Optical thickness effect on natural convection in a vertical channel containing a gray gas

The effect of radiation on natural convection heat transfer in a vertical parallel-plate channel with asymmetric heating, considering the radiation effects for both walls and participating air is presented. The channel is formed by one vertical wall heated by a uniform heat flux and by a vertical adiabatic plate. The governing equations of laminar natural convection and radiative transfer are solved by the finite volume method (FVM) and by the discrete ordinates method (DOM), respectively. The code was validated and verified with data reported in the literature. The effect of optical thickness (τ), channel width (b) and wall emissivity (εh) on the heat transfer and mass flow are investigated. The mass flow of the channel for τ=0.1 is up to 42% greater than that obtained for a transparent medium (τ=0.0). When τ=0.1, the average temperature difference between the air at the inlet and air at the outlet of the channel decreases up to 75% due to the increase of b from 0.02 to 0.10 m. Varying εh from 0.1 to 0.9 increases the radiative heat flux at the heated wall up to 72% and the mass flow rate increases up to 29%. A set of correlations were obtained for the mass flow, average convective Nusselt number and average radiative Nusselt number.