Ihtc14-22515 Experimental Characterization of Single and Multiple Droplet Impingement on Surfaces Subject to Constant Heat Flux Conditions

Spray cooling is one of the most promising technologies in applications which require large heat removal capacity in very small areas. Previous experimental studies have suggested that one of the main mechanisms of heat removal in spray cooling is forced convection with strong mixing due to droplet impingement. These mechanisms have not been completely understood mainly due to the large number of physical variables, and the inability to modulate and control variables such as droplet frequency and size. Our approach consists of minimizing the number of experimental variables by controlling variables such as droplet direction, velocity and diameter. An experimental study of single and multiple droplet impingements using HFE 7100 as the cooling fluid under constant heat flux conditions is presented. A monosized droplet train is produced using a piezoelectric droplet generator with the ability to adjust droplet frequency, diameter and velocity. In this study, heaters consisting of a layer of Indium Tin Oxide (ITO) as heating element, and silicon substrates are used. Film morphology was characterized using a Laser Induced Fluorescence (LIF) technique with a focus on the droplet impact zone by measuring variables such as film thickness and diameter of the impact zone. Infrared Thermography was used to measure surface temperature at the liquid-solid interface. The IR thermography technique was also used to characterize temperature gradients at the droplet impact zone. The results and effects of droplet frequency, fluid flow rate, and fluid temperature on heat flux are also presented. INTRODUCTION Current development in electronic devices places a significant demand on thermal management systems to dissipate high heat flux densities in an economic manner in order to maintain adequate junction temperatures at the chip level. The high heat fluxes require a phase change cooling methodology instead of traditional single phase options. Among the phase-change cooling technologies, spray and jet impingement cooling are the most promising options [1-2]. Liquid droplet sprays and jet impingement cooling have been widely used in the metal manufacturing industry and have been shown capable of high heat removal rates. Spray cooling and droplet impingement cooling can achieve peak heat fluxes several times superior to critical heat flux in pool boiling [2-3]. Most research on spray cooling has been experimental in nature focusing on the boiling regime with limited ability to vary experimental parameters as shown in the work of Toda [4], Yang [5-6], Tilton [7], and Sodtke and Stephan [8]. A common agreement among researchers is that the physics of spray cooling are insufficiently understood due to complexity of the interplay between multiple drop impingement events and bubble nucleation, rendering the heat transfer processes in the thin liquid film extremely chaotic. One promising avenue for uncovering the interplay between the dominant physical mechanisms in spray cooling is to remove the randomness in incident droplet size, frequency of arrival, and velocity at impingement. This can be achieved by carefully controlling the diameter and velocity of incident droplets, for instance, through the use of a piezoelectric droplet generator. Under this configuration, key non-dimensional quantities such as Weber and Reynolds numbers as well as the ratio of film height to droplet diameter, among others, can be manipulated at