Mixed convection over a backward-facing step in a vertical duct using nanofluids-buoyancy opposing case

Predictions are reported for buoyancy-opposing laminar mixed convection using nanofluids over a backward-facing step in a vertical duct, in which the upstream wall and the step simulated are adiabatic surfaces, while the downstream wall from the step is heated to a uniform temperature that is higher than the inlet fluid temperature. The straight wall that forms the other side of the duct was maintained at constant temperature equivalent to the inlet fluid temperature. The conservation equations are solved using the finite volume method. Eight different types of nanoparticles, Au, Ag, Al2O3, Cu, CuO, diamond, SiO2, and TiO2, with 5% volume fraction are used. Results presented in this paper are for a step height of 4.9 mm and an expansion ratio of 1.942, while the total length in the downstream of the step is 0.5 m. The Reynolds number is in the range of 33-3 ? Re ? 100. The downstream wall was fixed at uniform wall temperature in the range of 0 ?-T ? 30-C which is higher than the inlet flow temperature. Results reveal that the recirculation region that develops behind the backward-facing step in the case of forced convection and low buoyancyopposing level disappears when the buoyancy-opposing increases. An upward opposing flow is developed at the heated downstream wall from the exit to the step wall at high Reynolds number and high temperature difference, while at low Reynolds number and high temperature difference; the upward opposing flow is passed over the step wall and a shallow recirculation region is formed on the adiabatic upstream wall. It is found that SiO2 nanofluid has the highest Nusselt number at the highest buoyancy level. The skin friction coefficient decreases as Reynolds number and Prandtl number increase.