Experiments on the Onset of Longitudinal Vortices in Laminar Forced Convection Between Horizontal Plates

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Abstract
An experimental investigation is carried out to determine the onset of longitudinal columnar vortices due to buoyant forces for fully developed laminar forced convection between two infinite horizontal plates, each wall subjected to identical uniform axial temperature gradient but maintained at temperatures T1 and T2 (T1 > T2, T1 < T2, and T1 = T2) at lower and upper surfaces, respectively. The limiting case with vanishing axial temperature gradient and heating from below (T1 > T2) is known to have a critical Rayleigh number of 1708 and is used to check the accuracy of the testing apparatus. The onset of secondary flow is determined by a direct flow-visualization technique using cigarette smoke, and confirmed by a transverse temperature-profile measurement using a single thermocouple traverse. Experimental results for the critical Rayleigh number are compared with theory and the agreement is found to be good.

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