Unstable Aiding and Opposing Mixed Convection of Air in a Bottom-Heated Rectangular Duct Slightly Inclined From the Horizontal

Buoyancy-induced secondary vortex and reverse flows, flow transition, and the associated heat transfer processes in mixed convection of air through a bottom heated inclined rectangular duct were investigated experimentally. The local spanwise-averaged heat transfer coefficient and air temperature variations with time at selected locations were measured and the cross plane secondary flow was visualized for the Reynolds number ranging from 35 to 186, Grashof number up to 5 × 10⁶, and −20° ≤ Φ ≤ 26°. The results indicated that the heat transfer enhancement is due to the presence of the buoyancy-driven secondary vortex flow and/or reverse flow. The onset of thermal instability was found to move upstream for a larger negative inclined angle (opposing convection) and/or a higher Grashof number and to be delayed for a larger positive inclined angle (aiding convection) and/or a higher Reynolds number. At increasing Grashof number, the instantaneous flow visualizations clearly showed the changes of the vortex flow and/or reverse flow structures in the downstream section of the duct. At slightly supercritical Grashof numbers the secondary flow is in the form of two pairs of longitudinal rolls with the vortex flow ascending along the side walls. For higher Grashof numbers the vortex rolls rotate in the opposite direction with the secondary flow descending near the side walls and the flow was found to be time periodic. At even higher Grashof numbers in opposing convection the buoyancy induced reverse flow exists and the flow is quasi-periodic. Further raising the Grashof number or lowering the Reynolds number causes the flow to change from a transitional quasi-periodic state to a chaotic turbulent state.