Numerical Efforts of Aerodynamic Re-Design in a Transonic Centrifugal Compressor: Part 1 — Basic Approach
Growing demands for higher specific output power in turbomachinery applications have drawn attention to aerodynamic design philosophy for a single-stage transonic centrifugal compressor with higher pressure ratios. As Part 1 of numerical efforts, some fundamental approaches in aerodynamic design were carried out in a classical 6:1 pressure-ratio compressor design of 1970’s which was selected as a baseline. The effects of the impeller blade angle distribution, the addition of the splitter blade, the changes of the tangential divergence angle of the channel-wedge diffuser and some tweaks in diffuser vane shapes near the trailing-edge were investigated in steady-state RANS CFD solutions with the conventional mixing plane interface. New blade angle distributions together with the introduction of splitter blades in the impeller brought significant improvements in the compressor pressure ratio, efficiency and operability, thanks to reduced shock strengths and enhanced blade loadings in the spanwise direction. Helicity contours on the cross sectional planes in the impeller support the benefits observing a power balance among the shroud passage vortex, the blade vortices and the tip leakage vortex. With a reduced tangential divergence in the channel-wedge diffuser passage from the original design, an impressively extended surge margin was obtained. It was confirmed from the helicity contours that a streamwise vortex structure at the entrance region of the diffuser vane plays a key role in the range of operation. A diffuser vane shape with the curved pressure surface near the trailing-edge provided a slightly higher pressure ratio and efficiency around design flow than that with the original cut-off trailing-edge. An elliptical trailing-edge diffuser vane showed rather performance drops because of the counter-clockwise hub vortex breakdown near the suction surface, resulting in less flow diffusion. Through investigations of a set of design cases, two final compressor designs, differing in the diffuser vane shape near the trailing-edge, were obtained within the work scope of the present study. However, selecting one of the two will depend on design duties for the following component because of the level of exit swirls and their rate of changes over the flow rates.