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

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    Severity of quenching and kinetics of wetting of nanofluids and vegetable oils
    (ASTM International, 2010) Jagannath, V.; Prabhu, K.N.
    In the present work, the suitability of vegetable oil blends with mineral oil and alumina based nanofluids as quench media for industrial heat treatment was investigated. Sunflower oil, palm oil, and mineral oil were used for preparing the blends. Alumina based nanofluids of varying concentrations ranging from 0.01-4 % were used. The size of alumina particles was about 50 nm. The severity of quenching and heat transfer coefficients were estimated during quenching of copper probes. Heat transfer coefficients were estimated using a lumped heat capacitance model. The static contact angle was measured on copper substrates having a surface texture similar to the probes used for estimation of heat transfer coefficients. A dynamic contact angle analyzer was used for this purpose. The measured contact angles of nanofluids on copper were high compared to oils, indicating poor wetting by quench media that are polar in nature. Wetting characteristics had a significant effect on heat transfer coefficients estimated during quenching. Copyright © 2009 by ASTM International.
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    Heat transfer during quenching and assessment of quench severity - A review
    (ASTM International, 2010) Prabhu, K.N.; Fernandes, P.
    In the heat treatment of steel, quenching is done to prevent ferrite or pearlite formation and allows formation of bainite and martensite. For a particular grade of steel, the effectiveness of quenching depends on the cooling characteristics of the quenching medium. The cooling rate is not a constant throughout the quenching process; instead it varies depending upon the various stages that occur during the quenching process. Knowledge of heat transfer during various stages of quenching and kinetics of wetting of the quench medium is fundamental to the understanding of the relationship between material, quench medium, microstructure, and properties. In this paper the characteristics of various quench media, the effect of process parameters on quenching, mechanisms of thermal transport, methods of assessing severity of quenching, and techniques of estimation of heat transfer coefficients are reviewed. An attempt is also made to highlight the importance of wetting kinetics of liquid media on quenching. Copyright © 2009 by ASTM International.
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    Wetting characteristics of sn-0.7cu lead-free solder alloy on copper substrates
    (Trans Tech Publications Ltd ttp@transtec.ch, 2012) Satyanarayan, S.; Prabhu, K.N.
    In the present work, the effect of surface texture on wetting characteristics of lead-free solder Sn-0.7Cu on copper substrates have been investigated at 298°C. The wetting tests were carried out using FTA 200 (First Ten Angstrom) dynamic contact angle analyzer. The surface texture of copper substrate significantly affected the wetting properties of Sn-0.7Cu solder alloy. Contact angles of about 30° were obtained on Cu substrate having smooth surface texture (Ra = 0.0155μm). On other hand contact angles on rough copper surface texture (Ra = 1.1194μm) were reduced to 20°. The contact angles decreased with increasing surface texture of Cu substrate. For rough Cu substrate, it seems that the solder atoms dissolve into the substrate in the time period of 200-600s. © (2012) Trans Tech Publications.
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    Wetting behavior of lead-free solders on copper substrates
    (Institution of Engineering and Technology jbristow@theiet.org, 2013) Satyanarayan, S.; Prabhu, K.N.
    The effect of substrate surface roughness on the wetting behavior of Sn-0.7Cu and Sn-0.3Ag-0.7Cu solder alloys on copper (Cu) substrates was investigated. The contact angles of both solder alloys decreased with increase in substrate surface roughness. The exponential power law (EPL), φ = exp (-Kτn), was used to model the relaxation behaviour of solders. Solder spreading kinetics was successfully represented by the (EPL). EPL parameters (K and n) alloy decreased with an increase in surface roughness. Spreading of solder alloys on both substrates exhibited capillary, gravity and viscous regimes. High spreading rates in the capillary regime, moderate in gravity regime and almost constant rates in viscous regimes were observed.
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    Determination of multiple heat flux transients during quenching of inconel 600 probe
    (ASM International joanne.miller@asminternational.org, 2013) Ramesh, G.; Prabhu, K.N.
    The time temperature data at axial and radial locations were measured during immersion quenching oflnconel 600 probes in a mineral oil quench medium. The temperature data and thermo-physical properties were used as input to an inverse heat conduction model for estimating spatially dependent heat flux transients. The estimated temperature data agreed very well with measured temperature data for increasing number of unknown surface heat flux components. The peak heat flux value decreased to a minimum and then increased to a high value in the axial direction. The inverse analysis indicated non uniform nature of wetting front and boiling of mineral oil on the quench probe surface resulting in large temperature gradients within the quench probe. The present work clearly indicates spatial dependence of boundary heat flux transients even for a simple cylindrical probe and the need for their estimation during quenching.
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    Heat transfer during quenching and assessment of quench severity-A review
    (2009) Prabhu, K.N.; Fernandes, P.
    In the heat treatment of steel, quenching is done to prevent ferrite or pearlite formation and allows formation of bainite and martensite. For a particular grade of steel, the effectiveness of quenching depends on the cooling characteristics of the quenching medium. The cooling rate is not a constant throughout the quenching process; instead it varies depending upon the various stages that occur during the quenching process. Knowledge of heat transfer during various stages of quenching and kinetics of wetting of the quench medium is fundamental to the understanding of the relationship between material, quench medium, microstructure, and properties. In this paper the characteristics of various quench media, the effect of process parameters on quenching, mechanisms of thermal transport, methods of assessing severity of quenching, and techniques of estimation of heat transfer coefficients are reviewed. An attempt is also made to highlight the importance of wetting kinetics of liquid media on quenching. Copyright © 2009 by ASTM International.
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    Review of non-reactive and reactive wetting of liquids on surfaces
    (2007) Kumar, G.; Prabhu, K.N.
    Wettability is a tendency for a liquid to spread on a solid substrate and is generally measured in terms of the angle (contact angle) between the tangent drawn at the triple point between the three phases (solid, liquid and vapour) and the substrate surface. A liquid spreading on a substrate with no reaction/absorption of the liquid by substrate material is known as non-reactive or inert wetting whereas the wetting process influenced by reaction between the spreading liquid and substrate material is known as reactive wetting. Young's equation gives the equilibrium contact angle in terms of interfacial tensions existing at the three-phase interface. The derivation of Young's equation is made under the assumptions of spreading of non-reactive liquid on an ideal (physically and chemically inert, smooth, homogeneous and rigid) solid, a condition that is rarely met in practical situations. Nevertheless Young's equation is the most fundamental starting point for understanding of the complex field of wetting. Reliable and reproducible measurements of contact angle from the experiments are important in order to analyze the wetting behaviour. Various methods have been developed over the years to evaluate wettability of a solid by a liquid. Among these, sessile drop and wetting balance techniques are versatile, popular and provide reliable data. Wetting is affected by large number of factors including liquid properties, substrate properties and system conditions. The effect of these factors on wettability is discussed. Thermodynamic treatment of wetting in inert systems is simple and based on free energy minimization where as that in reactive systems is quite complex. Surface energetics has to be considered while determining the driving force for spreading. Similar is the case of spreading kinetics. Inert systems follow definite flow pattern and in most cases a single function is sufficient to describe the whole kinetics. Theoretical models successfully describe the spreading in inert systems. However, it is difficult to determine the exact mechanism that controls the kinetics since reactive wetting is affected by a number of factors like interfacial reactions, diffusion of constituents, dissolution of the substrate, etc. The quantification of the effect of these interrelated factors on wettability would be useful to build a predictive model of wetting kinetics for reactive systems. © 2007 Elsevier B.V. All rights reserved.
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    Nanoquenchants for industrial heat treatment
    (2008) Prabhu, K.N.; Fernades, P.
    The present work outlines the possibility of using nanofluids for industrial heat treatment. Development of nanoquenchants having (i) high quench severity for enhancement of heat transfer for thick sections with low quench sensitivity and (ii) low cooling severity for thin sections with high quench sensitivity would be extremely useful to the heat treating community. The temperature dependent heat transfer coefficient and the wettability of the medium are the two important parameters that can be used to characterize a nanoquenchant to assess its suitability for industrial heat treatment. © 2007 ASM International.
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    Effect of substrate surface roughness on wetting behaviour of vegetable oils
    (2009) Prabhu, K.N.; Fernades, P.; Kumar, G.
    Vegetable oils are mainly used in the heat treating industry due to their environmental friendliness. In the present work the effect of surface roughness on spreading of vegetable oils on stainless steel substrates was investigated. Spreading phenomenon was digitally recorded and analyzed. All of the oils under investigation exhibited power law spreading behaviour of the type: A = ktn, where A, t, k and n represent the drop base contact area, spreading time, constant and exponent, respectively. The coconut and sunflower oils exhibited accelerated kinetics owing to their lower viscosity as compared to palm and mineral oils while peanut oil showed intermediate behaviour. Viscous regime was dominant during spreading of mineral and palm oils as compared to that of coconut oil. All the oils took longer period of time on rough surfaces than on smooth surfaces to relax to the same degree of wetting. Oils spreading on rough surfaces had to overcome the additional barrier due to asperities of the rough surface. Contact angle decreased with increase in roughness supporting the Wenzel's proposition. The decrease was significant for increase in roughness from 0.25 ?m to 0.50 ?m for all oils. However, the effect was negligible with further increase in roughness particularly for high viscosity oils. A spread parameter (?) is proposed to account for the variation of contact angle with surface roughness of the substrate and momentum diffusivity of the spreading liquid. The result suggested that low viscosity liquids exhibit improved wetting characteristics during spreading on rough surfaces. A model is proposed to estimate dynamic contact angles on substrates having varying surface roughness. © 2008 Elsevier Ltd. All rights reserved.
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    Severity of quenching and kinetics of wetting of nanofluids and vegetable oils
    (2009) Jagannath, V.; Prabhu, K.N.
    In the present work, the suitability of vegetable oil blends with mineral oil and alumina based nanofluids as quench media for industrial heat treatment was investigated. Sunflower oil, palm oil, and mineral oil were used for preparing the blends. Alumina based nanofluids of varying concentrations ranging from 0.01-4 % were used. The size of alumina particles was about 50 nm. The severity of quenching and heat transfer coefficients were estimated during quenching of copper probes. Heat transfer coefficients were estimated using a lumped heat capacitance model. The static contact angle was measured on copper substrates having a surface texture similar to the probes used for estimation of heat transfer coefficients. A dynamic contact angle analyzer was used for this purpose. The measured contact angles of nanofluids on copper were high compared to oils, indicating poor wetting by quench media that are polar in nature. Wetting characteristics had a significant effect on heat transfer coefficients estimated during quenching. Copyright © 2009 by ASTM International.