Mario F. Trujillo 1 Assistant Professor Department of Mechanical Engineering, University of Wisconsin, Madison, WI 53706 e-mail: mtrujillo@wisc.edu Jorge Alvarado Associate Professor Department of Engineering Technology and Industrial Distribution, Texas A&M University, College Station, TX 77843 Eelco Gehring Graduate Research Assistant Department of Mechanical Engineering, University of Wisconsin, Madison, WI 53706 Guillermo S. Soriano Graduate Research Assistant Department of Engineering Technology and Industrial Distribution, Texas A&M University, College Station, TX 77843 Numerical Simulations and Experimental Characterization of Heat Transfer From a Periodic Impingement of Droplets In this combined experimental and simulation investigation, a stream of HFE-7100 drop- lets striking a prewetted surface under constant heat flux was studied. An implicit free surface capturing technique based on the Volume-of-Fluid (VOF) approach was employed to simulate this process numerically. Experimentally, an infrared thermogra- phy technique was used to measure the temperature distribution of the surface consisting of a 100 nm ITO layer on a ZnSe substrate. The heat flux was varied to investigate the heat transfer behavior of periodic droplet impingement at the solid–liquid interface. In both experiments and simulations, the morphology of the impact zone was characterized by a quasi-stationary liquid impact crater. Comparison of the radial temperature profiles on the impinging surface between the experiments and numerical simulations yielded rea- sonable agreement. Due to the strong radial flow emanating from successive droplet impacts, the temperature distribution inside the crater region was found to be signifi- cantly reduced from its saturated value. In effect, the heat transfer mode in this region was governed by single phase convective and conductive heat transfer, and was mostly affected by the HFE-7100 mass flow rates or the number of droplets. At higher heat fluxes, the minimum temperature, and its gradient with respect to the radial coordinate, increased considerably. Numerical comparison between average and instantaneous temperature profiles within the droplet impact region showed the effect of thermal mixing produced by the liquid crowns formed during successive droplet impact events. [DOI: 10.1115/1.4004348] Keywords: multiple droplet impingement, two-phase flow heat transfer, openFOAM, Volume-of-Fluid 1 Introduction One of the phase change cooling options for efficiently remov- ing heat in electronic devices and other surfaces at relatively low wall superheats is spray cooling technology [1,2]. This option has been the subject of a number of investigations [1,3–7] in recent years. Spray cooling has a complicated physical behavior, since it involves a combination of multiple droplet impingement events, droplet collisions, droplet splashing, bubble nucleation, dry-out, and rewetting periods in an extremely chaotic liquid film environ- ment. This has lead some authors [6] to conclude that the physics of spray cooling is still not well understood. In one of the latest review articles [7], the author summarizes a number of conflicting experimental findings, for instance, the conclusion that droplet ve- locity has a significant influence on the critical heat flux in the experiments of Chen et al. [8]; while, in the work of Mudawar and Estes [9], the argument made was that the volumetric flux (i.e., number of droplets) is the key physical parameter rather than ve- locity. Similar contradictions can also be noted in the role of sur- face roughness on heat performance. In the present work, our focus is on a detailed and local charac- terization of the heat transfer mechanisms stemming from multi- ple droplet impingements on a thin liquid film. This is motivated by the fact that sequential multiple droplet impacts are a key com- ponent in the study of spray cooling phenomena. Rather than try- ing to include all relevant aspects of the problem, which will likely lead to more confusing trends, our choice is to focus first on the signature droplet impingement process. As the results will show, one of our main findings is that the immediate region of impingement experiences significant cooling to the point, where the temperature field is drastically reduced from its saturated value. The literature on droplet impingement with heat transfer has focused mostly on single droplet impacts [10–15] or on two suc- cessive impacts [16,17], giving relatively little attention to the multiple droplet impingement case [18]. A common theme in their work was the solidification of molten droplets striking a cooler substrate [12,16,17,19] and their comparison to experi- mental images, as well as the temporal characteristics regarding droplet spread. Work by Pasandideh-Fard et al. [11] considered the cooling behavior of a water droplet striking a hot surface, where it was noted that much of the physics can be captured by a one-dimensional conduction model. The role of the evaporative layer formed between a droplet and a hot surface was numeri- cally studied by Nikolopoulos et al. [13]. The coupling between the fluid dynamics and heat transfer in single droplet impinge- ment on a dry surface were reported by Bhardwaj et al. [15], where their numerical technique consisted of a Lagrangian track- ing of the interface. From a purely theoretical modeling approach, Di Marzo et al. [10] investigated the evaporation of a droplet deposited on a hot surface. Experimental observations by Shen et al. [14] led to an identification of the hydrodynamic and thermal characteristics of the impingement of a droplet on smooth and nanostructured surfaces. For the numerical studies, most of the work incorporated the VOF method or Lagrangian interface tracking to follow the significant deformation of the droplet(s) during impingement. 1 Corresponding author. Contributed by the Heat Transfer Division of ASME for publication in the JOUR- NAL OF HEAT TRANSFER. Manuscript received December 23, 2010; final manuscript received May 26, 2011; published online October 5, 2011. Assoc. Editor: Louis C. Chow. Journal of Heat Transfer DECEMBER 2011, Vol. 133 / 122201-1 Copyright V C 2011 by ASME Downloaded From: http://heattransfer.asmedigitalcollection.asme.org/ on 01/28/2014 Terms of Use: http://asme.org/terms