Performance assessment in a heat exchanger tube with alternate clockwise and counter-clockwise twisted-tape inserts

The article presents an experimental study of turbulent heat transfer and flow friction characteristics in a circular tube equipped with two types of twisted tapes: (1) typical twisted tapes and (2) alternate clockwise and counterclockwise twisted tapes (C-CC twisted tapes). Nine different C-CC twisted tapes are tested in the current work; they included the tapes with three twist ratios, y/w = 3.0, 4.0 and 5.0, each with three twist angles, 0 = 30 degrees. 60 degrees and 90 degrees. The experiments have been performed over a Reynolds number range of 3000-27,000 under uniform heat flux conditions, using water as working fluid. The obtained results reveal that the C-CC twisted-tapes provide higher heat transfer rate, friction factor and heat transfer enhancement index than the typical twisted-tapes at similar operating conditions. The results also show that the heat transfer rate of the C-CC tapes increases with the decrease of twist ratio and the increase of twist angle values. Depending on Reynolds number, twist ratio and twist angle values, the mean Nusselt numbers in the tube fitted with the C-CC twisted tapes are higher than those with the typical ones and the plain tube around 12.8-41.9% and 27.3-90.5%, respectively. The maximum heat transfer enhancement indexes of the C-CC twisted tapes with 0 = 90 degrees for y/w = 3.0. 4.0 and 5.0, are 1.4, 1.34 and 1.3, respectively. In addition, correlations of the Nusselt number and the friction factor for using the C-CC twisted tapes are also determined. Both predicted Nusselt number and friction factor are within +/-15% and +/-15% deviation compared to the experimental data. (C) 2009 Elsevier Ltd. All rights reserved.