TO-98-19-1 -- Experimental Study of the Effects of the Spiral Angle on Evaporative Heat Transfer Enhancement in Microfin Tubes PDF

The in-tube evaporative heat transfer of the hydrofluorocarbon R-134a was examined for a smooth tube and five microfin tubes. The heat transfer coefficient in the smooth tube was compared with four correlations. The Yoshida correlation was found to be the best in annular flow, and the Kandlikar correlation with F_K = 2.33 was the best in stratified flow, but the latter needs clarification for its applicability to the low quality range because of the strong nucleate boiling effect caused by the large value of F_K. The effect of the spiral angle of microfin tubes was investigated to determine the optimal spiral angle. Spiral angles of 6, 12, 18, 25, and 44 degrees were tested at constant mean diameter, 8.71mm (0.343 in.), and number of fins, 60. The optimal spiral angle was found to be mainly dependent on the mass flux, and it was 18 degrees for a mass flux of G = 50 kg/m²·s (36,865 lbm/ft²·h), 6 degrees for G = 100 kg/m²·s (73,730 lbm/ft² ·h), and 6 to 18 degrees for G = 200 kg/m ·s (147,460 lbm/ft²·h).

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