Three-dimensional mixed convection in a horizontal rectangular duct heated from below was studied numerically for a non-Boussinesq fluid. The duct was of aspect ratio four, with cold side-wall boundary conditions. Results were obtained at a reduced temperature of 2.33, Rayleigh number of 130,700, and Reynolds numbers of 5 and 20. Using the Re = 20 solution as initial conditions, the Re = 5 solution developed asymmetries about the duct centerline plane, with a streamwise periodic generation and dissolution of the inner pair of vortices. The consequence of these coherent vortical structures was a complex time-dependent distribution of the Nusselt number on the lower wall, with implications in the area of chemical vapor deposition. Specifically, when compared with the steady flow case occurring at Re = 20, an increased spatial uniformity of the time-averaged Nusselt number along the lower duct surface resulted.

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