Journal Articles
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Item Item Heat transfer at the metal/chill interface during solidification of commercially pure aluminium square bar castings with cast iron chill at one end was investigated. Experiments were carried out for different chill thicknesses and superheats. The inner surface temperature of the chill initially was found to increase at a faster rate for higher superheats. The effect of chill thickness on the inner surface temperature of the chill was observed only after the heat from the solidifying casting had sufficient time to diffuse to the interior of the chill material. Inverse analysis of the non-linear one-dimensional Fourier heat conduction equation indicated the occurrence of peak heat flux at the end of filling of the mould. The effect of superheat on heat flux was minimal after filling. However, the effect of chill thickness had a significant effect on the heat flux after the occurrence of peak heat flux. Higher heat flux transients were estimated for castings poured at higher superheats. The corresponding heat transfer coefficients were also estimated and reported. The heat flux model presented in this work can be used for determination of casting/chill interfacial heat flux as a function of chill thickness and superheat. These heat flux transients could be used as boundary conditions during numerical simulation of solidification of the casting. © 2002 Elsevier Science B.V. All rights reserved.(Effect of chill thickness and superheat on casting/chill interfacial heat transfer during solidification of commercially pure aluminium) Gafur, M.A.; Haque, M.N.; Prabhu, K.2003Item Heat transfer during the solidification of an Al-Cu-Si alloy (LM4) and commercial pure tin in single steel, graphite, and graphite-lined metallic (composite) molds was investigated. Experiments were carried out at three different superheats. In the case of composite molds, the effect of the thickness of the graphite lining and the outer wall on heat transfer was studied. Temperatures at known locations inside the mold and casting were used to solve the Fourier heat conduction equation inversely to yield the casting/mold interfacial heat flux transients. Increased melt superheats and higher thermal conductivity of the mold material led to an increase in the peak heat flux at the metal/mold interface. Factorial experiments indicated that the mold material had a significant effect on the peak heat flux at the 5% level of significance. The ratio of graphite lining to outer steel wall and superheat had a significant effect on the peak heat flux in significance range varying between 5 and 25%. A heat flux model was proposed to estimate the maximum heat flux transients at different superheat levels of 25 to 75°C for any metal/mold combinations having a thermal diffusivity ratio (?R) varying between 0.25 and 6.96. The heat flow models could be used to estimate interfacial heat flux transients from the thermophysical properties of the mold and cast materials and the melt superheat. Metallographic analysis indicated finer microstructures for castings poured at increased melt superheats and cast in high-thermal diffusivity molds.(Effect of superheat, mold, and casting materials on the metal/mold interfacial heat transfer during solidification in graphite-lined permanent molds) Prabhu, K.; Suresha, K.M.2004Item Heat flow at the casting/mold interface was assessed and studied during solidification of Al-Cu-Si (LM 21) alloy in preheated cast iron molds of two different thicknesses, coated with graphite and alumina based dressings. The casting and the mold were instrumented with thermocouples connected to a computer controlled temperature data acquisition system. The thermal history at nodal locations in the mold and casting obtained during experimentation was used to estimate the heat flux by solving the one-dimensional inverse heat conduction problem. The cooling rate and solidification time were measured using the computer-aided cooling curve analysis data. The estimated heat flux transients showed a peak due to the formation of a stable solid shell, which has a higher thermal conductivity compared with the liquid metal in contact with the mold wall prior to the occurrence of the peak. The high values of heat flux transients obtained with thin molds were attributed to mold distortion due to thermal stresses. For thin molds, assumption of Newtonian heating yielded reliable interfacial heat transfer coefficients as compared with one-dimensional inverse modeling. The time of occurrence of peak heat flux increased with a decrease in the mold wall thickness and increase in the casting thickness. © ASM International.(Casting/mold thermal contact heat transfer during solidification of Al-Cu-Si alloy (LM 21) plates in thick and thin molds) Prabhu, K.; Chowdary, B.; Venkataraman, N.2005Item Wetting kinetics, kinematics and heat transfer characteristics of pongamia pinnata vegetable oil for industrial heat treatment(Elsevier Ltd, 2014) Ramesh, G.; Prabhu, K.The suitability of pongamia pinnata vegetable oil as cooling medium for quench heat treatment was investigated. Wetting kinetics, kinematics and heat transfer characteristics of pongamia oil during immersion quenching of hot Inconel probe were determined and compared with palm and mineral oils. A comparison of the relaxation of contact angle indicated early attainment of equilibrium contact angle for pongamia oil droplet on Inconel substrate. The equilibrium contact angle value of pongamia oil was in between mineral and palm oils. However, the spreading kinetics was faster with pongamia oil. Pongamia oil showed the formation of a stable vapour film on the probe surface during quenching. This phenomenon was not observed in palm oil and other vegetable oils reported in literature. Pongamia pinnata oil exhibits uniform nature of wetting front, intermediate rewetting temperature and less variations in rewetting temperatures during quenching. The hardening power of pongamia oil was higher than palm and mineral oils and the cooling of the probe was more uniform during quenching in pongamia oil. The heat transfer characteristics of pongamia oil were found to be superior to palm and mineral oils after film/transition boiling. Pongamia oil showed lower heat transfer but more uniform cooling during film boiling. © 2014 Elsevier B.V.Item Wetting and cooling performance of mineral oils for quench heat treatment of steels(Iron and Steel Institute of Japan, 2014) Ramesh, G.; Prabhu, K.In the present work, wetting kinetics, kinematics and heat transfer characteristics of mineral oils having varying thermo-physical properties sourced from different suppliers were investigated using contact angle, online video imaging and cooling curve analysis techniques. The relaxation behavior of mineral oils of low viscosity and surface tension on Inconel substrate indicated improved wettability and fast spreading kinetics while mineral oils of high viscosity and surface tension showed reduced wettability and slower spreading kinetics. Further, the spreading behavior of mineral oils of lower viscosity and density showed the absence of viscous regime. During rewetting, formation of double wetting fronts and more uniform nature of wetting front were observed with mineral oils of high viscosity and flash point whereas no additional wetting front was observed for mineral oils of low viscosity and flash point. Among the convectional/fast/hot mineral oils, higher wetting front velocity and cooling rate were obtained for low viscosity mineral oil. The heat extracting capability of high viscosity mineral oils was higher during vapour and nucleate boiling and lower during liquid cooling stage. Further, highly viscous mineral oils showed uniform heat transfer compared to mineral oils having low viscosity. © 2014 ISIJ.Item Spatial dependence of heat flux transients and wetting behavior during immersion quenching of inconel 600 probe in brine and polymer media(Springer Boston, 2014) Ramesh, G.; Prabhu, K.Cooling curve analysis of Inconel 600 probe during immersion quenching in brine and polymer quench media was carried out. Thermal histories at various axial and radial locations were recorded using a high-speed data acquisition system and were input to an inverse heat-conduction model for estimating the metal/quenchant heat flux transients. A high performance smart camera was used for online video imaging of the immersion quenching process. Solution to two-dimensional inverse heat-conduction problem clearly brings out the spatial dependence of boundary heat flux transients for a Inconel 600 probe with a simple cylindrical geometry. The estimated heat flux transients show large variation on axial as well as radial directions of quench probe surface for brine quenching. Polymer quenching showed less variation in metal/quenchant heat flux transients. Shorter durations of vapor film, higher rewetting temperatures, and faster movement of wetting front on quench probe surface were observed with brine quenching. Measurement of dynamic contact angle showed better spreading and good wettability for polymer medium as compared to brine quenchant. The solid-liquid interfacial tension between polymer medium and Inconel substrate was lower compared with that of solution. Rewetting and boiling processes were nonuniform and faster on quench probe surface during immersion quenching in brine solution. For the polymer quench medium, slow rewetting, uniform boiling and repeated wetting were observed. © 2014 The Minerals, Metals & Materials Society and ASM International.Item Cooling performance of select mineral oil and polymer quenchants(ASTM International, 2014) Tiwary, V.; Prabhu, K.In the present investigation, quench severity was determined for industrial oil quenchants and varying concentrations of PAG polymer. Viscosity, thermal conductivity, density, flash point, and fire point were measured for all quenchants. Cooling curve parameters were determined based on cooling curve analysis ISO/DIS 9950 technique. An Inconel 600 probe was used for this purpose. Severity of quenching was determined based on the Grossmann technique. Viscosity and thermal conductivity had a significant effect on quench severity. Heat flux and HTC at the metal/quenchant interface were computed by both lumped capacitance and Inverse modeling techniques. The effect of ultrasonic and Tensi agitation on cooling curve parameters was assessed. Polymer quenchants of lower concentrations showed marginally higher heat transfer rates compared to water particularly during unagitated condition. © © 2014 by ASTM International.Item Comparative study of wetting and cooling performance of polymer-salt hybrid quench medium with conventional quench media(Taylor and Francis Ltd., 2015) Ramesh, G.; Prabhu, K.Wetting kinetics, kinematics, and cooling performance of a polymer-salt hybrid quenchant were investigated. The rewetting phenomenon for brine, water, polymer, and polymer-salt hybrid solutions was characterized as rapid uniform, fast non-uniform, slow uniform, and fast uniform processes, respectively. A dimensionless rewetting time was proposed to assess the nature of the wetting front. The hybrid quenchant showed higher heat transfer during vapor and transition boiling and lower heat transfer during nucleate boiling and convective cooling. The presence of salt in the hybrid solution resulted in early destabilization of the vapor film and an increase in wetting front velocity and rewetting temperature. The polymer constituent delayed the rewetting phenomenon. © 2015 Taylor & Francis Group, LLC.