20th International Symposium on Application of Laser and Imaging Techniques to Fluid Mechanics • LISBON | PORTUGAL • JULY 11-14, 2022 Influence of Lubricant Addition on Heat Exchange Regimes during Spray Cooling Marija Gajevic Joksimovic ∗ , Ilia V. Roisman, Cameron Tropea, Jeanette Hussong Institute for Fluid Mechanics and Aerodynamics, Technical University Darmstadt, Darmstadt, Germany *Corresponding author: gajevic-joksimovic@sla.tu-darmstadt.de Keywords: Spray cooling, Lubricant solutions, Heat transfer regimes ABSTRACT Spray cooling is used in a variety of industries, including metallurgy, electronics, and medicine. Ease of spray genera- tion, 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 e vaporation. 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 m ulti-component m ixture with i mproved c ooling and lubricat- ing 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 par- ticles, etc.) can significantly a ffect t he s pray i mpact, as w ell as t he o utcome of c ooling r egimes. N evertheless, 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 (Woodfield et al., 2006) 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 e xperimental d ata 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 d ramatically, p articularly at r ather h igh w all t emperatures. 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. 1. Introduction Spray impact onto a hot surface is often used in a variety of industrial applications. These ap- plications include wall impact of a fuel spray in engines (Meingast et al., 2000; Lahane & Sub- ramanian, 2014) or gas turbines, in selective catalytic reduction (SCR) systems of diesel driven