Heat Transfer During Consecutive Impact of Two Droplets on a Heated Substrate

Spray cooling is an attractive solution to cool high heat flux dissipating surfaces. However, the modelling of its transport process is very challenging due to simultaneous interactions of multiple droplets with the heated wall. In this context, we have performed an experimental investigation of two consecutive droplets impacting on a heated substrate. First droplet (Droplet #1) impacts the hot surface at a certain We number, followed by oncoming droplet (Droplet #2) impacting the same substrate at the same We number, after a designated time (~ thermal diffusion time scale). Stainless steel foil heater is utilized to provide constant volumetric heat generation, along with a desired initial wall temperature boundary condition. The temperature field of heated SS substrate is altered due to the impact of Droplet #1. Afterwards, Droplet #2 merges with Droplet #1, spreads, recedes, and the combined droplet mass attains a quasi-equilibrium shape. The variation of peak heat flux during impact of Droplet #2 is distinctly lower compared to the initial impact of Droplet #1. Further spreading during impact of Droplet #2 beyond the region of Droplet #1 leads to interaction of liquid with region of higher temperature substrate and results in greater net heat transfer at the peripheral region of the spreading combined droplet mass. The key differences during Droplet #2 impact is delineated compared to Droplet #1 impact on a dry heated substrate. Additionally, the role of We number of Droplet #2 and the initial substrate temperature on the ensuing heat transfer to the droplet is also reported. This study helps in understanding the nuances of such consecutive droplet interaction and its influence on applications such as spray cooling so that better models may eventually be developed, incorporating the experimental details and outcomes.