Response to Comments by M. J. Assael and W. A. Wakeham on: D. P. H. Hasselman, “Can the Temperature Dependence of the Heat Transfer Coefficient of the Wire–Nanofluid Interface Explain the ‘Anomalous’ Thermal Conductivity of Nanofluids Measured by the Hot-Wire Method?”

Abstract

The word “wire” in the sentence in my paper, “The temperature of the wire is not affected by this discontinuity,” should be replaced by “fluid.” Although Assael and Wakeham reject my suggestion that the temperature dependence of the heat transfer coefficient at the hot wire–nanofluid interface can have a significant effect on the resulting value of the thermal conductivity of the nanofluid, the results of any study in which this effect is ignored should be treated with caution. Although Tertsinidou et al. published a correction of their paper, in which, as I pointed out, the values of the mass fractions of the particles were in error, they provided no explanation for why these were in error and did not describe how the corrected values were obtained. Also, because Tertsinidou et al. did not provide the values of the thermal conductivity of the particles in their calculation the thermal conductivity of the nanofluid by the theory of Hamilton and Crosser is another reason for their findings to be treated with a great deal of caution. But even if they were correct, their findings are valid only for the hot-wire method in in which the fluid is stationary, for which the rate of heat transfer as a function of degree of turbulence is a minimum. With increasing degree of turbulence, the rate of heat transfer will increase, with the thermal conductivity of the nanofluid playing an increasingly smaller and the heat transfer coefficient at the solid-nanofluid interfaces playing a increasingly larger role. For that reason, studies of the heat transfer characteristics of nanofluids for heat exchanger purposes should rely on methods, which closely resemble the conditions of turbulent flow encountered in practice.