Enhanced forced-convection from ribbed or machine-roughened inner surfaceswithin triangular ducts

An experimental investigation has been conducted to study the steady-state forced-convection heat-transfer characteristics of the hydrodynamic fully-d eveloped turbulent flow in air-cooled horizontal equilateral triangular duc ts (i.e. of 60 degrees apes angle), which were each fabricated with the sam e length of 2.4 m and hydraulic diameter of 0.44 m. The inner surfaces of t he triangular ducts were roughened by a milling process, shaping process or fixing uniformly-spaced parallel square ribs orthogonal to the mean air fl ow. The average surface roughness of the inner surfaces, which were produce d by milling and shaping processes, were 3.0 and 11.5 mum, respectively. Th e square-sectioned ribs, adopted to produce the roughened surface, had diff erent protrusions of 6.35, 9.525 and 12.7 mm, and the uniform separation be tween the centre lines of two successive ribs was kept constant at 57.15 mm . Both the triangular ducts and the square ribs were fabricated out of dura lumin. The experiments were performed with the hydraulic-diameter based Rey nolds numbers ranging from 4000 to 15000. The entire inner wall of the duct was heated uniformly, while its outer surfaces were thermally well insulat ed. By comparing the heat-transfer performances with these of a smooth tria ngular duct (i.e. average inner-surface roughness of less than 1.0 mum) hav ing the same geometry, it was found that forced convection was enhanced by the roughened surfaces. In addition, a much enhanced forced convection was obtained by fixing uniformaly-spaced parallel square ribs, rather than by f abricating random roughness on its inner surfaces by machining. However, th e heat-transfer enhancement was not proportional to the rib size; the maxim um forced convection heat-transfer augmentation was obtained using the smal lest (i.e. 6.35 mm) ribs of these tested. Non-dimensional expressions for t he determination of the steady-state heat-transfer coefficient of the equil ateral triangular ducts, which were fabricated with the various kinds of ar tificial inner-surface roughness, were also developed. (C) 2001 Elsevier Sc ience Ltd. All rights reserved.