Numerical prediction of the effects of free surface vortex on internal flow and energy dissipation characteristics in a bulb tubular pump

Bulb tubular pumps (BTPs) operating under low water levels are prone to free surface vortices (FSVs), which can induce unstable flow and seriously threaten the safe operation of pumps. In this paper, a gas–liquid two-phase flow method was adopted to numerically predict the FSV at the pump water intake, and the influence of FSV on the internal flow and energy performance in a BTP device at different submergence depths were investigated. The results indicate that FSV intensity increases as the submergence depth decreases. When the submergence depth is reduced to Hs = 2.14D, FSV evolves into a continuous air-entraining vortex. Compared to the condition without the air-entraining vortex, the unit efficiency decreases by 3.63% and the velocity distribution uniformity at the intake is reduced by 3.34%. The entrained air within the impeller primarily develops along the wall surface with a relatively low volume fraction, and internal flow and energy characteristics of the impeller are not significantly affected. In contrast, the air-entrainment vortex within the guide vane obstructs fluid motion, exacerbating the development of unfavorable flow patterns and leading to increased irreversible energy loss. Furthermore, the entropy production of the bulb and outlet passage varies the most when there is an air-entrainment vortex or not.