International Journal of Advanced Research Methodology in Engineering & Technology, ISSN 2456 6446
Volume 1, Issue 3, May 2017
CFD Analysis and Experimental Investigation on
the Performance of Double Pipe Heat Exchanger
using Dimples
[1]
Mr. B. Vijayaragavan., [2]Mr. S. Rajasundar., [3]Mr. C. Raju., [4]Mr. M. Hari Hara Sudhan
[1]
Assistant Professor., [2,3,4]UG scholar.,
Department of Mechanical Engineering.,
Mepco Schlenk Engineering College., Sivakasi.
Abstract—Double pipe heat exchangers are widely used
in various heat transfer applications starting from oil refineries
to automobile radiators because of simplicity in design. The rate
of heat transfer in a double pipe heat exchanger can be increased
by using various heat transfer augmentation techniques out of
which dimples is identified as a passive method with least value of
pressure drop in comparison with other techniques. In the
present work, the performance of double pipe heat exchanger
with dimples of various shapes and configurations are
investigated using the CFD package ANSYS® FLUENT 16.0 and
the arrangement providing efficient heat transfer is identified
through CFD results and experimentally validated along with the
plain tube model. The inline arrangement with counter flow is
chosen for the study with dimple dimension of depth to diameter
ratio 0.26. Out of the various pitches ranging from 300mm to
100mm and dimple shapes considered such as hemispherical,
square, triangular and elliptical, the hemispherical dimpled tube
with a pitch of 150mm arranged in two rows is identified to be
the most efficient. The experimental results are in good
agreement with the CFD results and hence the studies show that
performance of double pipe heat exchangers can be enhanced
with the selected dimpled configuration which improves the heat
transfer rate by creating turbulence in the flow at a minimum
pressure drop.
Keywords—Heat Transfer, Dimples, CFD, Double pipe Heat
Exchanger, Pitch.
I.
INTRODUCTION
Heat exchangers are equipment that is commonly used to
transfer heat between two fluids at different temperatures. The
double pipe heat exchanger consists of two tubes that are
concentrically arranged. One of the fluid (either hot or cold
fluid) flows through the tube and the other through the
annulus. For a Double Pipe Heat Exchanger the flow may be
either parallel or counter flow. In the parallel, the flow
direction of the hot fluid will be the same as that of the cold
fluid. In the counter flow, the flow directions of the hot and
the cold fluids are opposite to each other. Wide range of
researches are already done to study the flow characteristics
and heat transfer in heat exchanger tubes [1]. Studies
concerning the fundamental characteristics of heat transfer
equipments and the usage of passive methods in double pipe
heat exchangers for heat transfer augmentation have been
frequently cited [2], [9], [10]. Out of the various types of heat
exchangers double pipe heat exchanger has drawn many
attentions due to its simplicity and wide range of usages [3].
Among various heat transfer enhancers, a dimpled surface
shows a high heat transfer capacity with relatively low
pressure loss penalty compared to other types of heat transfer
enhancement techniques that are available [4]. Heat transfer
rate was enhanced considerably compared to smooth channel
values by the use of dimples. Also this reveals remarkably low
pressure losses that are nearly one-half the magnitudes
incurred with protruding element [5]. Heat transfer rate was
enhanced considerably by using staggered arrangements of
dimple having depth to diameter ratio 0.26 [6]. The distance
between dimples can have a considerable effect on heat
transfer enhancement. Also it was revealed that the in-line
dimple configuration resulted in better performance compared
to staggered arrangement [7]. Dimples provide pressure drop
penalties which are smaller than other augmentation
techniques by not protruding into the flow and therefore by
not increasing losses due to form drag. CFD simulations can
provide good predictions by using realizable k-ε turbulence
model, because of its improved predictive capabilities
compared to the standard k-ε model and because of its ability
to resolve portions of complex flows located closed to the
surface [8]. The presence of solid baffles results in significant
heat transfer enhancement and also the associated increase in
the pressure drop and higher local thermal stress at the roof of
the baffle structure [12].
Specification of heat exchanger
Total length of the double pipe heat exchanger is taken as
1500mm. Inner and outer diameter of concentric tubes are
taken as 25.4mm and 50.8mm [13]. Copper is selected as
materials for inner tube owing to its high thermal conductivity
and polyvinyl chloride(PVC) is selected for outer shell.
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International Journal of Advanced Research Methodology in Engineering & Technology, ISSN 2456 6446
Volume 1, Issue 3, May 2017
II.
CFD analysis
Computational Fluid Dynamics (CFD) has been
performed based on a 3-D finite volume method. K-epsilon (kε) turbulence model was used for the purpose of flow analysis
of all the CAD models. K-epsilon (k-ε) turbulence model is
the most common model used in Computational Fluid
Dynamics (CFD) to simulate mean flow characteristics for
turbulent flow conditions. It is a two equation model
which gives a general description of turbulence by means
of two transport equations (PDEs). The original impetus for
the K-epsilon model was to improve the mixing-length model,
as well as to find an alternative to algebraically prescribing
turbulent length scales in moderate to high complexity flows.
For a much more practical approach, the standard k-ε
turbulence model is used which is based on our best
understanding of the relevant processes, thus minimizing
unknowns and presenting a set of equations which can be
applied to a large number of turbulent applications [9].
Initially, we started analyzing plain double pipe heat
exchanger for both parallel and counter flow arrangements.
The inlet temperatures of cold and hot fluids are taken as
300K and 360K respectively. Mass flow rate of hot and cold
fluids are taken as 0.12kg/s and 0.1kg/s respectively. From the
fluent database water is selected as fluid which flows through
inner tube and outer shell. Copper is selected as wall material.
Table 1 shows the comparison of computational value and
Parallel Flow
compared with the results of plain heat exchanger. Thus we
inferred an enhancement in heat transfer due to the
introduction of dimples. Then the number of dimples has been
increased to 9 and 13 dimples in a row, with pitch values of
Number of Dimples
T1(K)
T2(K)
Plain tube HX
309.88
351.14
HX with 5 Dimples in 1 row
312.95
347.03
HX with 9 Dimples in 1 row
314.46
345.56
HX with 13 Dimples in 1 row
314.85
345.28
150
and
100mm
respectively. Table
2 shows the comparison of TABLE 2 outlet temperatures
of hot and cold fluids for various configurations.
Counter Flow
Result Type
T1(K)
T2(K)
T1(K)
T2(K)
CFD Results
Analytical Results
307
309
349
347
310
315
344
345
analytical calculation value of the plain double pipe heat
exchanger for both parallel and counter flows.
TABLE 1
Analytical calculation of Outlet temperatures of both
parallel and counter flows are calculated using NTU method
[NTU =UA/ Cmin]. Table 1 shows that counter flow
arrangement is giving better heat transfer rate. CFD results are
found to be accordance with analytical results. Then the
analysis is continued by using counter flow arrangements with
hemispherical shape dimple in the heat exchanger tubes.
CHART 1
From the above chart it is evident that heat transfer rate
increases appreciably for the configuration of nine dimples in
one row. Then the analysis is continued by increasing the
number of rows to two and four. Table 3 shows outlet
Number of Rows
T1(K)
T2(K)
1
314.46
345.56
2
316.09
343.81
4
316.78
343.24
temperature results of cold and hot fluids for different row
configurations.
TABLE 3
From the above table it is identified that heat transfer
rate increases appreciably with dimple configuration of nine
dimples in two rows.
Fig.1 SOLIDWORKS Model of Hemispherical Dimples
The analysis for configuration of five dimples in 1 row
with a pitch range of 300mm is done and the results were
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International Journal of Advanced Research Methodology in Engineering & Technology, ISSN 2456 6446
Volume 1, Issue 3, May 2017
CHART 3
CHART 2
Comparing the CFD values of dimples and baffles
Rate of heat transfer increases by providing baffles in the
heat exchanger tubes [2],[13] So the heat exchanger tubes with
semicircular baffles were designed and analyzed with a
appropriate boundary condition.
Chart 4 shows hemispherical shape dimpled heat
exchanger is having having heat transfer rate than the
semicircular baffled heat exchanger with minimum pressure
drop. Then the analysis is continued by changing the shapes of
the dimples such as square, triangle and ellipse with constant
dimensions of D/d ratio of 0.26 and pitch value of 150mm
with 2 rows.
SOLIDWORKS Model of various shapes of dimples
Fig.3 Square
Fig.4 Triangle
Fig.2 SOLIDWORKS Model of Baffles
Table 4 shows the comparison of outlet temperature results
of cold and hot fluids for dimpled and baffled heat exchanger.
Type
T1(K)
T2(K)
Dimpled HX
316.78
343.24
Baffled HX
314.6
348.7
TABLE 4
Fig.5 Ellipse
Figure 3, 4, 5 shows the SOLIDWORKS models of
various shaped dimple tubes. Table 5 shows the outlet
temperature results of cold and hot fluids for various shape
dimpled heat exchanger.
Shapes
Hemisphere
Triangle
Square
Shapes
T1(K)
316.09
313.06
313.47
T1(K)
T2(K)
343.81
348.76
346.44
T2(K)
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International Journal of Advanced Research Methodology in Engineering & Technology, ISSN 2456 6446
Volume 1, Issue 3, May 2017
Chart 4 shows the comparison of outlet temperatures of
cold and hot fluids of various shaped dimple tube heat
exchanger. Also it is inferred that heat transfer rate is higher
for hemispherical dimples.
TABLE 5
CHART 4
Figure 6 shows the CFD results for hemispherical
dimpled tube heat exchanger with the configuration of nine
dimples in two rows.
Fig.6 CFD temperature plot for 9 dimples in 2 row
III.
EXPERIMENTAL SETUP & TESTING
Experimental setup was fabricated as per the required
dimensions. Fig.7 shows the photography of experimental
setup. The apparatus is also equipped with two water flow
meters having flow ranges of 0- 20 LPM for continuously
measuring and maintaining the particular water flow rate .
Overhead tank tap is connected to the outer shell through
hoses for cold water circulation. 20L tank with flow controlled
valve were fitted to hot inlet and coil type 1.5 KW immersion
heater is used to heat the water present in the tank. The inlet
and outlet temperatures of the inner and outer tubes are
measured by using digital thermometer.
Fig.7 Experimental setup
Water is allowed to flow through the outer shell at 301K
while hot water at 333K is allowed to flow through the inner
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International Journal of Advanced Research Methodology in Engineering & Technology, ISSN 2456 6446
Volume 1, Issue 3, May 2017
Type
T2(K)
T1(K)
CFD
325.3
308.71
Experimental
326.1
307.5
Table 7 shows the comparison of outlet temperature values
of experimental and CFD results for dimpled tube heat
exchanger with the configuration of 9 dimples in 2 rows.
tube in the counter current direction. Mass flow rate of both
hot and cold fluid is kept as 0.07Kg/s. In the above said
conditions the testing of the setup has been carried out. Also
analysis is repeated with the above said conditions. Then the
experimental results of the modified boundary condition are
compared with CFD results. Table 6 shows the validation of
CFD and experimental results for plain tube heat exchanger.
TABLE 6
TABLE 7
Result Type
T1(K)
T2(K)
CFD
306.53
327.96
Experimental
305.7
329.2
Chart 5 shows the validation of CFD and
experimental results of both plain and dimpled tube heat
exchanger.
Then we found that 9 dimples in 2 rows is having
appreciable heat transfer enhancement with the modified
boundary condition through CFD results. Considering the life
and stress concentration on the heat exchanger pipe by
providing dimples, it is found that 2 rows of dimples at a pitch
value of 150mm is efficient than the other configurations
considered.
Since copper is a highly ductile material, 9 dimples in 2
row
with hemispherical shape of dimple structure is
impressed on the heat exchanger tube using mechanical press
forging process. Press forging is the mechanical method to
deform a material precisely at a particular place or point.
Initially the copper pipe is filled with green sand and core to
avoid bulging of hollow copper pipe. Die for the desired shape
of the dimple is fabricated. Then the die structure was fitted in
the mechanical press forging machine and the required force
were given to impress the die shape of on the heat exchanger
tube. Diameter and depth of hemispherical dimples are 8 and
2mm respectively (D/d=0.26).
IV.
CHART 5
CONCLUSION
Thus the performance of double pipe heat exchanger with
dimples of various shapes and configurations are investigated using
the CFD package ANSYS® FLUENT 16.0 and we concluded that
the heat transfer rate of double pipe heat exchanger is increased
nearly 1.5 times by the use of hemispherical dimpled tube with
the arrangement of in two rows.
Fig.8 Photography of dimpled copper tube
Nomenclature
HX– Heat Exchanger
d/D –Depth to diameter ratio of dimple
T1 – Outlet temperature of cold fluid in K
T2 – Outlet temperature of hot fluid in K
NTU – Number of transfer units
U – Overall heat transfer coefficient in W/m2K
A – Surface area in m2
C – Heat capacity rate in KJ/Kg-K
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Fig.9 Inner protrusion of dimpled tube
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