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

Falling film evaporation outside the horizontal tube has the advantages of high heat transfer efficiency,less working fluid consumption,and small heat transfer temperature difference.It is expected to be used in refrigeration and air conditioning,seawater desalination,chemical petroleum and other fields.Falling film evaporation of the working fluid outside the horizontal tube involves vapor-liquid two-phase flow and heat transfer.The liquid film flow characteristics of the liquid working fluid outside the tube,such as the formation and distribution of the liquid film(liquid film spreading along the axial and circumferential directions),the thickness and distribution of the liquid film,the behavior and distribution of bubbles in the liquid film,and so on,play a decisive role in improving the heat transfer performance outside the tube and avoiding local dry areas occurring outside the tube.The corrugated tube is put forward to replace the smooth tube,using the periodic variation along the axial direction of the corrugated tube,attempting to use the structural features of the corrugated tube to improve the process of the falling film evaporation outside the horizontal tube and to enhance heat transfer.Based on the VOF model,considering the turbulence effect of the liquid film flow outside the tube,the SST k-ω turbulence model and user-defined functions(phase transformation module,which reflects the heat and mass transfer outside the tube)are combined to establish a mathematical model to describe the falling film evaporation of the working fluid R134 a outside the horizontal tube.Numerical simulations have been carried out to find the characteristics of the falling film evaporation.By comparing the differences of the falling film evaporation process of R134 a between outside the horizontal corrugated tube and smooth tube,the flow and heat transfer characteristics of the liquid film along the axial and circumferential directions is focused.In detail,the behavior of the liquid film outside the tube,including the liquid film flow,thickness and distribution of the liquid film,nucleation and distribution of bubbles in the layer of liquid film.In the meantime,effects of variation of the working condition and the geometric structure of the corrugated tube on the liquid film flow outside the tube(such as liquid film formation and distribution,liquid film thickness,and so on)and heat transfer characteristics(average heat transfer coefficient outside the tube)are also analyzed and discussed.When the falling film keeps in the state of columnar flow,optimal structural parameters of the corrugated tube are recommended.The main work and conclusions are as follows:(1)Differences of liquid film flow and distribution of R134 a in the falling film evaporation between outside the horizontal corrugated tube and smooth tube are compared.It is found that the liquid film flow outside the corrugated tube can be improved and local dry areas in the back area of the tube can also be avoided.Along the axial direction,the liquid film outside the corrugated tube flows faster than the smooth tube.Along the circumferential direction,the flow speed of the liquid film outside the corrugated tube is slower than that of the smooth tube.The spreading speed outside the corrugated tube is relatively uniform,and the closer to the peak section of corrugated tube is,the faster the speed of the liquid film is.When the liquid film outside the corrugated tube is falling off,several new liquid columns can be formed.(2)Differences of the liquid film thickness and distribution of R134 a in the falling film evaporation between outside the horizontal corrugated tube and the smooth tube are compared.The thickness of the liquid film outside the corrugated tube is 0.120 mm,which is 7.00% thinner than the smooth tube.Among them,the liquid film thickness in the front area(circumferential angle=0°～90°)is 0.119 mm,which is 13.14% thinner than the smooth tube;the liquid film thickness in the back area(circumferential angle=90°～180°)is 0.121 mm,which is 0.24% thinner than the smooth tube.(3)Differences of the bubble behavior in the layer of liquid film(including the initial nucleation time,the number and distribution of nucleation sites)of R134 a between outside the horizontal corrugated tube and the smooth tube are analyzed.Compared with the smooth tube,the initial nucleation time of the bubble in liquid film outside the corrugated tube is 2.6ms earlier,and the thinner liquid film corresponds to more bubble nucleation sites.The number of nucleation sites outside the corrugated tube is 15.32% more than that of the smooth tube,and the distribution of nucleation sites is wider(circumferential angle=55°～135°),reflecting 10.17%higher average heat transfer coefficient.(4)Influences of the heat flux and the position of the impact point of the liquid column on the heat transfer of the falling film evaporation outside the horizontal tube are studied.Increasing the heat flux of the tube,the number of bubble nucleation sites in the liquid film outside the tube increases,while the average heat transfer coefficient outside the tube also increases.The better location of the impact point of the liquid column is in the peak section of the corrugated tube,which can eliminate the local dry area outside of the tube and show higher average heat transfer coefficient.(5)Influences of the different geometric structure parameters of the corrugated tube on the heat exchange performance outside the tube when the falling film Reynolds number Ref is in the range of 525 to 875 are analyzed.With the same pitch of wave,increasing the wave amplitude can reduce the ratio of pitch of wave to wave amplitude,the thickness of the liquid film outside the corrugated tube decreases,while the average heat transfer coefficient increases.With the same wave amplitude,reducing the pitch of wave can also reduce the ratio of pitch of wave to wave amplitude,the liquid film spreads faster,and the thickness of the liquid film outside the corrugated tube decreases,while the average heat transfer coefficient increases.Under the conditions of lower falling film Reynolds number(Ref=525,612),Once the ratio of pitch of wave to wave amplitude is too small(λp/A=18.75,15,18),local dry areas occurs on the back area of the corrugated tube.Within the range of Ref=525～875,No.#9 corrugated tube has the best structural parameters(λp/A=20,λp=20mm、A=1.0mm).