Flow Pattern Selection and Unsteady Effects for Mixed Convection in a Horizontal Rectangular Channel

The three-dimensional, time-dependent mixed convection in horizontal rectangular ducts, heated from below, is numerically investigated with a time-accurate, finite volume projection method. The accuracy of the numerical model is critically assessed by comparison of the computed results with the experimental measurements obtained in a channel with high width/height aspect-ratio, and values of Reynolds number Re = 42, Grashof} number Gr = 6884 and Prandtl} number Pr = 0.71. The thermally developing, mixed convective flow in a channel of width/height = 2 is computed for Re = 10, Pr = 0.71 and values of Gr/Re^2 in the range 50-1000. It is found that, irrespective of the Gr value considered, transverse traveling waves are always present at the startup of the heating process. However, for the lower values of Gr considered, Gr = 5000 and 10000, at later times they are eventually swept away, giving rise to a steady longitudinal rolls pattern. At higher values of the Grashof number, Gr = 15000, the persistance of such transverse waves, results in unsteady modulations of the longitudinal vortex rolls. For Gr = 1x10^5, the flow is characterized by a chaotic combination of transverse waves, and ascending/descending plumes and thermals.