Air-water Two-phase Flow Characteristics During Drainage Processes Under Various Conditions Of Orifice Shapes And Ventilation

Drainage processes is a common phenomenon in both daily life and industries.For a water storage container,it is necessary to set a proper outlet and ventilation condition,which is important for an adequate outflow efficiency,as well as the stability and safety during the drainage process.Most existing studies focus on the drainage processes of containers with circular exits under fully-sealed or fully-ventilated conditions,while few has been done on the conditions of non-circular exits and different degrees of ventilation.In this study,the drainage processes of a water-filled tank with a bottom orifice were investigated under different conditions of orifice geometries and ventilation conditions,based on physical model tests,theoretical analysis,and numerical simulation.The main research contents and important conclusions of this study are as follows.First,the drainage characteristics of water-filled tanks with round,elliptical and rectangular bottom orifices of different areas,under sealed and ventilated conditions were studied through physical model tests.The air-water two-phase flow characteristics,pressure fluctuations,bubble formation mechanism and drainage efficiency during the drainage process were investigated.According to the experimental observations,under the sealed condition,bubbles were generated at the bottom orifice,accompanied by periodic water pressure fluctuations.However,under the ventilated condition,no bubbles were generated and the water pressure gradually decreased during the whole drainage process.The development of bubbles could be divided into three stages:formation,deformation,and decomposition.In this study,the mechanism of bubble formation was reveled combining the pressure fluctuations,and an empirical formula for the bubble formation period with water depth was established.A prediction model for bubble volume was also developed.Based on the experimental results,the drainage efficiencies under circular orifices were slightly greater than the others under sealed conditions,while the effect of orifice shape was insignificant under venting conditions.It was probably caused by the different efficiencies of air supply of different orifices shapes.Secondly,the drainage patterns,pressure fluctuations,air supply ways,and drainage efficiency of the water-filled tank under different ventilation conditions were investigated through physical model tests and theoretical analysis.As increasing the tank vent area from zero to the fully-open status gradually,three modes of water drainage were observed,including the periodic draining mode with periodic bubble generation at the orifice,full-flow draining mode without bubble formation,and combined draining mode with bubble generation triggered until falling to a certain water depth.For different drainage modes,the variation of air pressure in the tank headspace,water pressure at the tank bottom,and water depth with time showed different patterns.The air supply during drainage processes is mainly through the bottom orifice in the form of bubbles,and through the top venting slit.Different drainage modes also correspond to different ways of air supply,resulting in different drainage efficiencies.According to the experimental results,the relationship between the discharge coefficient and velocity coefficient in full-flow draining mode was established.Based on the laws of mass conservation and energy conservation,the theoretical equation of the tank drainage characteristics under the full-bore drainage mode was proposed,which was verified to predict the variation of water depth with time well,and also the time required for finishing the drainage.In addition,the transition criteria of combined draining mode and full-flow draining mode under ventilated conditions were quantitatively identified.Finally,numerical simulation of the tank drainage was conducted with the computational fluid dynamics software OpenFOAM using the VOF method.The model simulated the water drainage process under fully-ventilated and fully-sealed conditions,which were verified with the experimental and theoretical results.The growth processes of bubbles in the tank under fully sealed conditions were presented in detail by the numerical simulation,as well as the corresponding outflow status and the variation of water pressure distribution in the tank.In addition,the characteristics of water velocity distribution and flowline accompanying the bubble development during the drainage process were simulated,for a better understanding of the air-water two-phase flow during the drainage process of the tank.In this study,the drainage process of a water-filled tank under different conditions of bottom orifices and ventilation were investigated.Different drainage patterns were identified,and theoretical analysis was carried out in terms of air-water two-phase flow characteristics,air supply ways and drainage efficiency.The results can deepen the understanding of the water drainage process,which is of scientific meaning,and also provides a reference for the optimized design of water tank in practical applications.