Rayleigh-Bénard convection in nanofluids: Effect of temperature dependent properties

In this work, heat transfer enhancement using CuO-water nanofluids in natural convection using the Rayleigh-Bénard convection problem is investigated. The main focus of the current study is on the effects of variable thermal conductivity and variable viscosity of nanofluids on heat transfer enhancement in natural convection. The results are presented over a wide range of Rayleigh numbers (Ra = 103-106) and volume fractions of nanoparticles (0 ≤ φ ≤ 9%). For Ra > 103, and using the variable properties of nanofluid, the average Nusselt number was reduced by increasing the volume fraction of nanoparticles. However, for Ra = 103, the average Nusselt number was enhanced by increasing the volume fraction of nanoparticles. To study the significance of effects of temperature dependence of viscosity and thermal conductivity of nanofluid, the results obtained by using variable properties of nanofluid are compared with those based on constant property simulations and it was found that the temperature influence is small compared to the influence of high viscosity brought by the presence of high concentration of nanoparticles. The variable thermal conductivity and variable viscosity models were compared to both the Maxwell-Garnett (MG) model and the Brinkman model. It was found that for Ra > 103 the average Nusselt number was much more sensitive to the viscosity models than to the thermal conductivity models.