A two-dimensional numerical analysis is made, concentrating on the effect of aspect ratio on heat transfer characteristics of natural convection in a slender fluid layer enclosed between two vertical plates of different temperatures. It is shown that the boundary layer instability induces the secondary and the tertiary hook-shaped flows, which cause the renewal of the boundary layer. Then it is suggested that there exists the transition region where the average Nusselt number increases with the increase of the relative height, the aspect ratio, even for natural convection in a vertical fluid layer, as is seen in the tendency that the Nusselt number in the transition region from laminar to turbulent convection along a single vertical plate depends on the 2/5 power of the height-based Rayleigh number.
