Effective Thermal Conductivity of Aqueous Suspensions of Carbon Nanotubes (Carbon Nanotube Nanofluids)

This work is concerned with the effective thermal conductivity of aqueous suspensions of multiwalled carbon nanotubes (nanofluids). Stable nanofluids were made using sodium dodecylbenzene sulfonate as the dispersant. The effects of concentration of carbon nanotubes and temperature on effective thermal conductivity were investigated. It was found that effective thermal conductivity increased with increasing concentration of carbon nanotubes, and the dependence was nonlinear even at very low concentrations, which was different from the results for metal/metal oxide nanofluids. The effective thermal conductivity increased with increasing temperature, and the dependence was also nonlinear. At temperatures lower than ∼30 ◦ C, approximately linear dependence of the thermal conductivity enhancement on temperature was seen, but the dependence tended to level off above ∼30◦C. A comparison between the results of this work and those of published studies showed a large discrepancy in the effective thermal conductivity of carbon nanotube nanofluids. Differences in the interfacial resistances and thermal conductivities of carbon nanotubes used in these studies were proposed to be the main reasons. The experimental results were also compared with some classical macroscopic models for thermal conductivity of homogenous mixtures containing micrometer- or millimeter-sized particles. It was shown that the macroscopic models were inadequate for the prediction of the effective thermal conductivity of nanofluids. Analysis of possible mechanisms for thermal conduction enhancement suggested that networking of carbonnanotubes was likely to be responsible for the observed high effective thermal conductivity of carbon-nanotube nanofluids. Experiments at a temperature above 60‐70 ◦ C showed that the dispersant failed, which led to destabilization of nanofluids.