Influence of flow coefficient on unsteady impeller loading induced by impeller-diffuser interaction

Unsteady Reynolds-averaged Navier-Stokes (URANS) simulation has been conducted to investigate how the flow coefficient affects unsteady impeller loading. Simulations have been carried out at three flow coefficients — near stall, design, and near choke conditions — for a centrifugal compressor with a radial gap of 1.04. For computational efficiency, the unsteady simulation has been conducted for two impeller and diffuser passages via the Fourier transformation method. Unsteady loading is the largest at the near stall condition; second largest at the near choke condition; and smallest at the design condition. Relative to the design condition, the near stall condition shows lower minimum loading, and the near choke condition shows higher maximum loading. Thus, both off-design conditions result in higher unsteady loading than at the design condition. Such increases at off-design conditions stem from the variations in the pitch-wise static pressure at the diffuser vane inlet caused by the diffuser vane incidence.