Effect of Twisted-tape Inserts on Heat Transfer in a Tube - The Joint ...

The 2 nd Joint International Conference on “Sustainable Energy and Environment (SEE 2006)” 

A-030 (P) 21-23 November 2006, Bangkok, Thailand 

<strong>Effect</strong> <strong>of</strong> <strong>Twisted</strong>-<strong>tape</strong> <strong>Inserts</strong> on Heat Transfer in a Tube 

Watcharin Noothong 1 , Smith Eiamsa-ard 2 and Pongjet Promvonge 1, * 

1 

Department <strong>of</strong> Mechanical Engineering, Faculty <strong>of</strong> Engineering, King Mongkut’s Institute <strong>of</strong> Technology Ladkrabang, Bangkok, 

10520, Thailand 

2 

Department <strong>of</strong> Mechanical Engineering, Faculty <strong>of</strong> Engineering, Mahanakorn University <strong>of</strong> Technology, Bangkok 10530, Thailand 

Abstract: Influences <strong>of</strong> the twisted <strong>tape</strong> insertion on heat transfer and flow friction characteristics in a concentric double pipe heat 

exchanger have been studied experimentally. In the experiments, the swirling flow was introduced by using twisted <strong>tape</strong> placed inside 

the inner test tube <strong>of</strong> the heat exchanger with different twist ratios, y = 5.0 and 7.0. The experimental results revealed that the 

increase in heat transfer rate <strong>of</strong> the twisted-<strong>tape</strong> inserts is found to be strongly influenced by <strong>tape</strong>-induced swirl or vortex motion. 

Over the range investigated, the maximum Nusselt numbers for using the enhancement devices with y = 5.0 and 7.0 are 188% and 

159%, respectively, higher than that for the plain tube. In addition, the effects <strong>of</strong> the twisted <strong>tape</strong> on the heat transfer enhancement 

efficiency are also investigated. 

Keywords: <strong>Twisted</strong> Tape, Heat Transfer, Heat Exchanger, Swirl Flow, Enhancement Efficiency 

1. INTRODUCTION 

Heat exchanger is the apparatus providing heat transfer between two or more fluids, and they can be classified according to the 

mode <strong>of</strong> flow <strong>of</strong> fluid or their construction methods. Heat exchangers with the convective heat transfer <strong>of</strong> fluid inside the tubes are 

frequently used in many engineering application. Augmentation heat transfer, in connection with fluid mixing or non mixing, is also 

involved which most heat exchangers have no contact between the fluids. At present, the technology <strong>of</strong> the twisted-<strong>tape</strong> insert is 

widely used in various industries. Insertion <strong>of</strong> twisted <strong>tape</strong>s in a tube provides a simple passive technique for enhancing the 

convective heat transfer by introducing swirl into the bulk flow and by disrupting the boundary layer at the tube surface due to 

repeated changes in the surface geometry. It has been explained that such <strong>tape</strong>s induce turbulence and superimposed vortex motion 

(swirl flow) causing a thinner boundary layer and consequently resulting in a high heat transfer coefficient and Nusselt number due to 

repeated changes in the twisted <strong>tape</strong> geometry. On consideration <strong>of</strong> the heat transfer enhancement, it can be considered through 

bringing the twisted-<strong>tape</strong> to insert while the pressure drop inside the tube is higher. Because <strong>of</strong> low assets and easy setting up, it is 

widely used, especially in a compact heat exchanger. A heat transfer enhancement concept in which swirl was introduced in the flow 

was proposed by Kreit and Margolis (1959). In this concept, part <strong>of</strong> the fluid enters axially while the remainder is injected 

tangentially at various locations along the tube axis. The radial pressure gradient results in thinning <strong>of</strong> the thermal boundary layer 

with an accompanying improvement in heat transfer. Most <strong>of</strong> the swirl flows were created by insertion <strong>of</strong> a twisted <strong>tape</strong> in the tube. 

<strong>Twisted</strong> <strong>tape</strong>s, when inserted into tubes, tend to promote turbulence as well as to intensity mixing <strong>of</strong> the hot fluid and cold fluid. 

Thin in turn improves the heat transfer process. Many researches have investigated the effect <strong>of</strong> geometry <strong>of</strong> twisted-<strong>tape</strong>s on heat 

transfer and friction in a circular or rectangular smooth pipe in both experimental and numerical studies. Date [1], Date and Saha [2] 

numerically predicted the friction and heat transfer characteristics for laminar flow in a circular tube fitted with regularly spaced 

twisted-<strong>tape</strong> elements that were connected by thin circular rods. Ray and Date [3] investigated experimentally correlations <strong>of</strong> heat 

transfer and flow frictions in a square duct with twisted-<strong>tape</strong> insert. Kumar and Prasad [4] reported the improved solar collectors <strong>of</strong> 

water heating types by means <strong>of</strong> twisted <strong>tape</strong>s inserted in the water flow tubes. Sukhatme et al. [5] studied experimentally the effect 

<strong>of</strong> the twisted-<strong>tape</strong> insert on heat transfer rate and pressure drop in laminar flow under uniform heat flux. On the other hand 

Duplessis and Kroger [6] made the experimental and numerical studies with the constant wall temperature in laminar flow. Lepina 

and Bergles [7] provide a correlation <strong>of</strong> the heat transfer inside the tube for turbulent flow which can be utilized in the heat exchanger 

industry. 

In the present investigation, the influence <strong>of</strong> twisted-<strong>tape</strong> inserts on enhancement heat transfer efficiency, Nusselt number and flow 

friction behaviors in a double pipe heat exchanger is studied. In the experimental condition, a twisted-<strong>tape</strong> was inserted into the inner 

tube at twist ratios: y = P/D = 5.0 and 7.0, respectively. All <strong>of</strong> the experiments were carried out at the same inlet condition with the 

Reynolds number <strong>of</strong> the inner tube, Re = 2000 to 12000. 

2. METHODOLOGY 

2.1 Experimental Apparatus 

Figures 1 and 2 show a schematic view <strong>of</strong> the experimental setup and a concentric double pipe heat exchanger fitted with a 

twisted <strong>tape</strong>. The test section consists <strong>of</strong> concentric straight pipes made <strong>of</strong> Plexiglas, which are joined at regular interval <strong>of</strong> 1.0 m by 

flanges. The inner diameter <strong>of</strong> the outer pipe is 50 mm and the annulus (flow passage) is <strong>of</strong> 20 mm in the radial direction throughout. 

Water is supplied to the pipe from the chilled water loop <strong>of</strong> capacity 0.3 m 3 , an electrical heater controlled by adjusting the voltage, a 

stirrer, and a cooling coil immerged inside a storage tank. Hot air from a 7.5 kW blower was directed through the inner tube, while 

cold water was pumped through the annulus. In the test run, the <strong>tape</strong>s were placed in two different twisted ratios: y = 0.6 and 0.8 as 

depicted in Fig. 2. <strong>Twisted</strong> <strong>tape</strong>s were made from stainless steel strips <strong>of</strong> thickness 1.0 mm and width 19.5 mm. They were 

fabricated by twisting a straight <strong>tape</strong>, about its longitudinal axis, while being held under tension. All the pressure readings are taken 

under isothermal conditions. Inclined and U-tube manometers with manometric fluid (SG = 0.826) as working fluid is used for 

measuring the pressure drop at each probe location. The least count <strong>of</strong> the pressure measurement manometer is 0.5 mm water is used 

as the working fluid for entire study. Liquid and air flow meters were used to measure both water and air flow rates. The volumetric 

Corresponding author: kppongje@kmitl.ac.th 

1



flow rates <strong>of</strong> the hot air and cold water were adjusted by control valves, situated before the inlet ports. The inlet air was heated by an 

adjustable electrical heater. Both the inlet and outlet temperatures <strong>of</strong> the hot air and the cold water were measured by multi-channel 

with iron-Constantan thermocouple (type K). It was necessary to measure the temperature at 15 stations altogether at the outer 

surface <strong>of</strong> the inner tube for finding out the average Nusselt number. 

PC Computer 

Data Logger with 

Thermocouple type K 

Flow out 

Concentric Tube Heat Exchanger 

Flow in 

(Hot air) 

Rotameter 

Flow out 

Rotameter 


(Cold water) 

Chiller 

U-Manometer 

with Water 

Water 

Reservoir 

Thermostat 

with 

Electrical Heater 

Centrifugal Pump 

Blower 

Fig. 1 Schematic diagrams <strong>of</strong> experimental apparatus 

2.2 Data reduction equations 

For fluid flows in a concentric tube heat exchanger, the heat transfer rate <strong>of</strong> the hot fluid (air) in the inner tube can be expressed as: 

while the heat transfer <strong>of</strong> the cold fluid (water) for the outer tube is 

whereas, 

and 

Q = MC (T − T ) 

(1) 

p 

o 

i 

Q = hA(T ~ T ) 

(2) 

w − 

b 

Tb = (To 

+ Ti 

) / 2 

(3) 

∑ 

T ~ 

w = Tw 

/ 15 

(4) 

Where T w is the local wall temperature and evaluated at the outer wall surface <strong>of</strong> the inner tube. It must be measured at the depth 

from the outer surface <strong>of</strong> 0.5 mm. The average wall temperatures are calculated from 15 points, lined between the inlet and exit <strong>of</strong> 

the inner tube. The average heat transfer coefficient is calculated from the energy balance are estimated as follows: 

h = MC (T T ) / A(T ~ 

p o − i w − Tb 

) 

(5) 

The mean value <strong>of</strong> the Nusselt number is calculated based on the mean wall temperature w T and the local mean bulk fluid 

temperature. 

2



hD 

Nu = (6) 

k 

Where, h and k are the mean heat transfer coefficient and mean thermal conductivity <strong>of</strong> the fluid, respectively, at all thermocouple 

location. The local thermal conductivity k <strong>of</strong> the fluid is calculated from the fluid properties at the local mean bulk fluid 

temperature. The Reynolds number is calculated based on the difference rate Q through the test section. 

Re = VD / v 

(7) 

Where v is the kinematics viscosity <strong>of</strong> the working fluid. Friction factor can be written as follows: 

Flow out 

(Hot air) 

Dc 

D 

f = 

⎛ 

⎜ 

⎝ 

L 

D 

∆P 

⎛ 2 

⎞ ⎞ 

⎜ 

V 

⎟ ρ ⎟ 

⎠ 

⎝ 

2 

⎠ 

Outer tube 

Inner tube 

Flow out 

(Cold water) 


(Hot air) 

(8) 

L 


(Cold water) 

Fig. 2 The inner tube fitted with twisted <strong>tape</strong> at different twist ratios (y = 5.0 and 7.0) 

3. RESULTS AND DISCUSSION 

In the case <strong>of</strong> the twisted <strong>tape</strong> inserts, the means Nusselt numbers (<strong>of</strong> the inner tube or hot air) increased about 188% for y = 5.0 

and 159% for y = 7.0, when compared with the plain tube. It was shown that the smaller <strong>of</strong> the twist ratio, the higher in mean 

Nusselts number as seen in Fig. 3(a). From the experimental results, it is clear that the twisted <strong>tape</strong> inserts caused swirl and pressure 

gradient in the radial direction. The boundary layer along the tube wall would be thinner with the increase <strong>of</strong> radial swirl and 

pressure resulting inn more heat flow through the fluid. Moreover, swirl caused the flow to be turbulent, which led to even better 

convection heat transfer. From the figure, it is depicted that the effect <strong>of</strong> the twisted <strong>tape</strong> inserts decreased at low Reynolds numbers 

due to the weak swirl and low flow velocity. Thus, the increase in Nusselt number was low at smaller Reynolds number, while it 

became greater at the higher Reynolds numbers. This phenomenon is related to the speed <strong>of</strong> the swirl-flow <strong>of</strong> the hot air and the 

results <strong>of</strong> the destruction <strong>of</strong> the boundary layer level. Throughout the experimental results, it is obvious that the narrow twist ratio (y 

= 5.0) yielded the higher values <strong>of</strong> heat transfer than those were at the greater twist ratio (y = 7.0). This was due to the more violently 

swirl with the narrow twist ratio. The friction factor in the inner tube varied with Reynolds number as shown in Fig. 3(b). It can be 

seen that the frictions are similar in trend for both the plain tube and the tube fitted with twisted <strong>tape</strong> inserts. It was observed that the 

twisted <strong>tape</strong> inserts caused swirling flow into the tube and leaded to high friction <strong>of</strong> 3.37 and 2.94 times <strong>of</strong> the plain tube, for y = 5.0 

and 7.0, respectively in comparison with the plain tube. It can be seen that for the y = 5.0 gave smaller value <strong>of</strong> friction than that y = 

7.0 because the intervals gave lower value <strong>of</strong> swirling flow than those in the case <strong>of</strong> small twist ratio. 

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120 

100 

(a) 

Nusselt number 

80 

60 

40 

20 

0 

0.5 

(b) 

Friction factor 

0.4 

0.3 

0.2 

y=6.0 

y=8.0 

Plain tube 

0.1 

03 

(c) 

Enhancement efficiency 

2.5 

2 

1.5 

η =0.836Re 0.17 y -0.3 

1 

0 3000 6000 9000 12000 15000 

Reynolds number 

Fig. 3 Nusselts number, friction factor and enhancement efficiency versus Reynolds number for the tube fitted with twisted <strong>tape</strong> at 

different twist ratios (y) 

A useful comparison between swirl and axial flow can be made by comparing heat transfer coefficients at equal pumping power, 

since this is relevant to the operation cost. For constant pumping power; 

( V∆ 

P ) p (V& 

∆P 

) t 

and the relationship between friction and Reynolds number can be expressed as: 

& = (9) 

3 

p 

3 

) t 

( f Re ) = ( f Re 

(10) 

4



The enhancement efficiency (η ) at constant pumping power is the ratio <strong>of</strong> the convective heat transfer coefficient <strong>of</strong> the tube with 

twisted <strong>tape</strong> to the plain tube which can be written as follows: 

The enhancement efficiency for twisted <strong>tape</strong>s can be written as: 

ht 

η = 

(11) 

h 

p 

pp 

h 

0.17 −0.38 

η = = 0.836 Ret 

y 

(12) 

h 

t 

p 

pp 

Throughout the results <strong>of</strong> the experimentation, it appears that use <strong>of</strong> the small twist ratio (y) leads to higher enhancement 

efficiency than that <strong>of</strong> the larger twisted ratio (y) as can be seen in Fig. 3(c). It can be seen that the enhancement efficiency increases 

as the twisted ratio (y) decreases which generally higher at high Reynolds numbers for all twist ratios. This suggests that twisted 

<strong>tape</strong>s are feasible in terms <strong>of</strong> energy saving at higher Reynolds numbers. It is assumed that the performance improvement in the 

intervals <strong>of</strong> Reynolds number and the sizes <strong>of</strong> the twisted <strong>tape</strong> is progressively developing. Thus it is possible to use the devices with 

the various styles <strong>of</strong> <strong>tape</strong>s inserts to reduce the pressure drop and possibly increase the heat transfer. 

4. CONCLUSION 

Experimental investigation <strong>of</strong> enhancement efficiency, heat transfer and friction factor characteristics <strong>of</strong> circular tube fitted with 

twisted <strong>tape</strong> inserts <strong>of</strong> different twist ratios has been studied. It can be observed that the swirl flow helps decrease the boundary layer 

thickness <strong>of</strong> the hot air flow and increase residence time <strong>of</strong> hot air in the inner tube. The enhancement efficiency and Nusselt number 

increases with decreasing the twist ratio and friction factor also increases with decreasing the twist ratio. The partitioning and 

blockage <strong>of</strong> the tube flow cross-section by the <strong>tape</strong>, resulting in higher flow velocities. Secondary fluid motion is generated by the 

<strong>tape</strong> twist, and the resulting twist mixing improves the convection heat transfer. 

5. REFERENCES 

[1] Date, A.W. (1974) Prediction <strong>of</strong> fully developed flow in a tube containing a twisted <strong>tape</strong>, International Journal Heat Mass 

Transfer, 17, pp. 845-859. 

[2] Date, A.W. and Saha, S.K. (1990) Numerical prediction <strong>of</strong> laminar flow and heat transfer in a tube fitted with regularly spaced 

twisted-<strong>tape</strong> elements, International Journal Heat Fluid Flow, 11, pp. 346-354. 

[3] Ray, S. and Date, A.W. (2003) Friction and Heat Transfer Characteristics <strong>of</strong> Flow through Square Duct with <strong>Twisted</strong> Tape Insert, 

International Journal Heat and Mass Transfer, 46, pp. 889-902. 

[4] Kumar, A. and Prasad, B.N. (2000) Investigation <strong>of</strong> twisted <strong>tape</strong> inserted solar water heaters-heat transfer, friction factor and 

thermal performance results, Renewable Energy Journal, 19, pp. 379-398. 

[5] Sukhatme, S.P., Gaitonde, U.N., Shidore, C.S. and Kuncolienkar, R.S. (1987) Forced convection heat transfer to visous liqiuid 

in laminar flow in a tube with a twisted <strong>tape</strong>, Proceding Ninth Natl. Heat Mass Transfer Conference, (Paper no. HMT 7-87) 

Indian Institute <strong>of</strong> Science, Bangalore, India, Part B, 1-7. 

[6] Duplessis, J.P. and Kroeger, D.G. (1983) Numerical prediction <strong>of</strong> laminar flow with heat transfer in tube with a twisted <strong>tape</strong> 

insert, Proceeding <strong>of</strong> the International Conference on Numerical Methods in Laminar and Turbulent Flow, pp. 775-785. 

[7] Lepina, R.F. and Bergles, A.E. (1969) Heat transfer and pressure drop in <strong>tape</strong>-generated swirl flow <strong>of</strong> single-phase water, ASME 

Journal Heat Transfer, 91, pp. 434-442. 

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Effect of Twisted-tape Inserts on Heat Transfer in a Tube - The Joint ...

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