Enhancement of heat transfer rate of high mass flux spray cooling by ethanol-water and ethanol-tween20-water solution at very high initial surface temperature

•Obligations in achieving high cooling rates by spray cooling are overcome.•Heat removal is enhanced by spray cooling with coolants-ethanol and tween20 in water.•Achieved CHF with Tween 20-ethanol-water mixture is 1.6 times higher than pure water.•Maximum obtained cooling rate is 141°C/s in case of ethanol-water-Tween 20 mixture. Spray cooling is an efficient cooling technology over conventional cooling methods such as jet cooling on run-out table. However, the achieved cooling rates are still not enough for some specific applications. The main obligation in achieving high cooling rates is the occurrence of film boiling phenomenon. In the absence of any information on the heat transfer augmentation techniques of spray cooling at very high initial surface temperatures (∼900°C), the present work deals with enhancement of spray cooling at the aforesaid initial temperature by using different coolants which enhance the heat removal rate by creating high heat transfer area and decreasing the stability of the vapour and liquid film on the hot plate. For the experimental investigation, spray cooling experiments were conducted at 900°C initial surface temperature on a 6mm thick AISI 304 steel plate (100×100mm) by using different coolants. The surface temperature and heat flux have been calculated using INTEMP software. For the understanding of heat transfer mechanism, the coolants properties at different concentrations and spray behavior at different flow rates were measured. The ethanol-water spray cooling demonstrates that the heat removal rate increases with increasing ethanol concentration by decreasing contact angle. The reduction in the contact angle results in increasing heat transfer area and decreasing the vapour-bubble coalescence rate. However, beyond ethanol concentration of 500ppm, the excessive occurrence of the foaming decreases the heat removal rate. Further, the heat transfer rate is tried to enhance by adding tween-20 surfactant which lowers the contact angle significantly with the controlled characteristics of foaming. In the case of ethanol-water-tween 20 mixture spray, the achieved critical heat flux (2.1MW/m2) is 1.6 times that of pure water (1.3MW/m2). Due to the above mentioned favorable conditions for fast cooling, a maximum cooling rate of 141°C/s is achieved.