Numerical Investigation of Centrifugal Compressor Tip Clearance

Abstract

In this paper, the effect of the axial tip clearance on the performance and flow fields of a centrifugal compressor is studied numerically. The compressor is equipped with a pinched vane-less diffuser. Six different axial clearances were modelled and the relative axial tip clearance was varied from 0.027 to 0.154. The tip clearance was changed by transferring the shroud in the axial direction. The modelled results are compared to measured results obtained in previous projects. The results indicate that the effect of tip clearance to the impeller performance is linear, even though the clearance is increased to such amounts which are seldom used in traditional compressors. The clearance has significant effect on the flow fields. Even though the mass flow averaged tangential velocity remained constant with the increasing clearance, the velocity fields show increase in tangential velocity in the passage wake between the suction side of the splitter blade and pressure side of the full blade and decreased in the other blade passage with increasing clearance. Radial velocity contours showed that the backflow region near the shroud increased in size when the clearance is increased. The growth was stronger on the suction side of the splitter blade. Even though there is quite a difference in the stage efficiency between the highest and lowest clearance, the difference in velocity triangles at the impeller exit is small. This should be taken into a consideration when compressors with high relative axial tip clearances are designed.