Experimental analysis of thermal instability in natural convection betweenhorizontal parallel plates uniformly heated

An experimental investigation of natural convection between horizontal, hea ted, parallel plates in air was carried out by visualizing the flow and mea suring the air temperature. Grashof numbers, based on the plate spacing, va ried in the 1.22X10(5)-1.06X10(6) range. Flow patterns and probable onset o f 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 r eversing its motion between the plates. When the lower plate was heated, fl ow visualization showed that secondary flows were added to the C loop main flow. Such secondary structures arose as thermals, then changed into longit udinal vortices and, in the upper region of the open-ended cavity, a chaoti c motion was detected. The existence of these structures was confirmed by m easurements of instantaneous temperature values. They showed that the great er the Grashof number the more chaotic the flow in the outflow branch of th e C loop when the lower plate was heated and the upper one was unheated.