Experimental study of aspect ratio effects on longitudinal vortex flow in mixed convection of air in a horizontal rectangular duct

Abstract

Combined flow visualization and temperature measurement were carried out to investigate the effects of the aspect ratio on the spatial and temporal characteristics of the longitudinal vortex air flow in a bottom heated horizontal rectangular duct. In the experiment, the Reynolds number was varied from 2.5 to 50, the Rayleigh number from 3000 to 20,000 and the aspect ratio from 2 to 12, covering the steady and unsteady longitudinal vortex flows. The results indicated that the spatial vortex flow structure in a low aspect ratio duct is similar to that in a high aspect ratio duct with the central portion of the wider duct around × = A/2 removed, but the unstable vortex flow at high Re and Ra can be stabilized when the aspect ratio is reduced. Besides, in a high aspect ratio duct ( A ⩾ 8) a reduction in the Reynolds number causes the flow to change from steady to unsteady state with more frequent roll splitting and merging. While in a low aspect ratio duct ( A ⩽ 6) at decreasing Re the roll cross-section exhibits cyclic expansion and contraction in time and the longitudinal rolls become snaking and time periodic in the downstream portion of the duct. At a higher buoyancy-to-inertia ratio the snaking motion of the rolls gets more intensive and is initiated at a shorter distance from the duct inlet. When the value of the aspect ratio is an odd number, it was found that there are still an even number of rolls induced in the duct. The transition from steady to time-dependent state at increasing buoyancy-to-inertia ratio is subcritical in a high aspect ratio duct ( A ⩾ 8). However, in a low aspect ratio duct ( A ⩽ 6) there exists a finite range of the buoyancy-to-inertia ratio for the appearance of the time-periodic snaking vortex flow. Finally, the measured oscillation frequencies of the time periodic snaking vortex flow were correlated.