Abstract
For a mixed flow of free and forced convection between vertical parallel plates kept at uniform but different temperatures, the relation of the superimposed forced flow on the instability was investigated experimentally and theoretically. Experiments were performed in a duct with a large aspect ratio at Reynolds numbers ranging from 0 to 100. A flow-visualization technique was used. The result shows that the instability occurs due to a disturbance of travelling-wave type, the phase velocity being 0.855<u*> where <u*> is the mean velocity. The critical value of Grashof number Grc increases in proportion to Rec2 and the associated wave number ac decreases gradually with increasing Rec. The characteristics of the disturbance and their changes with Rec were well explained on the basis of linear stability theory. In addition, the amplitudes of disturbances near the critical condition were determined from a quasi-linear approximation, and the resulting secondary flow and heat and momentum transfer processes were studied.
