Experimental and Simulated Investigation on the Droplet Impact Characteristics on Non-Isothermal Rough Surfaces

The phenomenon of droplet impact on non-isothermal solid surfaces widely exists in mechanical parts in the field of aerospace, and the study of droplet impact dynamic behavior is of great significance to improve the heat transfer and lubrication performance of mechanical components. For the sake of obtaining the impact characteristics of droplets on the non-isothermal rough surfaces, silicone oil and non-isothermal rough surface that similar to the surface of the inner cavity of engine were selected to carry out solid-liquid collision research. An experimental platform for droplet impact investigation was built and the dynamic behaviors of impacting, spreading, and retracting of silicone oil droplets on metal surfaces with different characteristics were observed via high-speed camera. The impact dynamic was interpreted by dimensionless numbers including Reynolds number (Re), Weber number (We) and Ohnesorge number (Oh), the effects of silicone oil viscosity, impact velocity, initial diameter of oil droplets, metal surface roughness and temperature on the impact characteristics were highlighted. The results showed that on the isothermal surface, the maximum spreading diameter of droplets was positively correlated with impact velocity, temperature of substrate surface and initial diameter of droplets, and negatively correlated with the roughness of substrate surface and the viscosity of silicone oil; the maximum spreading diameter was less affected by Weber number, and there was no direct correlation between Ohnesorge number and the maximum spreading diameter. In particular, compared with that before increasing the initial diameter, the relative deformation of oil droplets did not increase significantly, because the kinetic energy increment could not significantly offset the negative impact

of viscous dissipation, and the change of morphology was still limited. After changing the surface roughness of the substrate (6.3~25 μm), the spreading diameter and temperature at the forward end of each time node were similar. On the non-isothermal surface, when the temperature was between 50 °C and 200 °C, the maximum spreading diameter of droplets was similarly, but compared with the final stable state, it could be seen that the edge morphology of oil droplets gradually changed from sharp to smooth, a small amount of liquid remained at the spreading edge and formed a wake during droplet retraction, and the residual wake became more obvious with the increase of substrate temperature. This phenomenon was due to the fact that the average spreading time of each oil droplet on the substrate surface at different temperatures was about 1.5 ms, which was far less than the time scale of heat transfer. Therefore, the maximum spreading state had been reached while the heat of the substrate had no time to transfer to the interior of oil droplet. The impacting process of silicone oil on the non-isothermal rough surface was numerically investigated, and the effects of substrate temperature on surface tension and viscosity were analyzed quantitatively, while the spreading state of droplets on rough surface was deduced combined with Wenzel state model. The research results of this paper provided abundant theoretical and experimental basis for understanding the impact behavior of droplets on non-isothermal rough metal surfaces.