Influence of alcohol additive and surface temperature on impact and spreading characteristics of a single water droplet

Abstract

An experimental investigation is conducted to explore the effect of alcohol additive and surface temperature on the impact dynamics and spreading characteristics of a single water droplet on a heated copper surface. Non-azeotropic bi-component alcohol-water mixtures with 0–8 vol.% ethanol, 0–7 vol.% n-butanol and 0–2.5% n-pentanol are adopted as the working fluid covering wide ranges of surface temperature from 25 to 450 °C and Weber number from 5.8 to 71.9. The results of the visualization study on the instantaneous droplet behavior show that depending on the surface temperature and alcohol additive, the droplets exhibit noticeably different impact behaviors at the same impact height. A new phenomenon is found that some alcohol-water bi-component droplets with smaller Weber numbers show better spreading ability compared to water droplets with higher Weber numbers. This implies that the addition of alcohol additives can even counteract the negative influence of smaller impact energy. The variation of the maximum spreading factor is characterized by a contour map that is constructed over the tested ranges of Weber number and surface temperature for different droplets. The present paper also proposes a new piecewise correlation for the maximum spreading factor in different heat transfer regimes, which takes both the effects of alcohol additive and surface temperature into consideration.