Natural convection heat transfer from a ribbed vertical plate: Effect of rib size, pitch, and truncation

Buoyancy-induced flows over ribbed vertical surfaces involve complex thermal and dynamic interactions between the mainstream and the surface texture, yielding contrasting effects on the heat transfer performance of the heated plate; proper analysis of the overall effect on the heat transfer rate is essential for efficient operation and optimization purposes. The present work pursues an insight into the different factors controlling this problem. Natural convection heat transfer from a vertical plate of 0.5 m height, regularly roughened with wooden transverse square ribs, is experimentally investigated. The surface temperature of the baseplate is varied so that a range of the plate Rayleigh number (Ra) from 3.4 × 108 to 4.9 × 108 is covered. The density of the roughness pattern and the rib pitch-to-height ratio (P/e) are varied by changing the number of ribs attached to the surface (from 10 to 40 rib rows) and using three different square cross-sections (of side lengths 2, 3, or 5 mm). The experimental work relies on the schlieren optical technique, through which the thermal boundary layer is visualized and the Nusselt number distribution is acquired. Analysis of the results reveals that enhancement of the local Nusselt number, relative to a corresponding smooth surface, may be attained only at the central part of the inter-rib region; this occurs exclusively for relatively large values of P/e. At a later stage, the effectiveness of rib truncation in enhancing the heat transfer from the baseplate is explored. Three staggered arrangements are considered, by varying the number of rib segments per row, and heat transfer enhancement, sensitive to the number of rib segments per row, is found. This paper also provides insight into the role of thermal-field disturbances close to turbulent transition, and sheds light on the potential of truncated ribs to amplify such perturbations.