The stability of thermal convection of large Prandtl number fluids in a vertical slot (aspect ratio = 10, 15, or 20) was studied experimentally. Secondary cells began to appear from the center of the slot and then prevailed. The critical Rayleigh number showed an increasing trend as the aspect ratio was increased, and became higher than that obtained from a linear stability analysis owing to a possible traveling wave mode of instability. As the Rayleigh number was increased, either the onset of the tertiary cells or the traveling waves occurred depending on the Prandtl number. In the transition process from laminar to turbulent flow, the secondary cells around the end regions split into smaller cells and an unsteady flow structure appeared with an original secondary cell in the center region and some smaller cells along the sidewalls.

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