EXPERIMENTAL-STUDY OF ASPECT RATIO EFFECTS ON LONGITUDINAL VORTEX FLOW IN MIXED CONVECTION OF AIR IN A HORIZONTAL RECTANGULAR DUCT

Combined flow visualization and temperature measurement were carried o ut to investigate the effects of the aspect ratio on the spatial and t emporal characteristics of the longitudinal vortex air how in a bottom heated horizontal rectangular duct. In the experiment, the Reynolds n umber was varied from 2.5 to 50, the Rayleigh number from 3000 to 20,0 00 and the aspect ratio from 2 to 12, covering the steady and unsteady longitudinal vortex flows. The results indicated that the spatial vor tex 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 arou nd x = A/2 removed, but the unstable vortex how at high Re and Ra can be stabilized when the aspect ratio is reduced. Besides, in a high asp ect ratio duct (A greater than or equal to 8) a reduction in the Reyno lds number causes the flow to change from steady to unsteady state wit h more frequent roll splitting and merging. While in a low aspect rati o duct (A less than or equal to 6) at decreasing Re the roll cross-sec tion exhibits cyclic expansion and contraction in time and the longitu dinal 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 distan ce from the duct inlet. When the value of the aspect ratio is an odd n umber, it was found that there are still an even number of rolls induc ed in the duct. The transition from steady to time-dependent state at increasing buoyancy-to-inertia ratio is subcritical in a high aspect r atio duct (A greater than or equal to 8). However, in a low aspect rat io duct (A less than or equal to 6) there exists a finite range of the buoyancy-to-inertia ratio for the appearance of the time-periodic sna king vortex flow. Finally, the measured oscillation frequencies of the time periodic snaking vortex how were correlated. (C) 1997 Elsevier S cience Ltd.