The influence of peripheral u-cut twisted tapes and ring inserts on the heat transfer and pressure drop characteristics in the transitional flow regime

Abstract

The purpose of this study was to experimentally investigate the heat transfer and pressure drop enhancement characteristics in a smooth circular copper tube with peripheral u-cut twisted tape (PUCTT) inserts without and with ring (PUCTTR) inserts. The PUCTT inserts were fabricated from 1 mm thick and 18 mm wide copper strips. The twisted tapes contained peripheral cuts with depth ratios of 0.105 and 0.216 respectively and were twisted to obtain a twist ratio of 5. Ring inserts were soldered on the PUCTT inserts to form PUCTTR inserts with ring space ratios of 1.25, 2.5 and 5 respectively. A total of five 900 mm long PUCTT inserts and one 770 mm insert were connected longitudinally to form a single PUCTT insert with an overall length of 5.27 m. The PUCTT and PUCTTR inserts were placed in a smooth copper tube with an inner diameter of 19 mm. Water was used as the test fluid and experiments were conducted between Reynolds numbers of 315 and 11,404 at constant heat flux boundary condition. This Reynolds number range covered the transitional flow regime, as well as sufficient parts of the laminar and turbulent flow regimes. This study focussed on the identification of the transitional flow regime with the PUCTT and PUCTTR inserts, as well as to investigate the influence of the depth ratio of the peripheral cuts and the ring space ratio on the transitional flow regime. It was found that the start and end of the transitional flow regime were affected by both the depth and ring space ratios. An increase in depth ratio caused the transitional flow regime to occur earlier. Furthermore, the transitional flow regime occurred earlier with PUCTTR inserts than with PUCTT inserts and transition occurred even earlier as the ring space ratio was reduced. It was concluded that an increase in depth ratio and reduction in ring space ratio significantly enhanced heat transfer in the transitional flow regime. Heat transfer and pressure drop correlations were therefore developed to predict the experimental data in the laminar, transitional and turbulent flow regimes as a function of Reynolds number, depth ratio and ring space ratio.