Influence of inlet gas volume fraction on the flow characteristics of centrifugal pump under stall condition

Stall is an unstable flow phenomenon of centrifugal pump under low flow conditions. Under stall condition, energy conversion instability, pressure pulsation surge, vibration and noise intensification often occur in centrifugal pumps, it even leaded to fatigue failure of blades in severe cases. The efficient and stable operation of centrifugal pump and its system is affected by stall. And the pump was often in the working condition of gas-liquid two-phase flow, the existence of gas in the working medium caused a great change in the operating characteristics of the centrifugal pump. Especially, the internal flow of the gas-liquid two-phase flow centrifugal pump was more complex under stall condition. The influence of different inlet gas volume fractions (IGVF) value on the flow characteristics of centrifugal pump under stall condition is different. Based on Euler-Euler inhomogeneous flow model and SST turbulence model, the influence of different IGVF value on the flow characteristics of centrifugal pump under stall condition is analyzed from the aspects of external characteristics, internal flow field, gas distribution and flow monitoring. Under pure water condition, based on the performance curve and internal flow characteristics analysis, it was determined that the internal flow field of the impeller appeared alternately stall phenomenon at 0.70Q_d and 0.65Q_d, and the stall degree at 0.65Q_d was more serious, which were two conditions with different stall degrees in the stall development stage. Under both typical stall conditions, as the IGVF value increased, the pump performance decreased, but the decrease amplitude was smaller compared to the design condition and deep stall condition. Gas primarily adhered near the leading edge of the pressure side of the impeller blades, and the adhered area gradually increasing with the increase of IGVF value. The relative velocity angle at the leading edge of the pressure side of the stall channel increased, and the flow direction of liquid phase was close to the suction side as a whole. So, the attack angle near the leading edge of the suction side decreased, the fusion of the separation vortex near the leading edge and the vortex in the middle position of the suction side was inhibited, further stall was inhibited. The stall flow field was disrupted when the IGVF value is 2% at 0.70Q_d, but the stall flow field completely disappeared when the IGVF value is 2% at 0.65Q_d. The circumferential uniformity of the flow field was quantitatively analyzed by introducing the coefficient of variation (CV) of the flow rate between the channels. As the flow field improved, the CV value of the flow rate between channels gradually decreased. When the CV value was greater than 3.69%, the centrifugal pump was in the stall stage, and the stall disappeared when the CV value was less than 1.38 %. When the model pump was at the inception point of stall, the CV value was in the range of 1.38%, 3.69%. The addition of a small amount of gas resulted in a smaller decrease in centrifugal pump performance during stall development and inhibited the occurrence of stall. The paper focused on the influence of IGVF value on the performance and flow characteristics of centrifugal pumps under stall condition, and quantitatively analyzeed the circumferential uniformity of the flow field. It provided a reference for further exploring the performance balance of gas and centrifugal pumps under stall condition and the relationship between stall and cavitation, so as to expanded the high efficiency region of centrifugal pumps and improving safe and stable operation.