Study of Heat Transfer Enhancement in Tubular Heat Exchanger with Twisted Tape Inserts

Abstract

In the present work an experimental investigation has been carried out to study the convective heat transfer augmentation in a double pipe counter flow heat exchanger (HE) with twisted tape insert. Also, the results are compared with a basic heat exchanger (BHE) of similar length, diameter and flow rate. The HE consists of a 140 inches long copper tube with diameter of 1inch which is concentered in a PVC pipe having a diameter of 2inch. Hot fluid is allowed to flow through the inner copper tube and the cold fluid was allowed to flow through the annular passage between copper tube and the PVC pipe. Experiments were conducted at different mass flow rates of the hot fluid for both the BHE and MHEs. The effects of inserted twisted tapes and twist ratio on heat transfer rate, pressure drop and thermal performance factor characteristics have been investigated extensively. A twist ratio is defined as the ratio of twist length (y) to twisted tape width at the large end (W). The experiments were carried out by using twisted tapes with three different twist ratios (y/W) of 4.5, 6.0 and 7.5. All cases were tested under turbulent flow regime for Reynolds number between 20000 and 50000. The thermal performance indicators, i.e. heat loss from hot fluid, overall heat transfer coefficient, effectiveness, Nusselt Number etc. have been found to be enhanced for the modified HEs compared to that for the basic one. Also, the thermal performance factor tended to increase with decreasing tape twist ratio. The effectiveness of HEs is found to be increased with modifications with twisted tapes. However, after a certain limit of the mass flow rate of the hot fluid the variation in HE's effectiveness becomes less significant compared to that found up to that limiting mass flow rate.

Full text

International Conference on Mechanical, Industrial and Energy Engineering 2018
23-24 December, 2018, Khulna, BANGLADESH
* Corresponding author. Tel.: +88-01913776863
E-mail addresses: anjan05me@gmail.com sadman.me.aust@gmail.com
ICMIEE18-164
Study of Heat Transfer Enhancement in Tubular Heat Exchanger with Twisted Tape Inserts
Sadman Hassan Labib1, Md. Riad Arefin Himel1, Jobayer Ibn Ali 1, Anjan Goswami2, *
1 Department of Mechanical and Production Engineering, Ahsanullah University of Science and Technology (AUST), Dhaka-
1208, Bangladesh.
2 Assistant Professor, Department of Mechanical and Production Engineering, Ahsanullah University of Science and
Technology (AUST), Dhaka-1208, Bangladesh.
ABSTRACT
In the present work an experimental investigation has been carried out to study the convective heat transfer augmentation
in a double pipe counter flow heat exchanger (HE) with twisted tape insert. Also, the results are compared with a basic
heat exchanger (BHE) of similar length, diameter and flow rate. The HE consists of a 140 inches long copper tube with
diameter of 1inch which is concentered in a PVC pipe having a diameter of 2inch. Hot fluid is allowed to flow through
the inner copper tube and the cold fluid was allowed to flow through the annular passage between copper tube and the
PVC pipe. Experiments were conducted at different mass flow rates of the hot fluid for both the BHE and MHEs. The
effects of inserted twisted tapes and twist ratio on heat transfer rate, pressure drop and thermal performance factor
characteristics have been investigated extensively. A twist ratio is defined as the ratio of twist length (y) to twisted tape
width at the large end (W). The experiments were carried out by using twisted tapes with three different twist ratios
(y/W) of 4.5, 6.0 and 7.5. All cases were tested under turbulent flow regime for Reynolds number between 20000 and
50000. The thermal performance indicators, i.e. heat loss from hot fluid, overall heat transfer coefficient, effectiveness,
Nusselt Number etc. have been found to be enhanced for the modified HEs compared to that for the basic one. Also, the
thermal performance factor tended to increase with decreasing tape twist ratio. The effectiveness of HEs is found to be
increased with modifications with twisted tapes. However, after a certain limit of the mass flow rate of the hot fluid the
variation in HE’s effectiveness becomes less significant compared to that found up to that limiting mass flow rate.
Keywords: Basic Heat Exchanger, Modified Heat Exchanger, Twisted Tapes, Twist Ratio, Turbulent Flow.
1. Introduction
Heat exchangers are widely used device working as
heating and cooling system in different industries like
oil refineries, power plants and even residential areas
[1-2]. Thermal performance improvement of many heat
exchangers are needed for energy saving, lower
operating cost moreover for better efficiency. Heat
transfer augmentation methods are used in the heat
exchanger systems to enhance the heat transfer rate and
improve the thermal performances [1-13]. Bergles [1]
categorized 13 heat transfer enhancement techniques in
‘active’ and ‘passive’ methods. External power is
required for surface vibration, fluid vibration, injection,
suction and electric or acoustic fields which are active
methods of heat transfer enhancement. Passive
techniques employ different surface geometry for heat
transfer enhancement. Treated surface, rough surface,
swirling flow devices, coiled tubes and surface tension
devices are included in passive techniques of improving
thermal performance of heat exchangers [2].
Heat transfer enhancement in heat exchanger has been
subjected of many experimental and analytical
investigation. Generally turbulent promotor which is
called ‘turbulator’, one of the passive techniques widely
used in heat transfer enhancement in the form of vortex
flow or swirl flow devices such as rib, fin, baffle,
winglet, propeller, grove roughened surfaces [3-9].
These turbulators are inserted in pipe flow to increase
heat transfer surface area, to provide an interruption of
thermal boundary layer development and to cause
increased heat transfer by increasing turbulence
intensity or fast fluid mixing. A number of
investigations have been made using various turbulators
with different configurations to heat transfer
enhancement in the heated tube of heat transfer, such as
twisted tube [10], wire-coils [11], grooved tubes [12],
compound turbulators [13].
Guo at el [14] numerically studied the contribution to
thermal performance of the conventional, short-width
and center cleared twisted tapes. Configuration
optimization of regularly spaced short-length twisted
tapes in a circular tube for turbulent heat transfer was
carried out by Wang et al. [15] by using computational
fluid dynamics (CFDs) modeling. Eiamsa-ard et al. [16]
presented experimental study on convective heat
transfer in a circular tube with short-length twisted tapes
inserted under uniform heat flux. Eiamsa-ard et al. [17]
performed experimental works on heat transfer and
friction factor characteristics in a heat exchanger fitted
with twisted tape elements. They made their analysis for
both continuously placed twisted tape and twisted tape
placed with various free spaced in circular tube.
Jaisankar et al. [18] experimentally examined the heat
transfer, friction factor and thermal performance caused
by twisted tape for solar water heater. The tape width,
rod-diameter effects, and phase angle effects on heat
transfer and pressure drop were analyzed experimentally
in a circular tube fitted with regularly spaced twisted

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tape elements by Saha at el [19]. But they did not
compare their studies with a basic heat exchanger.
In the light of above circumstances, the present study is
focused to an extensive investigation of the thermal
performance enhancement in double pipe heat
exchanger with twisted tapes. For this, first of all,
thermal performance study has been conducted in a
basic double pipe heat exchanger (BHE). Then, twisted
tape of copper with different twist ratios of 7.5, 6 and
4.5 have been inserted, and the thermal performance of
the modified heat exchangers (MHEs) has been
explored in a range of Reynolds number from 20000 to
50000. Finally, a comparative study has been conducted
among the BHE and MHEs to explore the degree of the
heat transfer augmentation.
2. EXPERIMENTAL SETUP
Fig. 1 Schematic of the Experimental Setup (The setup
is installed at the Project Lab of Dept. of Mechanical
and Production Engineering, AUST)
A detail of the experimental setup is displayed
schematically in Fig. 1. This experimental setup is
installed at the project lab under the Dept. of
Mechanical and Production Engineering, AUST. In this
setup, both hot and cold stream is water and it is
supplied by two 0.5 hp power pumps. The flow rate of
hot and cold both fluids are controlled by ball valve and
the flow rate is measured by means of volumetric flow
meter. In double pipe heat exchanger copper pipe of
1inch external diameter is used as tube and PVC pipe of
2inch internal diameter is used as shell. There are total
eight pen type thermocouples used in the apparatus to
measure the temperature of both hot and cold fluids at
different points. The hot fluid is heated by a 1 kW
portable water heater. Twisted tape made of copper used
as turbulator to investigate the thermal performance
improvement of the heat exchanger. Positioning of
twisted tapes inside the copper tube and the
thermocouples on the upper surface of the copper tube
attached by solid seal are shown in Fig. 2. The twisted
tapes with the length of 25 cm, width of 2 cm and 0.3
cm thick are shown in Fig. 3. Total number of 13
twisted taped are inserted in copper tube. Twisted tapes
having twist ratio (y/W) of 7.5, 6 and 4 are inserted to
study the thermal performance of the heat exchanger for
different twist ratio.
Fig. 2 Positioning of twisted tape and the thermocouple
in heat exchanger: (a) & (b) truncated portion of the
heat exchanger to show fittings, and (c) a magnified
view to show the thermocouple probe’s fitting with the
outer wall of copper tube to measure the wall
temperature.
Two pressure gauges are used to measure the pressure
drop in both shell and tube explicitly, due to insertion of
twisted tapes in the pipes which interrupts normal flow
of fluid. The experiments are conducted by varying flow
rate in terms of Reynolds numbers ranging from 20000
to 50000. First hot water and cold water are brought into
desired temperature. Temperature of cold water is
generally room temperature. The hot water temperature
is about 30˚C more than cold water. Heater connection
is turned off before starting the procedure. Ball valves
are regulated to get desired flow. Ball valve for cold
water pipeline is always fixed. Two pumps are started at
the same time. Data is taken from eight (8)
thermocouples. Two for cold water inlet and outlet
temperature, two for hot water inlet and outlet
temperature and another four for wall temperature of the
hot water tube. Data is also taken from two pressure
gauges mounted on the hot water pipe at inlet and outlet.
The setup runs for 30 minutes for every data collection
session. Data is taken after 1minute of interval. For
modified heat exchanger 13 twisted tapes are inserted

ICMIEE18-164-
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inside the copper tube. Rests of the procedures are all
the same for varying twisted tapes with various twist
ratios.
Fig.3 Twisted tapes with different twist ratios
(y/W = 4.5, 6, 7.5 (from top to bottom))
3. DATA ANALYSIS
The purpose of the current work is to determine the
effect of using twisted tapes in heat transfer rate of the
heat exchanger. For investigation all calculations have
to done for basic heat exchanger and modified heat
exchanger with different twisted ratios.
The steady state of the heat transfer rate is assumed to
be equal to the heat loss from the hot fluid in tube can
be expressed as follows:
QH = Qconv. (1)
where,
QH = mCp, H (To – Ti) (2)
The convection heat transfer from tube can be written
by
Qconv.= hA (Tw – Tb ) (3)
in which the bulk temperature is found by
Tb = (To + Ti)/2 (4)
and, the average wall temperature is calculated as:
Tw = Σtw /4 (5)
where, tw local wall temperature maintaining equal
distance along the length of the hot tube. The average
wall temperature Tw is computed by using 4 points of
local wall temperatures.
The average heat transfer coefficient (h) and Nusselt
number (Nu) are estimated as follows:
h = mCp, H (To – Ti)/ A(Tw – Tb) (6)
the heat transfer is calculated from the average Nu
which can be obtained by
Nu = hD/k (7)
Reynolds number (Re) is estimated by
Re = VD/ ϑ
(8)
where, V is the velocity of hot fluid inside the tube.
Effectiveness and overall heat transfer coefficient is
obtained as follows:
ϵ = ΔT (of fluid with minimum heat rate) / ΔTmax (9)
and, the overall heat transfer coefficient is found by
U = QH /A ΔTm (10)
where, ΔTm is logarithmic mean temperature difference
(LMTD) and it expressed as
 
 
–
/
()
[( ) ]
H out C in H in C out
H out C in H in out
mC
T T T T
ln T T T
TT


(11)
All of thermo-physical properties of water are
determined at the overall bulk water temperature Tb , Eq.
(4).
4. RESULTS AND DISCUSSION
4.1 Validation of Experiments on Basic Heat Exchanger
In this study, the experimentally obtained Nusselt
number values for the basic heat exchanger (BHE) are
compared with the correlation of Dittus-Boelter as given
below:
Nu = 0.023 Re0.8 Pr 0.4 (12)
Fig. 4 shows the variation of Nusselt number as function
of Reynolds number for experimental study and that for
the Dittus-Boelter correlation. For both cases, with the
increase of flow velocity in terms of Reynolds number,
the heat transfer in terms of Nusselt number also
increases. This occurs due to the fact that the heat
transfer rate enhances with higher flow velocity of the
working fluid. The experimental results display a good
agreement with the results obtained from Dittus-Boelter
correlation.

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4
Re
10000 20000 30000 40000 50000 60000
Nu
20
30
40
50
60
Basic HE
Dittus Boelter correlation
Fig.4 Variation of Nusselt number as a function of
Reynolds number for Basic Heat Exchanger (BHE).
Re
10000 20000 30000 40000 50000 60000
Nu
30
40
50
60
70
80
90 Basic HE
Modified HE (TR=7.5)
Modified HE (TR=6.0)
Modified HE (TR=4.5)
Fig.5 Variation of Nusselt number with Reynolds
number for BHE and MHEs
4.2 Effect of inserting Twisted Tape and Twist ratio
Insertion of twisted tape in the copper tube causes
smaller annular flow area for the hot fluid than in basic
heat exchanger. With the decrease of flow area, the flow
velocity increases. Twisted tape generates secondary
flow inside the copper tube along with the interruption
of normal fluid flow. Turbulence caused by higher flow
velocity and twisted tapes, accompanying with
secondary fluid flow causes increase in the heat transfer.
Twisted tapes with lower twist ratio has more twist than
the twisted tapes with higher twist ratio. With higher
twists the tapes generate rapid secondary flow and
turbulence which results higher heat transfer than other
twisted tapes with high twist ratio. The variations of
Nusselt number with Reynolds for modified HEs with
three different twist ratio along with basic HE is shown
in Fig. 5. Reynolds number is a function of velocity and
velocity itself is a function of mass flow rate and
Nusselt number is also proportional to the flow velocity.
Both Reynolds number and Nusselt number are higher
in the MHE with TR = 4.5, than both the BHE and the
other MHEs with TR = 6.0, 7.5, for different mass flow
rate of hot water. Fig. 5 displays that, the insertion of
twisted tape increases turbulence of the flow and thus
increases the velocity of the flow which increases heat
transfer in terms of Nusselt number. It also shows the
linear behavior of Nusselt number variation along
Reynolds number for both BHE and MHEs. The rate of
heat loss from the hot fluid of the inner tube with mass
flow rate of hot fluid for both the BHE and MHEs is
displayed in Fig. 6. Insertion of twisted tape in copper
tube causes swirl flow of hot fluid as well as turbulence.
For these reasons heat loss increases with the increase
of mass flow rate of hot fluid which indicates
augmentation of heat transfer rate due to twisted tape
insertions. The heat loss is found to be the highest in
MHE with TR = 4.5 and the least in the BHE. Nusselt
number is related to the heat transfer rate. Higher the
Nusselt number means there is more convective heat
transfer rate. With the increase in mass flowrate, Nusselt
number increases in terms of heat transfer rate.
Variation
mass flowrate, mH (kg/s)
0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50
Heat loss, Q H (kW)
4
5
6
7
8
9
10
11
Basic HE
Modified HE (TR = 7.5)
Modified HE (TR = 6.0)
Modified HE (TR = 4.5)
Fig. 6 Variation of heat loss from hot fluid with its mass
flowrate
of Nusselt number as function of mass flowrate is
shown in Fig. 7. From Fig. 7, Nusselt number is higher
in the MHE with TR = 4.5 than the BHE and the other
MHEs (TR = 6.0, 7.5). As the mass flow rate increases,
the Nusselt number increases for all types of the heat
exchangers. Fig. 8 shows the variation of Reynolds
number with mass flow rate of hot fluid. Reynolds
number increases for constant mass flowrates and is the
highest for the MHE with lowest twist ratio than the
MHEs and the BHE. Twisted tape with lowest twist
ratio has denser twists than other which help the tape to
generate more turbulence. This results in higher
Reynolds numbers for the MHEs compared to that for
BHE. The variation of overall heat transfer coefficient
as function of mass flow rate of hot fluid for BHE and
MHEs is displayed in the Fig.9. The overall heat
transfer coefficient increases with the increase of mass
flow rate of hot fluid for all types of heat exchangers.
As heat transfer enhancement method (insertion of

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twisted tape) applied, the overall heat transfer
coefficient increases with constant mass flow rate of hot
fluid. The overall heat transfer co-efficient is found to
be higher in the MHEs
mass flowrate, mH (kg/s)
0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50
Nu
30
40
50
60
70
80
90
Plain Tube
TR = 7.5
TR = 4.5
TR = 6.0
Fig. 7 Variation of Nusselt number with mass flowrate
of hot fluid
mass flowrate, mH (kg/s)
0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50
Re
10000
20000
30000
40000
50000
60000
Basic HE
Modified HE (TR = 7.5)
Modified HE (TR = 6.0)
Modified HE (TR = 4.5)
Fig. 8 Variation of Reynolds number with mass
flowrate of hot fluid
mass flowrate, mH (kg/s)
0.1 0.2 0.3 0.4 0.5
Overall Heat Transfer Co-efficient, U (W/m2 oC)
1500
2000
2500
3000
3500
Basic HE
Modified HE (TR=4.5)
Modified HE (TR=6)
Modifeid HE (TR= 7.5)
Fig. 9 Variation of Overall heat transfer coefficient with
mass flowrate of hot fluid
than the BHE. The overall heat transfer co-efficient
depends on the surface area. Twisted tape with low twist
ratio has more twist that tends to more surface area. So
the overall heat transfer coefficient is the minimum in
the BHE and the maximum in the MHE with twisted
tape having twist ratio of 4.5. The variation of
effectiveness with mass flowrate of hot fluid for all heat
exchangers is shown in Fig. 10. The comparison of
effectiveness among the BHE and the MHEs displays
that the effectiveness increases along with the increase
in mass flow rate of hot fluid for all cases. The
effectiveness of the MHE with TR = 7.5 is higher than
the BHE, and the effectiveness of the MHE with TR =
6.0 is higher than both the BHE and the MHE with TR
= 7.5. For the MHE with TR = 4.5, the effectiveness is
found to be the maximum. In the present study, for
different mass flow rate of hot fluid and for both basic
and modified heat exchangers effectiveness is computed
from the recorded data. The lines of effectiveness of the
BHE and that of
mass flowrate, mH (kg/s)
0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50
Effectiveness
0.12
0.14
0.16
0.18
0.20
0.22
Basic HE
Modified HE (TR = 7.5)
Modified HE (TR = 6.0)
Modified HE (TR = 4.5)
Fig. 10 Variation of Effectiveness with mass flowrate of
hot fluid
the MHEs (with TR = 7.5, 6.0, 4.5) nearly follow the
same trend. But it is found that as the mass flow rate
increases the difference in effectiveness decreases
between the heat exchangers. This means, the
effectiveness achieved by applying twisted tape inserts
is limited to the mass flowrate of the fluid. Hence, it can
be inferred that beyond a limit of the mass flow rate, in
terms of effectiveness, the achievement in MHEs
becomes insignificant compared to that achieved in the
BHE.

ICMIEE18-164-
6
5. Conclusion
In this study, an experimental investigation on different
performance indicators in a tubular heat exchanger (HE)
of basic and modified with twisted tape inserts has been
carried out following variable flow of hot fluid and
constant flow of cold fluid in a closed circuit. According
to the experiments, the major findings can be
summarized as follows:
(i) Heat transfer rate was found to be enhanced in
modified heat exchangers compared to that of basic heat
exchanger. The rate of heat transfer enhancement was
~14.1% for HE with TR of 4.5, ~6% for HE with TR of
6.0, and ~2.5% for HE with TR of 7.5 in the modified
heat exchangers(MHEs) compared to that of the basic
heat exchanger (MHE).
(ii) Reynolds number increases for the MHEs as twisted
tape generate turbulence in the fluid flow.
(iii) Nusselt number is found to be increased in the
MHEs due to the augmentation in the heat transfer rate
through the inner copper tube wall.
(iv) The effectiveness of the MHEs is limited to the
mass flow rate of the fluid through the inner tube. After
a limit of mass flow rate, the effectiveness becomes less
significant compared to that occurs at lower mass rates.
6. NOMENCLATURE
cp
Re
Pr
h
U
T
t
k
ϑ
: specific heat at constant pressure, kJ･kg-1･K-1
: Reynolds number
: Prandtl number
: specific enthalpy, kJ･kg-1
: overall heat transfer coefficient, W/m2 oC
: temperature, K
: Celsius temperature, oC
: Thermal conductivity, W/mK
: Kinematic viscosity, m2/s
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