Research On Flow Characteristics And Energy Dissipation Mechanism For High Speed Air-Water Two-Phase Flow In Baffle-Drop Shafts

In recent years,the process of urban water circulation has been changing with the rapid development of urbanization.In addition,extreme weather caused by climate change occurs frequently.These bring the urban diseases such as waterlogging and water environment pollution particularly prominent.To effectively solve these urban water problems,the construction of the “sponge city” was proposed by the Chinese government to enhance the urban capability of resistance natural disasters and water ecological self-repair.However,the construction of sponge city in old urban area was limited by dense population,poor space,and complex pipeline network.Therefore,deep tunnel drainage projects with the concept of“sponge city” construction have become the main method to solve urban flood disasters and prevent overflow pollution.As an important part of a deep tunnel drainage system,the drop shaft is used to transport the rainwater and sewage from the shallow drainage network to the deep tunnel.Presently,the research on the baffle-drop shaft mainly focuses on the hydraulic characteristics and structural optimization.Although the existing researches have gained a comprehensive understanding into the baffle-drop shaft,little is known about the turbulence characteristics,energy dissipation mechanisms,baffle pressure,geyser phenomenon and so on.In this dissertation,theoretical analysis,hydraulic model test,and numerical simulation were performed to systematically analyze the flow characteristics and energy dissipation mechanism for high speed air-water two-phase flow in baffle-drop shafts.The details are summarized as follows:Firstly,a scale of 1:25 hydraulic model test was established according to the practical project and laboratory conditions.Based on the observation of the flow phenomena during the discharge process,the identification methods of different flow regimes including wall-impact confined flow,critical flow,and free-drop flow were proposed.The maximum discharge capacity of the baffle-drop shaft was redefined according to the variation law of“cavity area” phenomenon,and the prediction model of the maximum discharge capacity was established.Based on the maximum energy dissipation rate,the optimal size of the baffle-drop shaft was d/B = 0.485 and ζ = 10°(d,B,and ζ are the baffle spacing,width,and angle,respectively).The distribution laws and fluctuation characteristics of baffle pressure under the steady and unsteady flow conditions were experimentally investigated.The design criterion of baffle-drop shaft key parameters was established based on the maximum discharge capacity and optimal structural parameters.Secondly,a three-dimensional numerical model of the baffle-drop shaft was established based on the Realizable k-ε model and Volume of Fluid(VOF)method.The prediction model of the maximum discharge capacity of baffle-drop shafts was modified by analyzing the variation law of water surface and fullness in the shaft.The influence of factors such as inflow rate,baffle spacing and angle on flow velocity,baffle pressure and shaft wall pressure were explored.According to the average pressure distribution,the hydrodynamic load prediction model on the baffles was established,which could provide a reference for the baffle structural design.The cavitation numbers under the design criterion and maximum discharge prediction model on the baffle was greater than the cavitation incipient number,and cavitation erosion was difficult to occur in the shaft.Compared with other locations,the top of shaft was prone to cavitation,and effective measures should be taken to avoid the cavitation and cavitation erosion.Thirdly,the turbulence characteristics in the baffle-drop shaft were analyzed from the aspects of turbulent kinetic energy,turbulent dissipation rate,vortex structure and vorticity based on the three-dimensional numerical model.It was found that the minimum baffle spacing is the optimal structural design at the design flow rate when the flow regime is free-drop flow.The baffle with a design angle could improve the energy dissipation effect of the shaft,and effectively reduce the hydrodynamic load of the water flow acting on the baffle.The energy dissipation mechanisms of baffle drop shafts were expounded in detail through the energy dissipation types and modes(inlet energy dissipation,baffle energy dissipation,and shaft-bottom energy dissipation).The calculation model of baffle energy dissipation was established based on the free falling flow theory.Baffles play a major role in the energy dissipation of the drop shaft at low flow rates,but for high flow rates,the energy dissipation is mainly achieved via the baffles and the shaft bottom.Finally,the effects of water depth,inlet pressure,air volume and pressure gradient on the geyser height and hydrodynamic load on the baffle bottom were experimentally studied based on a geyser model test.The prediction formula of maximum geyser height and mathematical model of geyser critical condition were established by using the multiple linear regression models.According to the relationships between the connecting mode of adit and connected area of dry/wet sides and the geyser mechanism,the corresponding reasonable control measures were put forward.The scheme of adit connecting the wet side and increasing the connected area is more conducive to the normal operation of baffle-drop shafts,and orifice plates are used to control the geyser height.These research results about the flow characteristics and energy dissipation mechanism for high speed air-water two-phase flow could provide a theoretical basis for the key structural parameters design of baffle-drop shafts and supply a powerful technical guarantee for the safe operation of deep tunnel drainage systems.