Flow And Heat Transfer Characteristics Of Falling Film Evaporation Outside Horizontal Elliptical Titanium Grooved Tube

LiBr-H₂O solution in lithium bromide absorption refrigeration system has strong corrosion to stainless steel,copper and copper alloy,which directly affects the service life of the refrigeration system and causes the decrease of system performance.Titanium alloy has the advantages of excellent corrosion resistance,small density and high hardness,and becomes a good choice for manufacturing heat exchange tubes instead of copper alloy.But the thermal conductivity of titanium is only 18.7 W·m^-2·K^-1,much lower than that of copper 401 W·m^-2·K^-1.Therefore,it is of great significance to study the flow behavior of liquid film outside the titanium tube and related heat transfer characteristics,and develop titanium tube with high heat transfer performance.In this dissertation,the enhanced heat transfer performance of titanium tube had been explored through experimental and simulation studies on titanium tube in horizontal tube falling film evaporator.The specific research contents are as follows:1.By studying the research progress of horizontal tube falling film evaporation heat exchange tubes at home and abroad,elliptical titanium groove tube had been used to replace ordinary round titanium tube to improve the heat transfer performance of titanium tube in falling film evaporation system.2.The structural parameters of elliptical titanium grooved tube directly affect the heat transfer efficiency outside the tube.The influence of elliptical coefficient E,groove spacing a,groove width b and groove depth h on the proportion of liquid film dry wall outside the tube and the average heat transfer coefficient had been studied by numerical simulation.Through the orthogonal simulation analysis of four factors and four levels,the order of the influence of groove structural parameters on the heat transfer performance outside the tube the heat transfer performance outside the tube was a>E>h>b,and the optimal structural parameters of elliptical titanium grooved tube were E=1.2、a=1.0mm、b=0.3mm、h=0.3mm.3.Taking the optimal tube type as the experimental object,a self-designed experimental platform had been used to conduct a comparative experiment on the external heat transfer performance of ordinary round titanium tube and elliptical titanium groove tube.The influence of spray densityΓ,saturation temperature T_satand heat transfer temperature difference T_△on the external heat transfer performance of horizontal elliptical titanium groove tube had been studied.The following conclusions had been obtained:（1）The external heat transfer effect of elliptical titanium groove tube was better than that of ordinary round titanium tube,and the heat transfer coefficient increased by 61.28%～116.05%.（2）The average heat transfer coefficient of the outer wall of the tube increases first and then decreases with the increase ofΓ,and the optimalΓ=0.1091kg·m^-1·s^-1.（3）The average heat transfer coefficient outside the tube first increases and then decreases with the increase of T_sat,and the optimal T_sat=7℃.（4）The average heat transfer coefficient of the outer wall first increases and then decreases with the increase of T_△,and the optimal T_△=11.2℃.4.On the basis of experiments,the effects of spray densityΓ,saturation temperature T_satand heat transfer temperature difference T_△on the flow and heat transfer characteristics of liquid film outside the elliptical titanium groove tube had been studied by numerical simulation method.The following conclusions had been drawn:（1）Γmainly affects the spread of liquid film outside the tube,the average thickness of liquid film and the velocity of liquid film.With the increase ofΓ,the spread of liquid film outside the tube is more uniform,the thickness of liquid film is smaller,and the velocity boundary layer of liquid film is thinner,which is beneficial to heat transfer.However,excessiveΓwill increase the average thickness of liquid film and increase the heat transfer resistance,which is not conducive to heat transfer.（2）T_satmainly affects the phase change heat transfer of liquid film outside the tube.The lower the saturation temperature,the more intense the phase change heat transfer,the more conducive to improve the average heat transfer coefficient of the outer wall.However,too low saturation temperature will greatly increase the dry wall area,which will deteriorate the heat transfer and reduce the heat transfer effect.（3）T_△not only affects the convective heat transfer between the liquid film and the wall,but also affects the phase change heat transfer of the liquid film outside the tube.With the increase of T_△,the convective heat transfer between the liquid film outside the tube and the wall,and the heat transfer effect of phase change outside the tube become better,so the average heat transfer coefficient of the wall outside the tube becomes larger.However,excessive T_△will lead to excessive evaporation of the liquid film outside the tube,and the dry wall outside the tube,resulting in deterioration of heat transfer,which is not conducive to heat transfer.