Visualization and quantitative study on film boiling heat transfer of a salt droplet on the hydrophobic substrate

A droplet impacts on high temperature surface may lead to film boiling. Hydrophobic coating used for corrosion prevention is easy to cause film boiling at low temperature, which results in a remarkable decrease in evaporation efficiency and heat transfer coefficient. However, there is a lack of quantitative visual experimental data on the flow field around the droplet to understand the heat transfer characteristics during film boiling. In this study, schlieren photography combined with high-speed imaging technology is used to observe the evolution process of a droplet impact, rebound, oscillation, and stable boiling. The effects of temperature on lifetime, vapour velocity and heat transfer coefficient of a salt droplet with different concentrations are studied for aluminum and Teflon surfaces. The distribution characteristics of vapour velocity are quantitatively analyzed using cross-correlation algorithm, and the calculation formulas of heat transfer coefficient under nucleate and film boiling modes are proposed. It is found that increasing the concentration of a salt droplet can improve the heat transfer coefficient during film boiling. This work will provide a theoretical basis for the improvement of heat exchanger efficiency in areas such as sea water spray cooling.