Heat transfer and pressure-drop experiments have been performed for gas flow inside nine, 1/2-in-OD, 0.035-in. wall thickness, A-nickel, spirally corrugated tubes. The corrugations, which varied from 0.003–0.028 in. deep, were formed by pulling the tubes through a rotating head containing four embossing tools; corrugation-spacing-to-corrugation-depth ratios (P/e) ran from 16–41. The data, for heat transfer to nitrogen, at approximately 200 psig, were correlated by an expression of the form NNu,B (NPr,B)−0.4 × (Tw/TB)0.5 = A(NRe,B)m, where all the physical properties were evaluated at bulk gas conditions. The exponent, m, on the Reynolds number was observed to be consistently greater (0.854–0.900) than the value of 0.8 found for smooth tubes; the constant, A, varied from 0.0095–0.0195 with no apparent correlation with P/e. Friction factors, measured with adiabatic airflow, were found to be up to 1.7 times that for smooth tubes. Tubes of this geometry were found to be very effective in enhancing heat transfer. On an equal pumping power basis, for example, a tube with P/e = 22 had a heat transfer coefficient 22 percent greater than a smooth tube.
Average Friction Factors of Choked Gas Flow in Microtubes
