Off-design performance prediction for diffuser pumps

This paper presents a numerical methodology developed for predicting off-design performance of diffuser pumps. In off-design conditions, the effects of rotor/stator interaction and the pump system characteristics become significant, and both need to be taken into account. The pump computational fluid dynamics (CFD) model and the system characteristic equation were incorporated to allow for flow dynamics in the duct. Flow analysis was applied to an entire circumference to capture the asymmetric flow feature at part-flow caused by rotating stall. An arbitrary sliding interface was exploited to consider the influence of rotor/stator interaction. The methodology was applied to a diffuser pump stage which includes a suction pipe, impeller, diffuser, U-bend, return channel and discharge pipe. Predicted results of the head-flow curve, pressure field and impeller radial forces were validated by the experimental data over the entire flow range. Back flow in the shroud region at small flowrates (from 0 to 40 per cent of design flow) was realistically exhibited. Flow in the region of 81 per cent of design flow displayed unstable features being caused by the occurrence of rotating stall in the impeller, such unsteady phenomena manifest themselves as a drop in the pump head rise, and larger fluctuations in impeller inlet pressure and radial forces. The present study has demonstrated that incorporation of the pump system characteristic equation and the pump CFD model produces a good prediction of pump off-design performance. The developed methodology can provide an alternative to dealing with pump off-design performance.