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Interaction of multiple liquid droplets is ubiquitous in various engineering systems, for example, spray cooling in various forms, which is an attractive option to manage high heat flux applications. Thermal transport phenomena ensuing during drop-on-drop impact is numerically investigated here. One drop (Droplet #1) is kept stationary and in quasi-equilibrium on a heated superhydrophobic solid substrate, while the second drop (Droplet #2) is impacted vertically on it with a certain Weber number ( WeDroplet #2  = 1.0, 4.0, and 8.2). The evolution of both droplets in different phases of impact, such as spreading, receding, and bounce-off regimes, along with associated heat transfer is scrutinized. The dependence of transport characteristics on various control parameters such as impact velocity of drop, volume ratio, fluid properties, and wall temperature are investigated. Distinct heat transfer trends are observed with the considered  We  number of impacting droplet (Droplet #2) being = 1.0, 4.0, and 8.2, respectively. Smaller droplet impacting with higher impact velocity results in more effective heat transfer as compared to a large droplet with small velocity, both having identical  We  number. To ease the analysis, comparison of heat transfer during drop-on-drop impact with a single droplet impact, having an equivalent diameter and velocity scales of the former drop-on-drop case, is also presented. It is revealed that the resulting heat transfer, with velocity scale based on momentum balance, is underpredicted, as compared to drop-on-drop impact. If the actual impact velocity of impacting droplet is taken, then the heat transfer gets overpredicted. Accordingly, analysis suggests that the appropriate representative velocity scale will be in-between these two bounds, which will accurately predict the heat transfer during drop-on-drop impact.

Thermal transport during drop-on-drop impact on a heated superhydrophobic substrate