Dynamic Characteristics Experimental Study Of Droplets Impacting On Heated Solid Surface

The dynamic process of liquid droplets impacting on heated solid surfaces is of considerable importance in various projects such as catalytic cracking,internal combustion,and spray cooling.To understand the process and its internal mechanisms can not only enrich the theory system of gas-liquid flow with free interface and heat transfer,but also improve the mass transfer and heat transfer engineering technology.In this paper,the dynamic characteristics of droplets impacting on curved surfaces and microstructure surfaces was studied experimentally,and theoretical analysis was carried out to the special behaviors of the impacting process.The dynamic process of droplets impacting on heated curved surface were recorded by using a high-speed camera,and the variation of the characteristic parameters of droplet such as non-dimensional spreading and contact time are analyzed.The dynamic Leidenfrost temperature of the concave surface is lower than that of the convex surface when droplets impact them at the same impact velocity.The gravitational component of the liquid film promotes droplets to spread on the convex surface,but suppresses the spreading of the droplets on the concave surface.With the diameter of the curved surface decreasing,this gravitational component increases,so droplets spread more on the convex surface but smaller on the concave surface.The heat exchange generated by the local direct contact between droplet and curved surface enhances droplet temperature,and then droplets' surface tension and viscosity decreases,which promotes droplet to spread.After droplets impact the heated curved solid surfaces,they will bounce completely when the solid surface's temperature are high enough.The asymmetric structure of curved surface causes uneven distribution of the mass of the impacting droplets,which causes them to retract quickly in the axis direction on convex surface or the curved direction on concave surface,then resulting in the whole droplets detaching quickly from the curved surfaces.The contact time of droplets impacting on concave surface is smaller than that on convex surfaces.Droplet will break and bounce off directly on convex surfaces when impact velocity is large enough,however,it will always undergo a retracting process and then bounce off concave surface.In this case,the contact time of droplets impacting on convex surfaces is smaller.In addition,with the increasing of curved surface's temperature,the effect of the Weber number on the contact time of droplet impacting on the curved surfaces gradually decreases.A structure surface with a gradient of groove pitch and a smooth surface were fabricated,and comparative experiment that the dynamic characteristics of droplets impacting on the structure surface and the smooth surface were conducted.At room temperature,droplets are in hydrophilic state on the structure surface,and are basically symmetrically spread along the vertical direction to the groove.At high temperatures,droplets are also basically symmetrically spread under different boiling state on the smooth surfaces.Droplets bounce toward denser region at contact boiling state,but toward sparse region at Leidenfrost boiling state when impacting on the structure surface.During the dynamic process of droplets impacting on the heated structure surface,there are mainly capillary force and vapor pressure force between droplets and the structure surface.The capillary force between droplets and the structure surface is significant when droplets are in contact boiling state.And denser structure under the left half of droplets enhances the capillary effect on droplets,which induces a Young's force toward the denser structure region,resulting in droplets bouncing towards the denser region.However,when droplets are in the Leidenfrost state,the vapor pressure between droplets and the structure surfaces plays a major role because there is almost no direct contact between the droplet and solid surface.The vapor pressure generated by the denser structure region is greater,so entire droplet is subjected to the force toward the sparse region and eventually bounces toward the sparse region.The above exploration of the directional bouncing motion of droplets on heated structure surface has important reference value for the structural design of microfluidic chip and high-efficiency heat exchange device.