Study On Dynamic Characteristics Of Droplet Impact On A Heated Tube With Different Wettability

The phenomenon of droplet impingement on wall surface occurs widely in daily life and production activities.In the actual process,there are many phenomena of droplets impacting on the curved surface,especially on the tube surface.The droplet impact velocity,wall wettability and wall temperature have a great influence on the dynamic characteristics of the droplets after impact.Therefore,in this paper,we builds an experimental system for droplets to hit the pipe wall with different wettability.the tube walls with different wettability were prepared by chemical reagent spraying method.Different wettability tubes with apparent contact angles of 21°,46°,99°and 151° were obtained.The dynamic characteristics of droplets impacting on the tube wall were recorded by high-speed camera.The effects of impact velocity,wall wettability and wall temperature on the process of droplets impacting on the tube wall were mainly studied by Origin software.When the droplets hit the wall that temperature ranging from 20? to 155?and different wettability,they appear spread,oscillate,retract and rebound,etc.,but no splashing or central jet occurred.The wall temperature had little effect on the morphological changes of the droplets after impact.Under the same wettability,the maximum spreading coefficient of the liquid film increases with the increase of Weber number.In this temperature range,the great influence of liquid film breakage on the maximum spreading coefficient.After the droplet hits the hydrophilic tube wall that contact angles of 21° and 46°,there is a critical rebound temperature near 170?,and the liquid film rebounds after the wall temperature is higher than this temperature.When the droplet hits the hydrophilic tube wall that contact Angle of 21°and the wall temperature is 230?,the liquid film splashes at the moment of spreading.When We is small,with the increase of wall temperature,the edge of the droplet after contacting the wall surface bounces partly,but no splash occurs,and the maximum spreading coefficient increases.When We is larger,with the increase of wall temperature,droplets always splash and break back after hitting the wall,and the maximum spreading coefficient has a relatively small change with temperature at high temperature.When the contact Angle is 46° and the wall temperature is 245?,a stable gas film is generated on the contact surface between the wall and the liquid film,which promotes the liquid film spreading.At this time,the maximum spreading coefficient reaches the maximum.Under high wall temperature,when the Weber number is large,the spatter is more likely to occur and the maximum spreading coefficient decreases.When the droplet hits the hydrophilic pipe wall that contact Angle of 46° and the We is 31.1,After the droplet impinges on the tube wall,the variation trend of the maximum spreading coefficient with temperature is different from that under other We number conditions.Summarizes the wall wettability and temperature conditions of splashes and central jets after droplets with different Weber numbers hit the wall of the heating tube.The wall temperature is a key factor in determining whether the edge of the liquid film is splashed,but the impact speed will affect the temperature range where the splash occurs.The worse the wall wettability,the higher the wall temperature,and the larger We number,the easier it is for the central jet to occur.