A schematic of the problem under study is shown in Fig. 1a. It consists of an air cavity that can be vertical or inclined, bounded by two isothermal walls (top and bottom) and remaining walls adiabatic. The cavity length varies from L=0.5m to L=3m and the inter-wall spacing can vary from b=0.01m to b=0.05m, leading to aspect ratios of 10 to 300. The depth of the cavity varies from w=0.5m to w=2m. The inclination of the cavity 0 with respect to the horizontal goes from 0 =0 degrees ( heated from below) to 0 =90 degrees(heated from side). The hot wall temperature goes from Tft=0°C to Tft= 300°C. The temperature difference between the hot and the cold walls goes from 20°C to 200°C provided >0.

The Nusselt number in these problems is defined in terms of the inter-wall spacing, as follows:

firef

where, q= heat flux passing from the hot wall to the cold wall.

=heat flux assuming only conduction between the hot and cold walls with conductivity at the mean temperature.

_ (Th — Tc) * Xref

Figure 1: Schematic (a): The air layer between hot and cold surfaces, with other faces adiabatic, (b): mesh utilized, solid triangles indicate intensification at the boundaries, mesh not shown in the third direction.