Abstract
Heat transfer, pressure loss, and thermal performance assessment in a circular tube heat exchanger with modified-twisted tapes are reported. The rectangular holes are punched out from the general twisted tape to reduce the pressure loss. The influences of the hole sizes (l/D, LR = 0.30, 0.44, 0.78, and 0.88) and twisted ratios (y/D, TR = 1, 1.5, 2, and 4) for the single and double twisted tapes are investigated with a numerical method at turbulent regime, Re = 3000–10,000. The finite volume method and the SIMPLE algorithm are used to investigate for the current research. The numerical results are reported in terms of flow structure and heat transfer behavior and compared with the smooth tube and the regular twisted tape. It is found that the modified-twisted tape provides higher heat transfer rate than the smooth tube due to the longitudinal vortex flows, created by the twisted tape. The longitudinal vortex flows help to increase fluid mixing. The rectangular punched holes of the twisted tape can reduce the pressure loss of the heating system. In addition, the maximum thermal enhancement factor is around 1.39 and 1.31 for the double twisted tape and single twisted tape, respectively, at Re = 3000, LR = 0.78, and TR = 1.
Highlights
Twisted tape is a type of vortex generators, inserted in heat exchangers to improve heat transfer rate and thermal efficiency
The modified-twisted tapes are improved to augment the heat transfer rate and thermal performance with reducing of the pressure loss in the circular tube heat exchanger
The twisted tape is punched with rectangular holes at various length ratios and inserted in the middle of the test tube
Summary
Twisted tape is a type of vortex generators, inserted in heat exchangers to improve heat transfer rate and thermal efficiency. The effects of twisted ratios, 0.577–1.732, were studied on the heat transfer rate and pressure loss for the Reynolds numbers, Re = 500–6000 They found that the Nusselt number and friction factor are around 1.25–2.10 times and 1.0–1.75 times over the base case (plain absorber/receiver of parabolic trough collector), respectively, while the thermal performance is around 1.35–2.05. Salam et al [27] investigated the augmentations on both heat transfer rate and thermal efficiency in a tube heat exchanger with rectangular cut twisted tapes by an experimental method They concluded that the Nusselt number and friction loss tend to increase around 2.3–2.9 times and 1.4–1.8 times, respectively, over the smooth tube when using the rectangular cut twisted tapes in the tube heat exchanger. The understanding of the flow and heat transfer mechanisms may help to improve the new design of heat exchanger and save costs of the energy
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