Determination of heat transport mechanism using nanoparticle property and interfacial nanolayer in a nanofluidic system

Abstract

Elucidation of heat transport mechanism is the key for the preparation and application of nanofluids with high thermal conductivity. However, the effect of microscopic parameters on nanoparticle property, especially thermal conductivity of nanofluids has limited studied. To establish the relationship between nanolayer structure characteristics and thermal conductivity, Al2O3, CuO, and Cu nanoparticles were dispersed in deionized water (W). The thermal conductivity of nanofluids was measured at various temperatures (293–333 K) and volume fractions (0.5–2.0 vol%). The thermal conductivity of nanofluids decreased in the following order: CuO/W > Al2O3/W > Cu/W nanofluids. Nanoparticles with a high thermal conductivity do not always exhibit a high thermal conductivity of nanofluids. Then, number density (N), nanolayer structure, and its thickness (δ) were obtained from a molecular dynamics simulation. The simulation results show that two obvious interfacial nanolayers exist around the central nanoparticle; however, the interfacial potential of the first layer is much higher than that of the second layer. Therefore, the morphology of the first layer mainly determines the thermal conductivity in a nanofluidic system. However, the trend of number density differs from that of thermal conductivity. Moreover, the thickness of interfacial nanolayer is similar for different nanofluids. Thus, only the number density or thickness of nanolayer is not a determined parameter to evaluate the thermal conductivity. A new parameter, N/δ (ratio of number density and thickness), is described to quantitatively evaluate the relationship between nanolayer structure and thermal conductivity. The high value of N/δ indicates that denser and more ordered liquid molecules arrange around the nanoparticles, creating a higher heat transfer channel than other regions and thus exhibiting a higher thermal conductivity of nanofluids.