Blockage and distortion effects on a vane island type diffuser performance of centrifugal pumps for various flow rates

International audienceExperimental investigation of mean flow velocity and pressure inside the vaneisland type diffuser passage of a laboratory centrifugal pump model is presented. A directional threehole probe was used to investigate hubtoshroud flow properties at diffuser leading and trailing edge planes. In addition, the midsection between suction and pressure sides along the diffuser passage was also studied. All these measurements were performed at five different flow conditions. The flow structures at the inlet section of the diffuser and along with the passage were analyzed in detail, with a focus on the flow distribution and the pressure recovery evolution of different diffuser parts. Blockage and distortion effects were analyzed by using existing published particle image velocimetry (PIV) measurements in the outer part of the impeller and around the diffuser throat in order to get a better understanding of the overall performances and local flow structures of such a diffuser design. The results show that back flows occur on the shroud region from impeller exit plane up to the semivane less part of the diffuser at low flow rates. Aerodynamic blockage effects combining with blade thickness geometrical blockage tend to diminish the effects of theoretical high positive incidence. For high flow rates, negative incidence effects induce high local velocity on the pressure side of the diffuser just after the throat. Existing aerodynamic blockage associated with low flow distortions are found to have a beneficial effect on pressure recovery for low flow rates. On the contrary, high flow distortions are found for increased flow rates, resulting in a low pressure recovery inside the semivane less part of the diffuser and after its throat