Quantitative analysis of tip clearance effects on two-phase performance: Flow structure and surge characteristics in a centrifugal pump

When a centrifugal pump is used to transport gas-liquid two-phase flow, the surge at high gas volume fraction leads to head and efficiency decrease, as well as cause strong vibrations in the pump system. To improve the transport performance of centrifugal pumps, the closed impeller is redesigned into a semi-open impeller. The effect of the blade tip clearance size on the head and efficiency is explored. The similarities and differences in the internal gas distribution patterns between closed and semi-open impellers are analyzed. The suppression mechanism of the tip leakage flow on surge is clarified. At 5% inlet gas volume fraction, the closed impeller exhibits significant gas accumulation characteristics on the pressure surface, forming gas pocket flow patterns and gas vortex. Additionally, some gas bypasses the blade leading edge, forming gas leading edge overflow, and re-accumulates at adjacent blades, further exacerbating the gas pocket flow patterns. This results in high gas phase distribution zones and low liquid phase velocity zones, inducing surge. There is a proper tip clearance, which provides an additional flow path for the gas in the flow passage. The liquid and gas mixed tip leakage flow impacts and suppresses the gas accumulation. The gas pocket flow pattern and gas vortex are disrupted. The liquid and gas mixed fluids can smoothly flow out of the impeller. The surge is suppressed and the performance of gas-liquid mixed transport is significantly improved. The optimal tip clearance size of 1 mm increased the head and efficiency by 16.2% and 13.1% respectively under surge condition.