CHARACTERISTIC OF MICRO EMISSION BUBBLES IN A DROPLET IMPINGING ON A HEATING SAPPHIRE SURFACE

Abstract

A technique for cooling that has been found to have a higher heat transfer coefficient than one that involves circulating cooling water over a heated surface is the spraying of fine water droplets onto a surface. This study aims to examine and elucidate the mechanism behind the behaviors that resulted in the formation of bubbles in a droplet's core when a water droplet collided with a heated surface. To clearly see the bubble that forms at the solid-liquid contact interface, a transparent sapphire was utilized as the heating surface. A highspeed video camera captured the boiling behavior from the horizontal view. The fluctuation of the bubbles was examined to ascertain their behavior during the boiling process. To ascertain the impact of surface wettability, two distinct heating surfaces were also employed. Compared to the original sapphire surface, the heated sapphire surface with hydrogen peroxide plasma layer had improved wettability thus higher bubble point density. At 320°C, the lifetime of bubbles was the longest, and the vertical fluctuation of the fluid that could be perceived as the bubbles moved was also intense. Moreover, the maximum instantaneous velocity of the fluid determined from the bubble movement at this time exceeded 2 m/s.