Faculty Publications
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Publications by NITK Faculty
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Item Inverse modeling of heat transfer with application to solidification and quenching(2002) Prabhu, K.N.; Ashish, A.A.The inverse modeling of heat transfer involves the estimation of boundary conditions from the knowledge of thermal history inside a heat conducting body. Inverse analysis is extremely useful in modeling of contact heat transfer at interfaces of engineering surface during materials processing. In the present work, the one-dimensional transient heat conduction equation was inversely modeled in both cartesian as well as cylindrical coordinates. The model is capable of estimating heat flux transients, chill surface temperature, and total heat flow from the source to the sink for an input of thermal history inside the sink. The methodology was adopted to solve boundary heat transfer problems inversely during solidification and quenching. The response of the inverse solution to measured sensor data was studied by carrying out numerical experiments involving the use of varying grid size and time steps, future temperatures, and regularization techniques.Item Effect of surface roughness on metal/quenchant interfacial heat transfer and evolution of microstructure(Elsevier Ltd, 2007) Prabhu, K.N.; Fernandes, P.In the present work, the effect of surface roughness on heat transfer rates in various quenchants was determined. The heat flux transients at the probe/quenchant interface were estimated by inverse modeling of heat conduction during end quenching of stainless steel probes with three different surface roughness (grooved, Ra = 3.0 and 1 ?m). Heat transfer during quenching was correlated with the hardness obtained for medium carbon AISI 1060 steel specimens. The effect of surface roughness on heat transfer rate during quenching in water and brine was significant for rough surface whereas its effect on heat transfer rate is only marginal in high viscosity oil quenchants. A fully martensitic structure was observed with grooved surface subjected to water quenching. With a smooth surface a mixed microstructure was obtained. The oil quenched specimens were found to be less sensitive to surface roughness. © 2005 Elsevier Ltd. All rights reserved.Item Effect of section size and agitation on heat transfer during quenching of AISI 1040 steel(2007) Fernandes, P.; Prabhu, K.N.In the present work an attempt has been made to determine the heat flux transients during quenching of Ø28 mm × 56 mm height and Ø44 mm × 88 mm height AISI 1040 steel specimens during lateral quenching in brine, water, palm oil and mineral oil. The heat flux transients were estimated by inverse modeling of heat conduction. The variation of heat flux transients with surface temperature for different quenching media is investigated. Higher peak heat flux transients are obtained for 28 mm diameter specimen than 44 mm diameter specimen during quenching in aqueous media. However quenching with oil media shows opposite results. Agitation of quenching medium increases the peak heat flux during the quenching of steel specimen in all the quenching media. Peak hardness is obtained at the surface and with smaller diameter specimens during agitation. © 2006 Elsevier B.V. All rights reserved.Item Estimation of spatially dependent heat flux transients during quenching of inconel probe in molten salt bath(ASTM International, 2017) Pranesh Rao, K.M.; Prabhu, K.N.Several industrial heat treatment processes, such as martempering and austempering, require a quench bath to be maintained at a temperature ranging between 150°C–600°C. Molten salts, molten alkali, and hot oils are the preferred quenchants for these processes. Molten salts and molten alkali are preferred over hot oil because they possess properties like wide operating temperature range, excellent thermal stability, and tolerance for contaminants. In the present work, the performance of a molten potassium nitrate (KNO3) quench bath was analyzed with an Inconel probe that measured 60 mm in height and 12.5 mm in diameter. The probe was heated to 850°C and subsequently quenched in a bath maintained at 450°C. Cooling curves at different locations of the probe were recorded using the K-type thermocouples inserted into the probe. Spatially dependent transient heat flux at the metal/quenchant interface was estimated using inverse heat conduction technique. The existence of two stages of quenching—boiling stage and convection stage—was confirmed by analyzing the heat flux. The heat transfer coefficient was calculated based on heat flux obtained by the inverse method. The nonuniformity in heat transfer along the length of the probe was quantified by calculating the range of surface temperatures at each instance. The hardness distribution in an AISI 4140 steel was predicted using the temperature distribution in the Inconel probe and obtained using inverse method. Uneven distribution of hardness predicted in the probe was attributed to the nonuniform cooling of the probe during quenching. © © 2017 by ASTM International.Item Effect of Bath Temperature on Cooling Performance of Molten Eutectic NaNO3-KNO3 Quench Medium for Martempering of Steels(Springer Boston, 2017) Pranesh Rao, K.M.; Prabhu, K.N.Martempering is an industrial heat treatment process that requires a quench bath that can operate without undergoing degradation in the temperature range of 423 K to 873 K (150 °C to 600 °C). The quench bath is expected to cool the steel part from the austenizing temperature to quench bath temperature rapidly and uniformly. Molten eutectic NaNO3-KNO3 mixture has been widely used in industry to martemper steel parts. In the present work, the effect of quench bath temperature on the cooling performance of a molten eutectic NaNO3-KNO3 mixture has been studied. An Inconel ASTM D-6200 probe was heated to 1133 K (860 °C) and subsequently quenched in the quench bath maintained at different temperatures. Spatially dependent transient heat flux at the metal–quenchant interface for each bath temperature was calculated using inverse heat conduction technique. Heat transfer occurred only in two stages, namely, nucleate boiling and convective cooling. The mean peak heat flux (qmax) decreased with increase in quench bath temperature, whereas the mean surface temperature corresponding to qmax and mean surface temperature at the start of convective cooling stage increased with increase in quench bath temperature. The variation in normalized cooling parameter t85 along the length of the probe increased with increase in quench bath temperature. © 2017, The Minerals, Metals & Materials Society and ASM International.Item Effect of section thickness on heat transfer during quenching in vegetable oils(ASTM International, 2018) Nayak, U.V.; Prabhu, K.N.In the present work, mineral, sunflower, karanja, and neem oil were used as quench media. 304 stainless steel probes with diameters of 25 mm and 50 mm were quenched in these oils to assess the effect of section diameter on heat transfer during quenching. Cooling curve analysis was carried out by instrumenting the probes at various locations with thermocouples. The heat extraction ability of oil quench media was quantified using an inverse heat conduction method. Thermal data and the predicted hardness values showed the suitability of nonedible vegetable oils as potential quenchants to heat treat steels. The predicted hardness was higher during quenching in karanja oil compared to other oil media. © © 2018 by ASTM International.Item Heat transfer during quenching of inconel probe in non-edible vegetable oils; Wärmeübertragung während des Abschreckens der Inconel-Sonde in nicht essbaren Pflanzenölen(Carl Hanser Verlag Kolbergerstrasse 22 Munchen D-81679, 2018) Nayak, U.V.; Prabhu, K.N.Non-edible vegetable oils of Karanja and neem were used as quench media in the present investigation. The cooling characteristics of quenchants were obtained using Inconel 600 alloy probe and were compared with that of a fast-quenching mineral oil quench medium. Spatiotemporal heat flux was estimated using inverse heat conduction method. Heat removed from the probe during quenching showed higher and faster heat extraction by karanja oil quench medium compared to the mineral oil. Heat transfer characteristics of neem oil were comparable with the mineral oil. Rewetting time and temperature of vegetable oils were found to be higher than that of the mineral oil. © Carl Hanser Verlag GmbH & Co. KGItem Assessment of PCM-container interfacial heat transfer using a hot/cold probe technique(John Wiley and Sons Inc. P.O.Box 18667 Newark NJ 07191-8667, 2019) Sudheer, R.; Prabhu, K.N.A novel technique for assessing heat transfer characteristics of salt-based phase change materials (PCM) was proposed here. The method is based on solution to inverse heat conduction problem. Nanoparticles (Graphite, Graphene, and multi wall carbon nanotube [MWCNT]) were dispersed in the PCM (KNO3) to assess their respective influence on heat transfer in the PCM. Graphite added PCM offered highest heat flow values and heating rates, while the pure salt-PCM offered the least. The probe material had a significant influence on the heat transfer rates at the PCM-probe interface. © 2018 Wiley Periodicals, Inc.Item An experimental approach based on inverse heat conduction analysis for thermal characterization of phase change materials(Elsevier B.V., 2020) Agarwala, S.; Prabhu, K.N.A new method based on solution to inverse heat conduction problem for the assessment of solidification parameters of PCM salts has been proposed. The method estimates the mold -salt interfacial heat flux and it is used to calculate the latent heat of salt PCMs using calorimetry based energy balance equations. This method is more accurate compared to Computer Aided Cooling Curve Analysis (CACCA) techniques as it eliminates the drawbacks involved with base line fitting calculations and errors introduced due to the improper selection of solidification points. Pure salt PCMs such as KNO3 and solar salt were used for the validation of this technique. Both air and furnace cooling were adopted to demonstrate the effect of cooling rate on solidification characteristics. The wettability of salt samples on mild steel surface was analyzed to account for the difference in the thermal behavior of salts. © 2020 Elsevier B.V.Item A quantitative approach for thermal characterization of phase change materials(ASTM International, 2021) Agarwala, S.; Prabhu, K.N.A quantitative method for the calculation of phase change parameters of salt-based phase change materials (PCMs) has been proposed. This technique involves the estimation of mold-salt interfacial heat flux by solving Fourier's law of heat conduction within the salt and using it for the calculation of phase change enthalpy of salt PCMs. Radial heat transfer was ensured by keeping the length to diameter (L/D) ratio of the mold equal to 5. The proposed method eliminates any drawbacks involved with sample size, reference material, the baseline fitting calculations, and the errors introduced due to the selection of solidification points. Pure salt PCMs such as potassium nitrate (KNO3), sodium nitrate (NaNO3), and solar salt mixture (60 wt. % NaNO3 + 40 wt. % KNO3) were used for validation of this technique. The thermal behaviors of the salt and the mold during solidification of the salt sample were analyzed, and solidification characteristics such as cooling rate, solidification time, and phase change enthalpy of PCMs were determined. © © 2021 by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959