Radiation effects on natural convection in a vertical channel with an auxiliary plate

• Experimental investigation on combined natural convection and radiation heat transfer is carried out.
• The investigated configuration is a vertical channel with an unheated auxiliary plate in the centerline.
• A comparison among different positions of the plate is given in terms of channel wall temperature.
• At high Rayleigh numbers the auxiliary plate improves the channel thermal performance.
• Correlations among different dimensionless parameters are shown.

In this experimental study the effects of unheated auxiliary plate positions along the centerline of a vertical channel on combined natural convective in air and radiative heat transfer are investigated. The vertical channel is symmetrically heated and a uniform heat flux along the principal walls of the channel is assumed. Comparisons between experimental data carried out from different positions of the auxiliary plate and two surfaces emissivity values are given in terms of heated wall temperatures and radiative heat transfer. Average Nusselt numbers by means of experimental data are evaluated. Results are given for different thermal and geometrical dimensionless parameters. Results show that the lowest maximum channel wall temperatures are attained for the auxiliary plate placed at the channel exit for high channel Rayleigh numbers and heat flux values. Correlations for average Nusselt numbers, global average Nusselt numbers and dimensionless maximum wall temperatures in terms of channel Rayleigh numbers and auxiliary plate-to-channel height ratio, h/L, are evaluated in the ranges: 20 < Ra'< 1.5 × 105, 22 < Ra′∗ < 1.7 × 105 and 0 ≤ h/L ≤ 0.5 for inlet and outlet positions and walls emissivity equal to 0.9. The proposed correlations are in good agreement with correlation for simple channel.