Heat transfer enhancement using air-atomized spray cooling with watereAl 2 O 3 nanofluid Satya V. Ravikumar a , Krishnayan Haldar a , Jay M. Jha a , Samarshi Chakraborty a , Ishita Sarkar a , Surjya K. Pal b , Sudipto Chakraborty a, * a Department of Chemical Engineering, IIT Kharagpur, 721302, India b Department of Mechanical Engineering, IIT Kharagpur, 721302, India article info Article history: Received 31 July 2014 Received in revised form 15 April 2015 Accepted 18 April 2015 Available online xxx Keywords: Air-assisted atomizer Heat transfer enhancement Nanofluid Run-out table application Spray evaporative cooling abstract The study deals with the air-atomized spray cooling using nanofluid as the cooling media for high heat flux applications. The nanofluid has been prepared by commercial Al 2 O 3 particles of diameter less than 13 nm and water. Heat transfer study has been carried out on a pre-heated steel specimen of dimensions 100 mm  100 mm  6 mm. The initial temperature of the plate which was subjected to air-atomized spray cooling was over 900  C. Various coolants consisting of 0.1% volumetric concentration of water eAl 2 O 3 mixture, with or without a dispersing agent (surfactant) were used for the study. The dispersing agents used are sodium dodecyl sulphate (SDS) and polyoxyethylene (20) sorbitan monolaurate (Tween 20). Inverse heat conduction software INTEMP has been used for estimating the surface heat flux and temperatures taking into account the measured internal temperature histories by the thermocouples during the cooling process. The results obtained using nanofluid coolants are compared with that of the results where pure water (filtered potable water) is used as a coolant. The analyses reveal that the cooling rate, critical heat flux and heat transfer coefficients are significantly enhanced when nanofluids are used as coolants in air-atomized spray process. Also, the nanofluid coolants with dispersing agent shows a better enhancement of heat transfer over that of the nanofluid without the dispersing media. The nanofluid with dispersing agent Tween 20 is found more effective than that of its counterpart. Overall, the percentage enhancement in cooling rate of all these nanofluids compared with pure water (filtered potable water) is 10.2% for watereAl 2 O 3 , 18.6% for watereAl 2 O 3 eSDS, and upto 32.3% for watereAl 2 O 3 eTween 20. © 2015 Elsevier Masson SAS. All rights reserved. 1. Introduction Spray cooling plays crucial role in many industries including nuclear reactors, electronic chips, coal gasification, fire suppression, heat treatment of steel plates in the run-out table, etc. The ability of high momentum liquid sprays to extract high heat flux at controlled rates from the metals parts operated above 600  C has made them invaluable in these applications. As the temperature of the metal parts is above Leidenfrost point [1], heat transfer occurs through boiling of spray droplets which can be called spray evap- orative cooling [2]. On the basis of the evaporation period of the droplet on the hot surface, different boiling regimes namely film boiling, transition boiling, nucleate boiling and single-phase forced convection occur sequentially during cooling. In recent years, steel of high strength is an essential component in many industrial applications. In steel processing industries, the steel of high strength properties are generally achieved during cooling of steel strips in the run-out table (ROT) [3]. Here, the cooling rate plays an important role in the thermo-metallurgical phase transformations which eventually govern the final mechan- ical properties of the steel. The major temperature ranges of cooling intensity which are important for phase transformations in steels are 900  Ce600  C and 900  Ce200  C [4,5]. Upon applying a faster cooling intensity, in the former temperature range, steel of dual phase microstructure (ferriteepearlite, pearliteemartensite, mar- tensiteeaustinite, etc.) can be found which exhibits high tensile strength and the later temperature range gives a fully martensite phase microstructure which provides ultra high strength in steels. * Corresponding author. Tel.: þ91 3222 283942 (office). E-mail address: sc@che.iitkgp.ernet.in (S. Chakraborty). Contents lists available at ScienceDirect International Journal of Thermal Sciences journal homepage: www.elsevier.com/locate/ijts http://dx.doi.org/10.1016/j.ijthermalsci.2015.04.012 1290-0729/© 2015 Elsevier Masson SAS. All rights reserved. International Journal of Thermal Sciences 96 (2015) 85e93