Gas-filled tall rectangular cavities typically found in insulating glazing (IGUs) of fenestration systems, with constant temperatures at the side walls and zero heat flux at the top and bottom, were investigated. Critical Rayleigh numbers, at which multicellular flow begins to form were determined for aspect ratios from 10.7 to 80. Using a general-purpose fluid flow and heat transfer finite-element analysis computer program (FDI 1993), numerical calculations were performed over the range of aspect ratios, A, from 10 to 80 with Rayleigh numbers, Ra, varying within the laminar flow regime. The calculations revealed that for aspect ratios between 10.7 and 30, the multicellular flow pattern dies out before the flow enters the turbulent flow regime. In addition, the lowest aspect ratio at which multicellular flow patterns existed was 10.7, which is lower than the lowest limit (A=12) published by other researchers. The resulting critical Rayleigh numbers are plotted on a graph as a function of the aspect ratio and the Rayleigh numbers. The overall heat transfer results in terms of the average, or integrated, Nusselt numbers, Nu, are compared with available numerical and experimental data on multicellular flow in rectangular cavities, and good agreement was found. Streamline contour plots and temperature profiles are plotted for selected cases.

KEYWORDS: year 1997, Calculating, double glazing, windows, doors, heat flow, cavities, Rayleigh number, Nusselt number, turbulent flow, comparing, temperature distribution