Experiments were conducted to investigate roughness effects on flow characteristics and heat transfer of air and CO 2 flow 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

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... features were generated: (i) uniform roughness of 3.8 µm, (ii) uniform roughness of 1.8 µm, (iii) internal grooves 45 µm deep and 218 µm wide, and (iv) helical grooves 96 µm 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.