An experimental investigation of natural convection between horizontal, heated, parallel plates in air was carried out by visualizing the flow and measuring the air temperature. Grashof numbers, based on the plate spacing, varied in the 1.22×105-1.06×106 range. Flow patterns and probable onset of secondary motions were observed for three heating modes: (1) both plates heated, (2) upper plate heated and lower one unheated, and (3) upper plate unheated and lower one heated. The main flow pattern resembled a C shape (C loop) for all modes. In fact, the flow penetrated inside the cavity close to the leading edge of the lower plate and exited from the upper part, by reversing its motion between the plates. When the lower plate was heated, flow visualization showed that secondary flows were added to the C loop main flow. Such secondary structures arose as thermals, then changed into longitudinal vortices and, in the upper region of the open-ended cavity, a chaotic motion was detected. The existence of these structures was confirmed by measurements of instantaneous temperature values. They showed that the greater the Grashof number the more chaotic the flow in the outflow branch of the C loop when the lower plate was heated and the upper one was unheated. [S0022-1481(00)01801-6]

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