Numerical Simulation Of Flow And Heat Transfer Characteristics Of Falling Film Evaporation In Horizontal Tube

Seawater desalination technology is one of the important technologies to solve the problem of water shortage in China.Because low-temperature multi-effect evaporation technology has the characteristics of good heat transfer performance,simple structure,and low material performance requirements,it is currently the most widely used seawater desalination technology in China.The horizontal tube falling film evaporator has many advantages such as high heat and mass transfer efficiency,small liquid loading,negligible pressure drop,and so on.It is a key device in the low-temperature multi-effect evaporative seawater desalination technology.Therefore,it is of great significance to study the falling film flow and heat transfer characteristics of the horizontal tube falling film evaporator.In the horizontal tube falling film evaporation technology,the thickness and uniformity of the liquid film are considered to be important hydrodynamic characteristics,which have a great influence on the heat transfer of the fluid.In order to study the flow and heat transfer characteristics of a horizontal falling film evaporation tube,this paper established a CFD model of falling film evaporation heat transfer for round tubes,porous tubes,and elliptical tubes.And the formation and distribution of liquid film outside the tube and the heat transfer characteristics of falling film evaporation were investigated.The influence of factors such as spray density,evaporator tube diameter,liquid distribution height and other factors on the thickness of the water and seawater liquid film outside the circular tube was discussed.The results show that in the circumferential direction,the liquid flows downwards outside the tube to form a liquid film.The speed of the liquid film first increases and then decreases.On the contrary,the thickness of the liquid film shows a change law of first decreasing and then increasing.The thickness of the liquid film outside the evaporation tube decreases with the increase of liquid distribution height,increases with the increase of spray density,and decreases with the increase of pipe diameter.The changes in the distribution and thickness of the liquid film on the surface of the porous elliptical tube and the smooth elliptical tube are compared and analyzed,as well as the variation of the local heat transfer coefficient of the falling film evaporation outside the different tubular tubes.The results found that the local heat transfer coefficient of the outside evaporation of the porous tube and the smooth tube began to gradually decrease along the low ring direction,and the heat transfer performance gradually decreased.Moreover,the heat transfer coefficient of the porous tube is about twice that of the smooth tube,and its heat transfer performance better.The main reason is that the surface porous tube not only provides more heat exchange area,but the uneven surface structure also destroys the development of the boundary layer of the liquid film layer,reduces the heat transfer resistance,and enhances the heat transfer effect.