Heat transfer and friction factor characteristics in turbulent flow through a tube fitted with perforated twisted tape inserts

https://doi.org/10.1016/j.icheatmasstransfer.2013.05.012Get rights and content

Abstract

This work deals with the experimental investigation on Nusselt number, friction factor and thermal performance factor in a circular tube equipped with perforated twisted tape inserts with four different porosities of Rp = 1.6, 4.5, 8.9 and 14.7%. The experiments were conducted in a turbulent flow regime with Reynolds number ranging from 7200 to 49,800 using air as the working fluid under uniform wall heat flux boundary condition. The experimental results revealed that both heat transfer rate and friction factor of the tube fitted with perforated twisted tapes were significantly higher than those of the plain tube. Over the range investigated, Nusselt number, friction factor and thermal performance factor in the tube with perforated twisted tape inserts was found to be 110 –340, 110 –360 and 28–59% higher than those of the plain tube values, respectively. In addition, the empirical correlations of Nusselt number, friction factor and thermal performance factor were formulated from the experimental results of tape inserts.

Introduction

Heat transfer augmentation techniques are recurrently used in heat exchanger systems in order to enhance heat transfer and improve the thermal performance. Several methods are applied to improve the thermal performance of heat transfer devices such as treated surfaces, rough surfaces as well as incorporations of inserts (such as turbulators and swirl flow devices) [1], [2]. Among the passive techniques, insertion of twisted tape swirl generator is one of the most promising techniques. Tubes with twisted tape insert have been widely used as the continuous swirl flow devices for augmentation the heat transfer rate in heat exchanger tubes and applied in many industrial applications such as heat recovery processes, air conditioning and refrigeration systems, cooling of modern electronic appliances, chemical reactors, food and dairy processes [3]. The swirl flow induces the turbulence near the tube wall and increases the residence time of the fluid in the tube. The higher turbulence intensity of the fluid close to the tube wall associated with the twisted tape is responsible for an excellent fluid mixing and an efficient redevelopment of the thermal/hydrodynamic boundary layer which consequently results in the improvement of convective heat transfer [4], [5], [6], [7].

The studies on heat transfer enhancement by means of twisted tapes have been extensively reported. Twisted tapes are commonly installed in a tube heat exchanger to promote the fluid mixing between central region and nearly the wall region. Agarwal and Rao [8] studied the isothermal and non-isothermal friction factors and mean Nusselt numbers under uniform wall temperature condition heating and cooling of Servotherm oil in a circular ube with twisted tape inserts. Eiamsa-ard et al. [9] experimentally studied the heat transfer and fluid friction characteristics in a circular tube fitted with regularly spaced twisted tape elements, and showed that the heat transfer coefficient increased with the decrease of twist ratio. Bhuiya et al. [10], [11] experimentally studied the performance of heat transfer for turbulent flow through a tube with double and triple helical tape inserts. Naphon [12] experimentally studied the heat transfer and pressure drop characteristics in the horizontal double pipes with twisted tape insert.

Twisted tapes were used combined with several enhancement devices for further improving heat transfer rate in several research works such as twisted with spirally grooved tube [13], corrugated tube [14], dimpled tube [15], conical ring [16], wire coil turbulators [17] and twisted tape with wire coil [18]. An alternative approach to improve their performance is modifying their geometries to induce extra fluid flow disturbing. Murugesan et al. [19] used V-cut twisted tapes to analyze heat transfer and pressure drop in a circular tube. Some experiments were performed to obtain heat transfer enhancement in a tube using delta-winglet twisted tapes [5]. Thermal characteristics were investigated experimentally in circular tube fitted with serrated twisted tapes [7], [20], with twisted tapes consisting center wings and alternate axes [21] and with a peripherally cut twisted tape has an alternate axes [22]. Heat transfer and pressure drop characteristics in a tube heat exchanger fitted with dual twisted tape elements in tandem were analyzed [6]. Heat transfer characteristics in a helical ribbed-tube with double twisted tapes were investigated experimentally [23]. Heat transfer enhancement attributed to helically twisted tapes was experimentally investigated by Eiamsa-ard et al. [24]. The experiments were performed with different twist and pitch ratios for Reynolds number range between 6000 and 20,000. Ahamed et al. [25] experimentally studied the prediction of heat transfer in turbulent flow through a tube with perforated twisted tape inserts. Wazed et al. [26] experimentally studied the enhancement of heat transfer in turbulent flow through a tube with perforated twisted tape inserts.

Heat transfer and friction factor characteristics of CuO/water nanofluid and water in a circular tube fitted with modified twisted tapes has alternate axis were studied experimentally [27], [28]. The influences of twin-counter/co-twisted tapes on heat transfer, friction factor and thermal enhancement factor were experimentally interpreted [4]. The heat transfer and the pressure drop characteristics of turbulent flow through rectangular and square ducts with combined internal axial corrugations on all the surfaces of the ducts and with twisted tapes with oblique teeth were studied experimentally [29]. Experimental investigations of heat transfer and friction factor characteristics of circular tube fitted with trapezoidal-cut twisted tape were performed for the Reynolds number range of 2000 to 12,000 [30]. Heat transfer, friction and thermal performance factor characteristics of a double pipe heat exchanger fitted with square-cut twisted tape were investigated experimentally using the water as working fluid [31]. It is seen in most cases that the heat transfer rates associated by the combined techniques or the modified twisted tapes are higher than those given by the typical twisted tapes. However, it is a challenging task of researchers to modify twisted tapes with appropriate geometries with the aim of achieve an outstanding heat transfer results with reasonable pressure drop.

The available literature showed that numerous studies have been carried out on heat transfer enhancement and pressure drop characteristics in tubes with various geometrical configurations of swirl flow generators. However, augmentation of heat transfer and friction factor characteristics in turbulent flow through a tube fitted with perforated twisted tape inserts were limitedly reported. Therefore, in the present study, the effects of perforated twisted tape inserts on heat transfer performance with corresponding increase in friction factor through a circular tube equipped with perforated tape inserts for a wide range of porosities ranging from 1.6 to 14.7% were investigated. Moreover, new correlations were developed for predicting the heat transfer, friction factor and thermal performance factor.

In this study, some assumptions were made in order to make easy experiments, comparison and analysis which created some limitations in the actual results. These were:

  • i.

    Inside diameter of the tube (Di) was used instead of hydraulic diameter (Dh) in defining Reynolds number (Re), Nusselt number (Nu), and friction factor (f).

  • ii.

    All the fluid properties were calculated at local bulk temperature (Tbx) and at atmospheric pressure instead of local pressure in the test section which was slightly less than the atmospheric pressure.

  • iii.

    The heat transfer was considered only by forced convection from inside wall of the tube to the fluid. However, there were points of contact between the inserts and the inner wall of the tube. Thus there was the potential for heat transfer to occur through the inserts by conduction. It was not possible to quantify this, also heat was conducted through the ends of the test section to adjacent sections.

Section snippets

Data reduction equations

The experimental data were used to calculate the Nusselt number, friction factor and thermal performance factor at different Reynolds number in turbulent flow region for both the cases with and without using perforated twisted tape inserts.

Mass flow rate was calculated by,m˙=ρAxViwhere ρ is the density of air, Ax is the cross sectional area of test section and Vi is the mean inlet velocity.

In the test section the velocity of air was obtained from,V=m˙ρbAxwhere ρb is the density at bulk fluid

Experimental setup

The experimental facility consisted of an inlet section, a test section, an air supply system (Electric blower) and a heating arrangement. A schematic diagram of the apparatus with the basic components is presented in Fig. 1. The tube shaped inlet section, 533 mm long was made as an integral part of the test section to avoid any flow disturbances upstream of the test section and to get fully developed flow in the test section as well. The inlet section shape of the experimental setup was made as

Experimental results and discussion

In this study, the experimental results of the tube fitted with perforated twisted tape that effect on the heat transfer, friction factor and thermal performance factor behaviors were investigated. The experiments were performed using perforated twisted tapes with four different porosities of 1.6, 4.5, 8.9, and 14.7% in the range of Reynolds number between 7200 and 49,800. The heat transfer and friction factor results obtained from the present plain tube were first validated with those obtained

Conclusion

An experimental study has been carried out to investigate the flow friction and heat transfer characteristics in a circular tube fitted with perforated twisted tapes of different porosities (Rp = 1.6, 4.5, 8.9 and 14.7%). It has been found that the perforated twisted tape inserts enhanced the heat transfer rate significantly with corresponding increase in friction factor in comparison to that of the plain tube. Based on the experimental results, key findings of this study could be summarized as

Acknowledgements

The authors would like to gratefully acknowledge the Chittagong University of Engineering and Technology (CUET) for their support of this research.

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  • Cited by (0)

    Communicated by W.J. Minkowycz.

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