International Journal of Dynamics of Fluids.
ISSN 0973-1784 Volume 13, Number 2 (2017), pp. 295-308
© Research India Publications
http://www.ripublication.com
Numerical Investigation of Heat Transfer Enhancement
and Pressure Drop of a Double Tube Heat Exchanger
with Rectangular Fins in the Annulus Side
N Sreenivasalu Reddy *
Rajarajeswari College of Engineering, Bengaluru, Karnataka, India.
K Rajagopal
Sri Krishnadevaraya University, Anantapuramu, Andhra Pradesh, India.
P H Veena
Smt.V.G.College for Women, Gulberga, Karnataka, India.
Abstract
In the present study the design and performance of double pipe heat exchanger
with straight rectangular fins in the annulus side are analyzed numerically.
Computational fluid dynamics (CFD) model using free open source code has
been performed to study the fluid flow, heat transfer coefficient and pressure
drop in the annulus side of double pipe heat exchanger for different
configurations. A numerical investigation is carried out for different values of
mass flow rate and varying the number of fins. The numerical results first
validated with experimental results for a simple double pipe heat exchanger.
Then the CFD model results have been validated with rectangular fins. The
results of rectangular fins in the annulus side causes increased rate of heat
transfer and pressured drop compared to plain double pipe heat exchanger.
The numerical study is performed by varying mass flow rate of cold fluid in
the annulus side and kept mass flow rate of hot fluid in the inner pipe is
constant. The performance and increased pressure drop is a function of
number of fins and mass flow rate.
296 N Sreenivasalu Reddy, K Rajagopal and P H Veena
NOMENCLATURE
A heat transfer surface area (m2)
Cp Specific heat, KJ/kg oC
U Overall heat transfer coefficient KW/m2 oC
Q Heat transfer rate, KW
T Temperature, oC
M Mass flow rate, kg/s
LMTD Log mean temperature difference
K Thermal conductivity, KW/m oC
SIMPLE Semi Implicit method for pressure linked equations
INTRODUCTION
Double pipe heat exchangers have an important role in various engineering process
application. A double pipe heat exchanger consists inner pipe and outer pipe. Heat
flows between two fluids, which are flowing in inner and outer pipes respectively.
The fluids may flow in parallel or counter flow direction. Double pipe exchangers are
used in applications involving low volume flow rate. Recently there are many studies
have been done in improvements, enhance ment heat transfer rate, Yang et al. [1],
Akpinar [2], Ma et al. [3], their results in low cost, raising thermal rating and life of
the equipment.
Pourahmad and Pesteei [4] studied on double pipe heat exchanger by providing curvy
strip turbulators in the inside pipe, their findings are on improvements in enhancement
of heat transfer characteristics. Ibrahim [5] investigated the increase of laminar flow
and heat transfer plane tubes with helical tape inserts. Results of permeable baffles
and flow pulsation on a concentric tube heat exchanger effectiveness was studied by
Targui and Kahalerras [6], the authors proposed that addition of rotary machinery in
the inner pipe increases the heat transfer. An analysis of using plain and perforated
variable spacing with helical tabulators was studied by Sheikholeslami et al. [7], heat
transfer and fluid flow analysis were carried out for different area ratios and pitch
ratios. Results shows that effectiveness is an increasing function of open area ratio
and decreasing function of pitch ratio. Recently the fast growth of various
computational methods on double heat exchanger is done based on methods [8, 9].
These methods are complement to experiments and theory. Gorman et al. [10]
presented a numerical method for the thermal design. A detailed review was carried
out by Ahmed et al. [11], on star shaped finned tube heat exchangers. Further the heat
transfer enhancement methods are studied in [12-20].
Numerical Investigation of Heat Transfer Enhancement and Pressure Drop… 297
In the present paper a detailed numerical study of heat transfer for a water to water
concentric tube exchanger with rectangular fins at the annulus side is carried out. In
the literature baffles are almost used in shell and tube heat exchanger to increase heat
transfer rate and to decrease pressure drop, and no publication studying on thermo
hydraulic performance in the annulus side of the concentric tube heat exchanger could
be found. The mass flow rate in the inner pipe is kept constant as in conventional
concentric tube heat exchanger. The experiments were conducted for different cases
of number of fins in the annulus side. The numerical results of simple plain tube are
validated with experimental results.
NUMERICAL METHOD
Geometry Details
The main objective of this research is to compare different configuration of
rectangular fins in the annulus side of concentric tube heat exchanger. Addition of
these fins changes the pressure and velocity distribution along the annulus side of the
heat exchanger and thus changes in heat transfer and pressure drop. The configuration
of rectangular finned heat exchanger is as shown in Figure 1. The number of
rectangular fins varied from 6 to 8. In modified configurations the effect of geometric,
flow and thermal variables are numerically studied. Though the geometry is simple,
its numerical thermo fluid study is complex because of the flow regime in annulus
side.
Figure 1. Model of Inner Pipe of Double Pipe Heat Exhanger with 8 Fins and 6fins
The hot water flows in the inner tube while the cold water flows in the annulus side of
the double pipe heat exchanger. The material of the heat exchanger parts is copper
and its thermal conductivity is 401 W/m-k. Water is taken as a Newtonian and
incompressible fluid with constant thermo physical properties. In addition, the fluid is
considered as laminar and steady state. The viscous heating and compression work
298 N Sreenivasalu Reddy, K Rajagopal and P H Veena
terms are neglected in the energy equation. The heat exchanger is assumed newly
fabricated and fouling resistance is not considered. In the present study the parts of
the heat exchangers are modeled using Solid Edge software. The unstructured mesh is
generated using ANSYS Fluent software. And it is solved in open source codes
(OpenFoam).
Domain Definitions, Mess Sensitivity and Boundary conditions
In the present study there are three double pipe heat exchangers are modeled. One
simple unfinned double pipe heat exchanger, second double pipe heat exchanger with
6 fins, and third double pipe heat exchanger is with 8 fins. For each of the three
studied heat exchangers, three domains are defined. Two fluid domains (water in the
inner tube and water in the annulus side) and one solid domain (copper wall with
rectangular fins). The domains are meshed with a mix of unstructured tetrahedral and
prism grid. To ensure the accuracy of the results, the mesh sensitivity test was
conducted for 6 finned and 8 finned double pipe heat exchanger. The boundary
condition of no slip is set for all the solid walls. Zero heat flux is set for annulus side
wall, the walls of the inner tube and fins have the boundary condition of coupling heat
transfer. These walls considered as solid fluid interfaces between two fluid domains
and the solid domain. The inlet boundary conditions for the inner tube side and
annulus are set as mass inlet, the out let boundary conditions are set as pressure out let
is zero, so that inlet pressure is equal to pressure drop on both the inner tube side and
annulus. The open source codes are used to calculate fluid flow and heat transfer in
the computational domains. The governing equations are solved by finite volume
formulation with conjugate heat transfer SIMPLE algorithm. The numerical solution
is based on continuity, momentum and energy equations.
DATA REDUCTION
For the temperatures deviations, a log mean temperature difference (LMTD)
𝐿𝑀𝑇𝐷 = [(Tw,h,in−Tw,c,in)−(Tw,h,out−Tw,c,out)]
ln [Tw,h,in−Tw,c,in
Tw,h,out−Tw,c,out]
(1)
𝐿𝑀𝑇𝐷 = [(Tw,h,in−Tw,c,out)−(Tw,h,out−Tw,c,in)]
ln [Tw,h,in−Tw,c,out
Tw,h,out−Tw,c,in]
(2)
Numerical Investigation of Heat Transfer Enhancement and Pressure Drop… 299
For parallel flow and for counter flow is used. Heat transferred to the cold water in the
annulus, Qw,c, can be determined from
Qw, c = mw, c Cp, w(Tw, c, out – Tw, c, in) = UOAOLMTD (3)
where mw, c is the mass flow rate of cold water which passing through the annulus, Uo
is heat transfer coefficient, Ao is the surface area of the outside diameter of the inner
pipe, Cp, w is the specific heat of cold and hot water, Tw, c, in and Tw, c, out are the inlet
and outlet temperatures of cold water.Heat transferred from the hot water in the inner
pipe , Qw,h, can be determined as
Qw, h = mw, h Cp, w(Tw, h, in – Tw, h, out) = UiAiLMTD (4)
where mw, h is the mass flow rate of hot water which passing through the inner tube of
heat exchanger, Ui is heat transfer coefficient, Ai is the surface area of the inside
diameter of the inner pipe, Cp, w is the specific heat of cold and hot water, Tw, h, in and
Tw, h, out are the inlet and outlet temperatures of hot water.
The average heat transfer rate, Qave, is determined from the hot water side and cold
water side as
Qave = Qw,c+Qw,h
2 (5)
Qavg = UoAo LMTD (6)
The overall heat transfer coefficient Uo based on outer surface area of the inner pipe
can be determined as per the energy balance equation t, with negligible heat losses to
surroundings, from equations (1) and (2):
RESULTS AND DISCUSSION
To validate the numerical results a comparison is made with experimental data to
evaluate heat transfer for simple double pipe heat exchanger. Validation is done for a
hot water inlet temperature of 55 oC and 65 oC by varying the mass flow rate, from
0.01 kg/s to 0.03 kg/s in the annulus side of the heat exchanger, keeping constant
mass flow rate of 0.01 kg/s in the inner pipe. Validation of rate of heat transfer and
heat transfer coefficient at inlet temperature of 65oC for counter flow direction with
plain tube as shown in figure 2 and Figure 3 respectively. The numerical results are
good agreement with experiment results. Therefore it is concluded that present
300 N Sreenivasalu Reddy, K Rajagopal and P H Veena
numerical model produce a good prediction for heat transfer characteristics and
pressure drop.
Figure 2. Validation of rate of heat transfer at inlet temperature of 65oC for counter
flow direction with plain tube
Figure 3. Validation heat transfer coefficient at inlet temperature of 65oC for counter
flow direction with plain tube
Figure 4 and 6 shows the variation of rate of heat transfer in the annulus and mass
flow rate in the annulus side of different configuration of double tube heat exchanger
for counter flow direction at inlet temperature of 55 oC and 65 oC respectively. It can
be seen from the Figure 4 that maximum deviation for the heat transfer in the annulus
side is 12 %. Heat transfer coefficient on the annulus side is determined by Newton’s
law of cooling from the temperature field. Figure 5 and 7 shows the variation of heat
transfer coefficient using plain tube, 6 fins and 8 fins at inlet temperature of 55 oC and
65 oC respectively. It can be seen that there is an increment in heat transfer coefficient
as the increasing mass flow rate.
0
100
200
300
400
500
600
0 0.01 0.02 0.03 0.04
Rat
e o
f H
eat
tra
nsf
er
Q
Wat
ts
Mass flow rate Kg/s
Experimental plaintube Thi=65CNumerical Plain Tube
0
100
200
300
400
500
0 0.01 0.02 0.03 0.04
He
at t
ran
sfe
r co
eff
icie
nt
w/m
2 o
C
Mass flow rate Kg/s
Experimental plaintube Thi=65CNumerical Plain tube
Numerical Investigation of Heat Transfer Enhancement and Pressure Drop… 301
Figure 4. The variation of rate of heat transfer at inlet temperature of 55 oC
Figure 6. The variation of rate of heat transfer at inlet temperature of 65 oC
0
50
100
150
200
250
300
350
400
450
500
0 0.01 0.02 0.03 0.04
Rat
e o
f H
eat
tra
nsf
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Q W
atts
Mass flow rate Kg/s
Numerical Plain Tube
numerical 6 fins
Numerical 8 fins
0
100
200
300
400
500
600
700
0 0.01 0.02 0.03 0.04
Rat
e o
f H
eat
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nsf
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Q W
atts
Mass flow rate Kg/s
Numerical Plain Tube
Numerical 6 fins
Numerical 8 fins
302 N Sreenivasalu Reddy, K Rajagopal and P H Veena
Figure 5. The variation of heat transfer coefficient at inlet temperature of 55 oC
Figure 7. The variation of rate of heat transfer coefficient at inlet temperature
of 65 oC
From the above discussion it is concluded that numerical model as a reasonable
precision the same model is used for a similar double pipe heat exchanger with
rectangular fins. As it is mention in the boundary conditions, both side out lets are
zero pressure, therefore the pressure drop is equal to inlet pressures for both annulus
and inner pipe side. Variation of pressure drop in the annulus side and mass flow rate
of cold water as shown in Figure 8. The results obtained for 6 fins and 8 fins are same
trend that of plain tube. It is very clear from Figure 8 that the rise in the pressure drop
is a function of mass flow rate in annulus side. As it can be observed that there is an
increment in pressure drop as the mass flow rate increases.
0
100
200
300
400
500
600
700
0 0.01 0.02 0.03 0.04
He
at t
ran
sfe
r co
eff
icie
nt
w/m
2
oC
Mass flow rate Kg/s
Numerical Plain tubeNumerical 6 finsNumerical 8 fins
0
100
200
300
400
500
600
700
0 0.01 0.02 0.03 0.04
He
at t
ran
sfe
r co
eff
icie
nt
w/m
2 o
C
Mass flow rate Kg/s
Numerical Plain tube
Numerical 6 fins
Numerical 8 fins
Numerical Investigation of Heat Transfer Enhancement and Pressure Drop… 303
Figure 8. The variation of pressure drop at inlet temperature of 55 oC
Figure 9. The temperature distribution at inlet temperature of 65 oC using 6 fins
Figure 9 and 13 shows the temperature distribution at the center plane of the heat
exchanger, for a counter flow direction at a mass flow rate of 0.01 kg/s using 6 and 8
fins respectively.
The velocity stream lines for 6 fins and 8 fins are as shown in Figure 14 and 17
respectively. It can be observed that the flow patterns are in the annulus side is
irrotational. Figure 12 shows that flow patterns in the annulus side. Increasing the
velocity of the fluid is the one of the most enhance heat transfer performance for the
same mass flow rate. Out of three studied without fin, 6 fins and 8 fins the highest
velocity in the annulus side is located at the near the inlet at the entrance region. This
is because of the sudden decreasing flow area, velocity of the annulus have to increase
0
2
4
6
8
10
12
0 0.01 0.02 0.03 0.04
Pre
ssu
re d
rop
in P
a
Mass flow rate Kg/s
plain tube Thi=65C6 fins counterl flow8 fins
304 N Sreenivasalu Reddy, K Rajagopal and P H Veena
to keep mass flow rate through annulus side constant. Figure 16 shows turbulent
kinetic energy contours, the maximum values or identified near to the entrance region.
The highest value of turbulence and velocity at the entrance region indicate that large
local pressure drop could be generated. The distribution of pressure, using 6 fins and 8
fins at the mass flow rate of cold fluid is 0.01 kg/s in the annulus side for a inlet hot
water temperature of 65 oC as shown in figure 10 and 15 respectively.
Figure 10. The pressure variation at inlet temperature of 65 oC using 6 fins
Figure 11. The velocity distribution at inlet temperature of 65 oC using 6 fins
Numerical Investigation of Heat Transfer Enhancement and Pressure Drop… 305
Figure 12. The velocity vector at inlet temperature of 65 oC using 6 fins
Figure 14. The streamlines at inlet temperature of 65 oC using 6 fins
Figure 16. The variation turbulence kinetic energy distribution at inlet temperature of
65 oC using 6 fins
306 N Sreenivasalu Reddy, K Rajagopal and P H Veena
Figure 13. The temperature distribution at inlet temperature of 65 oC using 8 fins
Figure 15. The pressure variation at inlet temperature of 65 oC using 8 fins
Figure 17. The streamlines at inlet temperature of 65 oC using 8 fins
Numerical Investigation of Heat Transfer Enhancement and Pressure Drop… 307
CONCLUSIONS
In this study CFD model is used to investigate and compare the performance of
double pipe heat exchanger with rectangular fins in the annulus side. The effects of
rectangular fins and mass flow rate are studied. The results shows that compared to
plain annulus side, using rectangular fins in the annulus side enhance the heat transfer
on average with increasing pressure drop equal on average to 2,5,11 time pressure
drop for a plain annulus side for plain tube, 6 fins and 8 fins respectively. The
increase in pressure drop because of entrance region effect. The heat transfer
enhancement makes the double tube heat exchanger more compact without changing
it size and weight. The highest thermo hydraulic performance is obtained when 8 fins
are used in a laminar flow region.
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