Numerical Research On Flow And Heat Transfer Characteristics Of Liquid Column Spreading In Horizontal Falling Film Evaporator

The horizontal tube falling film evaporator has many advantages such as high heat transfer efficiency,low operating temperature,simple pretreatment of feed water,and the like,and has been widely applied in many industries such as seawater desalination and refrigeration technology.The research on the evaporation flow and heat transfer mechanism of the falling tube of the horizontal tube has attracted extensive attention from scholars at home and abroad.The researchers have tried to explore the complex flow field changes and heat transfer laws in the field of falling film falling film evaporation through experiments or numerical simulations.Provide guidance and theoretical basis for the design and improvement of the horizontal tube falling film evaporator.In this paper,the three-dimensional numerical model of falling film flow and evaporation heat transfer in horizontal tube is established.The flow and evaporation heat transfer process of fluid working water outside the tube during the falling film evaporation process are simulated by FLUENT software.The VOF model is used to accurately capture the dynamic process of the liquid film surface,and the numerical calculation results are visualized and processed.The distribution of the liquid film thickness,liquid film velocity and local heat transfer coefficient of the tube wall under different parameters are obtained.Membrane flow and evaporation heat transfer mechanism,the effects of three types of parameters such as spray density,evaporation temperature and tube spacing on flow and heat transfer performance were discussed.At the same time,the flow and heat transfer of liquid film near the upper stagnation point were studied.Focus on analytical research.The thickness of the liquid film gradually decreases and then gradually increases along the circumferential direction of the horizontal tube.Consistent with the mechanical analysis results,the liquid film formed by the adjacent liquid column overlaps at the midpoint of the transverse tube axial direction to form the axial maximum liquid film.The "annular protrusions" of the thickness converge at the bottom of the cross tube to form a secondary liquid column.As the spray density increases,the thickness of the liquid film at each point outside the tube increases,and the peak cross section becomes steeper and taller.When the tube spacing increases,the thickness of the liquid film decreases,but the difference in liquid film thickness caused by the increase in the tube spacing will gradually decrease,and the thickness of the liquid film tends to be uniform.The velocity stagnation occurs in the neck region where the top of the horizontal pipe and the incoming liquid column are connected,that is,the surface velocity distribution of the liquid film from the liquid column to the horizontal pipe surface first decreases and then increases,so the region is highly prone to generate fluid.Stacking phenomenon.For the impact zone with strong liquid membrane disturbance and the secondary liquid column formation zone,the local heat transfer coefficient is higher than the vicinity,and the liquid film velocity distribution is consistent with the change trend of the local heat transfer coefficient distribution on the outer wall of the pipe.As the spray density increases,the overall heat transfer effect outside the tube increases.The evaporation temperature of the fluid increases,the viscosity decreases,the liquid film spreads faster on the outer wall of the horizontal tube,and the local heat transfer coefficient increases.The increase of the pipe spacing causes the flow velocity to increase when the fluid reaches the top of the horizontal pipe,but the flow velocity increases gradually,and the heat transfer enhancement effect caused by the liquid film disturbance will also decrease.