1. An experimental and theoretical study was conducted on a four row heat pipe heat exchanger with distilled water as the working fluid. 2. Tests were performed at varying air flow rates and inlet evaporator temperatures to analyze the effect on effectiveness. 3. The maximum effectiveness occurred at a mass flow rate ratio of 2. A theoretical model was developed and showed good agreement with experimental results.

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International Journal of Mechanical Engineering and Technology (IJMET)
Volume 7, Issue 3, May–June 2016, pp.102–111, Article ID: IJMET_07_03_009
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
EXPERIMENTAL AND THEORTICAL
STUDY OF THE THERMAL
PERFORMANCE OF HEAT PIPE HEAT
EXCHANGER
Abdulsalam D.Mohamed
Mechanical Engineering, University of Waist
Aamer M.Al-Dabagh
Mechanical Engineering, University of Technology)
Duaa A.Diab
Mechanical Engineering, University of Waist
ABSTRACT
Heat pipe heat exchanger (HPHE) considers one of the most useful
devices for the recovery of waste heat energy. An Experimental study has been
carried out on air to –air HPHE constructed of thermosyphon heat pipes with
distilled water as the working fluid and a fill ratio of 75% from the evaporator
length. Its model was composed of 4 rows, each row contains 12 copper tubes,
each tube have ID= 9.5 mm, OD=10mm and length =950 mm and the rows of
tubes were arranged in a staggered manner. Aluminum wavy plate fins of
0.1mm thickness were fixed among the tubes to increase the heat transfer
area. Tests were conducted at various flow rates (air flow rate through
evaporator and condenser sections) ranged between 0.12 and 0.37 kg/s and at
different temperatures of air entering evaporator section (90, 100,110) to
indicate discontinuity in the effectiveness when the flow rate ratio equal to one
. The results show that the maximum value of effectiveness for the four heat
pipe heat exchanger occurs when = 2 . Theoretical model based on (ε-
NTU) has been developed by using visual basic language computer program
to analysis of the temperature distribution along heat pipe heat exchanger.
The results from this model were compared with experimental results. A
comparison between the experimental and theoretical results shows little
discrepancy.
Key words: Heat Pipe Heat Exchanger, Effectiveness, Mass Flow Rate Ratio,
Temperature Distribution.

2. Experimental and Theortical Study of The Thermal Performance of Heat Pipe Heat
Exchanger
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Cite this Article Abdulsalam D.Mohamed, Aamer M.Al-Dabagh and Duaa
A.Diab, Experimental and Theortical Study of The Thermal Performance of
Heat Pipe Heat Exchanger. International Journal of Mechanical Engineering
and Technology, 7(3), 2016, pp. 86–101.
http://www.iaeme.com/currentissue.asp?JType=IJMET&VType=7&IType=3
GENERAL TERMS
: Effectiveness, : actual heat transfer rate (w), : maximum heat transfer rate
(w), : total overall heat transfer (w/m2
.k), : total area (m2
) , : minimum heat
capacity rate (kJ/kg.k), : maximum heat transfer rate (kJ/kg.k), : number of
transfer unit, : effectiveness of evaporator, : number of transfer unit for
evaporator, : effectiveness of condenser, : number of transfer unit for
condenser, : overall heat transfer coefficient of evaporator (w/m2
.k), : heat
transfer area of evaporator (m2
) , :heat capacity rate of evaporator (w/k). :
Overall heat transfer coefficient of condenser (w/m2
.k), : heat transfer area of
condenser (m2
) , : heat capacity rate of condenser (w/k), : area of pipe (m2
), :
diameter of pipe(m) , : length of pipe (m) , : thickness of fin (m), : number of
fins per meter , : number of pipes, :pi (=3.14), : width of fin (m), : total
thermal resistance of evaporator (k/w), : external thermal resistance of evaporator
(k/w), : fouling thermal resistance of (k/w), : wall thermal resistance of
evaporator (k/w), : internal thermal resistance of evaporator , : total thermal
resistance of condenser(k/w), : external thermal resistance of evaporator (k/w),
: fouling thermal resistance of condenser (k/w), : wall thermal resistance of
condenser (k/w), : internal thermal resistance of condenser , : overall
effectiveness, j= number of row.
1. INTRODUCTION
Heat pipe is a high performance heat transfer device which is used to transfer a large
amount of heat at high rate with small temperature difference. This is achieved by
evaporation of working fluid in the evaporator section and condensing in the
condenser section and return of the condensate. If the wick is not used in the
construction of heat pipe, it is called thermosyphon, in the thermosyphon return the
working fluid from the condenser to evaporator is caused by gravity [1] Heat pipe is
used in several utilizations like heat exchanger working as recovery system, solar
energy communion system, cooling of electronic part, thermal control of space craft,
gas turbine rotor blade cooling, etc [2]. The operating Limits of heat pipe include
boiling limit, sonic limit, capillary limit, drag and viscous limit [3].
Thermosyphon Heat exchanger (THE) is one of the most productive devices for
waste heat recovery. A thermosyphon heat exchanger is composed of a number of
pipes arrayed in rows. It’s operating with condenser section with low temperature
fluid stream and evaporator section with high temperature fluid stream [4].
The advantages of Thermosyphon Heat Exchanger are Capable of operating even
when temperature difference between heat source and heat sink is very low, less
alimentation problems owing to containing no moving parts, large rate of
effectiveness, No need any external power, Less pressure drop for fluid flow, Full
disconnection of cold and hot fluids, Comparative economy and Large fidelity[5].
Zare et al. (2009) [6], studied the heat performance for air to water THE according to
method. They used cold water with 0.1 kg/s flow through condenser

3. Abdulsalam D.Mohamed, Aamer M.Al-Dabagh and Duaa A.Diab
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section and hot air in closed cycle was blown into evaporator section. The temperature
range of 125-225 From experimental results the effectiveness and heat transfer rate
of THE were affected by heat capacity ratio for high and low temperature fluid
streams .When the heat capacity ratio (Ce/Cc) of high and low temperature fluid
streams was higher than unity the effectiveness increased due to ability of streams to
release and absorb more heat. When the heat capacity ratio was equal to unity the
effectiveness was minimum because the releasing and absorption heat was less.
Danielewicz et al. (2014) [7], studied the effect of mass flow rate ratio between
evaporator and condenser sections on the effectiveness of air to air THE according to
method by using methanol as working fluid. The tested Heat exchanger
consisted of ten rows and each row contained ten thermosyphons made of carbon steel
while the fins were made from aluminum. They noticed that the effectiveness and heat
transfer rate increased when the mass flow rate ratio ( increased.
Vikramsinh and Mail (2015) [8], studied the thermal performance of THE for a lost
air heat recovery system by using nanofluid with variable source temperature. The
tested THE consisted of three rows arranged in staggered form. Each thermosyphon
had inner diameter of 14.90 mm and outer diameter of 15.88mm.The Experimental
tests were carried out to determine the characteristics of THE by using nanofluid.
They observed that the thermal performance of THE was enhanced by replacing the
conventional fluid in thermosyphon with nanofluid. A model of a multi –
thermosyphon heat exchanger predicted the energy savings.
2. EXPERIMENTAL TEST RIG
The heat pipe heat exchanger using thermosyphon was formed and constructed as
shown in figure 1. This module is composed of 4 rows; each row has 12 copper tubes
of 10 mm outside diameter. The rows of tubes were staggered and connected by
aluminum wavy plate fins. The fin density was 470 fins per meter. A copper tube
header of 22 mm nominal diameter was connected to the ends of each row of tubes.
Distilled water is utilized as working fluid in thermosyphon heat exchanger. The fill
ratio is chosen to be 75% from the volume of evaporator section. A duct and fan
system used for testing the performance of thermosyphon heat exchanger, Table (1.1)
indicate the specifications of this system Four air heaters of 3.5 Kw each are
established inside the inlet duct of evaporator section at different temperatures. All
heaters are connected directly to power supply except one is connected to variac
(voltage transformer), in order to control the air stream temperature.
Figure 1 Schematic of heat pipe heat exchanger

4. Experimental and Theortical Study of The Thermal Performance of Heat Pipe Heat
Exchanger
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Table 1.1 The specifications of duct and fan system:
4. THEORY
Formulas are dependent on developed (ε– NTU) method, which is more convenient to
indicate the temperatures distribution along thermosyphon heat exchanger and
capable of comparing theoretical prediction and experimental results. (ε-NTU) a
method depends on effectiveness of heat exchanger, (ε), which may be defined as
ratio of actual heat transfer, to maximum rate of heat transfer; therefore, the
effectiveness may be defined by Incropera [9]:
Applying conservation of energy, general exponential function for a counter –flow
heat exchanger is represented by incropera [6]:
The ratio is a dimensionless term called number of transfer unit which is
defined as:
For evaporator and condenser sections of heat pipe heat exchanger, their
effectiveness can be determined respectively by the following equations:
Where
The heat transfer areas or for evaporator and condenser associated with one
row of tubes for the evaporator and condenser can be calculated from the geometry of
thermosyphon and fins as shown in Fig(2).
Name of section
Length
m)
Area
(
Type of fan
Air flow rate
control type
Cond. section
Inlet duct 1.25 0.4 0.4
Centrifugal
fan (CFM)
without
Outlet duct 1 0.4 0.4 Without Grill
Evap. section
Inlet duct 1.25 0.4 0.4
Centrifugal
fan (CFM)
without
Outlet duct 1 0.4 0.4 Without Grill

5. Abdulsalam D.Mohamed, Aamer M.Al-Dabagh and Duaa A.Diab
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Or
Where the area of the heat is pipe without fins and is the area of the fins.
Figure 2 Schematic diagram of tubes and fins for one row evaporator or condenser
The overall heat transfer coefficients of evaporator and condenser sections are
required to calculate the NTU of the evaporator and condenser sections.
Where
An Effectiveness –NTU relationship for the jth
row can be evaluated for the evaporator
and condenser.
If , are given, then three equations can be written in convenient form
for solution following Azad and Geoola [10]:
 For Cc < Ce ( the condenser air streams is the minimum fluid)

6. Experimental and Theortical Study of The Thermal Performance of Heat Pipe Heat
Exchanger
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 For Ce < Cc (the evaporator air stream is the minimum fluid)
5. RESULTS AND DISSCUSSION
To describe the discontinuity in effectiveness when mass flow rate ratio is equal one
the tests were conducted.
1–In condenser section the flow rate is kept constant at minimum value (0. 132) kg/s
while the flow rate in evaporator section is changed gradually from minimum value to
maximum value.
2–In condenser section the flow rate is kept constant at maximum value (0. 132) kg/s
while the flow rate in evaporator section is changed from minimum value to
maximum value.
Figures (3and4) and results are interesting for the selection of flow rate in heat
recovery applications. If the processed fluid has a minimum flow rate, then higher
effectiveness in heat recovery system could be achieved by using largest amount of
exhaust gas flow. If the processed fluid has a higher flow rate, then the higher
effectiveness in heat recovering system is achieved by using smaller exhaust gas flow
rate. The right hand side curves (Fig 3) are obtained where the air stream in the
condenser section is kept at low flow rate and air stream in evaporator section is

7. Abdulsalam D.Mohamed, Aamer M.Al-Dabagh and Duaa A.Diab
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changing from low flow rate to high flow rate. The left hand side curves (Fig 4) are
obtained where the air stream in the condenser section is kept at high flow rate and air
stream in the evaporator section is changing from the low flow rate to high flow rate
for condenser section. Discontinuity in the effectiveness when the flow rate ratio
equal to one is shown clearly in (Fig5).
For predicting the temperature distribution of air across evaporator and condenser
sections of thermosyphon heat exchanger. A computer program written in visual basic
language as employed to generate the theoretical conditions by using iterative
processes.
The main purpose of the program is to calculate inlet and outlet temperature for
evaporator and condenser in each row of heat exchanger. The theoretical results
obtained from the program were compared with experimental data.
A comparison between predicted and measured temperatures in two air streams
for thermosyphon heat exchanger subjected to different values of air flow rate ratio
(1.31) and (0.45) at different temperatures are illustrated in Figures (6,7, 8,and9) .
Figures (7and9), illustrate that the difference between experimental and theoretical
results for both evaporator and condenser sections increases with increase of the inlet
temperature of air to evaporator section.
Figure 3 The effect of mass flow rate ratio on for a four heat pipe heat exchanger
subjected to constant flow rate in the condenser section at 0.1322 kg/s and varying evaporator
flow rate
Figure 4 The effect of mass flow rate ratio on for a four –row heat pipe heat exchanger
subjected to constant flow rate in the condenser section at 0.374 kg/s and varying evaporator
flow rate
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0.5 1 1.5 2 2.5 3
Effectivness
Mass flow rate ratio
T=90 T=100 T=110
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0 0.2 0.4 0.6 0.8 1 1.2
Effectivness
Mass flow rate ratio
T=90 T=100 T=110

8. Experimental and Theortical Study of The Thermal Performance of Heat Pipe Heat
Exchanger
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Figure 5 Discontinuity of the effectiveness at flow rate ratio equal one for a four row heat
pipe heat exchanger
Figure 6 Comparison of experimental and theoretical air temperature distribution for four-
row heat pipe heat exchanger at air flow rate ratio of 1.31 at inlet temperature=110
Figure 7 Comparison of experimental and theoretical air Temperature distribution for four-
row heat pipe heat exchanger at air flow rate ratio of 1.31at inlet temperature=120 ˚C
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 0.5 1 1.5 2 2.5 3
Effectivness
Mass flow rate ratio
T=90 T=100 T=110
0
25
50
75
100
125
0 1 2 3 4 5
Temperature(C)
Row number,n
Tc=Exp
Tc=Theo
Te=Exp
Tc=Theo
0
25
50
75
100
125
0 1 2 3 4 5
Temperature(C)
Row number,n
Tc=Exp
Tc=Theo
Te=Exp
Te=Theo

9. Abdulsalam D.Mohamed, Aamer M.Al-Dabagh and Duaa A.Diab
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Figure 8 Comparison of experimental and theoretical temperature distribution for four-row
heat pipe heat exchanger at air flow rate ratio of 0.45 at inlet temperature =110 ˚C
Figure 5.15 Comparison of experimental and theoretical temperature distribution for four-
row heat pipe heat exchanger at flow rate ratio of 0.45 at inlet temperature=120 ˚C
6. CONCLUSION
 The maximum value of effectiveness for the four heat pipe heat exchanger occurs
when = 2 .
 From solving the mathematical model there is little discrepancy between theoretical
and experimental results of temperature distribution of air along THE.
 The difference between theoretical and experimental results for evaporator section
increases with increasing evaporator air inlet temperature.
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0 1 2 3 4 5
Temperature(C)
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Te=Exp
Te=Theo
0
25
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125
0 1 2 3 4 5
Temperature(C)
Row number,n
Tc=Exp
Tc=Theo
Te=Exp
Te=Theo

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