Compound Heat Transfer Enhancement by a Combination of Ribbed Tubes with Wire Coil Inserts

An experimental investigation on compound heat transfer enhancement by a combination of ribbed tubes with wire coil inserts was performed. Two wire coils with different geometries were inserted in smoothed, straight ribbed and helically ribbed tubes to make six variations of enhanced tube, and the effects of heat transfer enhancement were examined. The heat transfer coefficient was obtained for Reynolds numbers ranging from 300 to 30,000 by means of Wilson’s method. The friction factor was also obtained from the measurement of pressure drop between both ends of the tube. Moreover, the performance of enhanced tubes were evaluated by comparing the heat transfer coefficient with that of the smoothed tube under the condition of the same pumping power. All enhanced tubes exhibit higher performance in the laminar region than in the turbulent region. The highest performance is accomplished by the helically ribbed tube with a wire coil having a pitch ratio P/e = 10.0 and a clearance NDC = 0.04 due to much higher heat transfer enhancement in comparison to the other five enhanced tubes and the smaller friction factor increase in comparison to the straight ribbed tubes.