Numerical Analysis of Overall Performance and Flow Phenomena of an Automatically Optimized Three-Dimensional Return Channel System for Multistage Centrifugal Compression Systems

For multi-stage compression systems, besides the aerodynamics of the impeller and the diffuser, the U-turn and return channel blades aerodynamics play an important role for the total stage efficiency of the compression system. Due to modern CAD and CAM methodology, three-dimensional blade surfaces of an impeller but also of return channel blades are easily designed and manufactured at a similar price as old-fashioned two-dimensional blade designs. This paper presents the numerical analysis and aerodynamic optimization of a three-dimensional return channel system for multi stage single shaft centrifugal compressor machinery. In a previous paper, a two-dimensional return channel blade system had been optimized by an automatic evolutionary algorithm [1]. This previous study showed further aerodynamic potential by utilizing a three dimensional return channel blade design. The three-dimensional blade comprises of ruling surfaces. In the present paper, for a three-dimensional blade design, both the blade angle and blade thickness distributions are allowed to be varied independently for the hub and the shroud. As a result, the total pressure loss of the three dimensional return channel blade can be relatively reduced by 6% compared with a classical two-dimensional return channel blade. This reduction in total pressure loss is partly caused by the matching of the leading edge angle to the non-uniform flow angle at the U-turn outlet. Furthermore, the different blade angle distribution helps to suppress flow separation at the blade suction side near the shroud and helps to reduce flow friction on the blade surface near the hub.