Fluid Flow Inside a Rectangular Duct With Two Opposite Walls Roughened by Deepened Scales

Laser-Doppler velocimetry (LDV) measurements supplemented by numerical simulation and flow visualization were performed to study flow characteristics and explain the reported heat transfer features in a rectangular channel with two opposite walls roughened by deepened scales. The study is lacking in the published literature. Ratios of scale print diameter to channel height, scale maximum depth to channel height and scale pitch to scale maximum depth were 1.0, −0.1, and 10 respectively. The scale-roughened section had a cross-sectional width to height ratio of 8. All measurements were undertaken at a fixed Reynolds number, based on hydraulic diameter and cross-sectional bulk mean velocity, of 10000 with air flows directed forward and downward. Results are documented in terms of distributions of mean velocity components, mean velocity vector field, fluctuation components, and turbulent kinetic energy. The distances attaining periodic fully developed flow condition are identified. Both LDV measurements and laser-sheet flow visualization unravel the presence of near-wall secondary vortex arrays in the cross-sectional planes. The fluid flow results are subsequently used to explain previously published heat transfer trends. The dominant flow dynamic factors are recognized to provide the logic for the differences in heat transfer enhancements attained by the forward and downward channel flows over the scaled walls. A comparison of the computed sizes of cavity trapped vortex illustrates the reported difference in heat transfer augmented by the scale and dimple roughened surfaces as well as by the turbulent and laminar flows.