Study On Water Air Two Phase Flow Under Annular Aerator

Large-scale hydropower development has become an important way to solve the problems of unbalanced distribution of water resources and energy shortage in China.Since the 21 st century,China's hydropower industry has developed vigorously and many hydraulic structures with high water head and large flow have been built.However,the problems of high-speed water flow such as cavitation,atomization and bank slope scouring have become more and more serious.Aeration technology is one of the important means to solve the problem that the overflow surface of water release structures is not damaged by cavitation erosion.More than half a century of research and practice has led to great development of aeration technology,from bottom aeration to three-dimensional aeration,from plane(inclined surface)overwater surface aeration to vertical full-section aeration of shaft,which has solved engineering problems well.Annular aeration sill is a new kind of aeration technology set in flood discharge shaft and has been successfully applied in horizontal swirl flood discharge tunnel of Gongboxia gorge.However,due to its shape determination based on a large number of model tests,it is found in prototype observation that the wind speed of annular aeration hole is too high and the scale effect is obvious,so the understanding of this new aeration facility is obviously lagged.In this paper,the suitability of different mathematical models for hydraulic parameters of water flow in flood discharge shaft with ring-shaped aeration sill,characteristics of water-gas two-phase flow in the lower part of ring-shaped aeration sill and excessive wind speed of ring-shaped air vent caused by scale effect are studied by combining numerical simulation with model test and prototype observation results.The main research results are as follows:(1)Different mathematical models have different suitability for simulating hydraulic parameters of water flow in flood discharge shaft with annular aeration tank.The specific manifestations are: local-VOF model is suitable for simulating aeration cavity;local-mixing model is suitable for simulating water-air mixing zone;and overall model is suitable for simulating ventilation volume.(2)When the upstream water level is below 2002 m,the discharge relationship of the flood discharge tunnel is typical weir flow relationship.When the upstream water level is above 2002 m,the flow is controlled by the section of annular aeration sill.Near the cavity area,the water flow is turbulent,the water-air mixing is sufficient,the doping concentration is high,the flow velocity,turbulent energy and turbulent dissipation ratio are high,and the pressure is low.With the decrease of elevation,the longitudinal turbulent diffusion of water flow decreases,the water-air mixing degree decreases,and the doping concentration decreases rapidly.Under the influence of swirl chamber top support and gravity,the water flow velocity decreases,pressure intensity increases,turbulent energy and turbulence increase.The dynamic dissipation rate decreases.The ventilation volume of annular vent is affected by discharge flow.With the increase of flow rate,the ability of water flow to carry gas is strengthened.The negative pressure of annular chamber is stronger,the ventilation volume is larger,and the more aeration is added in the shaft section.After the gate is fully opened,the discharge flow remains stable,the wind speed of annular vent is basically stable and the ventilation volume is relatively balanced.The energy dissipation rate of the shaft section is between 21.36% and 28.87%,accounting for about 1/4 of the total energy dissipation rate,which has the effect of energy dissipation by sections and greatly reduces the energy dissipation pressure of the flood discharge tunnel.(3)Comparing the three volumetric ventilation volumes and single-hole wind speed of 0.8m,1.0m and 1.2m annular vent diameters,it is found that the single-hole wind speed meets the specifications when the annular vent diameter is 1.0m.In this type,the water level rises in the shaft,the length of cavity is shortened to 2.5m,the flow velocity decreases and the pressure increases;the attenuation speed of aeration concentration decreases with the decrease of elevation,and the cavitation number of water flow on the shaft wall is greater than the critical value,which can effectively avoid the shaft wall being damaged by cavitation.Therefore,it is advisable to use1.0m diameter of air vent for annular aerator.