Influence of Lubricant Addition on Heat Exchange Regimes During Spray Cooling

Spray cooling is used in a variety of industries, including metallurgy, electronics, and medicine. Ease of spray generation, as well as the ability to cool relatively large surfaces or precise regions of interest, are among the key assets. Spray cooling can provide a relatively high heat flux, especially if it is accompanied by liquid evaporation. Additionally, it can be applied not only as a coolant but also as a transport medium for lubricating fluid in specific applications, such as hot die forging. Typically, the working fluid is a multi-component mixture with improved cooling and lubricating properties. Once the bulk liquid evaporates, particles or dissolved lubricant can settle on the hot solid substrate. Existence of components with various physico-chemical properties on the surface (binders, surfactants, dispersed particles, etc.) can significantly affect the spray impact, as well as the outcome of cooling regimes. Nevertheless, the accompanying thermal-hydraulic effects are still not well understood.  An experimental facility for observing the impact of the multi-component spray, and characterizing heat transfer was designed for this study at the Institute for Fluid Mechanics and Aerodynamics. As a working fluid, varied mixtures of water and industrial white lubricant were used in different ratios. The visualization of spray impact and identification of the main hydrodynamic regimes were achieved using a high-speed video system. The inverse heat conduction problem was used to calculate the heat flux during continuous cooling from 445°C to 100°C taking into account temperature readings from within the substrate. Since the substrate was entirely insulated on all sides (except on the sprayed surface), the boundary conditions were well defined. The experimental data for pure water drops were compared with the results obtained using lubricating additives. It is discovered that even minimal volumes of lubricant augment the heat flux dramatically, particularly at rather high wall temperatures. The spray cooling process is accompanied by an extensive foaming near the wall and as a result, the Leidenfrost point is moved to a higher temperature, thus suppressing film boiling regime.