The use of regularly spaced twisted tape inserts will obviously decrease the pressure drop and lead to lower pump power compared with full-length twisted tape inserts. Since the swirl flow generated by twisted tape decay in the free space between tape elements, some researchers considered to place twisted tapes with regular space in a tube or use increased tapered twisted tape. It is found that adding twisted tape insert in helical heat exchanger can enhance heat transfer performance by up to four times in comparison to conventional straight tubes. The flow in the tube is considered as laminar flow (Re = 100–2000). 9 numerically studied the flow behaviour and the heat transfer in helical tube with twisted tape insert. 8 developed a combined approach to predict friction coefficients and heat transfer for a wide range of Re (200 < Re < 10 5). 7 presented experimental flow visualization and computational modelling of single-phase laminar flows to clarify the mechanism of enhancement of heat transfer. This correlation predicts experimental data for wide range of fluids, flow conditions and tape geometry to within 10%. 6 developed correlation for the isothermal Fanning friction factors for circular tubes with twisted-tape inserts. 4, 5 conducted wide experiments to investigate heat transfer and pressure drop correlations for twisted-tape-inserts in isothermal tubes with Reynolds numbers ranged from laminar to transition and transient regions. Many researches related to the effect of twisted tapes on heat transfer, pressure drop and friction factor in a circular tube have been carried out. Among them, twisted tape can change the flow pattern in a channel, reduce thickness of the boundary layer and increase the heat transfer area. Passive techniques are applied widely to achieve higher energy efficiency, and examples include finned surfaces, twisted tape inserts, coiled tubes and so on. Detailed surveys have been made by Kareem et al. A variety of passive heat transfer enhancement techniques have been proposed, which do not consume additional energy other than that needed for circulating the working fluid. In order to improve the efficiency of heat exchangers, heat transfer should be increased while avoiding excessive pump power. The enhancement in heat transfer also leads to the reduction of heat exchanger size and temperature driving force etc. The need to reduce energy consumption has led to the development and usage of many heat transfer enhancement techniques. The thermo-hydraulic performance parameters for convective heat transfer of helium gas flowing in a circular tube are below unity for the calculated Reynolds region. However, the use of twisted tape inserts is not beneficial for energy saving. The increasing ratio for Nusselt number ranged from 3 to 18%. With a smaller tape width, a higher increasing ratio of Nu-wall/Nu-center is obtained. The heat transfer for circular tube with twisted tape attached to the wall shows better performance than that for the tube with center-placed twisted tape. The results indicate that the Nusselt number of the tube fitted with center-placed twisted tapes at various width is 7–51% higher than the plain tube, and performance in low Reynolds region was found more effective than that in high Reynolds region. The numerical analysis of the flow field, average Nusselt number, friction factor and thermo-hydraulic performance parameter of the tube are discussed with Reynolds number ranged from 2600 to 8760. The effect of tape width of 20, 18, 16, 14 and 12 mm on the heat transfer performance is discussed under the same twist ratio of 2.0. This study is conducted to investigate the effect of off-center placement of twisted tape on flow distribution and heat transfer in a circular tube.
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