TRANSVERSE VORTEX ROLLS IN MIXED CONVECTIVE AIR FLOW DRIVEN BY A HEATED CIRCULAR PLATE EMBEDDED IN THE BOTTOM OF A HORIZONTAL FLAT DUCT

摘要

Experimental flow visualization has been conducted here to unravel the structure of the buoyancy-driven transverse vortex rolls (T-rolls) at low-Reynolds number mixed convective air flow through a horizontal flat duct with a heated circular plate embedded in the bottom of the duct for the Rayleigh number ranging from 3506 to 23,485. The results suggest that regular transverse vortices prevail in the duct core only in a certain range of the Rayleigh number with a very low Reynolds number (Re ≤ 12.1) and the vortex flow is characterized by the T-rolls moving 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 are 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. These characteristics are very different from the transverse vortex rolls induced in a duct with a uniformly heated bottom plate.