In order to find out the better types of inserts in tubes for augmentation of heat transfer of gases in the lower Reynolds number range

flow visualization by means of hydrogen bubble technique and comparative tests of the friction drag and heat transfer of air in vertical tubes with uniform wall temperature were made. Six types of inserts were studied in the region of Reynolds numbers from 500 to 10

000. The f-Re and Nu-Re correlations of various inserts under the test condition were obtained. The heat transfer augmentation performance of these types of inserts was evaluated. On this basis

friction drag and heat transfer characteristics of the helical ribbon were investigated experimentally. The friction drag and heat transfer correlations of this kind of insert with e/d from 0.067 to 0.234 and S/e from 3.89 to 13.25 were obtained. Combining the results of the flow visualization with the performance of heat transfer augmentation

it was suggested that the spiral flow consumes less energy than the boundary layer separation to produce the same heat transfer augmentation in the laminar or transitional range.