Dynamic Behavior And Evaporative Heat Transfer Characteristics Of Droplet Impinging On A Heated Super-hydrophilic Cylindrical Wall

Low temperature multi-effect falling-film distillation technology has great development potential due to its advantages of small heat transfer temperature difference,high energy-utilization efficiency and available use of low grade energy such as solar energy,etc.As the key component of this technology,the heat and mass transfer between the falling film desalination tube and the spray fluid is an important factor affecting the desalination efficiency of seawater.The impacting process of spray droplets on the cylindrical wall will directly affect the formation of liquid film and then the heat transfer performance.Therefore,the research on the phenomenon of droplet impact on falling film desalination tube has a certain engineering reference value.In this paper,based on the volume of fluid(VOF)method,a three-dimensional numerical simulation was carried out to study the dynamic behavior and evaporative heat transfer characteristics of the droplets impact on a heated hydrophilic cylindrical wall.Firstly,the influence of characteristic and operational parameters in the case of vertical impact was considered.The parameters discussed include static contact angle,impact velocity,droplet diameter and wall superheat.The spreading process of liquid film,the contact area of liquid film,the average temperature of liquid film,the heat flow from the wall to the droplet and the evaporation volume ratio were analyzed and discussed.In the case of non-vertical impact,the influence of the incident direction and position of droplet was studied by introducing two parameters,i.e.,impact angle and horizontal offset.The dynamic behaviors and heat transfer characteristics were analyzed,respectively.The main conclusions of this paper are as follows:(1)For the influences of characteristic and operational parameters,the smaller the static contact angle,the better the wall wettability,the larger the contact area,the faster the average temperature rise of the liquid film,and the greater the heat flow,which promotes the evaporation phase change of the liquid film.However,when the wall wettability is poor,there will be partial rebound or even complete rebound,and the evaporation phase change rate of liquid film will decrease significantly.Based on this,a 10° of static contact angle was selected to keep the surface super-hydrophilic in the following discussion.Impact velocity reflects the kinetic energy of the droplet.The larger the impact velocity,the larger the contact area and the higher the average temperature of the liquid film.Impact velocity can improve the heat flow and evaporation rate,but with the increase of impact velocity,its influence gradually decreases,so it is not necessary to increase the impact velocity too much.The increase in the droplet diameter means an increase in the overall volume of the droplet,so the spreading area is also larger.The average temperature of the liquid film decreases due to the increase of the liquid volume,while the heat flow is opposite,proving that the increase of the droplet diameter cannot effectively increase the evaporation rate.The wall superheat reflects the magnitude of the heat transfer temperature difference.The liquid film in the central area becomes vapor due to its phase change,which breaks and forms a dry area and directly leads to a decrease in the contact area.The greater the wall superheat,the average temperature of the liquid film will be relatively higher,and the heat flow will be greater,then the evaporation rate will be significantly improved.(2)For the influence of the incident direction and position of droplet,the gravity assist effect caused by the geometric features of the cylindrical wall makes the droplet spread anisotropically.The greater the impact angle or the horizontal offset,the lower the positions of on the cylindrical wall,both the whole liquid film and its thickness peak.When the impact angle ? increases from 0° to 75°,the maximum contact area decreases from 76.5 mm2 to 45.6 mm2 with a reduction of 40.4%;when the horizontal offset ?x increases from 0 mm to 5 mm,the maximum contact area reduces from 76.5 mm2 to 61.4 mm2 with a reduction of 19.7%.Compared with a vertical impact without an offset(??0°and ?x?0 mm),for an impact with a certain impact angle or horizontal offset(??0° or?x?0 mm),the average temperature of the liquid film is lower,and the total heat flow is generally weakened,resulting in a decrease in the liquid evaporation rate.For the local heat flux distribution on the wall,it is found that the position of the higher heat flux is almost the same as the position of the peak of the liquid film thickness.