Spreading and heat transfer characteristics of droplet on a heated substrate

Abstract

The spreading characteristics of a droplet on a heated substrate have direct influences on its spreading area and heat transfer, so the exploration in this aspect is of important significance for cooling electronic and aerospace equipments. In the present paper, the evolution model of a droplet on a heated solid substrate is established based on the lubrication theory, and spreading processes are simulated respectively when the wall temperature is uniform and decreases exponentially from the center to both sides. A method of assessing the heat flux and heat transfer capacity of a two-dimensional liquid droplet is proposed. Influences of spreading characteristics and heat convective condition at the liquid-gas interface on heat transfer feature of the droplet are examined, and the results are in good agreement with the published ones in the literature. The simulated results show that in the case of uniform wall temperature, the evolution of the droplet is dominated mainly by gravity and illustrates symmetrical spreading characteristics, and the thickness profile presents a single-peak feature of which the value diminishes with time. The heat flux across the droplet surface decreases from both sides to the center, and the surface area of the droplet increases with time slightly, so the performance of heat transfer is strengthened to a certain extent. When the wall temperature decreases exponentially from the center to both sides, the spreading process of the droplet manifests three obvious stages, in which a single-peak feature of thickness profile gradually evolutes into a double-peak feature after surviving for a short period of time, and the peak values of the double-peak first increase firstly and then decrease, resulting from the complex game of gravity and thermocapillary force and their alternative dominance in the evolution. The variations of the dynamic contact angle and travelling speed of the contact line with time can also reflect the above characteristics. The heat flux in the center of the droplet increases, while its values at the double-peak and contact lines decrease with time. In addition, the heat flux at the contact line has a distinct jump feature compared with that at the adjacent position. The droplet surface area expands significantly with time, so the heat transfer capability is improved apparently. Enhancing heat convective condition at the liquid-gas interface, namely greater Biot number, slows the droplet spreading process, which inhibits the expansion of the droplet surface area. However, it enables the droplet to stay in a higher temperature region, resulting in the enhancement of heat dissipation of the droplet. Therefore, the comprehensive interactions of the above aspects strengthen the heat transfer capability, and this phenomenon tends to be increasingly significant over time. Greater Biot number delays the variations of the dynamic contact angle and the travelling speed of the contact line, without changing their general characteristics.