title: From Single Drop to Spray Cooling: Influence of Lubricant Addition on Heat Transfer
Creators: name: Gajević Joksimović, Marija
Publisher: name: UNSPECIFIED
Year: 2024

description: Spray cooling of solid substrates is one of the techniques used in various industrial processes such as forging, quenching or other metallurgical applications, electronics, pharmaceutical industry, medicine or for cooling of high power electrical devices. It is governed by various hydrodynamic and thermodynamic processes such as drop impact, heat conduction in the substrate and convection in the spreading drops, and different boiling regimes. Cooling performance is influenced by a wide range of parameters, including spray characteristics such as drop diameter, drop velocity, and mass flux, followed by surface temperature, surface material, and surface conditions. The problem of modeling spray cooling becomes even more challenging if the liquid is multicomponent. The presence of additives with various physicochemical properties (surfactants, binders, dispersed particles, dissolved phase etc.) can significantly affect the entire spray impact process and impact outcomes, and could lead to the formation of a thin deposited layer on the substrate. The present study attempts to understand the physics behind the process of spraying a multicomponent liquid onto a hot substrate.
A better understanding of the single drop impact is required for a more reliable modeling of spray cooling. In the present work, the single drop impact on a heated substrate is first experimentally investigated for different liquids under various thermodynamic and hydrodynamic conditions. The influence of two different industrial lubricants mixed with water in different ratios (suspensions and solutions) on drop impact and regimes is studied. The effect of additives on the outcome of drop impact, in particular solid graphite particles for the suspensions and organic salts for the solutions, is investigated for different impact conditions. High-speed visualizations of drop impact outcomes are classified according to known heat transfer regimes: nucleate boiling, thermal atomization, and film boiling. The influence of particles is taken into account in a theoretical model for heat transfer in the nucleate boiling regime and particle deposition.
Next, transient spray cooling of a hot thick target by a lubricant solution is experimentally investigated. The temporal evolution of the heat flux and surface temperature of an initially homogeneously heated substrate to temperatures significantly above the liquid saturation point is measured during continuous spray impact. The spray impact is additionally visualized using a high-speed video system, and the instantaneous local heat flux and surface temperature are temporally matched to the visualizations. Observations from the present work show that the presence of even very low concentrations of lubricant dramatically increases the heat flux, especially at high wall temperatures where film boiling is typically observed for spray cooling using distilled water. A significant increase in the drop rebound temperature, often associated with the Leidenfrost point, occurs. Mechanisms leading to the increase in heat flux and the shift of the Leidenfrost point are identified and discussed.
The findings presented in this thesis serve to a more comprehensive understanding of the underlying physics required for the modeling and prediction of spray cooling processes with industrially relevant liquids. The results of this study will provide a basis for modeling the formation of the lubricating layer by cooling sprays, which is necessary for predicting the evolution of its thickness and uniformity.