The Effect of Tip Leakage Flow on Part-Load Performance of a Mixed-Flow Pump Impeller

Abstract

The flow phenomena around the positive slope region of the head-flow characteristic were investigated experimentally on a mixed-flow pump impeller at various tip clearances for both shrouded and unshrouded cases, A positively sloped head-flow characteristic (abrupt decrease in pressure head) was caused by the onset of extensive flow separation in the impeller at the casing-suction surface corner. The corner separation in unshrouded cases appeared at a much lo werflow rate than the shrouded case due to the favorable effect of the tip leakage flow, which displaced the wake region away from the corner. The interaction between the tip leakage flows and secondary flows and the formation of the wake regions in shrouded and unshrouded cases were explained based on experimental observation and computations by the Dawes three-dimensional Navier-Stokes code. In the shrouded case, the flow rate at which an abrupt decrease in pressure head appeared was lowered substantially by introducing a leakage flow through a slit made between the shroud and the blade tip. Inlet recirculation was triggered by the corner separation and developed more gradually for larger tip clearances. Both the increased loss, due to the extensive flow separation, and the decreased Euler's head, due to the abrupt change in flow pattern caused by the inlet recirculation, were responsible for the generation of positively sloped head-flow characteristic in the unshrouded case when the tip clearance was small, while the increased loss was the primary factor in the shrouded case. IntroductionMixed-flow pumps often show an abrupt decrease in pressure head and positively sloped head-flow characteristic when the flow rate is reduced below a critical value. In the present paper, this condition when the slope becomes positive will be referred to as "stall onset." Since the occurrence of a positively sloped characteristic is more likely to cause instabilities in a pumping system, it is usually a contractual requirement to guarantee a substantial margin between operating and stall points or to achieve a head-flow curve with a monotonously negative slope. Despite a lot of experimental investigations (see review by Engeda and Rautenberg, 1988), the flow phenomena under part-load operating condition and the generation mechanism of positively sloped head-flow characteristic are not well understood.The performance difference between shrouded impellers (most commonly used for multistage configurations) and unshrouded impellers at various tip clearances is a matter of great interest, because they often show quite different characteristics, especially at partial flow rates. With regard to centrifugal impellers, Lenneman and Howard (1970), Kittmer (1975), Harada (1985), and Hamkins and Flack (1987) carried

A Review of Tip Clearance in Propeller, Pump and Turbine

“…Mixed-flow pump [7] Head drops by about 8% at design flow rate; tip-leakage reverse flow becomes stronger, and further thickens the casing boundary layer and intensifies vortexes in flow passage.…”

Influence of Tip Clearance on Pressure Fluctuation in Low Specific Speed Mixed-Flow Pump Passage

“…In an analysis of a shrouded mixed-flow pump with different tip clearances by Goto [15], tip-leakage flow displaced wake flow and increased the stall margin, however, the strong tip-leakage reverse flow thickened the casing of the boundary layer and developed a passage vortex. In Li et al's analysis [16] of the influence of tip clearance on pressure fluctuation in a mixed-flow pump, a large eddy simulation of pressure fluctuation at the shroud of the pump under small flow rate condition was performed, finding that the pressure fluctuation average near the wall region of the impeller decreases gradually as tip clearance gradually increases.…”

“…For tip-leakage flow, a study on the dynamic characteristics of rotating stall by Li et al [14] showed that vortexes were generated between the tip-leakage through clearance from the pressure surface to the suction surface and the mainstream, and flow separation occurs in the suction surface if flow rate decreases. In an analysis of a shrouded mixed-flow pump with different tip clearances by Goto [15], tip-leakage flow displaced wake flow and increased the stall margin, however, the strong tip-leakage reverse flow thickened the casing of the boundary layer and developed a passage vortex. In Li et al's analysis [16] of the influence of tip clearance on pressure fluctuation in a mixed-flow pump, a large eddy simulation of pressure fluctuation at the shroud of the pump under small flow rate condition was performed, finding that the pressure fluctuation average near the wall region of the impeller decreases gradually as tip clearance gradually increases.…”

Effects of a Nonuniform Tip Clearance Profile on the Performance and Flow Field in a Centrifugal Compressor

“…The influence of the tip clearance flow on the performance and flow structure in centrifugal compressors has been studied by many researchers [4][5][6][7][8][9][10][11][12][13][14]. In previous studies, it was found that the swirling flows and vortex motions within unshrouded impellers are sensitive to the tip clearance flow.…”