Numerical Simulation And Experimental Study On Heat Transfer Characteristics Of Droplet Impacting On High Temperature Solid Surfaces

Droplets impacting high-temperature surfaces exist in many daily life and industrial application scenarios,such as fire extinguishing,metal heat treatment and spray cooling,etc.It is of great significance to explore the heat transfer characteristics mechanism of droplets impacting high-temperature surfaces for industrial applications.In this paper,combining the advantages of experiments and numerical simulations,the mechanism of heat transfer characteristics of droplet impact on high-temperature surfaces is studied.The study is divided into two main parts,the first part elucidates the transformation of droplet boiling mode and the effect of vapor layer on heat transfer,as well as analyzes the mechanism of the effect of droplet impact parameters（impact velocity,surface temperature）on heat transfer.Based on the research content of the first part,the second part explores two methods to improve the droplet impact heat transfer（changing the material physical properties of the droplet and changing the characteristic structure of the impacted surface）and summarizes the results.The following are the specific research contents and conclusions:（1）Experimental research was carried out on the impact of droplets on the high-temperature solid surface.According to the results of the impact of droplets,the phase diagram of the droplet boiling mode was drawn,and the relationship expression between the dynamic Leidenfrost temperature(T_DL)and the Weber number was fitted,and it was found that T_DL has a power function relationship with Weber number.Using numerical simulation method to capture the change of vapor layer during droplet impact,it is found that the shape of vapor layer changes periodically during the impact process,and the thickness of vapor layer has a strict negative correlation with heat flow.The numerical simulation method is used to extract the heat transfer during the process of droplet impacting the high-temperature solid surface,and the influence of impact velocity and surface temperature on the heat transfer during the process of droplet impacting the high-temperature solid surface is explored.It was found that in the contact boiling state,increasing the impact velocity,or raising the surface temperature could substantially improve the heat transfer efficiency,while in the thin film boiling state,increasing the impact velocity or raising the surface temperature had a weaker effect on the heat transfer efficiency.（2）By customizing the material physical properties of liquid droplets（boiling point,viscosity,surface tension coefficient）in the numerical simulation method,the effect mechanism of the material physical properties of liquid droplets on heat transfer was studied.It was found that an increase in the boiling point facilitates heat transfer when droplets hit a hot surface,but this enhancement is affected by the surface temperature.This facilitation diminishes substantially as the boiling point approaches the surface temperature.An increase in viscosity increases the time required for a droplet to reach a state of maximum spread,impedes heat transfer between the droplet and the hot surface,and the surface temperature has little effect on this impediment.As the surface tension coefficient increases,the contact time of the droplet with the bottom surface during impact on the hot surface decreases.And the increase of the surface tension coefficient will weaken the heat transfer efficiency between the droplet and the bottom surface,but this effect on heat transfer will weaken with the increase of the surface temperature.（3）The impact process of liquid droplets on the high-temperature convex hull microstructure surface was simulated,and the influence mechanism of the convex hull microstructure on vapor diffusion and droplet impact heat transfer was studied.The research results found that the vapor accumulated at the bottom of the droplet during impact will diffuse from the center to the periphery in a ray-like manner,but the existence of microstructures will change the direction and speed of vapor diffusion,and then affect the formation of the vapor layer.In both the contact boiling and film boiling regimes,increasing the convex hull height enhances the heat transfer between the droplet and the bottom surface,but the enhancement is more effective in the film boiling regime.The effect of convex-hull spacing on heat transfer will be affected by the convex-hull height and surface temperature.When the convex-hull height is small,the most suitable convex-hull spacing for heat transfer is at a large value.As the convex-hull height increases,the most suitable the convex hull spacing suitable for heat transfer shifts from larger values to smaller values,and this shift speeds up on hotter surfaces.Among the 16 convex-hull microstructure surfaces with different parameters studied in this paper,under the same impact conditions,when the surface temperature is 413.15K,the heat transfer of the convex-hull microstructure surface with the best heat transfer effect is higher than that of the smooth surface,it has increased by 44.89%,which is134.97%higher than that of the convex-hull microstructure surface with the worst heat transfer effect.When the surface temperature is 573.15 K,the heat transfer of the convex-hull microstructure surface with the best heat transfer effect is increased by92.22%compared with the smooth surface,and the heat transfer of the convex-hull microstructure surface with the worst heat transfer effect is increased by 158.12%.