Improved return passages for multistage centrifugal compressors

Abstract

This thesis presents a design concept for return passages in multistage centrifugal compressors. Flow in a baseline return passage is analyzed to identify loss sources that have substantial potential for reduction. For the geometry analyzed, it is found that mismatched incidence of the return vane, separation on the hub surface at the exit of the return bend, and blockage due to separation on the shroud surface near the return bend inlet lead to losses which have such potential. Preliminary designs were developed and computationally assessed to determine effective geometries for eliminating separation on the hub at the return bend exit and for reducing losses. Then, based on assessment of the preliminary designs, other features of the loss generation were addressed. The geometry proposed includes an increased axial extent of the return bend, an increasing radius of curvature through the return bend, and lean of the return vane leading edge, mitigating the loss mechanisms identified in the baseline return passage. The three-dimensional calculations showed a cumulative loss coefficient that was 10% lower than the baseline. The design described was carried out with a fixed inlet condition, and a second return passage was thus developed to determine the potential loss reduction if the inlet geometry were modified. (The design of the impeller required to achieve the modified inlet flow was not considered.) The cumulative loss coefficient was reduced by 27% compared with the baseline, with the implication that an area for exploration is integration of the impeller and return passage flow fields.