Time- and space-resolved heat transfer characteristics of single droplet cooling using microscale heater arrays

Abstract

Heat transfer by phase change has long been an attractive method of cooling since large amounts of heat can be removed with relatively small temperature differences. The current study focuses on making detailed measurements of the heat flux and interfacial motion of an isolated droplet as it impacts an isothermal surface at low and moderate superheats. The heat flux measurements were made using a novel 96-element, feedback-controlled heater array, which allowed the local, instantaneous heat flux to be resolved. The dynamics of the droplet were visualized using a synchronized high-speed digital camera. The experiments were performed at three superheats (Tw−Tsat=9°C, 19°C and 29°C), with nominally constant droplet diameter (0.82 mm) and impact velocity (0.3 m/s). The accuracy of the measurement was checked by calculating the energy required to vaporize a droplet of a known mass, and show agreement to within 4%. The results show that the vaporization process can be divided into two parts; a first part which is characterized by a transient effective heat transfer coefficient, and a second part in which the heat transfer coefficient is constant. The details provided by the measurements show that the initial transient in h is not simply due to an unsteady conduction process, but is also affected by the drop deformation dynamics and external diffusive vapor region near the impact site. The second part of the evaporation process is compared to models proposed in the literature for droplets which maintain a constant contact angle, and is found to be in good agreement.