FORCED-CONVECTION AUGMENTATION OF TURBULENT FLOW IN A TRIANGULAR DUCT WITH ARTIFICIALLY ROUGHENED INTERNAL SURFACES

Abstract

Use of artificially roughened internal surfaces to enhance the steady-state forced convection in an air-cooled horizontal equilateral triangular duct (apex angle of 60°) has been investigated experimentally under a hydrodynamic fully developed turbulent flow condition with hydraulic diameter-based Reynolds numbers ranging from 4,300 to 15,000. The test duralumin triangular ducts were fabricated with the same axial length of 2.4 m and hydraulic diameter of 0.44 m, but four kinds of internal surface finishing. In addition to a smooth surface with an average surface roughness of less than 1 w m, the internal surfaces of the triangular ducts were roughened artificially either by milling/shaping processes, by cutting V-grooves, or by fixing square ribs with a uniform spacing along their axial length. The artificial surface roughness was fabricated in parallel and essentially in the orthogonal direction to the mean flow. By comparing the thermal performance of triangular ducts with the same geometry but different kinds of surface finishing, it was found that forced-convection augmentation could be achieved by the internally roughened surfaces. A much greater enhanced forced convection was obtained by cutting V-grooves or fixing square ribs at uniform spacing, rather than fabricating random roughness on the internal surfaces by machining processes. In addition, the best thermal performance was achieved with the ribbed surface. Nondimensional expressions for the determination of the steady-state heat transfer coefficient of the equilateral triangular ducts, which were internally fabricated with various kinds of artificial surface roughness under consideration, have also been developed.