Experimental Study of Two-Phase Air/Water Flow in a Centrifugal Pump Working With a Closed or a Semi-Open Impeller

The characteristics of a transparent centrifugal pump of radial type were investigated for different conditions when conveying two-phase (air/water) flows. A closed impeller and a geometrically similar semi-open impeller, both made out of acrylic glass, were employed for comparison purposes when increasing air loading. The performance of the pump was measured for either a constant gas volume fraction or a constant air flow rate at the pump inlet. Hysteresis effects were studied by considering three different experimental approaches to reach the desired operating conditions. A constant rotational speed of 650 rpm was set for all experiments. The whole system was made of transparent acrylic glass to allow high-quality flow visualization. A systematic experimental database was produced based on shadowgraphy imaging, so that the resulting two-phase regimes could be properly identified. The results show that for gas volume fractions between 1 and 3%, the deterioration of pump performance parameters is much lower in the semi-open impeller compared to that of the closed impeller. Nevertheless, in the gas volume fraction range between 4 and 6%, the trend is reversed; the semi-open impeller performance is reduced compared to the closed impeller, particularly in overload conditions. At even higher gas loading, the semi-open impeller shows again superior performance. Flow instabilities and pump surging were much stronger in the closed impeller. The main reason for that was the occurrence of alternating gas pockets on the blades of the closed impeller. Additionally, pump surging was observed only in a very limited range of flow conditions in the semi-open impeller. Comparing the different experimental procedures to set the desired flow conditions, no significant hysteresis effects could be observed in the closed impeller. However, in the semi-open impeller obvious hysteresis in the performance could be seen for gas volume fractions between 4 and 6%. All the obtained experimental results will be useful to check and validate computational models used for CFD in a comparison study.