Effect of gap thickness on a rectangular-cell compound-honeycomb solar collector

Abstract

Compound-honeycomb solar collectors employ a honeycomb to suppress natural convection and air gaps between the honeycomb and the absorber plate or the glazing to reduce conductive and radiative heat losses. Results of an experimental study on the effect of the thickness of these gaps on the total heat transfer across a compound honeycomb confined between two isothermal and low emissivity boundaries are presented. The honeycomb consists of rectangular cells with elevation aspect ratio 6.3 and plan aspect ratio 82 (15.8 mm thick, 206 mm wide and 2.5 mm deep), constructed from parallel glass strips. Plots of Nusselt number versus Rayleigh number are given for compound honeycombs with air gaps of thickness 1.6 mm, 3.3 mm, 6.4 mm and 9.6 mm above and below the honeycomb and for a compound-honeycomb layer with an air gap of thickness 6.4 mm below the honeycomb. Measurements are reported for tilt angles of 0°, 30° and 60° where the long dimension of the rectangular cells in the honeycomb is horizontal. As the air gaps' thickness increase, coupled conductive-radiative heat transfer is reduced, while the critical Rayleigh number is also reduced and convective heat transfer increases. However, even for relatively thick air gaps, the decrease in critical Rayleigh number is moderate, and the rectangular-cell compound honeycomb is found to be an effective convection suppressor. A compound honeycomb with air gaps above and below the honeycomb is shown to be superior in suppressing convection to a compound honeycomb of equal total thickness with only one air gap below the honeycomb.