A computational study on air entrapment and its effect on convective heat transfer during droplet impact on a substrate

This paper numerically investigates air entrapment behavior when a water droplet impacts on a solid substrate in the presence of ambient air. The dynamics of air entrapment is specifically described, in particular, retraction, contraction and toroid formation have been discussed. Volume-of-fluid method has been used in order to track the liquid-gas interface. The effect of surface wettability on the air entrapment and its evolution is studied. In line with experimental findings, we have observed a dimple formation on droplet surface at the impact point in the present study which has not been reported using a numerical study so far. It has been demonstrated using numerical study that the formation of dimple leads to the air entrapment. The surface wettability is demonstrated to have significant influence on the evolution of the air film into bubble. The processes during the evolution, such as inertial retraction, contraction and pinch-off are delayed on increasing the contact angle. Air bubble formed on the substrate gets detached from the substrate for low contact angle of 35° (hydrophilic surface) however, it remains attached to the substrate for higher contact angles: 90° and 120° (hydrophobic surface). Further, the influence of air entrapment on the heat transfer characteristics is studied. The deformation of the droplet as it approaches the substrate, influences the boundary layer on the substrate which eventually affects the wall heat flux. Post impact, the wall heat flux is primarily influenced by the liquid fraction and the air entrapped. In the region of only air, the wall heat flux is very low compared with that in the region of water or mixture of water and air. The present numerical results have been validated employing experimental results available in the literature.