Abstract
Droplet impact phenomena and thin liquid film flow are widespread in nature, industrial production and daily life. The spreading characteristics and temperature evolution of the liquid film after droplet impact are the key controlling factors in many industrial heat transfer processes. Constructing a thin micro-nano structured superhydrophilic surface on a metal surface is a promising approach to achieving heat transfer enhancement. Therefore, in this paper, we experimentally investigated the hydraulic characteristics and temperature distribution evolution of water droplet impact on cold superhydrophilic surface using high-speed imaging and infrared thermal imaging techniques. During the droplet spreading on superhydrophilic surface, there is an inertial-force-dominant rapid spreading regime followed by the friction-dominant slow spreading regime. It is observed that a precursor film forms in the radial direction. The results show that the droplet spreading diameter is positively correlated with the We number, increasing as the weber number becomes larger. The spreading diameter decreases as the wall temperature decreases, but the effect of temperature is not obvious compared with that of impact weber number. For temperature evolution, a low temperature center area forms at the impact center and a ring-shaped high temperature zone is observed first for droplet impact on cold superhydrophilic surfaces. Along spreading radial direction, the temperature distribution shows an uphill to downhill curve with its gradient inverted in sign near the high temperature zone. Then the high temperature ring disappears and the liquid film temperature shows a monotonically decreasing trend along the radial direction. The duration time of high temperature ring shortens with the increase of We number and decrease of wall temperature. Meanwhile, in order to reveal the reasons for the formation of special temperature distribution, CFD numerical simulation is adopted to analyze the mechanism of ring-shaped high temperature zone’s formation. CFD numerical simulation demonstrates that the temperature evolution law is in good agreement with the experiment results. The temperature distribution of high temperature ring is caused by uneven distribution of the liquid film thickness due to the superwetting properties of superhydrophilic surface. This work is of great significance for further understanding and provides new sights of the liquid film flow on superhydrophilic surface in heat transfer process. Furthermore, it has certain reference significance for the spray and heat transfer process in engineering practice.
Spray Cooling with Formation of Liquid Film. Distribution of Heat Transfer Coefficient in High-Temperature Region.