Abstract
Theoretical results derived in this article are combined with experimental data to conclude that, while there is no improvement in the effective thermal conductivity of nanofluids beyond the Maxwell's effective medium theory (J.C. Maxwell, Treatise on Electricity and Magnetism, 1891), there is substantial heat transfer augmentation via nanofins. The latter are formed as attachments on the hot wire surface by yet an unknown mechanism, which could be related to electrophoresis, but there is no conclusive evidence yet to prove this proposed mechanism.
Highlights
The impressive heat transfer enhancement revealed experimentally in nanofluid suspensions by Eastman et al [1], Lee et al [2], and Choi et al [3] conflicts apparently with Maxwell’s [4] classical theory of estimating the effective thermal conductivity of suspensions, including higher-order corrections and other than spherical particle geometries developed by Hamilton and Crosser [5], Jeffrey [6], Davis [7], Lu and Lin [8], Bonnecaze and Brady [9,10]
The theoretical results derived in this article are combined with experimental data [23] to conclude that, while there is no improvement in the effective thermal conductivity of nanofluids beyond the Maxwell’s effective medium theory [4], there is substantial heat transfer augmentation via nanofins
It is imperative to conclude that the only way, an excessively higher effective thermal conductivity of the nanofluid suspension as obtained by Eastman et al [1], Lee et al [2] and Choi et al [3] could have been obtained even in an apparent form, is if the wire was excessively exposed to the solid phase temperature
Summary
The impressive heat transfer enhancement revealed experimentally in nanofluid suspensions by Eastman et al [1], Lee et al [2], and Choi et al [3] conflicts apparently with Maxwell’s [4] classical theory of estimating the effective thermal conductivity of suspensions, including higher-order corrections and other than spherical particle geometries developed by Hamilton and Crosser [5], Jeffrey [6], Davis [7], Lu and Lin [8], Bonnecaze and Brady [9,10]. Further attempts for independent confirmation of the experimental results showed conflicting outcomes with some experiments, such as Das et al [11] and Li and Peterson [12], confirming at least partially the results presented by Eastman et al [1], Lee et al [2], and Choi et al [3], while others, such as Buongiorno and Venerus [13], Buongiorno et al [14], show in contrast results that are in agreement with Maxwell’s [4] effective medium theory. The explanation is based on the fact that the THW experimental method used in all the nanofluid suspensions experiments listed above needs a major correction factor when applied to non-homogeneous systems This time-dependent correction factor is of the same order of magnitude as the claimed enhancement of the effective thermal conductivity. The third new result is that their time dependent “effective thermal conductivity” converges at steady state to values that according to our calculations confirm the validity of the classical Maxwell’s theory [4] and its extensions [5,6,7,8,9,10]
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