The function of nano layer in enhancing the thermal conductivity of TiO2/water nanofluids

Abstract

Nanoparticles have the capability to effectively improve the thermal conductivity of base fluids, thus improving the heat transfer coefficient of heat transfer systems. In this study, a non-equilibrium molecular dynamics (NEMD) method based on the Fourier law is employed to study the thermal conductivity of TiO2 (r-TiO2)/water nanofluids with temperatures ranging between303K and 333K and volume fractions in the range of1-2%. The ordered layer structure as a shell is analyzed and its influence is surveyed by calculating the number density and radial distribution function (RDF).The results revealed that a clear, solid-like nanolayer of about 0.5 nm can be observed around the nanoparticle. In this regard, the thickness of the nanolayer is less affected by variations in volume fraction and temperature. The g(r) values and the number density decreased with the increase in temperature. Additionally, the g(r) values and the number density at the level of the nanolayer were much higher compared to those at other parts. This indicates the existence of more water molecules in the nanolayer, thereby reducing contact thermal resistance and improving thermal conductivity. Macroscopically, the thermal conductivity increases with the increase in volume fraction. It was found that the increase in the volume fraction from 1%to 2%at303Kresulted in an increase in the effective thermal conductivity from1.027 and 1.042, respectively. In other words, the thermal conductivity of the nanofluid was 2.7% and 4.2% higher than that of the base liquid under the same conditions.