Abstract
Heat transfer, pressure drop and thermal performance characteristics in ellipsoidal dimpled tubes fitted with twisted-tape swirl generators have been studied experimentally. Ellipsoidal dimpled surfaces function as the turbulence promoter near the tube wall while twisted-tapes act as the swirling flow generators. The experiments were performed by using twisted tapes with different twist ratios (y/W = 3.0, 4.0 and 5.0) and water as the working fluid for Reynolds numbers between 5000 and 15,000. The experimental results of the dimpled tubes equipped with twisted-tape were compared with those of the dimpled and plain tubes without twisted tape. Evidently, the dimpled tubes fitted with twisted-tapes consistently yield higher Nusselt numbers and friction factors than the dimpled and plain tubes without twisted tape. It is also found that the average Nusselt numbers of the dimpled tubes equipped with twisted-tapes at y/W = 3.0, 4.0 and 5.0 are higher than those of the dimpled tube alone up to 40.7%, 32% and 26.7%, respectively which correspond to the higher thermal performance factors up to 10.8%, 6.7% and 3.6%, respectively. Depending upon Reynolds number, the dimpled tube with twisted-tape at the smallest twist ratio, y/W = 3.0, give higher thermal performance factors than those at y/W = 4.0 and 5.0 by around 3.9% and 6.7%.
