Radiation effects on natural convection in air in a divergent channel with uniformly heated plates

Nowadays trends in natural convection heat transfer are oriented toward either the seeking of new configurations to enhance the heat transfer parameters or the optimization of standard configurations. An experimental investigation on air natural convection in divergent channels with uniform heat flux at both the principal walls is presented in this paper to analyze the effect of radiative heat transfer. Results in terms of wall temperature profiles as a function of the walls diverging angle, the interwall spacing, the heat flux are given for two value of the wall emissivity. Flow visualization is carried out in order to show the peculiar pattern of the flow between the plates in several configurations. Nusselt numbers are then evaluated and correlated to the Rayleigh number. The investigated Rayleigh number ranges from 7.0 × 102 to 4.5 × 108 . The maximum wall temperature decreases at increasing divergence angles. This effect is more evident when the minimum channel spacing decrease. A significant decrease in the maximum wall temperature occurs passing from ε = 0.10 to ε = 0.90, except in the inlet region. Flow visualization shows a separation of the fluid flow for bmin = 40 mm and θ = 10°. Correlations between Nusselt and Rayleigh numbers show that data are better correlated when the maximum channel spacing is chosen as the characteristic length.