Cooling Performance and Heat Flux Evolution of Sparse Sessile Droplets: An Experimental Study

Spray cooling is widely used in various fields to enhance heat transfer. However, there is scant research on sparse spray cooling under low-pressure and low-flow conditions. In this paper, the dynamic morphological evolution and heat transfer characteristics of sessile droplets generated by spray were experimentally investigated. The effects of the number of sprays, wall temperature, and wall wettability on cooling performance were explored. The results indicate that the morphology of the sessile droplets was jointly determined by the number of sprays and the initial wall temperature, transforming from uniform small droplets to a merged liquid film, which significantly affected the evaporation time and heat transfer efficiency. The wall temperature drop amplitude showed a non-monotonic variation trend of “first increasing and then decreasing” with the number of sprays. The optimal number of sprays increased as the initial wall temperature rose, specifically from 30 to 50 as the wall temperature is raised from 50 °C to 80 °C, and the overall cooling effect of hydrophilic surfaces was superior to that of hydrophobic surfaces. The heat flux distribution of sparse spray cooling indicated that, at the initial cooling stage, the latent heat of vaporization dominated, accounting for 53% to 82% of the total heat flux, and this proportion increased with the rise of the initial wall temperature and was jointly regulated by the number of sprays and wall wettability. As the droplets evaporated, the dominant factor of heat flux gradually reverted to natural cooling dominated by convection and radiation. The basic heat transfer performance of sparse spray cooling is clarified, and a theoretical basis as well as data support are provided for the design of high-efficiency spray cooling technology.