Enhancement of Spray Cooling in Transition and Nucleate Boiling Regimes by Using Novel Techniques

Abstract

Metals used in the manufacturing of submarine, multi-storey building, automotive industries and oil transporting pipelines require high tensile strength with moderate hardenability and these are directly related to the microstructure of the steel. The implementation of fast quenching technology in Run-Out Table (ROT) of a hot strip mill mitigates the above stated requirement. However, the development of fast quenching process is still a challenging task for the current generation researchers due to the early onset of Leidenfrost effect. Furthermore, the literature also does not disclose any fast quenching methodology which is able to mitigate the requirements. Therefore, in the current research, an attempt has been made to develop fast quenching techniques for the mitigation of the requirement of metal processing industries. In the current research, initially the role of surface, chemical and mechanical modification methodologies in heat extraction process have been separately investigated. In case of surface modification, PEG is added to modify the surface during cooling in the favourable direction of heat transfer. For this type of modification, initially, theoretical calculations have been performed to obtain the exact conditions. For the chemical modification, various additives such as Al2O3, dextrose, soapnut added water have been used for the attainment of fast quenching technology. In addition to the above, the third implemented technique for the achievement in enhancement is mechanical modification. For this, by altering the inertia of the plate, augmentation is focused. In case of mechanical modification, inertia is altered by conduction cooling on a moving plate. Then, the process depicting the combined modification has been investigated. For the combined modification, water with various additives are used to cool the plate which is in moving condition. For the experimentation, two experimental set-ups were fabricated i.e. one for static plate and another is for moving plate. In addition to the above, for the determination of heat transfer mechanism, dropwise experiments were carried out and for this purpose another separate experimental set-up was used. For the prediction of surface heat flux and temperatures, INTEMP software has been used. Among Al2O3 nanofluid, Dextrose and Soapnut added water, the heat extraction capacity is found to be maximum (CHF = 1.72MW/m2) in case of spray quenching conducted by nanofluid (0.15 % Al2O3). This is due to the alteration of thermo-physical properties in the favourable direction of heat transfer which enhances the heat transfer co-efficient, latent heat extraction period and the vapour film instability. The above stated value becomes 2.2MW/m2, when along with the chemical modification, mechanical modification is also performed. In case of Polyethylene glycol (PEG) added water spray, the surface morphology analysis clearly ensures the surface modification by PEG. According to the droplet dynamics, the exact condition (VF/CF ˃ 30×10-4) describing the hydrophilic behaviour which is essential for high heat removal rate are determined. The experimental result illustrates that the conditions indicated by the modelling lead to the enhancement and as a consequence, an average heat flux of 1.82 MW/m2 is obtained. However, in this case, the achieved critical heat flux is lower than the maximum value obtained in case of chemical modification. In case of mechanical modification, augmentation is noticed. However, the intensity of enhancement is not significant. Furthermore, the maximum enhancement (CHF = 2.23 MW/m2) is obtained in the presence of chemical, mechanical and surface modifications. Finally, a comparative study is performed to identify the appropriate coolant for various cases.