Effects of fluctuating heat flux on natural convection of Al2O3 and Cu nanofluids in a square enclosure: a lattice Boltzmann study

The natural convective heat transfer inside a square enclosure containing Al 2 O 3 and Cu nanofluids has been studied using Lattice Boltzmann method. A nonuniform sinusoidal heat flux was applied on one of the side boundaries. The calculated results and experimental data were in good agreement. Impacts of Rayleigh (Ra) number (10 5 –10 7 ), nanoparticles solid volume fraction or SVF (0.01–0.1), and local oscillation wavelengths for wall heat flux (0.1, 0.5, and 1) on thermal and flow patterns have been studied. It was shown that an increase in SVF (0.01–0.1) and Ra number (10 5 –10 7 ) for Cu nanofluids decreases the maximum wall temperature by 48.3 and 49.2%, respectively. In addition, for the same oscillation wavelength and Ra number, Cu-water nanofluid has about 30% higher rate of heat transfer compared to Al 2 O 3 -water. With an increase in SVF at low Ra number, the heat tends to be transferred at a constant rate. The enclosure containing Al 2 O 3 -water has lower absolute stream function values than Cu-water nanofluid by 26.6%.