Effects of splitter blade on vaned-diffuser stall and the corresponding warning method

The transient characteristic of the disturbance from impeller, which is largely influenced by the stagger angle of IGV (inlet guide vane), significantly determines the performance of the vaned-diffuser downstream. Thus, the understanding of these phenomena is critical to avoid the degradation of diffuser performance including the dreadful rotating stall. The paper adopts both the experimental and numerical methods to investigate the significant change of the diffuser transient characteristics under large pre-swirl effects. The whole process of diffuser rotating stall, which origins from the vortex at rear of diffuser, is comprehensively studied. It is noticed that the variation of pre-stall behavior within vaned-diffuser closely relates to the pre-swirl angle, which is mainly reflected by the characteristic change of separated vortex from diffuser blade surface. Under large negative pre-swirl condition, the flow non-uniformity between main and splitter blades of impeller significantly grows. It applies more influences on the transformation of vortex from rear to semi-vaneless region of diffuser and triggers the final stall precursor much earlier. Besides, a novel stall warning method based on the non-uniformity coefficient C has been developed and the prediction errors are controlled lower than 2 % in most cases.