Compound Heat Transfer Enhancement of a Converging-Diverging Tube with Evenly Spaced Twisted-tapes*

Pressure drop and compound heat transfer characteristics of a converging-diverging tube with evenly spaced twisted-tapes (CD-T tube) have been investigated experimentally. Swirl was generated by evenly spaced twisted-tape elements which vary in twist ratio and rotation angle. Space ratio also has an important effect on the characteristics. For comparison, experiments in a smooth circular tube and a converging-diverging (CD) tube without twisted-tapes were carried out. The results show that the twisted-tape with twist ratio y=4.72 and rotation angle θ= 180° has the best performance among the four types of twisted-tapes presented in this paper. At Reynolds number ranging from 3400 to 20000, when space ratio s=48.6, the heat transfer efficiency index, which increases as the Reynolds number increases, is 0.85–1.21 and 1.07–1.15 compared to that of a smooth circular tube and a CD tube without twisted-tape inserts, respectively.