Free convection suppression using honeycomb cellular materials

Abstract It is well-known that an undisturbed layer of air forms an effective insulating barrier against heat transfer because of its low thermal conductivity. In confined air spaces any initiation of free convection will inevitably lead to an increase in the rate of heat transfer across the gap for a fixed temperature difference. To delay the onset of convection for horizontal layers of fluid it has been shown theoretically and experimentally that the introduction of cell walls will effectively raise the critical Rayleigh number by providing more shear surfaces within the fluid. This paper reports on some of the earlier work dealing with the horizontal cell (this being vertical cellular constraining walls within a horizontal fluid layer) and extends the theory to cover the case of the inclined cell and the vertical cell. The photographic results of flow visualization studies are also presented as it is felt that an inspection of the flow patterns leads to a fuller understanding of the problem. Whereas the concept of the critical Rayleigh number is quite valid for the horizontal layer of fluid heated from below, it is shown to be inadequate for the inclined fluid layer as would be found in an inclined solar absorber of the flat plate type. Instead, the solutions of the equations show that convection must be initiated for any temperature gradient to be established between the absorber plate and the glass cover plate. Although the honeycomb may be effective in suppressing convection in the horizontal case, the effect of such constraints on the fluid in the inclined case is negligible as far as suppression of natural convection is concerned.