Heat transfer and pressure drop characteristics in the transitional flow regime with twisted tape inserts

Abstract

Studies on heat transfer and pressure drop characteristics in a tube with twisted tape inserts have been receiving research attention in the laminar and turbulent flow regimes since 1921. However, several gaps exist in the transitional flow regime. The purpose of this study was to experimentally investigate the heat transfer and pressure drop characteristics in a smooth circular copper tube with conventional twisted tape (CTT) inserts, alternating clockwise and counter clockwise twisted tape (CCCTT) inserts and peripheral u-cut twisted tape (PUCTT) inserts without and with ring (PUCTTR) inserts.

An experimental set-up was designed and constructed in this study. The set-up was validated by comparing the results of the heat transfer and pressure drop characteristics in a smooth tube (without twisted tape inserts) with literature. The smooth circular copper tube had a wall thickness, an inner diameter, and a length of 1.5 mm, 19 mm, and 5.27 m respectively. The twisted tape inserts considered in this study were fabricated from 1 mm thick and 18 mm wide copper strips. The strips used for the CTT inserts were twisted to form tapes with twist ratios of 3, 4 and 5. A total of five 900 mm long CTT inserts were connected longitudinally to an additional 770 mm insert to form a tape with overall length of 5.27 m. The strips used for CCCTT inserts were twisted to obtain a twist ratio of 5 and 12 tapes were joined longitudinally so that a clockwise direction twisted tape insert was connected to a counter clockwise direction twisted tape insert. The assembling was at connection angles of 0°, 30° and 60°, to form CCCTT inserts with an overall length of 5.27 m. For the PUCTT inserts, the peripheries of the strips were cut to achieve depth ratios of 0.105 and 0.216. The strips were twisted to form tapes with a twist ratio of 5 and ring inserts were soldered on the PUCTT inserts to form PUCTTR inserts with ring space ratios of 1.25, 2.5 and 5. A total of five 900 mm long PUCTT inserts were connected longitudinally to an additional 770 mm insert to form a tape with overall length of 5.27 m.

Water was circulated as test fluid and experiments were conducted at constant heat flux boundary condition, at Reynolds numbers of 300 – 11 404. This Reynolds number range covered the transitional flow regime, as well as sufficient parts of the laminar and turbulent flow regimes. This study focused on the identification of the transitional flow regime with the CTT, CCCTT, PUCTT and PUCTTR inserts. With the CTT inserts, it was the influence of twist ratio and heat flux on the transitional flow regime. For the CCCTT inserts, it was the influence of connection angle and heat flux on the transitional flow regime. Modified Grashof number which is a function of heat flux was used to describe free convection effects in the CTT and CCCTT inserts. The PUCTT and PUCTTR inserts it was the influence of depth ratio as well as ring space ratio on the transitional flow regime.

When twist ratios and heat fluxes of the CTT inserts were compared, a reduction in twist ratio and heat flux caused the transitional flow regime to occur earlier. When the CCCTT inserts were compared it was found that both the start and end of the transitional flow regime were influenced by the connection angle and heat flux. When different connection angles of the CCCTT inserts were compared it was found that an increase in connection angle enhanced the heat transfer in the transitional flow regime. An increase in heat flux significantly enhanced the heat transfer in the laminar flow regime and delayed transition. When depth ratios of the PUCTT inserts were compared, 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. An increase in depth ratio and reduction in ring space ratio significantly enhanced heat transfer in the transitional flow regime. It can be concluded that when the CTT, CCCTT and PUCTT inserts were compared, transition first occurred with the CCCTT inserts and delayed the most with the CTT inserts.

Heat transfer and pressure drop correlations were developed to predict the experimental data in the laminar, transitional and turbulent flow regimes. Where applicable the correlations were developed as a function of Reynolds number, twist ratio, modified Grashof number, connection angle, depth ratio and ring space ratio.