Unsteady flow structure and its evolution in a low specific speed centrifugal pump measured by PIV

The intense fluid-dynamic interaction associated with the complex flow structure in the centrifugal pump is detrimental to the stable operation of the pump. Based on the particle image velocimetry (PIV) measuring technique, flow structures in a low specific speed centrifugal pump are measured and presented in detail. Emphasis is laid upon the complex flow structures around the volute tongue region, where intense rotor-stator interaction appears. Results show that the typical jet-wake flow pattern at the blade outlet is observed for flow rates lower than the nominal working condition, and the inflection point of the jet-wake region is found to be at the center of the blade channel. At high flow rate, the jet-wake flow pattern is not apparent due to the high momentum fluid concentrating on the blade suction side. Within the pump, several typical vorticity sheets (both positive and negative) are captured on the blade pressure and suction sides at high flow rates, while at low flow rates, the negative vorticity sheets on the blade suction side disappear due to the separated and reverse flow structures occurring. When the vorticity sheet sheds from the blade trailing edge, it would impinge on the volute tongue with subsequent cutting and distortion. Three typical interacting modes are observed at different flow rates, and the cutting effect of the volute tongue on the vorticity sheet is dependent strongly on the local absolute velocity distribution. At high flow rates, the cutting effect is more significant, and the entire vorticity sheets (positive and negative) would be cut and torn by the volute tongue.