Experimental observations of the onset of unsteadiness for buoyant airflow along smooth and rough vertical isothermal walls

An experimental study of the buoyancy-induced air flow along a vertical isothermal wall is presented. A high and thick aluminum plate, electrically heated, was used as test section with the purpose of observing the thermal field in the convective fluid in conditions not far from the onset of the transitional regime. The schlieren technique and miniature thermocouples have been employed for both qualitative (flow unsteadiness) and quantitative (local air temperature and heat transfer coefficient at the wall) observations. For the smooth surface, the flow and heat transfer characteristics were found to be sensitive to the enclosure configuration surrounding the plate. The presence of roughness elements (staggered rows of segments of finite length) led to increased heat transfer relative to the smooth plate probably associated with the onset of unsteadiness close to the edges of segments when their dimensions were properly selected.