Abstract
Flow separation is undesirable and lowers the efficiency of centrifugal impellers. In this study, the evolution characteristics of separated vortices in a centrifugal impeller are studied under the off-designed flow rate condition. Unsteady Reynolds-Averaged Navier–Stokes (URANS) with standard k-ε turbulent model is applied to simulate the alternating stall in the six-blade centrifugal impeller. We present and analyze the distributions of pressure gradient (either adverse or favorable) and skin friction coefficients on both sides of the blade for the stalled and unstalled passages to study the relationship between pressure gradient and separation of boundary layer flow. The evolution of skin friction coefficient is also presented at various axial cross sections. Numerical results reveal that, for the stalled passage, the increase in adverse pressure gradient on the pressure surface near the middle of the blade (S/S0 = 0.4) is much larger than that of the suction surface during a vortex formation cycle. The skin friction coefficient on the pressure surface also increases in magnitude sharply and the variation shows a peak-valley trend, while the coefficient on the suction surface increases slowly. Comparing the distribution of skin friction coefficient on the pressure surface of the same blade at different axial cross sections, it is found that the skin friction coefficient notably increases at S/S0 = 0.6 on the middle axial cross section (Z/b2 = 0.5). For the unstalled passage, both the pressure and suction surfaces produce favorable pressure gradients. The skin friction coefficient on the pressure surface shows an increasing trend around S/S0 = 0.5, and a large vortex can be seen at the exit of the impeller. The variation of skin friction coefficient on the suction surface is relatively mild; thus, the flow is relatively stable. It is clarified that the effect of adverse pressure gradient and wall shear stress jointly cause separation of the boundary layer; thus, the separated vortices are generated in the rotating impeller and deteriorate the performance of the impeller.
Highlights
Separated flow is a complex phenomenon in fluid mechanics
The experimental results of Byskov et al [4] concluded that, for the centrifugal impeller operating under a quarter flow rate Q/Qn = 0.25, the flow is unstable and stall cells emerge in the passages
For the stalled passage I, a large vortex appears on the suction surface of the leading edge of the
Summary
Separated flow is a complex phenomenon in fluid mechanics. It is widely encountered in flow around various geometries such as bluff cylinders and hydrofoils and in turbo-machineries [1]. The physics of unsteady flow in the impeller and volute of centrifugal turbo-machineries have and turbulent statistics, were presented and analyzed to reveal their influences on the performance been extensively studied, as reviewed above. The flow characteristics, such as the evolution of of the impeller. We understand that the flow in a rotating centrifugal impeller experiences the effect vortices and turbulent statistics, were presented and analyzed to reveal their influences on the of pressure gradient on the surface of the blade passage, which is essential in triggering the initial performance of the impeller.
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