Numerical Simulation Of Heat And Mass Transfer In Evaporation Of Falling Film At Static And Swinging State

With the increasing development of maritime trade,the ship navigation has become an important carrier of modern transportation.However,with the increase of consumption of fossil fuels,the harmful gases have severely polluted the environment.Therefore,the exploration of clean and environmental friendly energy has become an important subject of current research.Absorption refrigeration technology due to its advantages of energy saving and environmental protection has become one of the ideal marine refrigeration technologies.The energy produced by the combustion of fuel oil in the marine diesel engine has not been fully utilized.Nearly half of the energy is dissipated into the atmosphere in the form of heat,which will cause waste and bring harm to the atmosphere.Absorption refrigeration technology uses exhaust gas and waste heat generated by diesel engine consumption.It can reduce the cost of additional operation and has no pollution to the environment.The generator is one of the important heat exchangers in the absorption refrigeration unit,and its performance determines the efficiency of the unit.Compared with the traditional immersion generator,the falling-film generator can ignore the flow pressure drop in the process of evaporation outside the tube and condensation inside the tube,reduce the thermal driving force and increase the heat transfer performance.Therefore,it is widely used in absorption refrigeration.However,absorption refrigeration has not been widely used in ships,mainly because it is necessary to consider the impact of different working conditions on the performance of refrigeration equipment during navigation.When the ship is sailing,it will shake and swing with the sea conditions,and different swing amplitude and swing periods will affect the performance of the refrigeration cycle.This paper mainly studies the effects of different swing amplitude and swing periods on heat and mass transfer performance of falling film evaporation.Firstly,the heat and mass transfer characteristics of falling film evaporation of Li Br-H₂O solution in a static vertical tube are analyzed.STARCCM+software is used to study the liquid film flow and the heat and mass transfer effect generated by evaporation.The finite volume method is used to solve the velocity,temperature,concentration and other parameters.The distribution of film thickness,concentration,temperature,velocity,the heat and mass transfer coefficient are calculated.The results show that in the initial stage of liquid film flow,there are small fluctuations and the overall flow is flat.When it flows to the middle of the tube,due to the effect of wall viscosity and surface tension,the liquid flows with a larger fluctuation range.The driving force of mass transfer is larger and the evaporation effect is better.With the flow of the fluid,the mass transfer force decreases,the liquid film concentration increases,the evaporation capacity decreases,and the velocity increases,the fluctuation range of liquid film thickness becomes smaller,and it is more flat when it is located at the exit.By changing the variables that can affect the falling film evaporation,such as heat flux,inlet concentration,wall temperature,spray density,etc,the influence on the liquid film surface concentration,temperature,velocity,heat and mass transfer coefficient are analyzed.When the spray density and the wall temperature are constant,the heat flux increases and the average heat and mass transfer coefficient increases linearly.When the inlet temperature and heat flow are constant,the increase of inlet concentration will reduce the thermal conductivity.The mass diffusion coefficient will decrease,the mass transfer performance will decrease.The spray density has a great influence on the evaporation heat transfer of falling film in the tube.With the increase of spray density,the liquid film thickness increases and the heat and mass transfer coefficient increases.By studying the influence of different swing period and swing angle on the heat and mass transfer of liquid film flow in the swinging state,and comparing with the static state,the influence of swinging on ship navigation is analyzed.The results show that the fluctuation of the liquid film makes the energy of the liquid film interface better transfer to the inside of the liquid film.When the swing period is constant,the larger the amplitude,the shorter the time the liquid film stays in the tube,which is not conducive to evaporation.When the swing period is constant,the larger the amplitude is,the greater the velocity is,and the shorter the time the liquid film stays in the tube,which is not conducive to evaporation.In this study,when the swing angle is 7°and the swing period is 5s,the swing evaporation effect is the best and the heat and mass transfer coefficient is the highest.When the swing angle is constant,the long swing period has the best effect on heat and mass transfer.At the same time,the changes in the waveforms of different swing angles are also studied.The surface waves generated by the liquid film promote the fluid flow and component diffusion,and strengthen the evaporation.