Transient Natural Convection in an Enclosure with Variable Thermal Expansion Coefficient and Nanofluid Properties

Transient natural convection is numerically investigated in an enclosure using variable thermal conductivity, viscosity, and the thermal expansion coefficient of Al2O3water nanofluid. The study has been conducted for a wide range of Rayleigh numbers (103≤ Ra ≤ 106), concentrations of nanoparticles (0% ≤ ϕ ≤ 7%), the enclosure aspect ratio (AR =1), and temperature differences between the cold and hot walls (∆T= 30). Transient parameters such as development time and timeaverage Nusselt number along the cold wall are also presented as a nondimensional form. Increasing the Rayleigh number shortens the nondimensional time of the initializing stage. By increasing the volume fraction of nanoparticles, the flow development time shows different behaviors for various Rayleigh numbers. The nondimensional development time decreases by enhancing the concentration of nanoparticles.