Numerical investigation of energy loss mechanism of mixed-flow pump under stall condition

Abstract

In order to further enrich the theory of rotational stall, this paper explored the mechanism of internal energy loss in the mixed-flow pump under stall condition based on the shear stress transport (SST) k-ω turbulence model, identified the vortices in the impeller by the Q-criterion method, and characterized the turbulence intensity by the turbulent kinetic energy (TKE). The results show that the SST turbulence model can well predict the positive slope characteristics of the energy performance curve of a mixed-flow pump, and there are obvious characteristics of rotating stall in impeller. The numerical results of the energy performance curve and internal flow field are in good agreement with the experimental results. Under stall conditions, there exist swirls and vortices at impeller inlet, which makes the flow inception angle increase by 38°. Meanwhile, the tip leakage flow causes certain energy loss in the rim region, but the high loss area caused by the leakage vortex does not coincide with the core region of the leakage vortex. The interaction between stall vortex and the main flow in the flow passage results in the large-scale blockage area, which is the most fatal effect on the head drop of the mixed-flow pump. The backflow appears on the suction surface of the blade outlet, which has a great impact on the main flow and leads to a large hydraulic loss. The typical characteristics of critical stall and deep stall are found. Under the critical stall condition, part of the leakage flow overflows the leading edge of the lower blade and affects the flow of the next passage. While under deep stall condition, the streamlines of the leading edge of stall vortex reaches the inlet of the next stage blade and overflows, causing a lot of energy loss. And the interference effect between tip leakage flow and stall vortex is strengthened. In general, under the deep stall condition, the average turbulent kinetic energy value in the passages is the highest, and the uniformity of flow rate distribution in the four passages is the worst, resulting in the largest energy loss, which is at the lowest head in the saddle area.