Experimental study on gas–liquid flow patterns and bubble size in a high-speed rotating impeller of a three-stage centrifugal pump

Head degradation and performance failure will occur when centrifugal pumps work under high gas volume fraction. The gas–liquid two-phase flow patterns and bubble size in a rotating impeller were experimentally studied by high-speed photography technology in a three-stage centrifugal pump. A new impeller was designed with double rings and a transparent shroud, overcoming the speed limitation below 1500 rpm in current studies and reaching the maximum speed of 3500 rpm. Four gas–liquid flow patterns are observed, which are discrete bubble flow, bubble flow, gas pocket flow and stratified flow. The result shows that the transition from bubble flow to gas pocket flow is the critical condition for the severe head degradation. Reducing inlet gas volume fraction, increasing rotational speed and reducing liquid flow rate lower than design value cause the transition from gas pocket flow to bubble flow and the delay of head degradation. By image processing technique, the probability density function (PDF) of bubble diameter in impeller is found to follow a lognormal distribution. Moreover, bubble Sauter diameter dSa has a linear relationship with inlet gas volume fraction λ, rotational speed n−0.75 and liquid flow rate Qw−0.6, respectively. A new correlation is established for predicting bubble Sauter diameter in impeller under high rotational speeds (1500–3500 rpm). The strong relationship between flow pattern transition and head degradation is revealed. Based on the same head performances of three booster stages in pump, the conclusions driven from the visual impeller in the first stage can also be extended to other stages.