We observe a dramatic enhancement of thermal conductivity in a nanofluid containing magnetite particles of average diameter of 6.7nm under the influence of an applied magnetic field. The maximum enhancement in the thermal conductivity observed is 300% (k∕kf=4.0) at a particle loading of 6.3vol%. The increase in thermal conductivity is attributed to the effective conduction of heat through the chainlike structures formed in the nanofluid. This finding is consistent with the theoretical prediction of enhanced thermal conductivity in nanofluid containing fractal aggregates [R. Prasher et al., Appl. Phys. Lett.89, 143119 (2006)].
We experimentally demonstrate the tunable thermal property of a magnetically polarizable nanofluid that consists of a colloidal suspension of magnetite nanoparticles with average diameter of 6.7nm. Controlling the linear aggregation length from nano- to micron scales, the thermal conductivity (TC) of the nanofluid has been enhanced up to 216%, using 4.5vol% of nanoparticles. Repeated magnetic cycling shows that the TC enhancement is reversible. It has been confirmed that the large enhancement in TC is due to the efficient transport of heat through percolating nanoparticle paths. Our findings offer promising applications in “smart” cooling devices.
The unusually large enhancement of thermal conductivity (k/k(f)∼4.0, where k and k(f) are the thermal conductivities of the nanofluid and the base fluid, respectively) observed in a nanofluid containing linear chain-like aggregates provides direct evidence for efficient transport of heat through percolating paths. The nanofluid used was a stable colloidal suspension of magnetite (Fe(3)O(4)) nanoparticles of average diameter 6.7 nm, coated with oleic acid and dispersed in kerosene. The maximum enhancement under magnetic field was about 48φ (where φ is the volume fraction). The maximum enhancement is observed when chain-like aggregates are uniformly dispersed without clumping. These results also suggest that nanofluids containing well-dispersed nanoparticles (without aggregates) do not exhibit significant enhancement of thermal conductivity. Our findings offer promising applications for developing a new generation of nanofluids with tunable thermal conductivity.
We investigate the temperature-dependent thermal conductivity (k) of aqueous and nonaqueous stable nanofluids with average particles size of 8 nm stabilized with a monolayer of surfactant. Iron oxide (Fe3O4) nanoparticles are synthesized by a coprecipitation technique and are characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), vibrating sample magnetometry (VSM), dynamic light scattering (DLS), and theromogravimetric analysis (TGA). The particles are functionalized with suitable surfactants and dispersed in aqueous and nonaqueous base fluids. The thermal conductivity and viscosity measurements are carried out using a transient hot wire and a rotational rheometer, respectively. The thermal conductivity of aqueous nanofluids increases with temperature while it shows a decrease in nonaqueous nanofluids. Interestingly, the ratio of thermal conductivity of both nanofluids with respect to base fluids (k/k
f) remains constant with an increase in temperature, irrespective of the nature of the base fluid. This observation is in sharp contrast to microconvection theory predictions of an increase in thermal conductivity with a rise in temperature. These results unambiguously confirm the less dominant role of microconvection on thermal conductivity enhancement. Although the viscosity of nanofluids decreases with increases in temperature, the viscosity ratio with respect to base fluid remains constant. These results show that the viscosity and thermal conductivity of nanofluids simply tracks those properties of the base fluids. Measurement of particle size with temperature shows that the average particle size remains constant with temperature.
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