Augmentation of Fast Cooling Operation of Steel Plate by Modifying the Orientation of Cooling System and Properties of Coolants

Abstract

The fabrications of light weight aircrafts and nuclear reactors need steel with high tensile strength and moderate hardenability. For the production of steel with aforesaid properties, high cooling rate is required and this is not possible to achieve using the conventional technology. The challenging task is the elimination of film boiling phenomenon at high surface temperature. Spray cooling is considered as a potential replacement of conventional cooling methods like jet cooling and air atomized spray cooling when uniformity in temperature distribution, quenching rate and economic are considered together. However, still, the desired cooling rates cannot be fulfilled by the conventional spray cooling process. Hence, in the current work, various experimental investigations have been carried out to reduce the vapor film formation and also to achieve significant augmentation required by the various steel industries dealing the production of above mentioned materials. In the current work, for the experimentation, two types of indigenously designed and fabricated experimental set up have been used: (i) High mass flux spray cooling and (ii) Dropwise evaporative cooling. Before experimentation, for the identification of heat transfer mechanism, the spray and coolant characterization were performed. The orientation of the plate and the thermo-physical properties of the coolants are the factors that influence the formation and growth of vapor film. Therefore, in the current work, by conducting spray cooling experimentation on an inclined plate, the quenching rate and uniformity in temperature distribution are tried to improve. The heat transfer analysis has depicted that heat removal rate enhances (CHF from 1.21 MW/m2 to 1.46 MW/m2) with the increasing plate inclination up to 30o due to in the augmentation of replacement rate of vapor and liquid layer from the hot substrate and further increment in inclination declines the heat transfer rate (CHF from 1.46 MW/m2 to 1.26 MW/m2) due to decrease in residence time and lowering of the replacement rate of vapor and liquid layer. The experimental result has also shown that the nozzle to plate distance and the water flow rate are the important factors that affect the heat transfer rate. The optimum nozzle to plate distance and water flow rate is found to be 45mm and 16.67x10-5 m3/s, respectively and the maximum heat transfer rate is accomplished at a plate inclination of 30o. The achieved critical heat flux (CHF of 1.46 MW/m2) for 30o inclination is 20% higher as compared to the horizontal condition (CHF of 1.21 MW/m2). The lowest heat flux was found for 0o plate inclination. Then, the factors such as viscosity and surface tension of the coolant, controlling factor for the heat transfer coefficients are altered for further identification of the physical conditions to obtain the highest augmentation of heat transfer at very high temperature. The result shows that critical flux of 1.46 MW/m2 and 1.58 MW/m2 are achieved at 30o plate angle using water and Tween 20 added water as a coolant, respectively. Furthermore, after experimentation, the additives and hot plate interaction has been tried to reveal. The SDS present in SDS added water decomposes at higher temperature due to deposition followed by pyrolysis, and the newly formed compounds react with the various elements of steel to produce various compounds. The kinetics for the formation or decomposition of the formed compounds have been proposed and validated by using the information revealed by EDS and XRD analysis. Further, the roughness of the heat treated steel plate has been measured which shows an increasing trend in the roughness with the SDS concentration in water. In an another quenching strategy, the heat transfer rate is augmented by altering the vapor bubble formation and their coalescence rates. In the current work, these two factors were changed during the cooling by using dissolved carbon dioxide as coolant. Among different combinations of dissolved carbon dioxide added water, the highest critical heat flux (CHF of 1.67 MW/m2) has been obtained for horizontal case by using 8000 ppm concentration of dissolved carbon dioxide in water. Further, the combination of salt and Thums-up is also used as the coolant and an enhanced CHF of 1.7 MW/m2 has been achieved. However, the inclined condition of the plate has shown no effect on quenching rate in case of dissolved carbon dioxide (8000 ppm) added water and of Thums up (40 %) - NaCl (0.1M) added water mixtures. The comparison among boiling curves, heat transfer coefficients, average heat fluxes (AHF), coolant consumptions, critical heat fluxes (CHF) and its corresponding temperature (TCHF) has clearly established that Tween 20 added water is the appropriate cooling methodology for the fast quenching operation. Furthermore, the hardness, tensile strength and microstructure analysis also ascertain the improvement in hardness and tensile strength after quenching which fulfils the requirement of metal industr ies. The absence of any deposition or surface modification is also considered a way to encourage the metal processing industries to adopt the above discussed quenching technology