Free Convection Heat Transfer across Rectangular-Celled Diathermanous Honeycombs

Abstract

Experimentally obtained Nusselt number-Rayleigh number plots are presented for free convective heat transfer across inclined honeycomb panels filled with air. The honeycomb cells were rectangular in shape with very long cell dimensions across the slope and comparatively short dimensions up the slope. Elevation aspect ratios, AE, investigated were 3, 5 and 10; angles of inclination, θ, measured from the horizontal, were 0, 30, 60, 75 and 90 deg. The effect on the Nusselt number, of the emissivities of the plates bounding the honeycomb, and of the emissivity of honeycomb material, was also investigated. The measurements confirmed that the critical Rayleigh number and the post-critical heat transfer depend on the radiant properties of the honeycomb cells. The critical Rayleigh numbers at θ = 0 were well predicted by the methods of Sun and Edwards. For 0 < θ ≤ 75 deg, the critical Rayleigh numbers and the Nusselt-Rayleigh relations were both found to be essentially the same as their horizontal counterparts provided the Rayleigh number was first scaled by cos θ. For θ = 90 deg, the form of the Nusselt-Rayleigh relation was found to be very different from that for θ ≤ 75 deg and similar to that observed for square-celled honeycombs for θ ≥ 30 deg. The θ = 90 deg data were found to be closely correlated by an equation of the form recently proposed by Bejan and Tien.