Intensification of thermo-hydraulic performance in heat exchanger tube inserted with multiple twisted-tapes

The present work presents the thermo-hydraulic characteristics in the heat exchanger tube inserted with multiple-twisted tapes (M-77). Multiple-twisted tapes consisted of a larger twisted tape and small twisted tapes which were placed along with the larger twisted tape. The study encompassed 6 different twist ratios of the small twisted tapes (y/w = 2.5, 5, 10, 15, 20 and 25) and 5 different numbers of the small twisted tapes (N = 2, 3, 4, 5 and 6). Experiments were conducted under uniform wall heat flux condition by using air as the testing fluid for Reynolds numbers from 6000 to 20,000. A large single twisted tape in a tight-fit form (T-TT) and that in a loose-fit form (L-TT) were tested as the reference cases for comparison. The experimental results found that the use of multiple-twisted tapes (M-TT) with N = 3, 4, 5 and 6 leads to the considerable increases in heat transfer enhancement and friction factor as compared to those of single twisted tape. For multiple twisted tapes (M-TT), Nusselt number (Nu), friction factor (eta) and thermo-hydraulic performance factor (II) increase with increasing number (N) and decreasing twist ratio (y/w) of small twisted tapes. The maximum thermo-hydraulic performance factor of 1.2 is obtained by the use of M-TT with N = 6 and y/w = 2.5. In addition, the numerical work on flow and thermal behaviors was also performed for a better understanding of heat transfer mechanisms.