A quantitative phase diagram of droplet impingement boiling

The traditional phase diagram of droplet impingement boiling on a heated substrate is drawn based on the boiling morphologies recorded with high-speed camera, which is useful to qualitatively determine the strength of droplet impingement heat transfer and the transition boundaries. However, the major shortcut of this method is that one cannot distinguish the heat transfer variation in the same region on phase diagram. To address this problem, this paper proposes a quantitative phase diagram which is determined with the measurements of temperature evolution in substrate. In the experiments, the two ends of a differential thermocouple are inserted in the substrate at different depth, and the recorded maximum temperature difference during the droplet impact is employed to quantify the intensity of heat transfer. Therefore, the contour of the maximum temperature difference in the space of the substrate temperature and the droplet Weber number provides a quantitative phase diagram. The comparisons of the two phase diagrams are conducted, and it shows that the new phase diagram could reveal richer information. For example, a second peak of the heat flux in the transition boiling could be observed under specific Weber numbers, which forms an island in the quantitative phase diagram. Finally, based on the captured images, the observed heat transfer characteristics were analyzed and explained.