Single phase flow heat transfer characteristics of quad-channel twisted tape inserts in tubes

doi:10.1016/j.icheatmasstransfer.2020.104835

International Communications in Heat and Mass Transfer

Volume 118, November 2020, 104835

https://doi.org/10.1016/j.icheatmasstransfer.2020.104835 Get rights and content

Abstract

Heat transfer and pressure drop of turbulent fluid flow were studied numerically and experimentally in a quad-channel with twisted tapes placed in a circular tube under uniform heat flux boundary condition. All the tests were conducted with water as the working fluid where the turbulent flow range of Reynolds numbers were maintained at 4250 to 11000. Quad-channel twisted tapes (QCTT) were manufactured by 3D printer with the material PLA, the lengths of 50 mm, 100 mm, 200 mm with the twist ratio of 4.48. The tapes were identified with their tape/pipe length ratios (TPR) of 0.025, 0.05 and 0.1. Validation of the experimental data was performed by the comparison of the experimental results with the data from the well-known empirical correlations. Alteration of the friction factors and Nusselt numbers with the Reynolds numbers are presented in this paper. The results showed that the Nusselt number and the friction factor increased with the increasing of the length of QCTTs. On the other hand, the performance evaluation criteria (PEC) should be declining with the increase of the lengths of the tapes. The evaluated temperature distributions inside the tested tubes with the inserts were plotted in order to investigate the observed data. Based on the obtained experimental data, the empirical correlations for the Nusselt number within the deviation of ±2% and the friction factor within the deviation of ±5% were developed for the practical usage.

Introduction

Heat exchangers are extensively used in the Industries. For enhancing the heat exchanger performance, the heat transfer enhancement had always been investigated by researchers in many ways. Mainly, there are two methods for heat transfer augmentation as the active and passive ones [1]. Sometimes, the fluid's physical properties were improved and thus the nanofluids were discovered from that idea and they were considered for application in heating and cooling systems [2]. Another option for heat transfer enhancement is the use of twisted tape inserts. Up to now, there have been many modified twisted tape inserts used and most of them were prepared from metals and metal oxides. Another common feature of them is that they have I-shaped cross-sectional areas whose production was too hard due to their complex geometry. Fortunately, 3D print technology has been emerged with the solution of these problematic issues.

Insertion of structures into the flow path favorably alters the flow pattern [3] by introducing swirl in a pipe flow which could be utilized specifically for heat transfer enhancement. Manglik and Bergles [4], Al-Fahed and Chakroun [5] and Suresh et al. [6] studied the theory of the thermo-hydraulic performance of twisted tape inserts in a large hydraulic diameter annulus. Bhattacharyya et al. [7] observed that the centre-cleared twisted tapes in combination with rib roughness provide better performance than the individual enhancement technique acting alone for laminar flow through a circular duct up to a certain amount of twisted tape centre-clearance. Sivashanmugam and Suresh [8] investigated experimentally the heat transfer and friction factor characteristics of turbulent flow through a circular tube fitted with helical screw-tape inserts and observed enhanced heat transfer. Eiamsa-ard and Promvonge [9] reported the enhancement of heat transfer in a tube with regularly-spaced helical tape swirl generator with Reynolds number between 2300 and 8800 using water as working fluid. They inferred that the full-length helical tape with rod provides the highest heat transfer rate about 10% better than that of without rod.

Bhattacharyya et al. [10] numerically studied heat transfer behaviour in a tube with inserted twisted tape swirl generator for different values of the twist ratio and diameter ratio and for the Reynolds numbers within the range of 100–20000. They used transition-SST turbulence model for the evaluation and observed general enhancement of heat transfer and generation of higher pressure drop with the use of twist tape. They ascertained that an improvement of the thermal-hydraulic performance can only be observed for certain configurations and Reynolds numbers.

Hasanpour et al. [11] experimentally investigated the heat transfer and friction factor of different types of twisted tapes (typical, perforated, V-cut, U-cut). A corrugated tube was also used in their study and they used water as the working fluid. The maximum Nusselt number was obtained for V-cut twisted tape with the twist ratio of 3 at Reynolds number of 5000 and all the experiments showed that the minimum values of the friction factor belong to the perforated twisted tapes.

Eiamsa-ard et al. [12] studied the heat transfer and friction factor of air flowing through a circular tube with four different twisted tape inserts of different lengths (length ratio, LR = 0.29, 0.43, 0.57 and 1.0) and the Reynolds number range was varied between 4000 and 20000. Experimental results showed that the Nusselt number of the flow with the short-length twisted tapes were 1.16 (for LR = 0.29), 1.22 (for LR = 0.43) and 1.27 (for LR = 0.57) times of the plain tube. Nusselt numbers of all the flow with tapes were increased and the friction factor of them were decreased with the increase of Reynolds number. The friction factor values of the flow with the twisted tape with LR = 0.29, 0.43, and 0.57 were about 21%, 15.3%, and 10.5% lower than those of the full-length values. In the results as expected, the maximum Nusselt number of the flow was obtained by using the full-length twisted tape.

He et al. [13] conducted an experimental study to detect the heat transfer and friction factor characteristics of single-phase flow in a circular tube which was heated uniformly and fitted with cross hollow twisted tape inserts. The inserts were of three different hollow widths of 6, 8 and 10 mm, in the fully developed turbulent flow regime. The results revealed that the cross hollow twisted tape performed better in terms of heat transfer enhancement under laminar flow than that of turbulent flow conditions. The Nusselt number of flowing fluid over cross hollow twisted tapes were varied from 84% to 120% higher than those of the data from plain tube. The highest to lowest heat transfer rates for the twisted tape with hollow width 6 mm, 10 mm and 8 mm, showed that the increase of heat transfer coefficient (Nusselt number) did not have a linear relationship with the increase of hollow width of the twisted tapes.

Esmaeilzadeh et al. [14] experimentally studied heat transfer and friction loss characteristics of nanofluids flow through circular tube with typical twisted tape inserts of different thicknesses (0.5 mm, 1 mm and 2 mm) under laminar flow regime. The results showed that the increase of thickness of twisted tape insert enhanced both the heat transfer coefficients and the friction factors. Convective heat transfer coefficients of the flowing water over the thickness of 0.5 mm, 1 mm and 2 mm were enhanced around 75.03%, 80.20% and 90.58% respectively compared with the corresponding data from the plain tube.

Man et al. [15] carried out an investigation on heat transfer and pressure drop characteristics in turbulent flow regime in a circular tube fitted with typical twisted tapes of different lengths and alternation of clockwise and counter-clockwise twists of tapes (ACCT tapes) of different lengths. The results indicated that ACCT tapes had a good heat transfer performance compared with the typical twisted tapes of the same length. They also observed that the heat transfer enhancement was getting better with the increasing of length of the twisted tapes.

Sivashanmugan and Suresh [16] did an experimental investigation on heat transfer and pressure drop characteristics of circular tube fitted with full-length helical screw element of different twist ratios and different twist orders under turbulent regime. The results showed that the maximum heat transfer coefficient was obtained with the twisted tape of smallest twist ratio.

Bhuiya et al. [17] conducted an experimental study to observe the convective heat transfer coefficients and the friction factor of air flow through a circular tube fitted with perforated twisted tapes of different porosities under turbulent flow conditions. For all the twisted tapes with porosity showed higher Nusselt numbers and friction factors than the data obtained for the plain tubes. But it is surprising that the maximum Nusselt number did not belong to the largest or smallest porosities. This means that, the reduction of the hole diameters on the twisted tape surface positively affected the heat transfer coefficient to a certain limit.

Bas and Ozceyhan [18] experimentally examined the effect of clearance and twist ratios of the twisted tape inserts on heat transfer coefficient and pressure drop characteristics of air flowing into a tube under uniform heat flow with the Reynolds number range of 5000 to 25000. The used twisted tapes had five different twist ratios (TR = 2, 2.5, 3, 3.5, and 4) and three different clearance ratios (CR = 0, 0.0178, and 0.0357). The experimental results showed that the Nusselt number was decreased with the increase of the clearance and the twist ratios. On the other hand, friction factor decreased with the increase of the clearance ratio, but it increased with the decrease of the twist ratio as expected. It was also observed that the Nusselt number was increased and the friction factor was decreased with the increase of the Reynolds number. The maximum heat transfer enhancement was obtained as 1.789 for the twisted tape at CR = 0 and TR = 2. It was mentioned that the twist ratio had a greater effect on the heat transfer enhancement than the clearance ratio.

Abdolbaqi et al. [19] carried out an experimental investigation to determine heat transfer coefficient and friction factor of water flowing through a horizontal flat tube fitted with twin counter-swirl and co-swirl twisted tapes with three twist ratios (TR = 5, 10, and 15) under turbulent flow boundary condition (7200 < Re < 32400). They observed the transfer rates of the twin counter-swirl twisted tapes was around 22.5% higher than that of the co-swirl one and 61% higher than that of the smooth horizontal flat tube. Further, the differences of the friction factor values obtained in counter and co-swirl twisted tapes were decreased with the increase of the twist ratios.

Piriyarungrod et al. [20] experimentally investigated the heat transfer rate and pressure drop characteristics of turbulent flowing fluid (Re 6000 to 20000) over twisted tapes of four different taper angles (θ = 0.0°, 0.3°, 0.6° and 0.9°) and each of them had three different twist ratios (TR = 3.5, 4.0 and 4.5). They observed that the Nusselt numbers were decreased with the increase of the taper angles. On the other hand, the friction factor was decreased with the increase of the taper angles. Further, the Nusselt number and friction factor were increased with the increase of the twist ratios as expected. The tube equipped with tape at the taper angle of θ = 0.9° and twist ratio of 3.5 had the maximum thermal performance factor of 1.05 at the Reynolds number of 6000, though all the variations at the taper angle of 0.9° had the lowest heat transfer rates.

Based on the above review, many geometric parameters of twisted tapes had been investigated by researchers for decades such as twist ratio, taper angle, length etc. However, almost all of them had used conventional twisted tapes which had I-shaped cross section, and those twisted tapes could be classified as dual-channel. After the exploration of 3D printers, there had the chance to produce twisted tapes which were more complex geometries and lighter as compared with conventional twisted tapes which were mostly made of metals. In the recent past, almost all the twisted tapes had been used in research were made of metal, due to that the ability to form twisted tapes were so hard. In the literature, there are no data about the effect of length of quad-channel twisted tapes (QCTT) on heat transfer and pressure drop characteristics of flow under turbulent regime. Therefore, the aim of this study is to investigate numerically and experimentally the effect of length of QCTT on heat transfer and friction factor characteristics. All the experiments had been done under turbulent flow conditions for the Reynolds number range of 4255 to 11000. Specifications of the previous studies and the present study can be seen in Table 1. In addition, the turbulent kinetic energy and the streamlines of flow velocity, temperature distributions inside the tested tubes with inserts were obtained by using the ANSYS software. Moreover, some correlations for Nu and f of the previous studies are presented in Table 2 along with the two new proposed correlations for the Nusselt numbers and the friction factors.

Section snippets

Experimental apparatus

The schematic diagram of the test apparatus is depicted in Fig. 1. The collecting tank had the capacity of 9 L. The chiller was consisted of a powerhead, a cooling coil and a water bath. It was used to obtain the desired inlet temperature. The powerhead was used to circulate water from the water bath for a homogenous heat distribution. There were two viewing glass of two sides of the test section to observe the flow. 7 T-type thermocouples were mounted on the surface of the test tube at the

Data reduction

Total heat generated by the DC power supply, represents by ${\dot{Q}}_{s}$ and that can be calculated by Eq. (1). ${\dot{Q}}_{s} = VI$

Where, V is the voltage, I is the current and both of them can be adjusted by an inverter.

Heat absorbed by the flowing water through the tube is estimated by Eq. (2). ${\dot{Q}}_{water} = {\dot{m}}_{water} c_{p_{water}} (T_{out} - T_{in})$

Where, ${\dot{Q}}_{water}$ is the heat transfer rate, ${\dot{m}}_{water}$ is mass flow rate, and c_pwater is the specific heat of distilled water.

The average heat transfer rate is calculated from Eq. (3). ${\dot{Q}}_{ave} = \frac{{\dot{Q}}_{s} + {\dot{Q}}_{water}}{2}$

Numerical study of heat transfer on plain tube and QCTTs

Apart from the experimental study, a numerical investigation on the heat transfer characteristics of the plain tube with and without twisted tape under uniform heat flux conditions also been carried out in order to observe the characteristics of heat transfer in the test tubes. Numerical simulations of the fluid flow inside the plain tube had been performed, then the work was expanded to consider the flow in the quad-channel twisted tapes. Heat transfer and friction loss of fluid flowing in

Validation of plain tube without twisted tape inserts by well-known correlations

The experimental apparatus was initially validated for plain tube. Experimental results for the Nusselt number was were calculated by using Eq. (6) and it was compared with the Gnielinski [23] and Dittus-Boelter equations [24] which are presented by Eqs. (13), (14), respectively. $Nu = \frac{(f / 8) (Re - 1000) Pr}{1 + 12.7 {(f / 8)}^{0.5} ({Pr}^{2 / 3} - 1)}$ $Nu = 0.023 {Re}^{0.8} {Pr}^{0.4}$

Experimental friction factor values were estimated by Eq. (7) and these values were compared with the data obtained by using Blasius Equation [25] which is presented

Results and discussion

Twisted tapes insert in tubes are well-known for enhancement of heat transfer. These are considered as additional type of turbulators and installed as retrofits or for novel projects of heat exchangers in the heat exchanger market. Shared effect in the flow of them could accomplish improvement by inducing swirl flow in the tube side liquid, causing higher near wall velocities and mixing of liquids which augments heat transfer coefficient as a result of its motion inside the tube. A suitable

Conclusions

The effect of the lengths of quad-channel twisted tapes, inserted in a circular tube, on the heat transfer and pressure drop characteristics were investigated under turbulent flow conditions by using water as the working fluid. Both the heat transfer enhancement and pressure drop were examined separately, and correlated with the variable parameters. The findings of this investigation can be summed as follows;

i.
Gnilenski and Dittus-Boelter equations were used for the comparison and validation of

Nomenclature

surface area of test tube (m²)

c_p

specific heat (J/kgK)

D_h

hydraulic diameter of tube (m)

friction factor

heat transfer coefficient (W/m2K)

electric current (A)

thermal conductivity (W/mK)

pipe length (m)

tape length (m)

length ratio

m ̇

mass flow rate (kg/s)

Nusselt number

PEC

performance evalution criteria

Prandtl number

radius (m)

Reynolds number

temperature (°C)

TPR

tape/pipe length ratio (l/L)

\dot{Q}

heat transfer rate (W)

QCTT

Quad-channel twisted tapes

velocity (m/s)

voltage (V)

Declaration of Competing Interest

None

Acknowledgements

All the authors are grateful to the management of Thailand Research Fund (TRF), the National Research University Project (NRU) and King Mongkut's University of Technology Thonburi through the “KMUTT 55th Anniversary Commemorative Fund”.

Declaration of Competing Interest

On behalf of all the authors, the corresponding author states that there is no conflict of interest.

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Single phase flow heat transfer characteristics of quad-channel twisted tape inserts in tubes

Abstract

Introduction

Section snippets

Experimental apparatus

Data reduction

Numerical study of heat transfer on plain tube and QCTTs

Validation of plain tube without twisted tape inserts by well-known correlations

Results and discussion

Conclusions

Nomenclature

Declaration of Competing Interest

Acknowledgements

Declaration of Competing Interest

Int. J. Therm. Sci.

Int. J. Heat Fluid Flow

Exp. Thermal Fluid Sci.

Chem. Eng. Process.

Sol. Energy

Int. Commun. Heat Mass Transfer

Int. Commun. Heat Mass Transfer

Appl. Therm. Eng.

Int. J. Therm. Sci.

Int. J. Heat Mass Transf.

Appl. Therm. Eng.

Int. Commun. Heat Mass Transfer

Exp. Thermal Fluid Sci.

Int. Commun. Heat Mass Transfer

Chem. Eng. Process. Process Intensif.

Int. Commun. Heat Mass Transfer