Clearance effects on centrifugal compressor characteristic and stability

The effects of impeller inlet tip clearance and diffuser width on centrifugal compressor characteristic and stability have been experimentally investigated in a centrifugal compressor with a vaneless diffuser. An increase in the impeller inlet tip clearance decreases the overall pressure rise across the compressor, mainly due to the tip clearance loss in the impeller. However, the effect of inlet tip clearance on diffuser pressure rise or compressor stability is weak. A decrease in the diffuser width significantly lowers the compressor pressure rise, especially at hight flow rates. At the component level, the impeller is insensitive to the diffuser width variation, and the pressure rise across the diffuser actually increases as diffuser width is decreased. Upon further investigation, it has been found that the overall compressor characteristic is strongly influenced by the region between the impeller exit and the diffuser inlet. Also, a decrease in the diffuser width delays stall inception by increasing the radial velocity of the flow in the diffuser. Thus, the stalling flow coefficient is more sensitive to the variation in the diffuser than the inlet tip clearance. In all cases, rotating stall consists of two or three cells rotating at about approximately one tenth of the compressor rotational speed. When the number of cells changes from three to two, the rotational speed drops. However, when the number of cells remains constant, the cells' rotational speed increases as flow coefficient is lowered. All of these trends agree well with predictions from a new stability model developed by the first author.