Thermal and flow analysis of a droplet heating by multi-walls

Abstract

Abstract A water droplet heating from the multi-wall surfaces is considered and the resulting heat transfer characteristics and flow field are analyzed. The water droplet is located on the hydrophobic surface and a hydrophilic pin is introduced on the top surface of the droplet. Initially, the droplet fluid is assumed at a 300\u202fK and temperature of the hydrophobic and pin surfaces is maintained at 304\u202fK. Temperature and velocity distributions in the droplet fluid are predicted and the flow field predicted are confirmed experimentally incorporating the particle image velocimetry (PIV). Functionalized nano-size spherical silica units are placed at the surface to achieve hydrophobic characteristics. The contact angle of the resulting surface is in the order of 154° ± 3° and the contact hysteresis is in the order of 2°. Experimental data reveals that flow predictions are in agreement with the results obtained from the PIV data. Heating from the pin, located at the droplet top surface, and from the hydrophobic surface, located at the droplet bottom, results in four circulating cell structures in the droplet. The heated fluid is redistributed in the droplet interior by the flow currents, which results in heat transfer enhancement in the droplet interior. The circulating cell structures act like a closed-pack flow structure preventing the heated fluid being carried by the flow current towards the rotating cell centers. The Nusselt and the Bond numbers enhance with increasing droplet size; nevertheless, the Bond number attains values less than one while showing the strong Marangoni influence on the flow field.