Study On Dynamic Process And Mechanism Of Air-entraining Vortices In Pump Sump

Pump sump is an important part of small and medium-sized pump stations,and its purpose is mainly to provide a good inlet condition for pumps.However,due to the unreasonable design of geometrical dimensions and changes in operating conditions,vortices always appear in the pump sump.The appearance of vortices can lead to the flow deterioration in the inlet of the pump,which can affect the hydraulic performance of the pump.As a kind of vortex,the free-surface air-entraining vortex can draw air into the pump resulting in obvious reduction of the efficiency of the pump.The free-surface air-entraining vortex in the pump sump is a main factor affecting the efficiency and safe operation of the pumping station.As a result,it is important to study the dynamics of the free-surface air-entraining vortex.In this study,the dynamic process and characteristics of the free-surface air-entraining vortex are analyzed by numerical simulations and experiments.The mechanism of the air-entrainment and its suppression are revealed.The main contents are as follows:1.The three-dimensional flow in the pump sump was calculated.The applicability of different turbulence models was analyzed.The dynamic process of the air-entraining vortex was simulated.The factors affecting the air-entrainment were analyzed.The mechanism of the formation of the air-entrainment and its suppression were revealed.The results show that vortices predicted by SST k-? turbulence model are in good agreement with the experimental data.The dynamic process of the air-entrainment is predicted well by adopting the VOF multiphase model and SST k-? turbulence model.Four stages,preparation,initiation,development,and completion,are acquired.The vortex strength and pressure difference are two main factors for the air-entraining vortex.As they reach threshold values,the air-entraining vortex appears.In the preparation and initiation stages,a helicity density tube appears around the entrance of the suction pipe and under the free surface,respectively.Then,the two tubes connect with each other.At last,a new helicity tube is formed connecting the free surface and the entrance of the suction pipe.This means the free-surface air-entraining vortex is formed.Moreover,the air-entrainment disappears by placing the circular plate between the free surface and the entrance of the suction pipe.2.The air-entraining vortex in pump sump was measured by experiments.The dynamic process of the air-entraining vortex was observed.The critical submergence,air-entrainment frequency and positions were measured.The effect of a circulate plate on the air-entrainment was studied.The measured data show that fours stages of the air-entrainment can be observed.As the Froude number decreases,the value of the critical submergence decreases linearly.When the Froude number is below the threshold,the air entrainment disappears.The air-entrainment frequency decreases as the submergence grows.When the Froude number decreases,the air-entrainment frequency also declines.Positions of air-entraining vortices are random on the free surface,but there are still relatively concentrated areas.Air-entraining vortices approach the suction pipe as the submergence decreases.