Investigations On The Numerical Simulation Of Air-water Two Phase Flow And The Optimization Of Ventilation System Of Long Spillway Tunnel With High Energy Head

The ventilation problem of a spillway tunnel,that unexpected high air velocity is usually detected in air duct,has always been a concern for engineers.The main reason for the unexpected high air velocity is that the air demand of a spillway tunnel is usually underestimated by previous empirical formulas,which cause unreasonable design of the air duct.Therefore,this paper focous on numerical calcultion for the vetilation system of a spillway tunnel,including the one-dimensional model and the three-dimensional numerical simulation.More accurate prediction of air demand for spillway tunnel is realized in this paper based on the numerical method.Futhermore,the caracteristic of ventilation system of the long spillway tunnel with high energy head is systematically analyzed,and the optimal design method of ventilation structure is proposed.The contents of this paper are summarized as follows:(1)A theoretical 1-D model for the air-water stratified flow in a spillway tunnel is proposed.The conservation equations of air and water are established separately and coupled by a new interfacial shear stress formulation.The proposed model is verified by using prototype data and numerical CFD results and proved to give better prediction of air demand for the spillway tunnel of Jinping-I hydraulic project in China.In additions,we conclude that the model proposed in this paper can give more accurate results than the previous empirical formulas for air demand and is more convenient than 3-D numerical CFD model for engineering designers.(2)The reliability of the 3-D numerical simulation result is systematically verified.An improved hybrid drag model is proposed based on the comparison of results from different drag model,which effectively improves the accuracy of three-dimensional numerical simulation of the spillway tunnel ventilation system.Although the 3-D numerical simulation based on the multiphase theory need more calculation time than the 1-D theoretical model,it can give us complete information of the air-water two phase flow field,which makes it irreplaceable.In this paper,the basic equation and its interphase interaction terms of the Euler-Euler two phase model is discussed in detail.Taking the spillway tunnel of Jingping-I project as an example,the reliability of the results of 3-D numerical simulation is verified in the aspects of grid convergence,time dependency of the transient calculation,flow regime and the air velocity.The preliminary calculation results show that the interphase drag model is the very factor that greatly affects the prediction of the air demand.Therefore,the flow field calculated from different drag model is compared and discussed in details.The reason why different drag models are appropriate for different gate opening condition of the spillway tunnel can be explained by the variations of the flow characteristic.Finally,an improved hybrid drag model is proposed and proved to predict the total air demand of the Jingping-I spillway tunnel in an accuracy of 20%.After the results of the 3-D numerical simulation is verified,the vector field of the air velocity is analyzed.The characteristic difference between different conditions is discussed.Two flow modes of air velocity field are summarized.(3)A combined optimization design method of the spillway tunnel and its ventilation system is proposed based on the equilibrium relationship between air ducts and the free space above water flow.The verified calculation methods involved in this paper are applied to analyze the flow velocity,air pressure and water level in the tunnel of Jingping-I project.In order to reduce the unfavorable high velocity of air in the air ducts,the model is also applied to optimize the design of air ducts of Jinping-I project.The theoretical calculations show that if the No.1 air duct is expended into8.38 m in diameter,the unfavorable high velocity can be reduced to an acceptable value even though the No.3 air duct is not considered.Finally,the theoretical model is also applied to analyze the influences of the structural factors to the ventilation characteristics of a spillway tunnel.