Browsing by Author "Prabhu, N.K."

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A comparative study on cooling performance of hot oil and molten salt media for industrial heat treatment
(ASM International, 2019) Rao, P.K.M.; Prabhu, N.K.
The present work presents a comprehensive comparative study on the cooling performance of hot oil and molten KNO3-NaNO2-NaNO3 eutectic mixture quench media. The study was conducted using a cylindrical Inconel probe of 16Ï† and 60mm length. Cooling curves at different locations in the probe were acquired using thermocouples-DAQ system. The temperature data was recorded in PC and was subsequently used to calculate spatially dependent transient heat flux at the metal quenchant interface. The heat extraction mechanism in hot oil and NaNO2 eutectic mixture was different. Quench heat transfer occurred in two stages namely boiling stage and convective cooling stage during quenching in molten NaNO2 eutectic mixture. In the case of hot oil, apart from these two stages, third stage of cooling namely vapor blanket stage was observed. A detailed study was conducted to compare magnitude and uniformity of heat extraction during each stage of quenching. Molten salt offered higher cooling rate and more spatial uniform cooling as compared to hot oil quench media. The non-uniformity in surface temperature during boiling stage in Inconel probe was 10 times lower in molten salt medium as compared to that observed in hot oil medium. However, the non-uniformity in surface temperature during convective cooling stage in both the media were comparable. Based on the distribution of characteristic cooling time (t85) calculated in quenched Inconel probe, higher and uniform hardness distribution is predicted in steel parts quenched in molten NaNO2 eutectic mixture media as compared. Â© Â© 2019 ASM InternationalÂ® All rights reserved.
Characterization of metals and salts-based thermal energy storage materials using energy balance method
(2019) Agarwala, S.; Prabhu, N.K.
Thermal energy storage technologies minimize the imbalance between energy production and demand. In this context, latent heat storage materials are of great importance as they have a higher density of energy storage as compared with the sensible heat storage materials. The present study involves the characterization of energy storage materials using an energy balance cooling curve analysis method. The method estimates the convective heat transfer coefficient in the solidification range to characterize the phase change materials for applications in energy storage. The method is more beneficial than the Computer Aided Cooling Curve analysis methods as it eliminates baseline calculations and the associated fitting errors. Metals (Sn) and salts (KNO3 and NaNO 3) were used in the present work. Phase change characteristics like the rate of cooling, liquidus and solidus temperatures, time for solidification, and enthalpy of phase change were estimated for both metals and salts. It was observed that the energy balance cooling curve analysis method worked very well for metals but not well suited for low conductivity salts. Salts could not be characterized since the thermal gradient existing within the salt sample was not considered in this method. 2019 Wiley Periodicals, Inc.
Characterization of metals and salts-based thermal energy storage materials using energy balance method
(John Wiley and Sons Inc. P.O.Box 18667 Newark NJ 07191-8667, 2019) Agarwala, S.; Prabhu, N.K.
Thermal energy storage technologies minimize the imbalance between energy production and demand. In this context, latent heat storage materials are of great importance as they have a higher density of energy storage as compared with the sensible heat storage materials. The present study involves the characterization of energy storage materials using an energy balance cooling curve analysis method. The method estimates the convective heat transfer coefficient in the solidification range to characterize the phase change materials for applications in energy storage. The method is more beneficial than the Computer Aided Cooling Curve analysis methods as it eliminates baseline calculations and the associated fitting errors. Metals (Sn) and salts (KNO3 and NaNO 3) were used in the present work. Phase change characteristics like the rate of cooling, liquidus and solidus temperatures, time for solidification, and enthalpy of phase change were estimated for both metals and salts. It was observed that the energy balance cooling curve analysis method worked very well for metals but not well suited for low conductivity salts. Salts could not be characterized since the thermal gradient existing within the salt sample was not considered in this method. © 2019 Wiley Periodicals, Inc.
Effect of surface treatment on wetting behavior of copper
(Elsevier Ltd, 2019) Kalgudi, S.; Pavithra, G.P.; Prabhu, N.K.; Koppad, P.G.; Venkate Gowda, C.; Satyanarayan, S.
Super-hydrophobic surfaces are very useful in cleaning activities. Surfaces with water contact angles above 150Â° are regarded as superhydrophobic surfaces. In the present study an attempt has been made to achieve superhydrophobicity on copper substrate by electrochemical etching and electro-deposition of Co-Ni alloy and Co-Ni-Graphene composite. A contact angle of about 105Â° was obtained on Cu surface with electro-deposited Co-Ni alloy and on electro-deposited Co-Ni-G alloy contact angle was found to be 106Â°. The contact angle was significantly higher at about 142Â° with electro etched surface. Corrosion test was carried out with electrochemically etched Cu. Electrochemical etching time was varied from 30 to 240 min. The electro-etched Cu substrate etched for 60 min. showed better corrosion resistance with a corrosion rate of 0.197 mm/year. The surface topography of both etched and electrodeposited samples was studied by atomic force microscopy (AFM) and the results were correlated with the wettability data. Â© 2019 Elsevier Ltd.
Wetting Behavior and Heat Transfer of Aqueous Graphene Nanofluids
(2016) Nayak, V.U.; Prabhu, N.K.
Aqueous graphene nanofluids having concentrations 0.01, 0.1, and 0.3 vol.% were used as heat transfer media during quenching of ISO 9950 inconel alloy probe. Contact angle measurements were carried out to assess the wettability of graphene nanofluids. Nanofluids showed better wettability compared to base water with over 16% reduction in their contact angles. The cooling performance of the quench media was assessed by cooling curve analysis during quenching of an instrumented inconel probe from 860��C into the quench medium. Recorded temperature readings showed longer vapor phase stage during quenching with nanofluids. The severity of nanofluids was found to be lower relative to water. During quenching with nanofluids, the estimated�spatiotemporal heat flux transients at the metal/quenchant interface showed that more heat was removed during the vapor phase stage of cooling. The present study brings out the possibility of using stable water-graphene nanoplatelet suspensions for quench heat treatment of�steel components requiring cooling severity between water and oil/polymer quenchants. � 2016, ASM International.