Augmentation of convective heat transfer inside tubes with three-dimensional internal extended surfaces and twisted-tape inserts

Experiments were carried out to study the heat transfer and friction characteristics for water, ethylene glycol, and ISO VG46 turbine oil flowing inside four tubes with three-dimensional internal extended surfaces and copper continuous or segmented twisted-tape inserts. During the experiments, Prandtl numbers ranged from 5.5 to 590 and Reynolds numbers from 80 to 50,000. The experimental results show that this compound enhanced heat transfer technique, a tube with three-dimensional internal extended surfaces and twisted-tape inserts, is of particular advantage to enhance the convective heat transfer for the laminar tubeside flow of highly viscous fluid. For the laminar flow of VG46 turbine oil, the average Stanton number could be enhanced up to 5.8-fold inside tubes with three-dimensional internal extended surfaces and continuous twisted-tape inserts compared with an empty smooth tube, and the friction factor was also increased by almost 6.5-fold. Inside the tubes with three-dimensional internal extended surfaces, replacement of the continuous twisted-tape inserts with the segmented twisted-tape inserts induced a greater decrease in the friction factor but a comparatively smaller decrease in the Stanton number. (C) 2000 Elsevier Science S.A. All rights reserved.