International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org
Volume 3, Issue 4, April 2014
ISSN 2319 - 4847
Experimental and Numerical Investigation for
Structural and Thermal Characteristics of
Externally Finned Double Pipe
Heat Exchanger
1
Sameer H. Ameen, 2Deyaa Mohammed N. Mahmood,
3
Laith Najim A. Alameer
1,2&3
Institute of preparation technical trainers
ABSTRACT
The heat exchanger of double pipes was constructed in the present paper from a copper alloy with inclined
parabolic fins fixed over the outer surface of its inner pipe with different angles. The experimental part of the current
work was carried out and it gave a coefficient of convection equal to 304.5 W/m 2.oC, while the numerical part
was achieved using ANSYS PACKAGE. Both structural and thermal investigations were considered in the
numerical analysis which required developing a factor was called "thermal to structural performance coefficient"
which was maximum at 60o inclination angle among other inclinations, namely, 24.72 W/MN. The Parabolic fins
improved the local heat convection to about 2.42 more than pipes without fins.
Keywords : structural, thermal, heat exchanger, fin.
Nomenclature
A Surface temperature of inner pipe ( m2 ).
D Outer diameter of inner pipe ( m ).
Cp Specific heat at constant pressure ( J/kg.oC ).
h
heat transfer coefficient ( w/m2.o C ).
L
Length of double pipes ( m ).
mo Mass flow rate ( kg/s ).
P
Pressure ( Pa ).
Q
Heat transfer ( W ).
Vo Volumetric flow rate ( m3/s ).
t
Thickness of pipe (m).
Tin Inlet temperature ( oC ).
Tout Outlet temperature ( oC ).
Ts
Surface temperature ( oC ).
T∞ Fluid temperature ( oC ).
σH Hoop stress ( N/m2 ).
σL Longitudinal stress ( N/m2 ).
ρ
Density ( kg/m3 ).
ζ
Thermal to structural performance coefficient ( W/MN ).
1. INTRODUCTION
High performance heat exchangers are widely used in many industries such as petrochemical industry, automotive
industry, energetic industry and many others. The name of this kind of heat exchangers is in consistency with their
structure. Large heat transfer area leads to high thermal efficiency of the device. Its working principle is to cool rapidly
large amount of gaseous or liquid medium. Because of its compact size, it is possible to use it for easy installation in
various systems, like the heating, drying, air conditioning and the other systems [1].
The reliability of several compact enhanced heat transfer designs including internally finned bayonet tube, internally and
externally finned bayonet tube and CW primary surface sheet was investigated. The results indicate that for the bayonet
tube, the high stress is caused by high temperature, while for the thin CW primary surface sheet; it is caused by big
pressure difference. The big stress exists in the joint of inner fins and inner tubes, so the welding quality of the joint must
be ensured [2]. A finite element solution for the strain as a function of the tube material and geometry was determined for
several geometries and a family of design curves for elliptical tubes was created [3]. The inner fin and inner tube should
Volume 3, Issue 4, April 2014
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International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org
Volume 3, Issue 4, April 2014
ISSN 2319 - 4847
not be welded together and the gap between them should be less than 1 mm for considered bayonet tube with 6 mm
annulus height according to the coupled consideration of heat transfer and stress performances [4].
The optimization of a finned double pipe with trapezoidal fins augmented to the outer surface of the inner pipe has been
carried out by using genetic algorithm and Trust Region Method [5]. The different heights of triangular fins were laminar
forced convection in the fully developed flow through the finned annulus subjected to boundary conditions of constant
heat transfer rate per unit axial length with uniform peripheral temperature distribution [6].
The four different types of fluids and for different fin heights and location were also can be carried out for heat exchanger
[7]. Helical fins were used on inner tube to generate some vortex along the centerline of the helical channel [8]. A
longitudinal finned vertical pipe was used to represent the air-heating vaporizer in the CFD model, where nitrogen gas
was used as the working fluid inside the vertical pipe to made flow upward [9].
Heat exchanger was numerically analyzed, using ANSYS 14 CFX software, by putting extended surfaces in the annular
space and superior pressure drop was obtained with fins [10].
2. THEORETICAL ANALYSIS
The heat exchanger considered is of two pipes, each pipe has two pressures ( inside and outside ); therefore from
elementary structural analysis which states a pipe is merely a thin tube without fins develop hoop ( at circumferential )
and longitudinal ( along pipe ) stresses under inner pressure only as follows [11] :
P.D
2t
P.D
=
4t
H =
L
(1)
(2)
The above equations don't give real results due to firstly the pressure loading is not only on the inner surface of the tube
but also on the outer surface too; and secondly the effect of fin is not taken into consideration. Generally, elementary
solution does not exist and needs more complicated analysis which is the ANSYS PACKAGE.
In thermal analysis, the heat transfer is done by convection using the two basic equations as follows :
Q = h A ( Ts - T∞ )
(3)
Q = mo Cp ( Tout – Tin )
(4)
Where,
A=ЛDL
(5)
mo = ρ Vo
(6)
3. CASE STUDY
The case study of the current work is a double pipe with parabolic fin on the outer surface of the inner pipe that has a
cross – sectional area with dimensions as shown in Figure 1. The distance between the two adjacent fins is taken 40 mm
in the direction perpendicular to the paper plain with 0.6 mm fin's thickness. The two pipes are made of copper alloy with
inner pressure for each pipe of 5 bar. Structural and thermal mechanical properties of the heat exchanger material are
listed in Table 1.
Figure 1 Dimensions of cross – sectional area of pipes (mm)
Modulus of
elasticity
GN/m2
110
Table 1 : Structural and thermal mechanical properties of copper alloy [12]
Coefficient
Yield
Thermal
Poisson's
Density
Of thermal
Stress
Conductivity
3
ratio
Kg/m
Expansion
MN/m2
W/m.oC
1/oC
0.34
8300
280
1.8*10-5
401
Volume 3, Issue 4, April 2014
Specific
Heat
J/kg.oC
385
Page 202
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ISSN 2319 - 4847
4. EXPERIMENTAL PART
The double pipe heat exchanger, of dimensions shown in Figure 1, is constructed from its main parts as clearly seen in
Figure 2 which has 2 m length and 2 mm thickness. Number of fins are 50 fins distributed over pipe length equally by 4
cm spacing distance. The experimental result which is required from the mentioned rig is parameter h, using the
equations 3 to 6.
The flowing of cold water in the inner pipe has 15oC measured temperature, while the hot water in the outer pipe has
45oC temperature is flown in the opposite direction of the inner pipe. The flow rate of the cold and the hot water is 4
lit/min for both pipes. The outer surface temperature of the inner pipe is measured to be equal to 25oC.
By substituting all measured temperatures and dimensions in equations 3 and 4 and after solving them, the convection
heat transfer coefficient is determined equal to 304.5 W/m2.oC.
Figure 2 Heat exchanger rig
5. NUMERICAL PART
5.1 Structural results
The numerical structural results of the inner pipe with 60o inclined parabolic one fin are shown in Figures 3, 4, 5, and 6 for
Von – Misses stress, Shear stress, total deformation, and strain respectively. The results are calculated by applying pressure
of 5 bar at the inner and the outer surfaces of the inner pipe including surface of parabolic fin. Other, numerical results with
different fin's angle ( 90o, 75o, 45o, 30o ) are evaluated and listed in Table 2 in addition to angle 60o .
Figure 3 Von-Misses stress for 60o inclined fin (Pascal)
Figure 4 Shear stress for 60o inclined fin (Pascal)
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Figure 5 Total deformation for 60o inclined fin (Pascal)
Figure 6 Elastic strain for 60o inclined fin (Pascal)
Angle
90o
Table 2 : Structural results of finned inner pipe with different angles
Von-Misses stress
Deformation
Shear stress
(MPa)
(mm)
(MPa)
Strain
64.13
2.67 * 10-5
36.16
0.00058298
57.39
2.61 * 10
-5
30.89
0.00052173
3.02 * 10
-5
31.17
0.00050220
-5
39.52
0.00063585
52.68
0.00084173
75
o
60
o
45
o
69.94
4.24 * 10
30o
92.59
6.57 * 10-5
55.24
5.2 Thermal results
The temperature distribution , total heat flux, and temperature gradient for the inner pipe with 60o inclination angle of
fin are shown in Figures 7, 8, and 9 respectively.
Figure 7 Temperature distribution for 60o inclined fin (o C)
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Figure 8 Total heat flux for 60o inclined fin (W/m2)
Figure 9 Temperature gradient for 60o fin's angle ( oC/m )
30 degree
3250
o
Temperature Gradient( c/m)
The path which is shown in Figure 9 along pipe in z – direction is selected to plot deviation of the temperature gradient
and the heat flux over the outer surface as shown in Figures 10 and 11 respectively for all inclinations of fin under study
in the current work. The heat flux for the first three adjacent fins at 60o inclination is seen in Figure 12.
The investigated thermal to structural performance coefficient ( heat flux to Von-Misses stress ratio ) for all inclinations
of fin are calculated and assessed as shown in Figure 13.
45 degree
60 degree
2750
70 degree
90 degree
2250
1750
1250
0
0.5
1
1.5
2
2.5
3
3.5
4
Fin's Pitch (cm)
Figure 10 Temperature gradient along fin's pitch ( oC/m )
Heat Flux (W/m 2 )
1400
1300
30 degree
1200
45 degree
1100
60 degree
70 degree
1000
90 degree
900
800
700
600
500
0
0.5
1
1.5
2
2.5
3
3.5
4
Fin's Pitch (cm)
Figure 11 Total heat flux along fin's pitch ( w/m2 )
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Volume 3, Issue 4, April 2014
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1500
1400
2
Heat Flux (W/m )
1300
1200
1100
1000
900
800
700
600
500
0
2
4
6
8
10
12
Pipe Length (cm)
Coeffecient of Structural to Thermal (W/MN)
Figure 12 Total heat flux along pipe length at 60o fin angle ( w/m2 )
24
22
20
18
16
14
12
10
30
40
50
60
70
80
90
Fin's Angle (degree)
Figure 13 Variation of ζ factor to fin's angle (W/MN)
6. DISCUSSION
In the current work for structural numerical investigation, four items are evaluated for five different inclination angles of
fin to investigate which angle gives a better structural performance of lower stresses and deformation developed in fin.
Maximum value is located at the middle near root of fin for Von – Misses stress, shear stress, and strain cases as
shown in Figures 3, 4, and 6 respectively; while the maximum deformation is seen on the tip of the parabolic fin as
shown in Figure 5.
Table 2 clearly shows minimum Von – Misses stress of 55.24 MN/m2 developed in the pipe at 60o fin's angle only which
represent lowest value than against angle by about 13.9 %, 3.7 %, 21.0 %, and 40.3 % for 90o, 75o, 45o, and 30o angles
respectively. Minimum deformation is located at 90o and 75o fin's inclinations which is approximately equal or slightly
more than 2.6 * 10-5 mm, and these values are in increased in sequence with reduction in fin's angle.
Maximum shear stress obtained at 30o angle while minimum value was at 60o and 75o angles of fins are about 31 MN/m2.
Minimum strain is determined at 60o fin's angle of 0.00050220 which represents the lowest amount among other angles
of 90o, 75o, 45o, and 30o by identical percentage reduction as for Von – Misses stress case.
Thermal solution for 60o fin's inclination, namely the temperature distribution, the total heat flux, and the temperature
gradient are shown in Figures 7, 8, and 9 respectively. Figure 7 shows the maximum temperature at the outer diameter of
the pipe of 43o C, while the temperature is 15oC in the inner pipe leaving intermittent temperature at approximately 23oC.
The maximum heat flux and the gradient of temperature are located at rounded edges at the root of fin with magnitude of
4.84 GW/m2 and 103120 o C/m as seen in Figures 8 and 9 respectively.
Figures 10 and 11 represent the values of the temperature gradient and the heat flux along the pipe ( as seen the path in
Figure 9 ) starting from root of fin for all combination of fin's angles under consideration.
Clearly, figures show also that existing fin in general improve and increase both items mentioned before to higher
amounts from 563 W/m2 heat flux, for 60o fin's angle, to about 1365 W/m2 with enhancement factor of 2.42. First three
adjacent fins affect the performance of heat exchanger convection and that is seen in Figure 12 for the same angle of
previous figure.
The thermal to structural performance coefficient ( which is equal to total heat flux to Von – Misses stress ) at the root of
fin as shown in Figure 13 with maximum value of 24.72 W/MN at 60o inclination angle of fin.
7. CONCLUSIONS
The major points concluded from present work are listed as follows :
1) All combinations of fin's angle have enhanced performance of heat exchanger concerning the heat flux and the
temperature gradient to about 2.42.
2) Heat exchanger inner pipe which has 60o fin's angle has proved to be the best among other angles due to its higher ζ
factor of 24.72 W/MN for both structural and thermal analyses.
Volume 3, Issue 4, April 2014
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International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org
Volume 3, Issue 4, April 2014
ISSN 2319 - 4847
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