Experimental study of heat transfer enhancement in a pipe using twisted tapes

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –

6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME

100

EXPERIMENTAL STUDY OF HEAT TRANSFER ENHANCEMENT IN

A PIPE USING TWISTED TAPES AND WIRE COILS

Dr. A. G. Matani1, Swapnil A. Dahake

2

1Associate Professor, Mechanical Engineering Department, Government College of

Engineering, Amravati, (M.S.) India 2M.Tech. [2

nd Year Thermal Engineering], Government College of Engineering, Amravati,

(M.S.) India.

ABSTRACT

In the proposed work, the heat transfer enhancement using twisted tapes and wire coil

on pressure drop, Nusselt number (Nu), friction factor (f) and thermal enhancement index (�)

are experimentally determined. The twisted tapes are used as swirl flow generators while wire

coil along with twisted tapes used as co-swirl flow generators in a test section. The tests are

conducted using the twisted tape with three different twist ratios (y/w = 3.5, 2.66 and 2.25)

and wire coil along with twisted tapes, pitch ratio of 1.17 & 0.88 for Reynolds numbers range

between 5000 and 18,000 under uniform heat flux conditions. Also double twist generating

counter swirl are compared. The experiments using the twisted tape and with wire coil

performed under similar operation test conditions, for comparison. The experimental results

indicate that the tube with the various inserts provides considerable improvement of the heat

transfer rate over the plain tube. The experimental results demonstrate that friction factor (f)

and thermal enhancement index (�) increase with decreasing twist ratio (y/w) and Reynolds

number. The results also show that the wire coils along with twisted tapes are more efficient

than the twisted tapes for heat transfer enhancement.

1. INTRODUCTION

Research is going on to investigate the level of heat transfer enhancement that can

achieved by forced convection in which air is flow inside horizontal pipe. The comparison

between bare tube and enhanced tube configuration are made on the basis of forced

convection with air instabilities. These types of inserts increase the heat transfer coefficient

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ISSN 0976 – 6340 (Print)

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101

with respect to smooth tube. Differences among the enhanced configuration are also

determined to observe which of them most stable and unstable one is. These inserts induced

swirl flow heat transfer due to exponentially increasing heat input with exponential periods.

In the last decades, significant effort has been made to develop heat transfer

enhancement techniques in order to improve the overall performance of heat exchangers. The

interest in these techniques is closely tied to energy prices and, it is expected that the heat

transfer enhancement field will go through a new growth phase with the present increase in

energy cost. Although there is need to develop novel technologies, experimental work on the

older ones is still necessary. The knowledge of its performance shows a large degree of

uncertainty which makes their industrial implementation difficult [6]. The efficiency of heat

transfer equipment is essential in energy conservation. Furthermore, a more efficient heat

exchanger can reduce the size of the heat exchanger, thus reducing the costs associated with

both material and manufacturing of the heat exchanger [2]. Heat transfer enhancement

technology has been widely applied to heat exchanger applications in refrigeration,

automobile, process industries etc. There have been numerous attempts to reduce the size and

cost of heat exchangers. Hence, there have been continuous attempts to improve the

efficiency of heat exchangers by various methods. One of the best methods to achieve this is

the use of augmented heat transfer surfaces. Improved heat transfer can make heat exchangers

smaller and more energy efficient. The tube insert technology is one of the most common

heat transfer enhancement technologies for shell and tube heat exchangers.

Tube side enhancement techniques can be classified according to the following

criteria: (1) additional devices which are incorporated into a plain round tube (twisted tapes,

wire coils) and (2) non-plain round tube techniques such as surface modification of a plain

tube (corrugated and dimpled tubes) [5] or manufacturing of special tube geometries

(internally finned tubes) [6]. The dominant literature study usually mentions five types: wire

coils, twisted tapes, extended surface devices, mesh inserts and displaced elements [7]. The

main advantage of these types in respect to other enhancement techniques such as the

artificial roughness by mechanical deformation or internal fin types is that they allow an easy

installation in an existing smooth-tube heat exchanger. The various single and double twisted

tapes are also used to generate swirl/co-swirl [9]. Due to its low cost, the insert devices which

are most frequently used in engineering applications are wire coils and twisted tapes.

Heat transfer enhancement techniques have been extensively developed to improve

the thermal performance of heat exchanger systems with a view to reducing the size and cost

of the systems. Swirl/vortex flow is the one of the enhancement techniques widely applied to

heating/cooling systems in many engineering applications. The vortex flows can be classified

into two types: continuous swirl and decaying swirl flows. The former represents the swirling

motion that persists over the entire length of the duct for example helical/twisted tape and

coiled wires inserts while the latter means the swirl created at the duct entrance and then

decays along the flow path such as the tangential injection, the rib/baffle and the winglet

vortex generators [10]. Swirl flow has been used in a wide range of applications from various

engineering areas such as chemical and mechanical mixing and separation devices,

combustion chambers, turbo machinery to pollution control devices. It is commonly known

that the swirl flow enhances the heat transfer mainly due to the increased velocity in the swirl

tube and the circulation of the fluid by forced convection [4].



102

2. TWISTED TAPES & WIRE COIL TO GENERATE SWIRL

The knowledge of heat transfer in a short circular tube with the twisted-tape insert

tube is important to investigate the influence of the twist ratio of twisted tape on enhancement

of critical heat flux. A lot of the experimental investigations were conducted with the heat

transfer characteristics of the swirl flow. It was shown that the twisted-tape insert tube

provided considerable enhancement of turbulent heat transfer for heating/cooling of fluid.

The twisted-tape insert tubes are encountered in a number of important engineering and

science systems such as plasma facing components in fusion experimental facilities, high

power laser systems and etc. We believe that the enhancement of heat transfer for the twisted-

tape insert tube would be due to reduction of viscous sub-layer thickness on heated surface of

test tube with an increase in liquid flow velocity from straight flow to swirl one even at a

fixed mass velocity.

Wire coil inserts are devices whose reliability and durability are widely contrasted. In

extreme applications such as the tube-side of fuel pyrotubular boilers with great fouling

problems and with high variations in temperature that produce great dilatations, wires are

used without any problem. This is a cheap enhancement technique and it is completely viable

for many industrial applications. This fact hinders a widespread use of wire coil inserts in

industrial heat exchangers.

The twisted tapes are made of mild steel and have tape width (w) of 10 mm, 15 mm &

20mm shown in Figure 1, tape thickness (d) of 0.8 mm, and tape length (l) of 900 mm. All

tapes were prepared with different twist ratios, y/w = 3.5, 2.66 and 2.25 respectively where

twist ratio is defined as twist length (l) to tape width (w). The double twisted tapes are

manufactured by combining two single twisted tapes, which generate counter swirl in pipe

shown in Figure 2. On the other hand, to avoid an additional friction in the system that might

be caused the thicker tape. To produce the twisted tape, one end of a straight tape was

clamped while another end was carefully twisted to ensure a desired twist length. Another

insert, wire coil is made up of aluminum wire of 2 mm diameter having pitch of 30mm & 40

mm shown in Figure 3. These twisted tapes are fixed one by one inside the pipe having wire

coil to generate co-swirl.

Figure 1: Single twisted tapes (y/w = 3.5, 2.66 & 2.25)

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976

6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, Mar

Figure

Figure

3. TEST SECTION

The test section is surrounded by nichrome heating wire, which is wrapped around the

test section with a pitch distance of 5 mm. This pitch is good enough to provide a nearly

uniform heating on the outer surface of the test section

by a variable AC power supply. The overall electrical power added to the heating section, Q,

was calculated by measuring the voltage (0

control the convection losses from the

glass wool used. Four thermocouples are to be embedded on the test section to measure

surface temperature of pipe and two thermocouples are placed in air stream at entrance and

exist of test section to measure air temperature.

thermocouple bead is insulated from the electrically heated tube wall surface with a very thin

sheet of mica between the thermocouple and the tube surface so as not to be effected from

electricity. Fig. 2 shows the schematic view of experimental set

Figure

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976

6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME

103

2: Double twisted tape (y/w = 2.66)

Figure 3: Wire coil (p/d = 0.88)



uniform heating on the outer surface of the test section tube. The heating wire was powered


was calculated by measuring the voltage (0–200 V) and the electrical current (0

control the convection losses from the test section and other components, foam insulation and



measure air temperature. To avoid floating voltage effects, the



icity. Fig. 2 shows the schematic view of experimental set-up.

Figure 4: Experimental set-up


April (2013) © IAEME



tube. The heating wire was powered


200 V) and the electrical current (0–2 A). To

test section and other components, foam insulation and



To avoid floating voltage effects, the





104

4. DATA REDUCTION

In the experiments, the heat transfer rate in the tube is taken into account under a

uniform heat flux (UHF) condition by using air as the test fluid. The heat transfer given by

the hot surface to fluid (i.e. air) at any Reynolds number is

Rate at which air is heated

Qa = mCp (To - Ti)

The convection heat transfer from the test section is given as

Qc = hAs(Ts - Tm)

At steady state condition, the heat transfer is assumed to be equal to the heat loss from the

test section that can be drawn as

Qa = Qc

The mean temperature of the fluid in the test tube is given by

Tm = (To + Ti) / 2

Ts is the mean temperature of surface wall temperature of the test tube. The average wall

temperature is calculated from 4 points of local wall temperatures lined between the inlet and

the exit of the test tube. The average heat transfer coefficient (h) and the mean Nusselt

number (Nu) are estimated by

h = mCp (To - Ti) / hA(Ts - Tm)

The Nusselt number in terms of average heat transfer coefficient is defined as

Nu = hD / k

The Reynolds number is written as

Re = ρUD / µ

The experiment pressure losses, ∆p across the test tube are arranged in non-dimensional form

by using the following equation

� � ∆��

� 2 �

in which U is mean velocity in the test tube and L is the test tube length. All of thermo-

physical properties of the air are determined at the overall mean air temperature (Tm).

5. EXPERIMENTAL RESULTS AND DISCUSSIONS

In this section, the pressure drop, friction factor characteristics, heat transfer and

thermal enhancement index in a tube fitted with twisted tapes, double twisted tape and wire

coil (counter/co-swirl tape) are presented.

The experiments are performed in the range of Reynolds number between 5000 and

18,000. The results obtained for the tube fitted with the single twisted tapes (ST), wire coil

and the empty tube are used as the reference data for the performance evaluation of the

modified tubes.



105

5.1 Heat transfer result

Experimental results of the Nusselt number (Nu) in plain tubes combined with a

twisted tape (y/w = 3.5, 2.66, 2.25), double twisted tape (y/w = 3.5, 2.66) and wire coil (p/d =

1.17, 0.88) are presented in Figure 5. The Nusselt numbers for the plain tube acting alone are

also plotted for comparison. The data show that the Nusselt number (therefore, the heat

transfer coefficient) increases with increasing Reynolds number for the conventional

turbulent tube flow. This is the most likely caused by a stronger turbulence and better contact

between fluid and heating wall. It is noted that the increasing Nusselt number in the plain

tube in common with a twisted tapes and wire coil is caused by the generating of pressure

gradient along the radial direction, and this leads to redeveloping of thermal/hydrodynamic

boundary layer. The higher increase of the Nusselt number in this style of both turbulence

and swirl flows is a consequence of the higher reduction of boundary layer thickness and

increase of resultant velocity.

The variations of Nusselt number with Reynolds number for single and double twisted

tapes with wire coil of pitch ratio (p/d = 1.17) shown in Figure 6 and with wire coil of pitch

ratio (p/d = 0.88) shown in Figure 7. Nusselt number increases with the decrease of twist ratio

and the increase of Reynolds number. The highest Nusselt number is achieved for twist ratio

(y/w = 2.25) and pitch ratio (p/d =0.88).

However, the heat transfer enhancement by twisted tapes is less efficient than that

offered by wire coil indicated by the lower Nusselt number depending on operating condition.

Figure 5: Single twisted tape (y/w = 3.5, 2.66, 2.25), double twisted tape (y/w = 3.5, 2.66)

and wire coil (p/d = 1.17, 0.88)

20

30

40

50

60

70

80

90

5000 7000 9000 11000 13000 15000 17000

Nu

Re

smooth tube

STT(y/w)=3.5

STT(y/w)=2.6

6STT(y/w)=2.2

5DTT(y/w)=3.5

DTT(y/w)=2.6

6WC(p/d)=1.1

7



106

Figure 6: Single and double twisted tapes along with wire coil of pitch ratio (p/d = 1.17)

Figure 7: Single and double twisted tapes with wire coil of pitch ratio (p/d = 0.88)

5.2 Friction factor results

Experimental results of the friction factor (f) characteristics in plain tubes combined with

a twisted tape (y/w = 3.5, 2.66, 2.25), double twisted tape (y/w = 3.5, 2.66) and wire coil (p/d

= 1.17, 0.88) are presented in Figure 5. The friction factors of the plain tube acting alone are

also plotted for comparison. Figure shows the influence of a plain tube combined with a

twisted tape and wire coil on pressure loss, which indicates the friction in a heat exchanger.

30

40

50

60

70

80

90

100

110

5000 10000 15000

Nu

Re

STT(p/d)=1.17,(y/w)=

3.5STT(p/d)=1.17,(y/w)=

2.66STT(p/d)=1.17,(y/w)=

2.25DTT(p/d)=1.17,(y/w)=

3.5DTT(p/d)=1.17,(y/w)=

2.66

30

40

50

60

70

80

90

100

110

120

5000 7000 9000 11000 13000 15000 17000

Nu

Re

STT(p/d)=0.88,(y/w)=3.

5STT(p/d)=0.88,(y/w)=2.

66STT(p/d)=0.88,(y/w)=2.

25DTT(p/d)=0.88,(y/w)=3.

5DTT(p/d)=0.88,(y/w)=2.

66



107

The relationship between pressure loss in terms of friction factor and Reynolds number

for the tube with twisted tape (STT & DTT) and wire coil inserted and also for the plain tube

is presented in Figure 5. It is found that using twisted tape and wire coil gives higher friction

factor values than those from the plain tube as expected. The friction factor decreases with

the increase of twist ratio and Reynolds number.

The variations of friction factor with Reynolds number for single and double twisted

tapes with wire coil of pitch ratio (p/d = 1.17) shown in Figure 9 and with wire coil of pitch

ratio (p/d = 0.88) shown in Figure 10. The highest friction factor (or pressure loss) is obtained

in case p/d = 0.88 & y/w = 2.66.




0

0.05

0.1

0.15

0.2

0.25

0.3

5000 7000 9000 11000 13000 15000 17000

Fri

ctio

n f

act

or

(f)

Re

smooth tube

STT

(y/w)=3.5STT

(y/w)=2.66STT

(y/w)=2.25DTT

(y/w)=3.5DTT

(y/w)=2.66

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

5000 7000 9000 11000 13000 15000 17000

Fri

ctio

n f

act

or

(f)

Re

STT

(p/d)=1.17,(y/w)=3.5

STT

(p/d)=1.17,(y/w)=2.66

STT

(p/d)=1.17,(y/w)=2.25

DTT

(p/d)=1.17,(y/w)=3.5



108


5.3 Performance factor results

Siva Rama Krishna, Govardhan Pathipaka, P. Sivashanmugam [3] proposed a

performance evaluation analysis for the same pumping power and this method used for

present study, the performance ratio is defined as

� � ��/��.��

The performance analysis was made by above Eq. and the results are shown in Figure

11, 12, 13. In the present work, a thermal performance factor is evaluated since the factor is

an important parameter indicating the potential of a twisted tape for practical applications.

The thermal evaluation is considered under constant pumping power for each twisted tape

with respect to the case without twisted tape (plain tube). The thermal performance factors

for single twisted tape, double twisted tape and wire coil are presented in Figure 11.

Apparently, a thermal performance factor decreases with increasing Reynolds number for all

tape inserts. A larger pressure loss at a higher Reynolds number is responsible for the

mentioned result. This suggests that the geometry of wire coil is more appropriate for

practical use than the others in the view point of energy as well as operating cost savings.

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

5000 7000 9000 11000 13000 15000 17000

Fri

ctio

n f

act

or

(f)

Re

STT

(p/d)=0.88,(y/w)=3.5STT

(p/d)=0.88,(y/w)=2.66STT

(p/d)=0.88,(y/w)=2.25DTT

(p/d)=0.88,(y/w)=3.5DTT

(p/d)=0.88,(y/w)=2.66



109




0.95

1

1.05

1.1

1.15

1.2

1.25

1.3

1.35

5000 7000 9000 11000 13000 15000 17000

Pe

rfo

rma

nce

fa

cto

r

Re

STT (y/w)=3.5

STT

(y/w)=2.66

STT

(y/w)=2.25

DTT (y/w)=3.5

DTT

(y/w)=2.66

WC(p/d)=1.17

0.95

1.05

1.15

1.25

1.35

1.45

1.55

1.65

5000 7000 9000 11000 13000 15000 17000

Pe

rfo

rma

nce

fa

cto

r

Re

STT

(p/d)=1.17,(y/w)=3.5STT

(p/d)=1.17,(y/w)=2.66STT

(p/d)=1.17,(y/w)=2.25DTT

(p/d)=1.17,(y/w)=3.5DTT

(p/d)=1.17,(y/w)=2.66



110


6. CONCLUSIONS

Thermal characteristics in a tube fitted with twisted-tapes in co-swirl arrangement with

wire coil are presented in the present study. The work has been conducted in the turbulent flow

regime, Reynolds number from 5000 to 18,000 using air as the test fluid. The findings of the

work can be drawn as follows:

� For the inserted tube, the pressure drop tends to increase with the rise in mass flow rate

while the friction factor and performance factor give the opposite trends.

� The compound enhancement devices of the tube and the co-swirl show a considerable

improvement of heat transfer rate and thermal performance relative to the smooth tube

acting alone, depending on twist ratios.

� The co-swirl tube yields higher friction factor and performance factor than the smooth

tube at low Reynolds number.

� The various inserts in pipe mixes the bulk flow well and therefore performs better in

laminar flow.

� The result also shows twisted tape insert is more effective, if no pressure drop penalty is

considered. Twisted tape in turbulent flow is effective up to a certain Reynolds number

range.

� Swirl flow heat transfer is higher than non swirling flow.

NOMENCLATURE d wire diameter

f Friction factor

� Performance factor

Nu Nusselt number

p pitch

p/d pitch ratio

Re Reynolds number

w Width of tape

y Twist length

(y/w) twist ratio

0.95

1.05

1.15

1.25

1.35

1.45

1.55

1.65

1.75

5000 10000 15000

Pe

rfo

rma

nce

fa

cto

r

Re

STT

(p/d)=0.88,(y/w)=3.5STT

(p/d)=0.88,(y/w)=2.66STT

(p/d)=0.88,(y/w)=2.25DTT

(p/d)=0.88,(y/w)=3.5



111

REFERENCES 1. Shou-Shing Hsieh, Feng-Yu Wu, Huang-Hsiu Tsai, “Turbulent heat transfer and flow

characteristics in a horizontal circular tube with Shou-Shing Hsieh, Feng-Yu Wu,Huang-Hsiu

strip-type inserts”, International Journal of Heat and Mass Transfer (2003),Vol. 46, pp. 823-

835.

2. Leonard D. Tijing, Bock Choon Pak, Byung Joon Baek, Dong Hwan Lee, “A study on heat

transfer enhancement using straight and twisted internal fin inserts”, International

Communications in Heat and Mass Transfer (2006), Vol. 33, pp. 719-726.

3. Siva Rama Krishna, Govardhan Pathipaka, P. Sivashanmugam, “Heat transfer and pressure

drop studies in a circular tube fitted with straight full twist”, Experimental Thermal and Fluid

Science (2009), Vol. 33, pp. 431–438.

4. H. Gül a, D. Evinb, “Heat transfer enhancement in circular tubes using helical swirl generator

insert at the entrance”, International Journal of Thermal Sciences (2007), Vol. 46, pp. 1297-

1303.

5. Chinaruk Thianpong, Petpices Eiamsa-ard, Khwanchit Wongcharee, Smith Eiamsa-ard,

“Compound heat transfer enhancement of a dimpled tube with a twisted tape swirl generator”,

International Communications in Heat and Mass Transfer (2009), Vol. 36, pp. 698–704.

6. Alberto Garcia, Pedro G. Vicente, Antonio Viedma, “Experimental study of heat transfer

enhancement with wire coil inserts in laminar-transition-turbulent regimes at different Prandtl

numbers Experimental study of heat transfer enhancement with wire coil inserts in laminar-

transition-turbulent regimes at different Prandtl numbers”, International Journal of Heat and

Mass Transfer (2005), Vol. 48, pp. 4640-4651.

7. Alberto Garcia, Juan P. Solano, Pedro G. Vicente, Antonio Viedma, “Enhancement of

laminar and transitional flow heat transfer in tubes by means of wire coil inserts”,

International Journal of Heat and Mass Transfer (2007), Vol. 50, pp. 3176-3189.

8. Kumbhar D.G., Dr. Sane N.K., “Heat Transfer Enhancement in a Circular Tube Twisted with

Swirl Generator: A Review”, Proc. of the 3rd International Conference on Advances in

Mechanical Engineering, SVNIT Surat, January 4-6, 2010.

9. Panida Seemawute, Smith Eiamsa-ard, “Thermohydraulics of turbulent flow through a round

tube by a peripherally-cut twisted tape with an alternate axis”, International Communications

in Heat and Mass Transfer (2010), Vol. 37, pp. 652–659.

10. Pongjet Promvonge, Somsak Pethkool, Monsak Pimsarn, Chinaruk Thianpong, “Heat transfer

augmentation in a helical-ribbed tube with double twisted tape inserts”, International

Communications in Heat and Mass Transfer (2012), Vol. 39, pp. 953-959.

11. S. Eiamsa-ard, C. Thianpong, P. Eiamsa-ard, “Turbulent heat transfer enhancement by

counter/co-swirling flow in a tube fitted with twin twisted tapes”, Experimental Thermal and

Fluid Science (2010), Vol. 34, pp. 53–62.

12. Halit Bas, Veysel Ozceyhan, “Heat transfer enhancement in a tube with twisted tape inserts

placed separately from the tube wall”, Experimental Thermal and Fluid Science (2012), Vol.

41, pp. 51–58.

13. S. Eiamsa-ard , P. Seemawute, “Decaying swirl flow in round tubes with short-length twisted

tapes”, International Communications in Heat and Mass Transfer (2012), Vol. 39, pp 649–

656.

14. Er. Pardeep Kumar, Manoj Sain and Shweta Tripathi, “Enhancement of Heat Transfer using

Wire Coil Insert in Tubes” International Journal of Mechanical Engineering &

Technology (IJMET), Volume 3, Issue 2, 2012, pp. 796 - 805, ISSN Print: 0976 – 6340,

ISSN Online: 0976 – 6359.

Experimental study of heat transfer enhancement in a pipe using twisted tapes

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