Numerical investigation of a high-speed centrifugal compressor with hub vane diffusers

Abstract

Hub vane diffusers can obtain higher pressure recovery and efficiency compared with the vaneless diffuser and a wider operating range compared with the vane diffuser. To improve the performance of high pressure ratio centrifugal compressors it is necessary to understand the flow phenomena in the hub vane diffusers. Although some research work on hub vane diffuser flow has been published, many unknown flow physics and disputed questions still remain. A computational analysis of the flowfield in a high-speed centrifugal compressor with a backswept unshrouded impeller and wedge vane channel diffuser is presented. In order to obtain the optimum hub vane height, the pressure and velocity fields were numerically simulated for eight kinds of hub vane, which are varied as follows: h/b = 0 (vaneless), 0.2, 0.3, 0.4, 0.5, 0.6, 0.8 and 1 (vane). The advanced Navier-Stokes solver EURANUS/TURBO is applied with a Baldwin-Lomax model for closure. The calculations are performed with a central space discretization scheme, with second- and fourth-order artificial dissipation terms. The numerical procedure applied a four-stage explicit Runge-Kutta scheme, coupled to local time stepping. The good agreement of the computed stage performance and the experimental data of the stage with the vane diffuser shows that the solver is effective and reasonable. Numerical results show that the maximum flowrate attained by the stage with the vane diffuser is about 12.5 per cent less than with the hub vane diffusers. The stage efficiency reaches its maximum value for the hub vane diffuser with an h/b ratio of 0.5 at lower flowrate and for the hub vane diffuser with an h/b ratio of 0.4 at higher flowrate. The hub vane diffusers have a wider operating range than the vane diffuser. The hub vane diffuser with an h/b ratio of 0.3 shows a higher mass-averaged static pressure coefficient up to a flow coefficient φ = 0.16. The hub vane diffuser with an h/b ratio of 0.4 shows a slightly higher mass-averaged static pressure coefficient over an operating range of φ = 0.18–0.28, but it falls beyond φ = 0.3 below that of the vaneless diffuser. The performance of the centrifugal compressor with the shroud vane diffuser is better than that with the hub vane diffuser for the same height of the vane.