Study On Calculation Model Of Discharging And Energy Dissipation Of Baffle-Drop Shaft In Urban Deep Tunnel Drainage System

With the acceleration of urbanization and global warming,the problems of frequent waterlogging,serious overflow pollution and inadequate capacity of urban drainage system have become increasingly prominent.It is difficult to transform the shallow drainage pipe network in China because of the density of urban buildings and the large population.Therefore,the deep tunnel drainage technology with the construction concept of sponge city comes into being,which has been widely concerned at home and abroad,and has been applied in many cities.As an important part of deep tunnel drainage system,the drop shaft plays an important role in transporting rain and sewage from shallow drainage pipe network to deep tunnel and dissipating fluid energy in the process.Unreasonable structural design easily leads to poor hydraulic conditions,and then causes structural damage of shafts and deep tunnels,ground uplift and even explosion accidents,which seriously threatens the development of cities and the safety of people's lives and property.Therefore,it is of great practical significance to carry out a study on the energy dissipation problem of the baffle-drop shafts commonly used in the drainage system of deep tunnel in the city,to improve the design theory of deep tunnel drainage system,and to reasonably formulate the design,operation and maintenance method of the shaft of deep tunnel drainage system in line with the needs of current social and economic development.On the basis of studying and summarizing the deep tunnel engineering at home and abroad,this paper deeply studies the hydraulic characteristics and energy dissipation of deep tunnel baffled-drop shaft through the combination of hydraulic model test and numerical simulation.The main research results are summarized as follows according to the chapter arrangement:(1)In the process of discharge,with the decrease of inflow,a"cavity area"will first appear in the middle of the shaft,and then the cavity area will gradually expand upstream and downstream,respectively.According to the drop distance and shape of the fluid on the baffle,it can be divided into three flow regimes:wall-impact confined flow,critical flow and free-fall flow.(2)The energy dissipation mechanism of fluids in baffled-drop shaft mainly includes five aspects:the collision of fluids with the side wall causes the fluid to break up and disperse;the impact of fluids with water cushion forms the energy dissipation by hydraulic jump;the mixing of gas and liquid forms a local strong turbulent pattern with the energy loss of turbulence;the shape resistance and friction resistance of eddies are accompanied by the energy loss in the votex region;friction resistance between fluid and wall causes energy loss.(3)The variation rule of the velocity in the shaft is roughly as follows:with the decrease of the inflow,the velocity distribution gradually changes from the state of big at both ends and small in the middle to the state of gradual decrease along the shaft.The maximum velocity generally appears at the bottom of the interface between shaft outlet pipe and shaft wall.(4)The variation rule of the pressure in the shaft is roughly as follows:the average pressure increases with the decrease of the height of the shaft;the pressure in the same section fluctuates slightly.The maximum pressure occurs at the bottom of stilling well and the minimum pressure occurs at the top of wet zone of shaft.(5)The influence of baffle spacing on energy dissipation rate is shown as follows:at the same flow rate,the energy dissipation rate first has a"gentle slope"zone with the increase of baffle spacing,and the energy dissipation rate does not change significantly with the increase of folded plate spacing,or even increases slightly.Then,the energy dissipation rate decreases significantly with the increase of baffle spacing.at different flow rates,when the flow rate is less than about 30m~3/h,the influence of baffle spacing on the energy dissipation rate is not obvious,when the flow rate is greater than about 30m~3/h,the influence is significant.(6)The effect of baffle inclination on energy dissipation rate is as follows:at the same folding plate spacing,with 30m~3/h as the boundary flow rate,the change of inclination angle has little effect on energy dissipation rate below this flow rate.Above this flow rate,the influence of inclination on the energy dissipation rate increases slightly,but the change range is only within 2.48%.(7)The energy dissipation rate decreases in a"two-stage"manner with the increase of flow rate.When the flow rate is less than 30 m~3/h,the energy dissipation rate decreases slowly.When the flow rate is greater than that,the energy dissipation rate decreases rapidly;the diameter of the top vent has little effect on the energy dissipation rate.(8)Based on the existing theory and experimental results,the calculation of discharging energy dissipation of Baffle-drop shaft is discussed,the calculation formula of discharging velocity of folded plate shaft is given,and the calculation model of discharging energy dissipation is established.The results of calculation are compared with those of experiment and numerical simulation.