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http://www.iaeme.com/IJMET/index.asp 225 [email protected]
International Journal of Mechanical Engineering and Technology (IJMET)
Volume 7, Issue 3, May–June 2016, pp.225–239, Article ID: IJMET_07_03_021
Available online at
http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=7&IType=3
Journal Impact Factor (2016): 9.2286 (Calculated by GISI) www.jifactor.com
ISSN Print: 0976-6340 and ISSN Online: 0976-6359
© IAEME Publication
HEAT TRANSFER AUGMENTATION OF
LAMINAR NANOFLUID FLOW IN
HORIZONTAL TUBE INSERTED WITH
TWISTED TAPES
Prof. Dr. Qasim S. Mahdi
Mechanical Engineering Department,
College of Engineering, Al–Mustansirya University, Iraq
Noor AM. Mohammed
Mechanical Engineering Department,
College of Engineering, Al–Mustansirya University, Iraq
ABSTRACT
In this paper, an experimental study on the heat transfer enhancement and
friction factor characteristics for fully developed laminar CuO/distilled-water
(DI-water) nanofluid flow through horizontal tube inserted with different
geometries of twisted tapes under constant heat flux condition ranged from
4483 to 10000 W/ . =0.08% and 0.35% volume concentrations of CuO
nanoparticles are suspending in distilled water to prepare nanofluid. Twisted
types made from copper material with twist ratios Y=2.6 and 5.3 twist ratios,
thickness t=1 and 2mm and with semicircular and triangular cuts shape were
used to study their effect on twisted tape performance. Results showed that
both convective heat transfer in terms of average Nusselt number and
friction factor have significantly increasing with inserting twisted tape with
nanofluids as working fluid comparing with nanofluids or DI-water in smooth
tube case and this enhancement increases as both Reynold number and
volume concentration increases. Triangular cut twisted tape (TCTT) at Y=2.6
and t=2mm with CuO nanofluid at =0.35% showed the best preformance
among the other twisted tapes on heat transfer enhancement where
increased by 73% than smooth tube with DI-water, while friction factor
increased by 62%. New Empirical correlations have been developed for both
average Nusselt number and friction factor in the terms of the parameters
mentioned above.
Key words: Nanofluid, Twisted Tape, Heat Transfer Enhancement, Friction
Losses.
Prof. Dr. Qasim S. Mahdi and Noor AM.Mohammed
http://www.iaeme.com/IJMET/index.asp 226 [email protected]
Cite this Article: Prof. Dr. Qasim S. Mahdi and Noor AM.Mohammed, Heat
Transfer Augmentation of Laminar Nanofluid flow in Horizontal Tube
Inserted with Twisted Tapes. International Journal of Mechanical
Engineering and Technology, 7(3), 2016, pp. 225–235.
http://www.iaeme.com/currentissue.asp?JType=IJMET&VType=7&IType=3
1. INTRODUCTION
Enhancement the heat transfer process play an important role in increasing the
efficiency of the most important energy industry applications in heat transfer
including, power generation, chemical production, air conditioning, Transportation
and microelectronics, heating of circulating fluid in solar collector, heat transfer in
compact heat exchangers and many other industrial sectors associated with different
processes depends on the heating or cooling fluid inside tubes. During the last decades
many researchers have been experimentally, investigate the effects of nanofluid
technology and the tabulators' passive techniques on heat transfer enhancement and
pressure drop. Lazarus et al. [2009] experimentally investigated the effect of
convective heat transfer of de-ionized water with a low volume fraction =0.003% of copper oxide (CuO) nanoparticles to form nanofluid flows through copper tube under
laminar flow and heat flux conditions. The results has shown 8% enhancement for
convective heat transfer coefficient of the nanofluid even with a low volume
concentration of CuO nanoparticles. The heat transfer enhancement increased
considerably as the Reynolds number increased. They predicted a new correlation for
local Nusselt number variation along the flow direction of the nanofluid. Alimullah
Anwar [2014] studied experimentally the heat transfer augmentation and friction
factor characteristics through circular tube fitted with full-length helical screw insert
device for laminar and turbulent flow under constant heat flux condition. The results
showed that with this type of inserts a high swirl flow generates which increases the
convection heat transfer, thus Nusslet number increases as twisted ratio decreasing as
compared with plain tube. Akeel Abdullah [2011] studied experimentally the heat
transfer enhancement and pressure drop in turbulent flow of air for Reynolds number
range=5000 to 23000 in a horizontal circular tube under constant wall heat flux
condition fitted with combined conical-ring tabulators and a twisted-tape swirl
generator. It noticed from experimental results that temperature values increases along
the tube length and decrease as twist ratio decreases while the average Nusslet
number increases as Re increases and decreasing as twisted ratio decrease in case of
combined twisted tape and conical ring. The results showed a significant enhancement
in heat transfer process with conical ring tabulator than empty plain tube and much
better enhancement in case of combined twisted tape and conical ring. It's noticed that
the fanning friction factor decreases as Re increases and the values of friction factor
become higher when using conical ring in combined with twist tape than using
conical ring alone and especially at smaller twisted tapes ratio due to increase swirl
flow which leads to higher contact between secondary flow and tube wall. He also
predicted new empirical relationships for Nusslet number and friction factor for
combined conical ring and twisted tape. Esmaeilzadeh et al. [2014] studied
experimentally the characteristics of heat transfer and friction factor enhancement of
ɣ- /water nanofluid in laminar flow region flowing through uniform heated circular tube fitted with twisted tapes inserts with various thicknesses. They noticed
from results that the performance of convective heat transfer becomes better with the
addition of ɣ- /water nanofluid compared with water and the values of
convection heat transfer coefficient increases with increasing volume concentration
Heat Transfer Augmentation of Laminar Nanofluid flow in Horizontal Tube Inserted with
Twisted Tapes
http://www.iaeme.com/IJMET/index.asp 227 [email protected]
and becomes much higher when used in combined with twisted tape inserts,
especially with the thicker twisted tape. They indicated that the friction factor values
increases as twisted tape thickness increases when these values compared with the
friction factor values for pure water or with nanofluid, thus the use of twisted tape
inserts leads to larger surface contact and reduction the free flow area causes high
speed swirl flow and increasing the pressure drop.
Many researches attempt to improve the efficiency of heating systems and heat
exchanger by selecting different, geometries, working fluids or operational mode and
boundary condition. To overcome these problems, nanofluids and swirl flow devices
insert are the best techniques used to reduce size and costs of heat exchangers and
achieve a high heat transfer rate with minimum pumping power. Therefore, in this
study the effect of changing the twist ratio, thickness and cutting shape of twisted tape
with the flowing of CuO nanofluids at different volume concentrations through
horizontal uniform heated tube will be investigated in order to get to the desired
efficiency for heat transfer enhancement with less friction losses.
2. EXPERIMENTAL APPARATUAS AND PROCEDURE
2.1. Test Rig Description
Straight copper tube with 14.2mm inner diameter, 0.9mm thickness and 1000mm
length was used as the test section. Ten thermocouples were soldered, five on the
outer upper and five on outer lower surface tube along the test section in opposite
position with an equal distance between them in order to increasing the temperatures
readings accuracy. Thermocouples heads were well insulated. A 0.5mm thickness
asbestos heat resistance insulation wrapped around the tube to electrically isolated it
from the heater coils. Electrical heater coils with rating 1000W, resistivity 4.9
ohm/meter, (2*0.16) mm cross section and 3200mm length are wound tightly around
the tube to heated the test section with the desired heat flux by connecting it to a
Variac voltage transformer that supply an electric AC power to regulate the input
voltage across the heater coils to give a constant heat flux boundary condition along
the test tube. The test tube covered with a layer of fire resistance asbestos insulation
(30mm width and 5mm thickness) and another layer of fiberglass insulation with
50mm thickness to prevent heat losses. Two 4mm pressure taps inserted at the inlet
and outlet of the test section. The test tube has an entrance length before section part
and it's long enough to make sure that the flow is hydro dynamically fully developed
when it's enter the heated section. Figure (1) shows a 3D schematic diagram of
experimental test rig. Figure (2) shows a photograph for the test rig.
2.2. Twisted Tapes Geometries
Twisted tapes were made from copper straight tape with length 1m and width 12mm.
all the types and dimensions of twisted tapes used during the experimental work are
demonstrated in table (1).
Twisted tape manufacturing by clamping one end of the tape and twisting the
other end carefully to reach to the desired twist ratio. The two different cutting shapes
along the twisted tape edge has been extruded out by manufactures special pieces for
each cut shape and then these tapes inserted inside the core tube along the test section
by moving passage (flange) equipment at the end of the test tube. Figure (3) shows the
geometrical details of twisted tapes.
Prof. Dr. Qasim S. Mahdi and Noor AM.Mohammed
http://www.iaeme.com/IJMET/index.asp 228 [email protected]
2.3. Data Deduction
Heat transfer calculation
The electric power applied at the tube wall to achieve heating effect was determined
by:
where, I is the electric current d and V the electric voltage deliverers through
heater coils. Assumed well-insulated outer surface tube (no heat losses) therefore the
energy heat transfer to be absorb by the fluid:
where, ṁ is the mass flow rate of water through the horizontal tube, Cp the specific heat
of water and are the inlet and outlet water temperatures for the test tube.
In order to calculate the average heat transfer coefficient inside the tube the well-
known Newton’s law of cooling used as follows, Holman and John [2010]:
where, is the average heat transfer coefficient inside tube, the surface area calculated
from:
and the mean fluid temperatures estimated by:
surface temperature Sami .et al. [2014].
then, the average inner Nusselt's number (Nu) calculated as:
The Internal flow for heated tube is laminar fully developed flow and the Reynold
number values are ranging from 290 to 2000 and its estimate by the relation:
)
The thermal resistant
value across tube wall is
too small thus, the inner surface temperature equal to the outer surface temperature
[ .
Friction factor
The friction factor coefficient ( ) which is related to the pressure drop ( ) across
the heated tube length can be calculated by equation:
where:
Heat Transfer Augmentation of Laminar Nanofluid flow in Horizontal Tube Inserted with
Twisted Tapes
http://www.iaeme.com/IJMET/index.asp 229 [email protected]
where, is the average velocity inside the tube, is the Cross section area of
tube. The thermal conductivity ( ), the dynamic viscosity ( ) and the density of
base fluid are based on (mean bulk temperature).
3. RESULTS AND DISCUSSION
Experimental results for local Nusselt number and friction factor for smooth tube
were compared with the well-known Shah's correlation for the constant heat flux
condition in tubes and Hagen-Poiseuille's equation for pressure drop in laminar flow
respectively to sureness the accuracy of the experimental work results as shown in
figures (4) and (5). The results showed logical agreement with the results from the
mentioned equations.
3.1. Effect of Twist Ratios of Twisted Tape
The variation of average Nusslet number and friction factor at different water
Reynold's number for twisted tape ratios (Y=2.6 and 5.3) are clarified in figures (6)
and (7). From the resulting curves, it can be realized that the enhancement of heat
transfer in terms of average Nusselt number increases by decreasing the twist ratio
and by increasing the Reynold number value. The strength of swirling flow generated
from the twist of the tape depends on the twist ratio thus the swirling flow generates
from lower twist ratio is much higher than for higher twist ratio. Lowering the twist
ratio of twisted tape generates a stronger swirling flow which increases the turbulent
intensity of the main flow in order to improving the viscous boundary layer mixing
near the inner tube wall to augment the heat transfer process. Nusselt number for the
present study improved by 50.6% for Y=5.3 and by 55% for Y=2.6 at Re= 1923 than
for smooth tube case. In basic case for smooth tube the friction factor decreases as Re
increases due to the increasing in pressure drop, but it's found to be with inserting
twisted tape there is a considerable augmented in friction losses and whenever the
twist ratio decreased the friction factor increased and become much higher than for
smooth tube at the same values for Re due to the increasing in shear forces near the
tube wall. The friction factor for the present results increased by 38.47% for Y=2.6
and by 27% for Y=5.3 than those for smooth tube case.
3.2. The Effect of Twisted Tape Thickness
Figure (8) clarify the variation of heat transfer enhancement in terms of average
Nusselt number for twisted tapes thicknesses (t=1mm and 2mm) at different Reynold
number. It's clear from resulting figures that the twisted tape thickness has a
significant effect on the heat transfer process and as the tape thickness increases the
Nusselt number increases. Increasing the tape thickness narrowing the swirling path
flow that enhances the tangential velocities for better mixing to the viscous boundary
layer near the tube wall region. In addition, the tape edge effect which acting like a fin
dissipates heat from the inner tube surface to the working fluid thus increasing this
area improving the convective heat transfer. Nusslet number increased by 62% for
t=2mm and by 54% than t=1mm and for smooth tube respectively, while there was
increasing in friction losses by 23% for twisted tape at t=2mm comparing to twisted
tape at t=1mm as shown in figure (9).
Prof. Dr. Qasim S. Mahdi and Noor AM.Mohammed
http://www.iaeme.com/IJMET/index.asp 230 [email protected]
3.3. The Cut Shape Effect of Twisted Tape
The effects of triangular cut twisted tape (TCTT) and the semicircular cut twisted tape
(SCTT) on the variation of average Nusselt number and friction factor are clarified in
figures (10) and (11). As shown, at the same operation condition the heat average
Nusselt number is much higher for TCTT by 4% and by 6% than SCTT and typical
twisted tape respectively and by 65% than for smooth tube case. Generally, the typical
twisted tape (TTT) generates only a swirling flow but in case of adding several cuts
along the twisted tape edge it will produces many local vortices at each cutting section
that provides an excellent mixing for the viscous boundary layers in all direction
along the tube leads to higher improvement in heat transfer enhancement. Also, the
heat transfer enhancement rate depends on the cutting shape which controls the
strength of vortex generated which mean the vortices preformed behind the triangular
cut is more stronger than those preformed through the semicircular cut. On the other
hand, the friction factor enhances more with TCTT by13 % than SCTT and by 27%
than TTT because the addition of these local vortices promotes an additional shear
stress due to increasing in flow mixing between the viscous boundary layers of fluid
at the tube wall and twisted tape.
3.4. Combined Effect of Twisted Tape and Nanofluid on Heat Transfer
Figures (12) and (13) clarified the variation of average Nusslet number ( ) and
friction factor of CuO nanofluid for = 0.08% and 0.35% volume concentrations, flowing at various Reynold numbers through the inserted tube with triangular cut
twisted tape (TCTT) and typical twisted tape for 2.6 twist ratio and 1mm thickness.
It's observed that the heat transfer enhancement reached the highest level during this
study through the joint use of TCTT with CuO nanofluid. Where, the increased by
8% than TCTT without CuO nanofluid and by 22%, 21% than TTT with CuO
nanofluid for the same operation condition and volume concentrations. Also, this
enhancement increases with increasing both Reynold number flow and nanoparticles
volume concentration. Random motion of nanoparticles even at low volume
concentration become more active in convective heat transfer and accelerated due to
swirling flow and local vortices generated by the TTT and TCTT that provides perfect
mixing for the viscous layer for the working fluid. The friction factor for TCTT with
CuO nanofluid increased by 2% for = 0.08% and by 3% for = 0.35% than TCTT
without CuO nanofluide and for the TTT with CuO nanofluid increased by 1.5% for
=0.08 and by 2.7% for =0.35% than TTT without CuO nanofluid for the same operation conditions. The friction losses along the tested tube increases due to
increasing in the turbulent intensity of the flow that accelerating nanoparticles motion
through the swirling flow, which enhances shear stress forces near the inner tube wall.
4. DEVELOPING OF EMPIRICAL EQUATION
The Nusselt number and friction factor experimental results have been correlated by
the following equations:
Nusselt number and friction factor correlation for Twist ratio:
= 1.1 (16)
= 868.8 (17)
Valid for 451<Re< 2100, 2.6<Y<5.3, 5.34<Pr<7.1.
Nusselt number and friction factor correlation for TTT thickness:
Heat Transfer Augmentation of Laminar Nanofluid flow in Horizontal Tube Inserted with
Twisted Tapes
http://www.iaeme.com/IJMET/index.asp 231 [email protected]
= 0.515 (18)
= 294.43 (19)
Valid for 451<Re< 2100, 1mm <t< 2mm, 5.34<Pr<7.1.
Nusselt number correlation for TCTT with nanofluids:
= 0.943 (1+we/ (1+ (20)
Valid for 286<Re< 1772, 0 <de/ < 1.272, 5.34<Pr<7.1, 0 <we/ < 1.363, 0< <0.35.
5. CONCLUSIONS
Generally observation, high heat transfer enhancement and pressure drop occurs with
using the twisted tapes which is depends on the twist ratio, thickness and the cutting
shape for twisted tapes. Twisted tape twist ratio and thickness have greater impact on
heat transfer enhancement instead of adding different cutting shape on twisted tape
body, where increased by 54% and 38% with increasing thickness and twist ratio,
while increased by 27% with adding triangular cutting shape comparing with TTT
with the same dimensions. Triangular cuts showed better performance in heat transfer
enhancement than semicircular cuts which confirms that the fluid velocity accelerates
more through sharp cuts. The CuO nanofluid give better performance on heat transfer
enhancement when it's flowing through the inserted tube with twisted tape than the
flowing in smooth tube and this performance become more efficient with increasing
the nanoparticles volume concentration and Reynold number. Where the highest value
for was for TCTT with 0.35% volume concentration of CuO nanofluid.
Table 1 Characteristic dimensions of the twisted tapes inserted tubes
Insert set Revolution
No.
Thickness
mm
(t)
Pitch(H)
(Y*di)
mm
Width
mm
Twisted
ratio
(Y)
Metal Cut
dimension
Typical twisted
tape (TTT) 30 1 37 12 2.6 Copper ………..
Typical twisted
tape (TTT) 30 2 37 12 2.6 Copper ………..
Typical twisted
tape (TTT) 15 1 75 12 5.3 Copper ………..
Triangular cut
twisted tape
(TCTT)
30 2 37 12 2.6 Copper
width cut
(we=4mm)
depth cut
(de=3mm)
Semicircular cut
twisted tape
(SCTT)
30 2 37 12 2.6 Copper Radius cutting
(re=4mm)
Table 2 Properties for the two types of the nanosized particle at temperature 25C0
Al2O3 Cuo Property
710 535 Cp(kJ/kg.K)
3700 6400 ρ(kg/m3)
46 69 K(W/m.K)
180 120 α x10-7(m2/s)
80 40 Di (nm)
Prof. Dr. Qasim S. Mahdi and Noor AM.Mohammed
http://www.iaeme.com/IJMET/index.asp 232 [email protected]
Figure 1 3D schematic diagram of experimental test rig
1) Test section, 2) Water chiller system, 3) Insulated cold water tank, 4) flow meter, 5)
Thermocouples, 6) Valves, 7) Variac, 8) Temperature, 9) water pump.
Figure (2) Photograph for experimental test rig
Thermometer
Manometer
Variac
Heat Transfer Augmentation of Laminar Nanofluid flow in Horizontal Tube Inserted with
Twisted Tapes
http://www.iaeme.com/IJMET/index.asp 233 [email protected]
0
5
10
15
20
25
0 10 20 30 40 50 60 70
Present Experimental Work Shah Equation [1978]
q=6725 W/𝑚
Figure 3 Twisted tapes types and geometries
Figure 4 Comparison between experimental work and Shah Equation at =6725 W/ ,
Re=944
Prof. Dr. Qasim S. Mahdi and Noor AM.Mohammed
http://www.iaeme.com/IJMET/index.asp 234 [email protected]
Figure 5 Comparison between experimental work and Hagen equation at =6725 W/ ,
Re=944
Figure 6 Variation of Nusselt number with Reynold number for different twist ratio at =
10000 W/
0
0.05
0.1
0.15
0.2
0.25
0 500 1000 1500 2000 2500
Fric
tio
n f
acto
r
Reynold number
Hagen-Poiseuille equation[1996]
Present Experimental Work
0
5
10
15
20
25
30
35
40
45
50
0 500 1000 1500 2000 2500
Ave
rage
Nu
sse
lt n
um
be
r
Reynold number
Tube with TTT at T.W=2.6
Tube with TTT at T.W=5.3
smooth Tube
Heat Transfer Augmentation of Laminar Nanofluid flow in Horizontal Tube Inserted with
Twisted Tapes
http://www.iaeme.com/IJMET/index.asp 235 [email protected]
Figure 7 Variation of friction factor with Reynold number for twisted tapes at different twist
ratios at = 10000 W/
Figure 8 Variation of Nusselt number with Reynold number for twisted tapes at different
thicknesses at = 10000 W/
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0 500 1000 1500 2000 2500
Fric
tio
n f
acto
re
Reynold number
Tube with TTT at T.W=2.6
Tube with TTT at T.w=5.3
smooth Tube
0
10
20
30
40
50
60
0 500 1000 1500 2000 2500
Ave
rage
Nu
sse
lt n
um
be
r
Reynold number
Smooth Tube
Tube with TTT at thickness =1mm
Tube with TTT at thickness =2mm
Prof. Dr. Qasim S. Mahdi and Noor AM.Mohammed
http://www.iaeme.com/IJMET/index.asp 236 [email protected]
Figure 9 Variation of friction factor with Reynold number for different thicknesses at =
10000 W/
Figure 10 Variation of Nusselt number with Reynold number for different cut shape of
twisted tapes at = 10000 W/
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0 500 1000 1500 2000 2500
Fric
tio
n f
acto
r
Reynold number
Smooth Tube
Tube with TTT at thickness =1mm
Tube with TTT at thickness =2mm
0
10
20
30
40
50
60
70
0 500 1000 1500 2000 2500
Ave
rage
Nu
sse
lt n
um
be
r
Reynold number
Tube with TTT
Tube with SCTT
Tube with TCTT
Heat Transfer Augmentation of Laminar Nanofluid flow in Horizontal Tube Inserted with
Twisted Tapes
http://www.iaeme.com/IJMET/index.asp 237 [email protected]
Figure 11 Variation of friction factor with Reynold number for different cut shape of twisted
tape at = 10000 W/
Figure 12 Variation of Nusselt number with different volume concentrations of CuO
nanaofluid with TTT and TCTT
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0 500 1000 1500 2000 2500
Fric
tio
n f
acto
r
Reynold number
Tube with TTT
Tube with SCTT
Tube with TCTT
0
10
20
30
40
50
60
70
0 500 1000 1500 2000 2500
Ave
rage
Nu
sse
lt n
um
be
r
Reynold number
TCTT with DI-water
TTT with DI-water
TCTT with CUO 0.08%
TTT with CUO 0.08%
TCTT with CUO 0.35%
TTT with CUO 0.35%
Prof. Dr. Qasim S. Mahdi and Noor AM.Mohammed
http://www.iaeme.com/IJMET/index.asp 238 [email protected]
Figure 13 Variation of friction factor with different volume concentrations of CUO nanofluid
with TTT and TCTT
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0
0.05
0.1
0.15
0.2
0.25
0.3
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NOMENCLATURE
Cross section area, Average wall temperature,
Surface area, Mean bulk fluid temperature, Specific heat, kJ/kg.K Inlet fluid temperature,
de Cutting depth, m Inner surface tube temperature, Inside tube diameter, m Outer surface tube temperature, Outer tube diameter, m Average inlet velocity, m/sec
Friction factor V Electric volte, Voltage H Pitch, m w Width, m
Inside heat transfer coefficient, W/ . we Cutting width, m I Electric current, Amp Y Twist ratio
K Thermal conductivity, W/m.K Z Axial distance, m L Length, m Viscosity, kg/m.sec Mass flow rate, kg/sec Density, kg/
Nu Nusselt number Volume concentration of nanofluid
Average Nuseelt number
Pr Prandtl number
Pressure drop, Pas
Q Electric power, Watt Adsorbed heat energy, Watt
Heat flux, W/
Re Reynold number
re Cutting Radius, m
t Thickness, m
Surface temperature,
Outlet fluid temperature,