Enhanced Thermal Conductivity of Ethylene Glycol-Based Suspensions in the Presence of Silver Nanoparticles of Various Sizes and Shapes

Engineered suspensions in the presence of highly-conductive nanoparticles, coined as nanofluids, have been studied extensively as a novel family of advanced heat transfer fluids. Attention has been paid primarily to the enhanced thermal conductivity of the suspensions that depends significantly on the material, size, shape, dispersion and loading of the nanoparticles. In this paper, the effects of adding silver (Ag) nanoparticles of various sizes and shapes on the thermal conductivity of ethylene glycol (EG)-based suspensions were investigated experimentally. These included Ag nanospheres (Ag NSs), Ag nanowires (Ag NWs) and Ag nanoflakes (Ag NFs). The suspensions were prepared at concentrations of 1, 5 and 10 mg/mL. The size and shape of the various Ag nanoparticles were observed by means of microscopy techniques. The dispersion and stability of the suspensions were also inspected. Measurements of the thermal conductivity of the suspensions were performed on a Hot Disk Thermal Constants Analyzer, which is based on the transient plane source technique, at elevated temperatures from 10 to 30 °C at an increment of 5 °C. It was shown that the thermal conductivity of the EG-based suspensions increases with raising the temperature. The Ag NWs of a very high aspect ratio (∼400) caused greatest relative enhancement up to 15.6% at the highest loading of 10 mg/mL (∼0.1 vol.%). The other two types of nanoparticles, Ag NSs and Ag NFs with much smaller aspect ratios, only led to enhancements up to 5%. The formation of a network of Ag NWs that facilitates heat conduction was likely responsible for their better performance. In addition, the relative enhancement was predicted by the Hamilton-Crosser (H-C) equation that takes the shape effect of the particles into consideration. It was shown that the predictions far underestimate the thermal conductivity enhancements but are qualitatively consistent with their shape dependence.