Transverse recirculations in low Reynolds number mixed convective gas flow over a model heated substrate

Combined flow visualization and temperature measurement are conducted to investigate the structure of the buoyancy driven transverse vortex rolls (T-rolls) at low Reynolds number mixed convective gas flow through a horizontal flat duct with a 0.3 m (12-in.) heated circular plate embedded in the bottom of the duct for the Rayleigh number ranging from 3506 to 23,485. The results suggest that the regular transverse vortices prevail in the duct core only in a certain range of Rayleigh number with very low Reynolds number (Re ⩽ 12.1) and the vortex flow is characterized by the moving T-rolls over the heated plate enclosed by an incomplete circular roll. More specifically, the regular transverse vortex rolls are curved at the early stage of their initiation and deformed to some degree due to the presence of the incomplete circular roll around the upstream edge of the heated plate. But in the exit half of the duct the T-rolls are nearly straight and they are almost spanwisely symmetric with respect to the vertical central plane. Besides, the transverse rolls get stronger and bigger during the downstream moving and they do not travel downstream at a constant speed. The data from the transient temperature measurement reveal that the flow oscillation of the T-rolls driven by the circular heated plate is slightly space dependent for given Ra and Re. Moreover, the frequency of the flow oscillation decays substantially with the decrease in the Reynolds number. Furthermore, the amplitude of the temperature oscillation can be reduced for a raise of Ra. These characteristics are very different from the transverse vortex rolls induced in a duct with a uniformly heated bottom plate.