Enhancement of heat transfer rate in air-atomized spray cooling of a hot steel plate by using an aqueous solution of non-ionic surfactant and ethanol

Air-atomized spray cooling, where compressed air atomizes water into fine droplets, is an efficient alternative to conventional cooling techniques. The present work deals with the air-atomized spray cooling of a 6 mm thick stainless steel plate having an initial surface temperature of 900 °C, using surfactant Tween 20 and ethanol additives. The main difficulty in achieving a high cooling rate at elevated surface temperatures is the Leidenfrost phenomenon. The metallurgical properties of steel are highly affected by the run-out table cooling rate between the temperature range of 900–600 °C. Another important cooling region, particularly to achieve the high strength martensite microstructure in steel, is 900 °C–200 °C. Therefore, in this study, the heat transfer studies have been done over those temperature regions. The physical properties of the coolant mixture were measured to understand the heat transfer enhancement mechanism. The results show that increasing the ethanol fraction in pure water (with or without surfactant) enhances the critical heat flux, heat transfer coefficient and cooling rate of a hot surface in the nucleate and transition boiling regimes. A maximum cooling rate of 183 °C/s has been obtained with the ethanol–water mixture; whereas ethanol–water–surfactant mixture gives a cooling rate of 235 °C/s, both of which lie in ‘ultrafast cooling’ regime.