A numerical and experimental study has been conducted on two-dimensional laminar natural convection heat transfer in an undivided and divided vertical isothermal channel. For the divided channel configuration, an isothermal plate at the same temperature as the channel walls was located on the channel centre line. The study examined the effect of Rayleigh number, plate-to-channel length ratio, vertical plate position, and plate thickness on the heat transfer from the channel walls, the dividing plate and the channel as a whole.

Solutions to both the full elliptic and parabolic forms of the Navier-Stokes and energy equations have been obtained for Prandtl number Pr = 0.7 (air). Closed form expressions were derived for the limiting case of fully developed flow in the divided channel. Experimental measurements were made in air with a Mach-Zehnder interferometer.

Positioning the plate at the bottom of the channel gave the highest average Nusselt numbers for the dividing plate and the overall channel. At low Rayleigh number, when the plate is at the bottom of the channel, extending the channel walls above the plate increased the plate heat transfer by as much as a factor of four. At higher Rayleigh number, plate average Nusselt numbers as much as two times the isolated plate Nusselt number were predicted numerically.

In the fully developed regime, a short dividing plate located at the channel inlet reduces the wall average Nusselt number by about a factor of two, compared with the undivided channel. At moderate and high Rayleigh number, the wall average Nusselt number was almost independent of both the vertical plate position and the plate-to-channel length ratio.

At low Rayleigh number, increasing the dividing plate thickness caused the plate and wall average Nusselt numbers to decrease significantly. At high Rayleigh number the effect of plate thickness was small.

Average Nusselt number correlations are presented for the dividing plate, the channel wall and the overall channel.