Spray cooling is a high-performance technology, used in various industrial applications, such as hot die forging, hot mill rolling, cooling of powerful electronic or electrical systems, etc. In some applications, for example, during hot die forging, sprays are used not only as a coolant, but also as a transport medium for lubricating liquids. In these processes a multi-component mixture of water and lubricants are used because of their enhanced cooling and lubricating capabilities.

The main objective of the present experimental study is the characterization of a single multi-component drop impact onto a hot substrate. In particular, the influence of organic-salt-based industrial lubricants added to water drops in determining the drop residence time and drop impact regimes is investigated. The configuration of a experimental setup provides a side view of the drop and allows characterization of the drop impact, its spreading, splash and evaporation. Image processing is used for identifying impact regimes and measuring the drop residence time from drop collision to complete rebound or evaporation. The drop residence time is measured for a range of lubricant concentrations and surface overheat.

Measurements show that the residence time of the impacting drop reduces with the increase of the surface temperature. Residence time is in the most cases inversely proportional to the square of the substrate overheat, as in the case of distilled water impact. However, significant increase in residence time for large lubricant concentrations was observed, with intensive foaming near the wall region in the range of temperatures associated with nucleate boiling regime. This effect can be explained by the decrease of the rate of drop splashing and thus an increase in the residual drop mass which evaporates at the substrate. With an increase of the substrate temperature, a decrease of residence time of lubricant solutions was observed with intensive rebound, thus moving the threshold of the foaming boiling to a lower temperatures, in comparison to pure water case. On the other hand, thermal atomization was observed in the range of temperatures associated with film boiling regime. Namely, due to the presence of salts and surfactants in the lubricant solutions, the drop lamella is disrupted, leading to thermal atomization. Finally, an empirical correlation for the residence time is proposed as a function of the lubricant concentration.