Unsteady RANS simulation of three-stage centrifugal pumps with different impeller-diffuser gaps

In this study, unsteady RANS simulation is attempted for a three-stage centrifugal pump, a target pump for the workshop "Single- & Multi-Stage Pump Flow Prediction" which is being held in 29th IAHR Symposium on Hydraulic Machinery and Systems. A commercial code SCRYU/Tetra developed by Software Cradle Co. Ltd. is adopted for the unsteady RANS simulation. Our primary interest is to understand the effect of the gap between impeller trailing edge and diffuser leading edge on the axial thrust characteristics of the multi-stage centrifugal pumps. To do so, in addition to the simulation for the original pump model, the impeller-diameter cut model is also simulated. The diffuser inlet to the impeller outlet diameter ratio is 1.05 for the cut model against 1.02 for the original model. The good agreement is obtained for the hydraulic performance of the original model at the design flow rate, but only the fair agreement is obtained for the axial thrust force. From the pressure distributions inside the front and back side gap of the impeller, it is found that the discrepancy is due to that of the pressure distribution inside the back side gap of the second (and also perhaps the first) impeller. The effect of impeller-diffuser gap on the hydraulic performance and axial thrust force is predicted to be small at the design flow rate through the present computations. At the low flow rate, the balancing flow rate is significantly over-predicted by the present simulation. The reason for this remains unclear and will be hopefully made clear in our future study.