Alcohol-induced elevation in the dynamic Leidenfrost point temperature for water droplet impact

In the present study, an experimental investigation is conducted to explore the alcohol additive effect on water droplet impact on a smooth red copper surface and to construct impact regime maps. The observations show that the alcohol additives can effectively facilitate droplet atomization and breakup, and increase the dynamic Leidenfrost point temperature. An interesting phenomenon is noted that the dynamic Leidenfrost point temperatures of alcohol-added water droplets at lower impact velocities are obviously elevated compared with pure water at higher impact velocities. In other words, the addition of a small amount of alcohol additive can effectively compensate for the decrease in the dynamic Leidenfrost point temperature caused by smaller impact momentum. A new triggering mechanism of the dynamic Leidenfrost effect is proposed with the surface roughness effect taken into consideration. An approximation model based on the pressure balance criterion is then developed to discuss the underlying physics of the alcohol-induced elevation in the dynamic Leidenfrost point temperature. Combining the scaling analysis of vapor layer thickness, a scaling relation of the dynamic Leidenfrost point temperature is derived, and the predictions agree well with present and previous data. It is concluded that the present findings provide an effective method to expedite the transformation from film boiling to transition boiling for heat transfer improvement with an increased dynamic Leidenfrost point temperature, simply by adopting alcohol additives to regulate the droplet thermophysical properties.