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    A micro-convection model for thermal conductivity of nanofluids
    (01-11-2005)
    Patel, Hrishikesh E.
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    Sundararajan, T.
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    Increase in the specific surface area as well as Brownian motion are supposed to be the most significant reasons for the anomalous enhancement in thermal conductivity of nanofluids. This work presents a semi-empirical approach for the same by emphasizing the above two effects through micro-convection. A new way of modeling thermal conductivity of nanofluids has been explored which is found to agree excellently with a wide range of experimental data obtained by the present authors as well as the data published in literature. © Indian Academy of Sciences.
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    Model for thermal conductivity of CNT-nanofluids
    (01-06-2008)
    Patel, H. E.
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    Anoop, K. B.
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    Sundararajan, T.
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    This work presents a simple model for predicting the thermal conductivity of carbon nanotube (CNT) nanofluids. Effects due to the high thermal conductivity of CNTs and the percolation of heat through it are considered to be the most important reasons for their anomalously high thermal conductivity enhancement. A new approach is taken for the modeling, the novelty of which lies in the prediction of the thermal behaviour of oil based as well as water based CNT nanofluids, which are quite different from each other in thermal characteristics. The model is found to correctly predict the trends observed in experimental data for different combinations of CNT nanofluids with varying concentrations. © Indian Academy of Sciences.
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    A cell model approach for thermal conductivity of nanofluids
    (01-02-2008)
    Patel, Hrishikesh E.
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    Sundararajan, T.
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    This work presents a cell model for predicting the thermal conductivity of nanofluids. Effects due to the high specific surface area of the mono-dispersed nanoparticles and the micro-convective heat transfer enhancement associated with the Brownian motion of particles are addressed in detail. Novelty of the paper lies in its prediction of the non-linear dependence of thermal conductivity of nanofluids on particle volume fraction at low particle concentrations. The model is found to correctly predict the trends observed in experimental data over a wide range of particle sizes, temperatures and particle concentrations. © 2007 Springer Science+Business Media, Inc.
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    An experimental investigation into the thermal conductivity enhancement in oxide and metallic nanofluids
    (01-03-2010)
    Patel, Hrishikesh E.
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    Sundararajan, T.
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    One of the reasons for the controversy on the thermal conductivity enhancement of nanofluids is the lack of extensive data over a wide range of parameters. In the present study, a comprehensive experimental dataset is obtained for thermal conductivity of nanofluids with variation in nanoparticle material, base liquid, particle size, particle volume fraction and suspension temperature. Transient hot wire (THW) equipment as well as Temperature Oscillation equipment are developed for the measurement of thermal conductivity of liquids. The measurements show that, in general, thermal conductivity values of all the nanofluids are higher than that of the equivalent macro-particle suspensions. Metallic nanofluids are found to give higher enhancements than that of oxide nanofluids. Particle size is found to have a tremendous impact on the thermal conductivity of nanofluids with enhancement in the thermal conductivity increasing almost inversely with reduction in the particle size. Increase in temperature significantly increases the thermal conductivity of a nanofluid. It is also observed that the thermal conductivity of nanoparticle suspensions is relatively higher at lower volume fractions, thereby giving a non-linear dependence on the particle volume fraction.
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    Enhanced heat conduction characteristics of Fe, Ni and Co nanofluids influenced by magnetic field
    (01-11-2016)
    Katiyar, Ajay
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    Dhar, Purbarun
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    Nandi, Tandra
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    Magnetic nanofluids have enormous potential to improve thermal conductivity under the influence of magnetic fields. Magnetic field induced thermal transport capabilities of metallic magnetic nanoparticles viz. Fe, Ni and Co based stable magnetic colloids under the influence of magnetic field has been reported for the first time (detailed survey of literature supports the claim). Experimental investigations reveal highly enhanced thermal conductivity of such colloids under the influence of external magnetic field. The highest magnitude of thermal conductivity enhancement ∼106% and 284% is achieved for the Fe/HTO magnetic-nanofluids w.r.to the base nanofluid and pristine base fluid (in the absence of magnetic field) respectively at 0.05 T magnetic fields and 7.0 vol.% particle concentration. Ni and Co based nanofluids demonstrate less enhancement in the thermal conductivity magnitude compared to Fe based nanofluids due to lower values of saturation magnetic moments. However, the reduction in performance is not as drastic as expected since Ni and Co possesses better thermal conductivities than Fe, leading to compensation of the reduced field response. The underlying mechanism of enhanced conduction has been explained based on the formation of stable nanoparticle chains along the magnetic field lines which act as ‘short circuits’ for the thermal waves to travel faster. The enhancement in the thermal conductivity drops gradually beyond a critical magnetic field due to zippering/self-aggregation of the chained structure. The magnetic fluids have also been observed to be reversible with low thermal hysteresis and prove to be potential candidates as smart fluids in micro-scale devices.
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    Pool boiling characteristics of metallic nanofluids
    (01-01-2011)
    Krishna, K. Hari
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    Ganapathy, Harish
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    Sateesh, G.
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    Nanofluids, solid-liquid suspensions with solid particles of size of the order of few nanometers, have created interest in many researchers because of their enhancement in thermal conductivity and convective heat transfer characteristics. Many studies have been done on the pool boiling characteristics of nanofluids, most of which have been with nanofluids containing oxide nanoparticles owing to the ease in their preparation. Deterioration in boiling heat transfer was observed in some studies. Metallic nanofluids having metal nanoparticles, which are known for their good heat transfer characteristics in bulk regime, reported drastic enhancement in thermal conductivity. The present paper investigates into the pool boiling characteristics of metallic nanofluids, in particular of Cu - H2O nanofluids, on flat copper heater surface. The results indicate that at comparatively low heat fluxes, there is deterioration in boiling heat transfer with very low particle volume fraction of 0.01%, and it increases with volume fraction and shows enhancement with 0.1%. However, the behavior is the other way around at high heat fluxes. The enhancement at low heat fluxes is due to the fact that the effect of formation of thin sorption layer of nanoparticles on heater surface, which causes deterioration by trapping the nucleation sites, is overshadowed by the increase in microlayer evaporation, which is due to enhancement in thermal conductivity. Same trend has been observed with variation in the surface roughness of the heater as well. © 2011 by ASME.
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    Survey on nucleate pool boiling of nanofluids: The effect of particle size relative to roughness
    (01-10-2008) ;
    Prakash Narayan, G.
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    Baby, Anoop K.
    Pool boiling heat transfer using nanofluids (which are suspensions of nano-sized particles in a base fluid) has been a subject of many investigations and incoherent results have been reported in literature regarding the same. In the past, experiments were conducted in nucleate pool boiling with varying parameters such as particle size, concentration, surface roughness etc. and all sort of results ranging from heat transfer enhancement, deterioration and no effect were reported. This work tries to segregate a survey on pool boiling of nanofluids with respect to particle concentration. This is due to the fact that a major drift in heat transfer behavior is observed at higher and lower particle concentration. But upon deep perusal it has been found that deterioration in heat transfer coefficient are mainly observed at higher particle concentrations (4-16% by weight) and enhancements mainly at lower particle concentrations (0.32-1.25% by weight). Moreover, the relative size of the particle with respect to the surface roughness of the heating surface seems to play an important role in understanding the boiling behaviour. Also, recent works have reported that change in 'surface wetting' of the heating surface due to nanofluids and the formation of a porous layer modifiying nucleation site density can be of importance in predicting nucleate pool boiling characteristics of nanofluids. In the present paper, attempts are made to make systematic analysis of results in literature and try to bring out a common understanding of the results in literature. © 2008 Springer Science+Business Media B.V.
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    Magnetic field induced augmented thermal conduction phenomenon in magneto-nanocolloids
    (01-12-2016)
    Katiyar, Ajay
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    Dhar, Purbarun
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    Nandi, Tandra
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    Magnetic field induced augmented thermal conductivity of magneto-nanocolloids involving nanoparticles, viz. Fe2O3, Fe3O4, NiO and Co3O4 dispersed in different base fluids have been reported. Experiments reveal the augmented thermal transport under external applied magnetic field. A maximum thermal conductivity enhancement ∼114% is attained at 7.0 vol% concentration and 0.1 T magnetic flux density for Fe3O4/EG magneto-nanocolloid. However, a maximum ∼82% thermal conductivity enhancement is observed for Fe3O4/kerosene magneto-nanocolloid for the same concentration but relatively at low magnetic flux density (∼0.06 T). Thereby, a strong effect of fluid as well as particle physical properties on the chain formation propensity, leading to enhanced conduction, in such systems is observed. Co3O4 nanoparticles show insignificant effect on the thermal conductivity enhancement of MNCs due to their minimal magnetic moment. A semi-empirical approach has been proposed to understand the mechanism and physics behind the thermal conductivity enhancement under external applied magnetic field, in tune with near field magnetostatic interactions as well as Neel relaxivity of the magnetic nanoparticles. Furthermore, the model is able to predict the phenomenon of enhanced thermal conductivity as a function of physical parameters and shows good agreement with the experimental observations.
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    Colloidal graphite/graphene nanostructures using collagen showing enhanced thermal conductivity
    (10-03-2014)
    Bhattacharya, Soumya
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    Dhar, Purbarun
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    Ganguly, Ranjan
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    Webster, Thomas J.
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    Nayar, Suprabha
    In the present study, the exfoliation of natural graphite (GR) directly to colloidal GR/graphene (G) nanostructures using collagen (CL) was studied as a safe and scalable process, akin to numerous natural processes and hence can be termed "biomimetic". Although the exfoliation and functionalization takes place in just 1 day, it takes about 7 days for the nano GR/G flakes to stabilize. The predominantly aromatic residues of the triple helical CL forms its own special micro and nanoarchitecture in acetic acid dispersions. This, with the help of hydrophobic and electrostatic forces, interacts with GR and breaks it down to nanostructures, forming a stable colloidal dispersion. Surface enhanced Raman spectroscopy, X-ray diffraction, photoluminescence, fluorescence, and X-ray photoelectron spectroscopy of the colloid show the interaction between GR and CL on day 1 and 7. Differential interference contrast images in the liquid state clearly reveal how the GR flakes are entrapped in the CL fibrils, with a corresponding fluorescence image showing the intercalation of CL within GR. Atomic force microscopy of graphene-collagen coated on glass substrates shows an average flake size of 350 nm, and the hexagonal diffraction pattern and thickness contours of the G flakes from transmission electron microscopy confirm # five layers of G. Thermal conductivity of the colloid shows an approximate 17% enhancement for a volume fraction of less than approximately 0.00005 of G. Thus, through the use of CL, this new material and process may improve the use of G in terms of biocompatibility for numerous medical applications that currently employ G, such as internally controlled drug-delivery assisted thermal ablation of carcinoma cells. © 2014 Bhattacharya et al.
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    Experimental investigation of the dielectric and cooling performance of colloidal suspensions in insulating media
    (05-03-2009)
    Chiesa, M.
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    A new class of colloidal dielectric fluids was formulated with the objective of enhancing the dielectric and cooling performance of the constitutive base fluids a mineral and a synthetic oil for application in electro thermal environment. Colloidal particles with different dielectric properties were suspended by means of surfactants and sonication. Dielectric strength and thermal conductivity measurements were carried out for a number of different colloidal systems with varying particle volume fraction. An alternative non-destructive breakdown testing approach that limits the amount of electrical energy transferred to the test sample was employed to characterize the dielectric properties of suspensions. In general the addition of particles to the base fluids was detrimental for the overall dielectric strength except when iron oxide nanoparticles were added to mineral oil. On the other hand the thermal conductivity of the base fluids was increased by suspending the particles. Dielectric strength and thermal conductivity measurements of the different colloidal systems are interpreted and compared in light of the physical properties of the suspended particles and their base fluids. © 2008 Elsevier B.V. All rights reserved.