An experimental study of radiative effects on natural convection in air in convergent channels

An experimental analysis on natural convection in air in a convergent channel uniformly heated at the principal walls has been carried out, in order to analyze the effects of the radiative heat transfer. Results in terms of wall temperature profiles as a function of the walls inclination angle, the interwall spacing, the heat flux are given for two values of the wall emissivity. Flow visualization has been carried out, in order to show the peculiar pattern of the flow between the plates in several configurations. The comparison between two wall emissivity values, 0.10 and 0.90, shows that the effect of thermal radiation is more marked for larger convergence angles. For a wall emissivity equal to 0.90 and for small values of the minimum channel spacing heat transfer in slightly convergent vertical channels is stronger than in a vertical parallel channel. Flow visualization points out a recirculating zone in the upper part of the channel for small values of the minimum channel spacing and for converging angles equal to 5° and 10°. Nusselt numbers and dimensionless maximum temperatures are then evaluated and correlated to the Rayleigh number, in the investigated range from 5 to 5×108. A very good agreement between experimental data and correlations has been observed for the dimensionless parameters based on the maximum channel spacing.