Implementation of a high-frequency phosphor thermometry technique to study the heat transfer of a single droplet impingement

Abstract

Contributing to a better understanding of spray cooling systems, the heat transfer process underlying the event of a droplet impinging onto a uniformly heated stainless steel surface (SS304) was experimentally investigated. Since the heat transfer process is linked to the droplet’s hydrodynamics, high-speed videos were recorded to measure the deformation of the droplet. A series of isothermal and non-isothermal impacts were performed for Weber numbers ($We$) within the range $17.7\le We\le 58.2$. A strong relationship between the maximum spreading ratio reached by the droplet and its initial kinetic energy was found. The surface temperature directly affects the droplet hydrodynamic during the impact by promoting an oscillatory behavior of the droplet after the maximum spreading is reached. Given the spatial–temporal resolution of the heat transfer process, a high-frequency phosphor thermometry technique was implemented, finding that the temperature drop upon droplet impact was independent of impact velocity. The sharp temperature drop results in an intense thermal interaction that occurred during the first 10 ms of the impact. The maximum average heat flux registered was 98.56 $W/c{m}^2$ with a cooling effectiveness of 3.5%.