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It has been well established that there are no differences between microscale and macroscale flows of incompressible liquids. However, surface roughness has been known to impact the transport phenomena. This work aims to systematically quantify the effect of structured roughness geometries on friction factor in the laminar and turbulent flows as a precursor to the detailed heat transfer studies on these geometries. Experiments were conducted by varying the pitch (150–400 μm) and height (36–131 μm) of transverse rib roughness structures in rectangular channels such that the pitch-to-height ratio ranged from 2 to 8. The channel width was fixed at 12.70 mm and the length at 152.4 mm, while the channel gap was varied (230–937 μm). Tests were conducted over a Reynolds number range of 5–3400. The results are compared to the existing models, which do not account for specific roughness features such as pitch and height. A theoretical model is developed to predict the effect of roughness pitch and height on pressure drop along the channel length. Validation of the proposed theory is carried out by comparing the predictions with the experimental results. The model and the experimental results provide an understanding of the effect of two-dimensional structured roughness on the frictional losses in fully developed laminar flow. [ABSTRACT FROM PUBLISHER]

Roughness elements affect internal flows in different ways. One effect is the transition from laminar to turbulent flow at a lower Reynolds number than the predicted Re = 2300. Initial work at RIT in the subject area was performed by Schmitt and Kandlikar [1] and Kandlikar et al. [2], and this study is an extension of these efforts. The channel used in this study is rectangular, with varying separation between walls that have machined roughness elements. The roughness elements are saw-tooth in structure, with element heights of 107 and 117 μ m for two pitches of 405 μ m and 815 μ m, respectively. The resulting hydraulic diameters and Reynolds numbers based on the constricted flow area range from 424 μ m to 2016 μ m and 210 to 2400, respectively. Pressure measurements are taken at sixteen locations along the flow length of 88.9 mm to determine the local pressure gradients. The results for friction factors and transition to turbulent flow are obtained and compared with the data reported by Schmitt and Kandlikar [1]. The roughness elements cause an early transition to turbulent flow, and the friction factors in the laminar region are predicted accurately using the hydraulic diameter based on the constricted flow area. [ABSTRACT FROM AUTHOR]

Experiments were conducted to investigate roughness effects on flow characteristics and heat transfer of air and CO2flow in four circular micr-tubes of approximately 1 mm inner diameter. The tubes were made by electrodepositing nickel on an aluminum sacrificial substrate. The desired roughness structures were machined or etched on the substrate before depositing nickel to generate a replica of the aluminum substrate on the internal surface of the nickel tubes. Four different surface roughness features were generated: (i) uniform roughness of 3.8 m, (ii) uniform roughness of 1.8 m, (iii) internal grooves 45 m deep and 218 m wide, and (iv) helical grooves 96 m deep with 1.9 mm pitch. Friction factor and Nusselt number data for the smooth tube are in good agreement with the conventional correlations in both the laminar and the turbulent flow regimes. In the rough tubes, the friction factors are significantly higher than that of the smooth tube. Heat transfer coefficients in the laminar flow regime for both smooth and rough tube agree well with the conventional correlation. However, in the turbulent flow regime, heat transfer was enhanced by the roughness features and the enhancement increased with increasing Reynolds number. [ABSTRACT FROM AUTHOR]