The Numerical Simulation Of Aeration Flow In Pre-Aerator Stepped Spillways

With the development of numerical simulation of gas-liquid two-phase flow field,the advantages of solving high-speed hydraulics problems caused by high head and large flow rate become more and more obvious in engineering.Because the numerical simulation of gas-liquid two-phase interface flow and mixing phenomenon is still a difficult point in engineering research,many related problems need to be solved.In this paper,aiming at the numerical simulation of water and gas two-phase flow,the formation mechanism of aeration on free water surface and the physical structure of two-phase flow in free water surface are studied from the mathematical model of two-phase flow which is suitable for numerical solution.Combined with the medium spillway of a large hydropower station,Based on the Reynolds average N-S equation,the finite volume method is used to discretize the governing equation,Selected the Air entrainmnt model,the Multiphase flow model and the Turbulent model(RNG k-?)to simulated.The GMRES algorithm is used to solve the pressure problem.The two phase flow of high speed water,the gas on the discharge wall of pre-aerator stepped spillways is simulated under different experimental conditions,and the distribution law of hydraulic elements is summed up,which provides the basis for the selection of optimal shape of aerator in the futre.It can also calculate the hydraulic factors of water conservancy projects which are difficult to pass in other physical tests and provide certain technical support for engineering design and application.In this paper,the aeration flow is numerically simulated under 8 different working conditions,such as different flow rate,gradient of aeration sill,high aeration level of aerated sluice,in pre-aerator stepped spillways and traditional stepped spillway.The results show that the distribution trend of flow patterns and hydraulic factors are the same.Under the condition of other factors,the air concentration and pressure distribution at the first few steps of the water cover are fluctuated under the shape of different aerated sill.With the increase of aeration slope,the aeration cavity,the maximum negative pressure at the front steps and the aeration concentration increase,but the difference in the latter half is small.With the increase of aeration height,the maximum negative pressure decreases and the aeration cavity and aeration concentration increase.From the point of view of steady velocity,the efficiency efficiency of the front aerated stepped spillway with different shapes is considerable.With the increase of the flow rate,the aeration concentration and energy dissipation rate on the step surface decrease,and the maximum negative pressure value increases.However,the pre-aerated stepped spillway can meet the requirements of reducing cavitation damage and increasing energy dissipation rate in the whole stepped surface under the maximum single width flow rate.The simulation results of the traditional stepped spillway show that under the condition of large single wide flow rate,it is difficult to ventilate on the first steps of the stepped spillway,the aeration concentration is low,the negative pressure is too large,and the cavitation erosion may occur.The addition of the front aerator can not only increase the aeration concentration of the first several steps of the spillway,but also make the water flow reach the water vapor balance ahead of time,so as to relieve the negative pressure of the step level and the vertical near wall,nor reduce the energy dissipation rate of the stepped spillway.A new way of thinking is provided for solving these problems because of pre-aerated stepped spillway.After comprehensive analysis,when the flow rate is 76m2/s,the slope of pre-aerated is 1:10,and the height is 0.7m is the optimal scheme of the aerator connected the WES surface and the stepped spillway.