Infrared Thermography of Dropwise Evaporative Cooling of a Semi-Infinite Solid Subjected to Radiant Heat Input

A single droplet is gently deposited on the surface of a semi-infinite body and the evaporative cooling transient is recorded. This study is limited to the evaporative phenomena, therefore, the temperature range selected is such that nucleate boiling or film boiling are not observed. The solid (a glass-like material) is heated from above by two radiant electric panels and it is placed on a chilled plate held at near ambient, constant and uniform temperature. The transient temperature distribution over the semi-infinite solid surface is monitored by infrared thermography. Image processing techniques are used to eliminate undesired information and to retain the data which are then converted into surface temperature readings. Droplet sizes in the range of 10 to 50 μl are used with initial solid surface temperatures between 90 and 180 ̊C. The results are compared with similar experiments performed with the semi-infinite solid heated from below by conduction to gain insight into the competing mechanisms of evaporation by direct radiation from above and by conduction at the solid-liquid interface. The droplet aspect ratio is a dominant parameter in the conduction controlled evaporative component. Therefore, particular care is taken in assessing the behavior of the droplet shape during the transient. The direct radiation from above strongly reduces the surface tension of the liquid and thus allows the drop to spread on the surface more than for the conduction case.