Experimental Study on Cavitation in Centrifugal Pump Impellers

Investigations concerning cavitation in several impellers with three dimensional blades were carried out using a 200 mm bore single suction pump installed in a closed circuit system. Flow inside the impeller was observed with strobo-light, and the NPSH at the condition of cavitation incipience measured for each impeller was found to show a peculiar variation against capacity taking a minimum at or near the designed flow rate and a peak at a partial capacity. The flow at the impeller inlet was measured with a Pitot tube and it was clarified that the flow pattern which accompanied remarkable swirling and back flow at partial capacities was responsible for the above NPSH variation. The cavitation aspects in the impellers are presented by photographs and their correlations with the various local pressure drops are discussed. The impellers were compared in terms of pressure drop coefficients as well as the suction specific speed plotted versus capacities with a view to applying them to the practical fields.