Faculty Publications

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Author: search.filters.author.Prabhu, K.N.Subject: search.filters.subject.Cooling

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    Effect of thermal contact heat transfer on solidification of Pb-Sn and Pb-free solders
    (Elsevier Ltd, 2007) Chellaih, T.; Kumar, G.; Prabhu, K.N.
    The effect of thermal contact heat transfer on the solidification of spherical droplets of four solder alloys, namely, Sn-37Pb, Sn-9Zn, Sn-0.7Cu and Sn-3.5Ag, was studied using SOLIDCAST simulation package. A significant drop in the arrest time was observed for increase in heat transfer coefficient from 1000 to 2000 W/m2 K. Effect of contact conductance and thermal diffusivity of solder alloys on arrest time is quantified by the power relation, ? = m(?{symbol})n where ? and ?{symbol} are defined as arrest time and heat transfer parameters, respectively. Experiments were also carried out to investigate the effect of cooling rate on solidification behaviour of the solder alloys used in simulation. The results indicated the significant effect of mould material on interfacial heat flux and metallurgical microstructure. © 2005 Elsevier Ltd. All rights reserved.
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    Effect of modification melt treatment and chilling on eutectic arrest temperature and time during solidification of A357 alloy
    (2011) Prabhu, K.N.; Hegde, S.
    Thermal analysis technique has been recognised as an efficient non-destructive tool to assess the degree of modification in Al-Si alloys. Apart from chemical modification, chilling refines the microstructure. This is particularly significant as majority of Al-Si alloys are cast in metallic moulds. In the present study, the interaction between chilling and modification melt treatment is investigated to assess their effect on thermal analysis parameters using computer aided cooling curve analysis. For modified alloys, the depression of the eutectic arrest temperature was significant at higher cooling rates. The eutectic arrest temperature and time were correlated with the cooling rate using a power law. High cooling regime in thermal analysis plots was attributed to the combined effect of chilling and modification melt treatment on heat transfer. © 2011 Institute of Materials, Minerals and Mining.
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    Characterisation of water base copper nanoquenchants by standard cooling curve analysis
    (2011) Ramesh, G.; Prabhu, K.N.
    Water base copper nanofluids having concentrations varying from 0?001 to 0?1 vol.-% were prepared and used as quench media for immersion quenching. Cooling curve analyses were carried out by using a standard ISO/DIS 9950 quench probe. An inverse heat conduction model is employed to estimate the metal/nanoquenchant interfacial heat flux transients from the measured temperature field and thermophysical properties of the quench probe material. The addition of copper nanoparticles had a significant effect on the occurrence of the vapour blanket stage and nucleate boiling stage. Furthermore, all six cooling curve parameters were found to be altered by adding nanoparticles to water. The contact angle of water decreased from 67 to 39° by adding 0?1 vol.-% of copper nanoparticles indicating the improved wettability of nanofluids. The heat flux curve shows a maximum initially then drops rapidly during quenching. The peak cooling rate and heat flux of water increased by adding copper nanoparticles up to 0?01 vol.-%. Both parameters decreased with further increase in concentration of nanoparticles. The results suggest that the quench severity of water could be altered by adding copper nanoparticles. © 2011 IHTSE Partnership.
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    Effect of cooling rate during solidification of Sn-9Zn lead-free solder alloy on its microstructure, tensile strength and ductile-brittle transition temperature
    (Elsevier Ltd, 2012) Prabhu, K.N.; Deshapande, P.; Satyanarayan, S.
    Solidification rate is an important variable during processing of materials, including soldering, involving solidification. The rate of solidification controls the metallurgical microstructure at the solder joint and hence the mechanical properties. A high tensile strength and a lower ductile-brittle transition temperature are necessary for reliability of solder joints in electronic circuits. Hence in the present work, the effect of cooling rate during solidification on microstructure, impact and tensile properties of Sn-9Zn lead-free solder alloy was investigated. Four different cooling media (copper and stainless steel moulds, air and furnace cooling) were used for solidification to achieve different cooling rates. Solder alloy solidified in copper mould exhibited higher cooling rate as compared to other cooling media. The microstructure is refined as the cooling rate was increased from 0.03 to 25 °C/s. With increase in cooling rate it was observed that the size of Zn flakes became finer and distributed uniformly throughout the matrix. Ductile-to-brittle transition temperature (DBTT) of the solder alloy increased with increase in cooling rate. Fractured surfaces of impact test specimens showed cleavage like appearance and river like pattern at very low temperatures and dimple like appearance at higher temperatures. The tensile strength of the solder alloy solidified in Cu and stainless moulds were higher as compared to air and furnace cooled samples. It is therefore suggested that the cooling rate during solidification of the solder alloy should be optimum to maximize the strength and minimize the DBTT. © 2011 Elsevier B.V.
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    Effect of addition of aluminum nanoparticles on cooling performance and quench severity of water during immersion quenching
    (ASTM International, 2012) Ramesh, G.; Prabhu, K.N.
    In the present work, the effect of the addition of aluminum nanoparticles in concentrations varying from 0.001 to 0.5 vol. % on the cooling performance and quench severity of water during immersion quenching is investigated. The results of cooling curve analyses show that an increase in nanoparticle concentration increased the cooling rates at critical temperatures up to 0.05 vol. % and decreased them thereafter. The transition from the vapor blanket stage to the nucleate boiling stage was also altered by quenching in nanofluids. A finite difference heat transfer program was employed to generate cooling curves at different values of heat transfer coefficient from thermo-physical properties of the quench probe material. A Grossmann H quench severity versus cooling rate curve was established, and from this curve, the H factors of prepared nanofluids were estimated. An increase in nanoparticle concentration up to 0.05 vol. % resulted in an increase of the H value of water from 63 m -1 to 93 m-1, and any further increase in the concentration of nanoparticles resulted in a decrease in H. The results suggest both the enhancement and the deterioration of the cooling performance of water by the addition of aluminum nanoparticles. Copyright © 2012 by ASTM International.
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    Effect of addition of aluminum nanoparticles on cooling performance and quench severity of water during immersion quenching
    (2012) Ramesh, G.; Prabhu, K.N.
    In the present work, the effect of the addition of aluminum nanoparticles in concentrations varying from 0.001 to 0.5 vol. % on the cooling performance and quench severity of water during immersion quenching is investigated. The results of cooling curve analyses show that an increase in nanoparticle concentration increased the cooling rates at critical temperatures up to 0.05 vol. % and decreased them thereafter. The transition from the vapor blanket stage to the nucleate boiling stage was also altered by quenching in nanofluids. A finite difference heat transfer program was employed to generate cooling curves at different values of heat transfer coefficient from thermo-physical properties of the quench probe material. A Grossmann H quench severity versus cooling rate curve was established, and from this curve, the H factors of prepared nanofluids were estimated. An increase in nanoparticle concentration up to 0.05 vol. %resulted in an increase of the H value of water from 63 m 1 to 93 m 1, and any further increase in the concentration of nanoparticles resulted in a decrease in H. The results suggest both the enhancement and the deterioration of the cooling performance of water by the addition of aluminum nanoparticles. Copyright © 2012 by ASTM International.
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    Effect of boundary heat transfer coefficient and probe section size on cooling curves during quenching
    (ASTM International, 2012) Ramesh, G.; Prabhu, K.N.
    In the present work the effect of boundary heat transfer coefficient and section size of quench probe material on cooling curves was investigated by using finite difference heat transfer based SolidCast software. Simulations were carried out at different combinations of heat transfer coefficient and quench probe diameter and thermal history at the geometric center of the probe was estimated to generate cooling curves. Simulation results show that both boundary heat transfer coefficient and quench probe diameter had a significant effect on the average cooling rate. A relationship between Grossmann quench severity (H), thermal conductivity of material, size of the probe, and average cooling rate was established. By using this model, for a known quench medium, probe size, and material it is possible to predict the average cooling rate of the probe. On the other-hand, for a given material and required cooling rate, cooling severity required from the quench media could be predicted and accordingly an appropriate quench medium can be selected. © 2012 by ASTM International.
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    Wetting Behavior and Heat Transfer of Aqueous Graphene Nanofluids
    (Springer New York LLC barbara.b.bertram@gsk.com, 2016) Nayak, U.V.; Prabhu, K.N.
    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.
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    Effect of Polymer Concentration on Wetting and Cooling Performance During Immersion Quenching
    (Springer Boston, 2016) Ramesh, G.; Prabhu, K.N.
    The effect of varying concentrations (0 to 100 vol pct) of glycol polymer solution on wetting kinetics, kinematics, and cooling performance during immersion quenching was studied by using goniometry, online video imaging, and cooling curve analysis techniques. An increase in concentration of the polymer solution resulted in improved wettability and accelerated spreading kinetics of the quench medium. The quench medium showed medium-fast-nonuniform, fast-uniform, slow-uniform, explosive/rapid, repeated, and slow-nonuniform rewetting phenomena depending on the concentration of the polymer solution. The collapse of the vapor film was by an instantaneous rupture process in the quench medium containing more water and by nucleation of bubbles caused by the selective rupture process in the quench medium enriched with polymer. The quench medium consisting of an equal amount of water and polymer showed an explosive collapse of the vapor film on the quench probe surface. The nature of the wetting front was uniform with polymer quench media except at 100 vol pct concentration of polymer quenchant. There was enhancement in the cooling performance of the quench medium, which was enhanced for a lower volume concentration of the polymer solution. However, an increase in the concentration of the polymer resulted in a decreased cooling performance. The cooling of the probe was more uniform with polymer quenchants (5 to 25 vol pct), which exhibited fast and uniform rewetting. Polymer quenchants (75 to 100 vol pct) that exhibited repeated and slow-nonuniform rewetting showed large variation in heat transfer over the quench probe surface. © 2015, The Minerals, Metals & Materials Society and ASM International.
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    Wetting and Cooling Performance of Vegetable Oils during Quench Hardening
    (John Wiley and Sons Inc. P.O.Box 18667 Newark NJ 07191-8667, 2016) Ramesh, G.; Prabhu, K.N.
    Wetting kinetics, kinematics, and cooling performance of vegetable oils (sunflower, gingelly, palm, and coconut oils) during quenching of Inconel 600 probe were studied using goniometry, online video imaging, and cooling curve analysis. The results were compared with a conventional mineral oil quench medium. Improved wettability was obtained for vegetable oils with lower viscosity. Cooling curve analyses showed three stages of cooling for both mineral and vegetable oils. Video imaging of the quenching process and differential scanning calorimetry analysis confirmed that the first stage of cooling was caused by the formation of vapor film in mineral oil and due to the occurrence of a heated liquid layer around the quench probe surface in vegetable oils. Vegetable oils showed continuous boiling phenomenon during the convective cooling stage of quenching. The cooling performance of vegetable oils was found to depend on the concentration of mono-unsaturated fatty acid. The heat extracting capability of vegetable oils with lower mono-unsaturated fatty acid oils was found to be higher. However, no correlation was observed between fatty acid composition and uniformity of heat transfer. When compared to mineral oil quenching, vegetable oil quenching produced faster wetting kinematics and better cooling performance. © 2016 Wiley Periodicals, Inc.