Flow And Heat Transfer Characteristics Of The Liquid Film On The Horizontal Tube Falling Film Evaporation

China is now suffering from the shortage of fresh water.It is an efficient way to solve this problem by taking advantage of desalination in order to produce fresh water for drinking or industrial manufacture.Low temperature multi-effect distillation technology is one of the most widely used and studied seawater desalination technology in China owing to its high heat transfer coefficient,simple pretreatment,low corrosion and scaling,and low material requirement.Horizontal tube falling film evaporator is a key component of low temperature multi effect evaporation seawater desalination technology.It has many advantages,such as high heat transfer coefficient,better liquid distribution and easy production of uniform liquid film.For this reason,the heat transfer and flow characteristics of horizontal tube falling film evaporation are mainly studied in this paper.The conclusion on the rules of heat transfer and flow may give a reference for the design and optimization of horizontal tube falling film evaporator.A two-dimensional CFD model of falling film on the horizontal circular tube is established to study the formation process of the liquid film.The characteristics of the velocity and thickness of the falling film are studied.The factors which affect the velocity and thickness are discussed,including spray density,the height of spraying and tube diameter.It shows that along the tube circumference,the film velocity increases first and then decreases,reaching the maximum value at a certain location in the middle,while the film thickness decreases first and then increases,reaching minimum value at a certain location in the middle.The thickness of the falling film decreases while increasing the spray height and tube diameter.In order to study the flow and heat transfer characteristics and the mechanism of heat transfer enhancement of the liquid film on the special-shaped tube,falling film evaporation CFD models of three tubes,including circular tube,combined tube and elliptical tube,have been established.The liquid film formation and heat transfer characteristics have been calculated.The thickness and velocity of the liquid film outside the different tube and the characteristics of the excess temperature and local heat transfer coefficient in the different tube type liquid film are analyzed.The results show that the distribution of film thickness of special-shaped tube is similar to the circular tube.Along the tube circumference,velocity increases first and then decreases,and the film thickness first decreases and the increases.The thermal boundary thickness constantly increases along the tube circumference.The thermal boundary thickness of special-shaped tube is smaller than that of circular tube at the same position,which results in a better heat transfer effect of the shaped tube.The increment of the spray density and spray height are able to reduce the thermal boundary layer thickness.