Experimental investigation on the gas-liquid flow patterns in a centrifugal pump during self-priming process

Self-priming pumps are widely used for irrigation, water treatments, and industrial areas due to the self-priming capacity. As we know, the gas-liquid flow pattern during the self-priming process is very complex and still not fully understood. In this study, a visualization test was performed to reveal gas-liquid two-phase flow patterns in a transparent pump during the self-priming process by using high-speed photography. The dynamic change of water column in the inlet-pipe and outlet-pipe was recorded by a digital camera, and the internal flow states of the test pump were recorded by a high-speed camera under different positions and different frame rates. First, the transient gas-liquid flow patterns during the self-priming process were revealed. In the initial stage, a large amount of gas was sucked into the impeller and the water level rose fast in the inlet pipe and then kept steady. With the increase in volume of gas in the impeller, plenty of bubbles were generated rapidly because of the mixing of gas and liquid inside the impeller. Some small bubbles were transported to the gas-liquid separation chamber by the centrifugal force, while rest of bubbles still circulated together with the impeller. In addition, the light gas was discharged to the outlet pipe and the heavy water was reserved inside the chamber due to the effect of gravity force. Afterwards, part of the water and gas flowed back to the impeller through the reflux hole. Second, the dynamic evolution of bubble formation was captured. Many bubbles appeared as the gas pocket was crushed by the blade surface or the jet flow. In general, the self-priming capacity was determined by the mixing rate of gas and liquid, the transporting rate of a gas-liquid mixture from the volute to the gas-liquid separation chamber, and the separating rate of gas from liquid.