impeller by trimming the blades axially may be used to reduce the pressure ratio of a compressor while maintaining flow range; however, the amount of pressure ratio reduction was limited to between 9 and 13% before choking in the passage reduced the choked flow rate. Impeller performance was found to respond differently to axial trimming based on the diffusion ratio of the baseline impeller.