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International Journal of Mechanical Engineering and Technology (IJMET)
Volume 7, Issue 3, May–June 2016, pp.125–138, Article ID: IJMET_07_03_012
Available online at
http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=7&IType=3
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ISSN Print: 0976-6340 and ISSN Online: 0976-6359
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ENHANCEMENT OF HEAT TRANSFER IN
SHELL AND TUBE HEAT EXCHANGER
WITH TABULATOR AND NANOFLUID
Qasim S. Mahdi
Mechanical Engineering Department, College of Engineering,
Al–Mustansiriyah University, Baghdad, Iraq
Ali Abdulridha Hussein
Mechanical Engineering Department, College of Engineering,
Al–Mustansiriyah University, Baghdad, Iraq
ABSTRACT
The present work reported the use of variant twisted tapes fitted in a
double pipe heat exchanger to improve the fluid mixing that leads to higher
heat transfer rate with respect to that of the plain-twisted tape. Heat transfer,
flow friction and thermal enhancement factor characteristics in a double pipe
heat exchanger fitted with plain and variant twisted tapes using water as
working fluid are investigated experimentally. Tests are performed for laminar
flow ranges. The experimental data for a plain tube and plain-twisted tapes
are validated using the standard correlations available in the literature. Two
different variant twisted tapes which include V cut-twisted tape and Horizontal
wing cut-twisted tape with twist ratios of y = 2.0, 4.4 and 6.0 are used. In
addition, the variation of heat transfer coefficient of copper–nanofluids with
different of Reynold's number and volume concentration of nanoparticles in
plain tube without twisted tape. Based on these studies, the major conclusion
has been arrived the Nusselt number, friction factor and thermal enhancement
factor of variant twisted tapes are higher than that of plain twisted tape for the
twist ratios of 2.0, 4.4 and 6.0 respectively so among the variant twisted tapes
used in the present work, the horizontal wing cut-twisted tape give better
performance due to the effect of increased turbulence which improves the fluid
mixing near the wall of the test tube. By increasing volume concentration of
nanoparticles, thermal conductivity increases while the thermal boundary
layer thickness decreases. The Maximum thermal enhancement factor for P-
TT, V-TT and HW-TT are 3.903, 4.269 and 4.488 respectively and
enhancement plain twisted tape is better than CuO-nanofluid be three times.
Key words: Double Pipe Heat Exchanger, Twisted Tape Insert, Swirling,
Passive Methods, Heat Transfer Enhancement, Nanofluid, Turbulent, Laminar
Flow, Twist Ratio, Cuo Nanoparticles.
Qasim S. Mahdi and Ali Abdulridha Hussein
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Cite this Article: Qasim S. Mahdi and Ali Abdulridha Hussein, Enhancement
of Heat Transfer In Shell and Tube Heat Exchanger with Tabulator and
Nanofluid. International Journal of Mechanical Engineering and Technology,
7(3), 2016, pp. 125–138.
http://www.iaeme.com/currentissue.asp?JType=IJMET&VType=7&IType=3
NOMENCLATURE
Symbol Description Units
Ac Tube cross-sectional area m2
As Tube surface area m2
Cp Specific heat J/kg.K
D Diameter of outer tube m
de Depth of cut mm
Dh Hydraulic diameter m
f Friction factor -------
F Correction factor -------
h Heat transfer coefficient W/m2.K
H Pitch length based on 180° m
h Heat transfer coefficient W/m2.K
kf Fluid thermal conductivity W/m.K
L Length m
m Mass kg
Mass flow rate kg/s
Nu Average Nusselt number -------
Pr Prandtl number -------
Q Heat transfer rate W
Re Reynolds number -------
Sw Swirling conductivity W/m.K
T Temperature C, K
t Time sec
u Velocity vector m/s
W Width of twisted tape mm
w Width of the cut mm
y Twist ratio -------
ΔP Pressure drop Pa
Enhancement of Heat Transfer In Shell and Tube Heat Exchanger with Tabulator and
Nanofluid
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Subscripts
Symbol Title
a annular
bf Base fluid
in inlet
m mean
nf Nanofluid
o Smooth tube
out outlet
p particle
pp Pumping power
ref Reference value
S Surface
w Wall
Greek Symbol
Symbol Title Units
Fluid Density kg/m
3
Dynamic viscosity kg/m.s
Kinematics viscosity m2/s
φ Volume concentration percentage
Ф Heat dissipation term
∆h Difference in level of manometric fluid m
ƞ Thermal enhancement factor
1. INTRODUCTION
Nowadays, thermal systems are some of the most important systems used in
engineering applications. Therefore, different methods have been researched and
developed to enhance heat exchange in these systems and reach a high performance
thermal operation. Heat transfer rate in conventional heat exchangers can be improved
through a variety of augmentation techniques that employs surface enhancements.
This improvement in heat transfer rate occurs as a result of the following conditions
that are created by the use of enhanced surfaces. These conditions are Interrupting of
boundary layer development and rising degree of turbulence, increasing heat transfer
area and Generating of swirling and/or secondary flows. Recently, many industrial
applications such as refrigeration, automotive and process industries have been
employing heat transfer enhancement techniques in order to improve the performance
of heat exchangers.
Enhancing heat transfer in heat exchangers could lead to many economic and
environmental benefits. Energy, material and cost savings are achieved through better
heat exchanger designs that reduce its size and improve its efficiency. Watcharin et
al. [2006] have studied the heat transfer and pressure drop in a concentric double pipe
heat exchanger with twisted tape insertion. The twist ratios used are Y = 5.0 and 7.0.
It was observed that the maximum Nusselt numbers over the range studied for using
the twisted tapes with ratios Y = 5.0 and 7.0 are 188% and 159%, respectively, when
Qasim S. Mahdi and Ali Abdulridha Hussein
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compared to the plain tube. Yadav [2009] has studied the heat transfer and pressure
drop in a U-bend double pipe heat exchanger with half-length twisted tape insertion.
Half-length twisted tape was placed inside the inner tube of the heat exchanger in
order to introduce swirling flow. It was observed that the tape-induced swirl causes a
40% increase in the heat transfer coefficient of the half-length twisted-tape inserts
when compared to plain heat exchanger. However, the thermal performance of plain
heat exchanger was found to better than half-length twisted tape by (1.3-1.5) times.
Kapatkar et al. [2010] has examined the influences of fitting full length twisted tape
inserts in a plain tube for laminar flow on the heat transfer and friction factor. The
Reynolds number range was taken to be from 200 to 2000. Showed that full length
twisted tapes results in the following Nusselt number improvement are aluminum
tapes (50% to 100%), stainless steel tapes (40% to 94%) and insulated tapes (40% to
67%). The isothermal friction factor for the flow with the twisted tape inserts are
340% to 750 % higher as compared with those of smooth tube flow, in the given
range of twist ratios. A double pipe heat exchanger fitted with coil wire insert was
tested by Shashank and Taji [2013]. The wire is made up of three different
materials which are copper (Cu), aluminum (Al) and stainless steel. The study was
conducted over a Reynolds number of 4000 to 13000. The results showed that heat
transfer enhancements were 1.58, 1.41 and 1.31 for copper, aluminum and stainless
steel coils respectively. Moreover, the different coil wire inserts resulted in higher
friction factor than plain tube by 5.4 to 6.7 times for aluminum, 4.8 to 5.9 times for
stainless steel and 4.3 to 5.4 times for copper. Senthilraja and Vijayakumar [2013]
utilized a double pipe heat exchanger to experimentally measure the heat transfer
coefficient of CuO/Water nanofluid. A CuO nanoparticles were dispersed in a
deionized water to create a nanofluid. At room temperature, the nanofluid has a
diameter of 27nm at different volume concentrations (0.1% and 0.3%). It was found
that as time passes, the heat transfer coefficient increases while increasing the liquid
flow rate will result in an increase in the Nusslet number. The nanofluid with
concentration of 0.3% provided the highest heat transfer coefficient.
In the present study the effect of using twisted tape inserts and nanofluid
CuO/water will be investigated experimentally. Twisted tapes with variant twisted
tapes cut section and twist ratios as well as nanofluid with different volume
concentration were used for enhancement of heat transfer in double pipe heat
exchanger. Finally, an empirical correlations based on the experimental results of the
present study will be given for prediction the heat transfer (Nusselt number).
2. EXPERIMENTAL APPARATUAS AND PROCEDURE
2.1. Description of Test Rig
The external pipe: It is an insulated pipe which has been manufactured from copper
material of (51.78 mm) inner diameter, (1.5 m) length and (1.17 mm) thickness. It is
insulated from outside by glass wool. Insulation are used to reduce the heat losses to
the surrounding. A small hole was made in the external pipe for the thermocouples
wires that were installed on the external surface of the inner pipe. The hole was
patched with asphalt.
Internal pipe: It has been manufactured from copper material of (20.4 mm) inner
diameter (1530 mm) length and (0.88 mm) thickness. The pipe contains small vertical
(6 mm) ports at its inlet and outlet which are used to measure the difference in
pressure. The pressure sampling ports were welded using sliver brazing. The
thermocouples are fixed under these ports using metal support and screws to measure
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the inlet and outlet temperatures as shown in Figure (1). A valve is installed on the
outlet in order to control and stabilize the flow.
Twisted Tapes: In the test run, tapes are used with three different twist ratios y = 2.0,
4.4 and 6.0. Twisted tapes are made from copper strips of thickness 0.9 mm and width
17 mm as shown in table (1) and figure (2).
Table 1 Characteristic dimensions of the turbulators inserted tubes
Twist set δ
mm H mm
W
mm WR DR
No.
turn y=H/di Metal
P-TT 0.9 40.8 17 - - 37 2 Copper
P-TT 0.9 89.76 17 - - 17 4.4 Copper
P-TT 0.9 122.4 17 - - 13 6 Copper
V-TT 0.9 40.8 17 0.352 0.47 37 2 Copper
V-TT 0.9 89.76 17 0.352 0.47 17 4.4 Copper
V-TT 0.9 122.4 17 0.352 0.47 13 6 Copper
HW-TT 0.9 40.8 17 0.294 0.47 37 2 Copper
HW-TT 0.9 89.76 17 0.294 0.47 17 4.4 Copper
HW-TT 0.9 122.4 17 0.294 0.47 13 6 Copper
Figure 1 Schematic diagram of experimental test section
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Figure 2 Shapes Twisted Tapes: (a) Plain-Twisted Tapes (P-TT), (b) V cut-Twisted Tapes
(V-TT), (c) Horizontal wing cut-twisted tape (HW-TT)
2.2. Test Procedure
In order to evaluate the thermal performance of double pipe heat exchanger, a series
of experiments was carried out at different operational and conformational
parameters. Operational parameters demonstrate: hot water flow rate of (0.008,
0.0109, 0.0137,0.0164,0.0192,0.019) kg/sec, cold water flow rate of (0.18) kg/sec,
inlet hot water temperature of ( C and inlet cold water temperature of
( C.
Runs heater tank of hot water after the water situation and wait for a while, then
we take our pump and control a flow rate on flow meter, it put exist after pump, and
in the mean time we take our water pump and wait by flow meter existing then, and
there many thermocouples at the inlet and outlet of the test tube for both hot and cold
tubes, starts registered record temperatures as well as the differential pressure
manometer score and when you reach a state of stability takes values recorded after
the transfer of the calculator by a small memory, and restore the same steps when
insert twisted tapes inside the tube.
2.3. Performance Parameters
Present study consists two fluid flow inside heat exchanger in counter flow
arrangement as shown in Figure (1). cold water is forced to flow through annuli and
hot de-ionized water is passes through inner tube. Steady state condition, insulated
outer surface of heat exchange and no phase changer have been assumed during the
analysis of present heat exchanger. Under these conditions the heat dissipation of both
sides Eiamsa-ard et al. [2006]:
Heat transferred to the cold water in the test section
Qc = c Cpc (Tc2 -Tc1) (1)
Heat transferred from the hot water in the test section
Qh = h Cph (Th1 -Th2) (2)
The percentage of heat loss
ɛ =
(3)
b a c
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The average heat transfer rate for hot and cold water side is taken for internal
convective heat transfer coefficient
Qavg =
(4)
The surface area of the inner tube
Ai = π di L (5)
Logarithmic mean temperature difference
(6)
The overall heat transfer coefficient
U =
(7)
The annulus side heat transfer coefficient annulus side heat transfer coefficient
(ha) is estimated using the correlation of Dittus –Boelter equation
Nua =
= 0.023 Rea0.8
Prc0.3 (8)
The inner tube side heat transfer coefficient (hi) is determined by neglecting the
conduction thermal resistance of copper tube wall:
(9)
The inner tube side Nusselt number
Nui =
(10)
The Reynolds Number is based on the different flow rates at the inlet of the test
section
Rei =
(11)
Friction factor and is related to pressure drop in the test section
f =
(12)
The thermal enhancement factor (ɳ)
ɳ =
= a Re-b
y-c
(13)
3. EXPERIMENTAL RESULT
3.1. Comparison of Experimental Results
The heat transfer data for the plain tube is compared with literature data obtained
using Sieder and Tate (1936) Equation (14) Cengel [2008]. The plain tube data are
matching with Sieder and Tate equation with the discrepancy of ± 3% as shown in
Figure (3).
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(14)
The plain tube friction factor is shown in Figure (4) and these data are compared
with the data obtained using the Equation (15) Cengel [2008].
The experimental friction factors are matching with the deviation of ± 4% with the
results of the Equation (15).
(15)
In addition, the experimental data of the plain tube are correlated for Nusselt number
and friction factor respectively through Equations (16) and (17) as follow;
(16)
(17)
The Equations (16) and (17) are found to represent the experimental data within
±1% for Nusselt number ± 2% for the friction factor also.
3.2. Effect of Plain-Twisted tape
The experimental results of the tube fitted with P-TT are compared with the plain tube
and its results are validated using the correlations available in the literature for the
laminar flow at the inlet to test section. Otherness of Nusselt number, heat transfer
coefficient and friction factor with Reynolds number at the inlet to test section for the
tube fitted with P-TT of different twist ratios (y = 2.0, 4.4 and 6.0) and plain tube are
depicted in Figures (5) and (6) respectively.
By referring to the Figure (5) it can be observed that Nusselt number, heat transfer
coefficient increases with the increasing Reynolds number and also the result showed
that the use of lower twist ratio yields higher Nusselt number than that of the higher
twist ratio. This happens because the lower twist ratio creates stronger swirl flow
which makes the thinner boundary layer along the pipe wall. Therefore more heat transfer through the thinner boundary layer. The swirl flow also creates the fluctuation
of the energy between fluid layers and as a result the heat energy readily moves across
Figure 4 Experimental data verification of
friction factor for plain tube
Figure 3 Experimental data verification of
Nusselt number for plain tube
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the fluid layers. Moreover, the residential time of flow increases with stronger swirl
level which causes, the flow to have more time for exchanging the heat between the
core and the wall. Over the range studied, the mean Nusselt number for P-TT with
twist ratios y = 2.0, 4.4 and 6.0 are respectively 3.6467, 2.7001 and 2.4878 times
better than that for the plain tube.
Figure (6) shows that the friction factor decreases continuously with Reynolds
number and friction factor for lower twist ratio (y = 2.0) is significantly more than
that of the higher twist ratio (y = 4.4 and 6.0) due to stronger swirl flow offered by the
P-TT with lower twist ratio. From the experimental results, it can be observed that the
friction factors for the P-TT with twist ratio y = 2.0, 4.4 and 6.0 are respectively
6.80586, 4.3509 and 3.6079 times than that for the plain tube.
The experimental data are fitted by the following correlations:
(18)
(19)
The fitted values are agreeing with the experimental data within ±12% and -10%
for Nusselt number and friction factor respectively.
3.3. Effect of V Cut-Twisted Tape
This section is focused on the experimental study on the heat transfer and friction
factor for the horizontal concentric tube fitted with V-TT with different twist ratios y
= 2.0, 4.4 and 6.0 for laminar flow at the inlet of test section. The experimental results
of V-TT are compared with plain tube and P-TT.
Figure (7) shows the comparison between Nusselt number obtained from the tube
fitted with V-TT, P-TT and plain tube. It can be observed from the Figure (7) that, the
Nusselt number obtained from the V-TT is higher than those from the P-TT and plain
Figure 5 Nusselt number versus Reynolds
number for P-TT with different twist ratio(y)
and plain tube
Figure 6 Friction factor versus Reynolds
number for P-TT with different twist ratio(y)
and plain tube
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tube. This means that V-TT generated the secondary flow along with main swirl flow
produced by P-TT. Mean Nusselt number for the tube fitted with V-TT of twist ratios
y = 2.0, 4.4 and 6.0 are respectively 4.1519, 3.005 and 2.7410 times better than that
for the plain tube and similarly 1.1385, 1.113 and 1.101 times higher than that of P-
TT.
The comparison of the friction factor between the V-TT, P-TT and plain tube with
the variations of inlet Reynolds number is shown in Figure (8) respectively. It can be
clearly seen that the friction factor continues to decrease with increase in hot water
Reynolds number. At a given Reynolds number, the friction factor for the tube with
V-TT is consistently higher than that of the P-TT and plain tube. This is because of
the additional disturbance to the main swirl flow in the form of turbulence which
increases the tangential contact between the fluid and the wall. Mean friction factor
for the tube fitted with V-TT of twist ratios y = 2.0, 4.4 and 6.0 are respectively 8.817,
5.7521 and 4.783 times higher than that for the plain tube and 1.29562, 1.322 and
1.325 times higher than that of P-TT with the same twist ratios. The experimental data
of heat transfer and friction factor for the tube with V-TT with different twist ratios (y
= 2.0, 4.4 and 6.0) are correlated as the function of Reynolds number, Prandtle
number and twist ratios are as follows:
(20)
(21)
The deviation between the predicted and experimental Nusselt number and
friction factor are respectively ±4% and ±8%.
3.4. Effect of Wing Cut-Twisted Tape
This section is mainly focussed on the study of the heat transfer enhancement effect
by fixing the depth and width ratios and the position of the wing-cut from horizontal
direction. The heat transfer and friction factor characteristics of the HW-TT is studied
for the twist ratios y = 2.0, 4.4 and 6.0 and the results are compared with those tube
Figure 7 Nusselt number versus Reynolds
number for V-TT, P-TT with different twist
ratio(y) and plain tube
Figure 8 Friction factor versus Reynolds
number for V-TT, P-TT with different twist
ratio(y) and plain tube
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fitted with and without P-TT. The experimental results from the HW-TT are
correlated for Nusselt number and friction factor.
Figure (9) shows the relationship between the Nusselt number and Reynolds
number for the tube with HW-TT, P-TT with different twist ratios, y = 2.0, 4.4 and
6.0 and also the plain tube respectively. In general, the HW-TT provide higher
Nusselt number than that of P-TT and plain tube depending on the twist ratios used.
HW-TT provides additional vortices to the fluid in the vicinity of the tube wall in
addition with swirl flow generated by the P-TT and thus leads to a higher heat transfer
enhancement in comparison with plain tube and P-TT.
On the other hand the use of HW-TT, mean Nusselt numbers for the twist ratios
2.0, 4.4 and 6.0 are respectively 4.3889, 3.242 and 2.895 times of that for the plain
tube and 1.1978, 1.2 and 1.1636 times of that for the tube fitted with P-TT. This may
be a consequence of better mixing between the core and the fluid wall due to the more
efficient turbulence offered by the HW-TT.
The friction factor characteristics at various Reynolds number based on the inlet
side of the tube fitted with HW-TT, P-TT and the plain tube is displayed in Figure
(10). At a given Reynolds number, the friction factors of all the HW-TT are
consistently higher than that of the tube with P-TT and plain tube due to an additional
blockage provided by the wings to the flowing fluid in the tube. For a tube with HW-
TT, the mean friction factors with twist ratios of 2.0, 4.4 and 6.0 are respectively
9.277, 6.419 and 5.207 times of those in the plain tube and 1.363, 1.4753 and 1.4432
times of those in the tube with the P-TT.
The experimental data of heat transfer and friction factor for a tube with the HW-
TT with different twist ratios are correlated as follows:
(22)
(23)
The deviation of the multiple regressions of Nusselt number and friction factor are
+12% and ±12%, respectively.
Figure (9) Nusselt number versus
Reynolds number for HW-TT, P-TT with
different twist ratio(y) and plain tube
Figure (10) Friction factor versus
Reynolds number for HW-TT, P-TT with
different twist ratio(y) and plain tube
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3.5. Comparison of Variant Twisted Tape Inserts
The variant twisted tapes performances have been compared based on the thermal
enhancement factor because it relates the Nusselt number and friction factor
characteristics. Therefore, it is worthy to compare the performance using thermal
enhancement factor instead of making the comparisons of the Nussselt number and
friction factor of the variant twisted tape used in the present work separately.
Figure 11 Thermal enhancement factors versus Reynolds number for PTT, V-TT and HW-TT
Figure (11) present the comparison of the variant twisted tapes used in the present
work for the twist ratios y = 2.0, 4.4 and 6.0 respectively.
The thermal enhancement factor (ƞ for the P-TT, V-TT and HW-TT is expressed
in the equation (29), (30) and (31) respectively:
(24)
(25)
(26)
3.6. Inner Nusselt's Number with Nanofluids
Performance of double pipe heat exchanger with nanofluids has been studied to show
the effect of concentration on heat transfer enhancement. Water cold Reynold's
number has been selected as 3300 during nanofluids experiments. Figures (12) show the variation of the Nusselt's number with a Reynold's number of CuO nanofluids.
These figures clearly indicate that Nusselt's number increases with increasing both
Reynold's number and volume concentration of nanoparticles. The main reason of this
enhancement due to the increase in both thermal conductivity and heat transfer
coefficient of nanofluid.
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Figure 12 Variation Nusselt number of CuO/water Nanofluid with Reynolds Number for
various volume Concentration
Table (2) show the enhancement of Nusselt number with variation in volume of
copper-nanofluids.
Table 2 Enhancement in Nusselt number of CuO nanofluid
Concentration %
(lit/hr)
Enhancement
%
Concentration
%
(lit/hr)
Enhancement
%
0.2 30 7.92 0.2 60 5.3
0.4 30 10.2 0.4 60 6.41
0.6 30 12.38 0.6 60 8.41
0.2 40 7.31 0.2 70 4.48
0.4 40 9.78 0.4 70 6.49
0.6 40 11.32 0.6 70 7.65
0.2 50 5.87
0.4 50 8
0.6 50 9.68
4. CONCLUSIONS
The investigation on heat transfer and friction factor characteristics for variant twisted
tapes (P-TT, V-TT, HW-TT) fitted in the double pipe heat exchanger, with twist ratios
y = 2.0, 4.4 and 6.0 have been studied and presented. According to the past studies, it
is observed that modifications on the P-TT i.e. small cuts on the tape, will give an
assurance for enhancement of both heat transfer and thermal enhancement. The
variant twisted tapes are used based on the concept of introducing a small cuts on the
peripheral region of the tape (V-TT and HW-TT). The conclusion arrived the plain
tube data of Nusselt number and friction factor were verified with the standard
correlations in order to ensure the performance of the experimental set up for laminar
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flow. The maximum deviation observed for both the experimental Nusselt number
and friction factor is ±4% and ±5% with the standard correlations values respectively.
The tape with lower twist ratio (y = 2.0) offered a better thermal enhancement than
the tape with higher twist ratio (y = 4.4 and 6.0). The V-TT provides improved heat
transfer enhancement than that of P-TT. In the group of variant twisted tapes, HW-TT
yields better thermal performance. The Nusselt number enhancement are (240, 183,
159)% for P-TT to y=(2.0, 4.4, 6.0) respectively also (326, 211, 181)% for V-TT to
y=(2.0, 4.4, 6.0) respectively in final the enhancement are (348, 232, 196)% for HW-
TT to y=(2.0, 4.4, 6.0).
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