Effects of Flow Coefficient on Unsteady Impeller Loading

Abstract

Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulation has been conducted to investigate how flow coefficient affects unsteady impeller loading. Simulations have been carried out at three flow coefficients - near stall, design, and near choke conditions - for two radial gaps (1.04 and 1.10). For computational efficiency, the unsteady simulation has been conducted for two impeller and diffuser passages via Fourier Transformation method. Both steady and unsteady simulations have been validated against experimental data. For the radial gap of 1.04, the unsteady loading (the difference between the maximum and minimum loadings) is the largest at the near stall condition

second largest at the near choke condition

and smallest at the design condition. Flow coefficient effects on the unsteady impeller loading are mostly due to the variations in pressure fluctuations on the pressure side of the impeller blade. Relative to the design condition, the near stall condition shows lower minimum loading and the near choke condition shows higher maximum loading. Therefore, both off design conditions result in higher unsteady loading than the design condition. Such differences stem from the variations in the pitch-wise static pressure at the diffuser vane inlet caused by the flow incidence onto the diffuser vanes. For the radial gap of 1.10, unsteady impeller loading decreases at three flow coefficients compared to that of the radial gap of 1.04. In addition, flow coefficient has a negligible effect on the unsteady impeller loading. Such independence stems from the attenuated potential effect of the diffuser vane with the radial gap increase.