Rotor-stator interaction in radial flow pumps and fans at shut-off conditions
The prediction of shut-off head within a centrifugal pump or fan has historically relied on the use of a combination of empirical formulae, together with experience. This reliance has been forced on designers due to both a lack of information regarding the flow at shut-off and the available tools with which to gain more accurate insights. To improve understanding the following investigation examines the capability of the commercial CFD package, FLUENT, to model the flow in a centrifugal pump at shut-off conditions. The computational model was validated using experimental measurements from a purpose built two-dimensional centrifugal pump rig. The rig used air as the working fluid and was similar in design to that used by Miner. Measurements were made of both the fluctuating velocities, using LDA, and the fluctuating pressures, using microphones, within the volute of the pump. The CFD model uses a sliding mesh which enables the full time-dependent rotor/stator interaction of the pump to be modelled. The results show the volute flow contains two patterns, a recirculating eddy in the outlet duct and a volute flow circulating around the rotor. This volute flow separates partway around the volute, with re-attachment on the discharge side of the tongue. The major effect of the volute at shut-off is to act as a diffuser with a strong circumferential pressure gradient over approximately the first 1200 after the tongue. A comparison of the experimental and computational results showed that good qualitative agreement was obtained at most positions at shut-off and considerable insight was gained into the flow mechanisms. However, the results showed that the CFD model over predicted the measured shut-off head by 25%. This was attributed to an over prediction of the effective viscosity due to the use of the k-s turbulence model.