Effect of Nanofluid Concentration on Heat Pipe
Thermal Performance
Wei-Chiang Weia,*, Sheng-Hong Tsai, Shih-Yu Yanga, Shung-Wen Kanga
Department of Mechanical and Electro-Mechanical Engineering Tamkang University
#151, Ying-Chuan Rd., Tamsui, Taipei, 25137, Taiwan.
b
Department of Mechanical Engineering, St. John’s & St. Mary’s Institute of Technology,
499, Sec. 4, Tam King Road, Tamsui, 25137 Taipei, Taiwan
*wapili@yahoo.com.tw
http://www.me.tku.edu.tw/
a
Abstract: - The thermal density of electronic parts and systems have been increased continuously as high
speed and high density are required for them. The higher heat generation of the CPU of desktop PC than the
Pentium-Ⅳ grade has been recently increased to more than 130W, and the available packaging space has been
compacted. Therefore, in present study the nanofluid is employed as working medium for conventional 211
m wide grooved circular heat pipe. The nanofluid used in the present case is an aqueous solution of
various-solution sliver (Ag) nanoparticles. The average diameters of Ag nanoparticles is 10 nm. The
experiment was performed to measure and compare thermal resistance of pure water and nanofluid filled heat
pipes. At the same charge volume of 0.51mL, test result showed the average decrease of 28%-44% in thermal
resistance of heat pipe with nanofluid as compared with pure water. As a result, the higher thermal
performances of the nanofluid have proved its potential as substitute for conventional pure water in
the grooved heat pipe.
Key-Words: - Nanofluid, heat pipe, thermal resistance, nanoparticles.
1. Introduction
Over the past decade, the use of heat pipes
in electronic cooling applications has increased
dramatically, primarily in notebook computers.
In fact, virtually every notebook computer
manufactured today uses at least one heat pipe
assembly. Typically used to carry less than 25
W of power, these parts are low in cost and
highly reliable. Use of heat pipes in high-power
(>150 W) cooling applications has been limited
to custom applications requiring either low
thermal resistance or having a severely
restricted enclosure area. The cost of these
larger diameter heat pipes was high due to a
limited number of manufacturers and handmade
assembly times.
With progresses of nanotechnology and
thermal engineering, many efforts have been
devoted to heat transfer enhancement. The usual
enhancement techniques for heat transfer can
hardly meet the challenge of ever increasing
demand of heat removal in process of high
energy devices. However, traditional fluids have
poor heat transfer properties compared to most
solid. Therefore, Argonne National Laboratory
has developed the concept of a new class of heat
“Nanofluids,” which are
engineered by suspending ultrafine metallic or
nonmetallic particles of nanometer dimensions
in traditional fluids such as water, engine oil,
ethylene
glycol
(Choi,
1995).
Some
experimental investigations have revealed that
the nanofluids have remarkably higher thermal
conductivities and greater heat transfer
characteristic than those of conventional pure
fluids [1-6]. A theoretical model and an
experimental are proposed to describe heat
transfer performance of nanofluid flowing in a
tube, the experimental results illustrate that the
thermal conductivity of nanofluids remarkably
increases with the volume fraction of ultra-fine
particles [7].
In 2001, nanofluid consisting of copper
nanoparticles dispersed in ethylene glycol has a
much higher effective thermal conductivity than
either pure ethylene glycol or ethylene glycol
containing the same volume fraction of
dispersed oxide nanoparticles [8]. The
convective heat transfer feature and flow
performance of Cu-water nanofluids in a tube
have experimentally been investigated by Xuan
and Li [9]. Sarit et al. go detailed into
investigating the increase of thermal
conductivity with temperature for nanofluid
with water as base fluid and particles of Al2O3
or CuO as suspension material, and the results
indicated an increase of enhancement
characteristics with temperature, which makes
the nanofluids even more attractive for
applications with high energy density than usual
room temperature measurements reported
earlier [10]. You and co-works performed
boiling experiments, varying conditions of
fluids. The measured pool boiling curves of
nanofluids saturated at 60℃ have demonstrated
that the critical heat flux increases dramatically
(~200% increase) compared to pure water case
[11].
In 2004, some researchers investigated
gold nanofluid on meshed heat pipe thermal
performance. The circular meshed heat pipe has
a length 170 mm and an outer diameter of 6 mm.
The thermal resistance of heat pipe ranges form
0.17 to 0.215 ℃/W. The measured results show
that the thermal resistance of the heat pipes with
nanofluids is lower than with pure water [12].
2. Experimental Procedure
The nanoparticle used is silver particles of
10 nm in sizes. The base working fluid used is
pure water. The test nanofluid was obtained by
dispersing the nanoparticles (NANOHUBS
TECHNOLOGY CO., LTD.) in pure water. The
nanofluid concentration of 1 mg/l, 5 mg/l, 10
mg/l, and 50 mg/l, 100 mg/l (ppm) were used in
the study. Figure.1 showed a TEM image of Ag
nanoparticles with an average diameter of 10
nm.
Fig.1 TEM micrograph of Ag nanoparticles
In the experiment, the outer diameter and
length of the heat pipe are 6 mm and 200 mm,
respectively (Asia Vital Components CO., LTD).
The heat pipe contained 211 m wide grooves.
An experimental system was set up to measure
the wall temperature of circular heat pipes
(Figure.2).
Data Logger
Thermocouple
Computer
Water out
Cooling Block
Heating Block
Heat Pipe
Water in
Thermostat
Reservoir
Power Supply
(a)
15
27
70
100
187
Uni t : mm
(b)
Fig.2 A schematic view of the system for
measuring the wall temperature of heat pipe
with nanofluid
The experimental system was composed of
a cooling system, the test section, a power
supply and measurement system, and a data
acquisition system (IMC Spartan-L). A PC was
used to monitor and process the experimental
data. The cooling system included a
constant-temperature thermal reservoir and a
cooling chamber. The condenser section of the
heat pipe was inserted horizontally into the
cooling chamber. The coolant circulated
through the cooling chamber, where heat was
removed from the condenser section by forced
convection, and then to a constant-temperature
reservoir. The constant-temperature reservoir
was set to the required temperature and held at a
constant temperature through the tests. The
temperature variation of the cooling fluid was
held to within 40°C.
The local temperature on the heat pipe was
measured by five isolated type-T thermocouples.
Two thermocouples were attached to the
evaporator; one was attached to the adiabatic
section, the others were attached to the
condenser section. All thermocouples were
calibrated against a quartz thermometer. The
uncertainty in temperature measurements was
 0.1 ℃.
The power supply was turned on and the
power incremented. At this point in the tests, it
took approximately 15 to 20 minutes to reach
steady-state. Once the steady-state condition
had been reached, the temperature distribution
along the heat pipe was measured and recorded,
along with the other experimental parameters.
The power input was then increased
incrementally, and the process repeated until
dryout occurred as determined by rapid spikes
in the evaporator thermal couple farthest from
the condenser. Once dryout was reached, the
temperature difference between the evaporator
and condenser rapidly increased. The power
input at this point was assumed to be the
maximum heat transport capacity of the heat
pipe at this power level and operating
temperature, which is defined as the adiabatic
vapor temperature.
Two heater bars (maximum 120 W) were
used as a heat source in the heating section.
Thus, the heating load ( Q ) and temperature
difference ( dT  Tevaporator  Tcondenser ) were
measured, and the thermal resistance (R) was
calculated by the equation, R  dT / Q . For
present study, the heat pipe was horizontally
fixed during measurement.
3. Analysis
Although heat pipes are very efficient heat
transfer devices, they are subject to a number of
heat transfer limitations. There are various
parameters that put limitations and constraints
on the steady and transient operation of heat
pipes. These limitations determine the
maximum heat transfer rate a particular heat
pipe can achieve under certain working
conditions. For high heat flux heat pipes
operating in a low to moderate temperature
range, the capillary and boiling limits are
commonly the dominant factors that govern the
operation of heat pipes. The calculation of the
operational limitations for heat pipes can be
found as follows.
Capillary Limit. For a given capillary
wick structure and working fluid combination,
the pumping ability of the capillary structure to
provide the circulation for a given working
medium is limited. This limit is usually called
the capillary or hydrodynamic limit. In order to
maintain the continuity of the interfacial
evaporation, the capillary pressure head must be
greater than or equal to the sum of pressure
losses along the vapor-liquid path. The pressure
balance can be expressed as [13]
Pcap,max  Pl  Pv  Pe ,  Pc ,  Pg (1)
Boiling Limit. Nucleation within the
capillary wick is undesirable for wicked heat
pipe operation since the bubbles can obstruct
h fg  v , sat T
If the nucleus is smaller than the critical size, it
is unstable and the nucleus will disappear after a
short period of time. In 2004, C.Y. Tsai et al. [9]
explained the reason for reducing the thermal
resistance of heat pipe, revealing that the
nucleation size of vapor bubble is smaller for
fluid with suspended nanoparticles than without
them.
Pure Water
30W
41.2
5ppm
1ppm
41
100ppm
40.8
50ppm
T(
℃
)
10ppm
40.6
40.4
40.2
0
50
100
150
200
Position of thermocouples (mm)
(a)
42.5
Pure Water
40W
5ppm
1ppm
42.3
10ppm
T( ℃ )
the liquid circulation and cause hot spots on the
heated wall. If the boiling is severe it dries out
the pipe wall, which is defined as the boiling
limit. However, under a moderate heat flux, low
intensity stable boiling is possible without
causing dryout. It should be noted that the
boiling limitation is a radial heat flux limitation
as compared to an axial heat flux limitation for
the other heat pipe limits.
The boiling limit is directly related to
bubble formation in the liquid. In order that a
bubble can exist and grow in a liquid, a certain
liquid superheat is required. On the other hand,
in order that a bubble can exist and grow in a
superheated liquid, its size must be larger than a
critical value. The relation for the critical size of
a bubble or nucleus under a certain liquid
superheat and physical properties can be
expressed as [13]
2Tsat
(2)
Rb 
100ppm
42.1
50ppm
41.9
41.7
41.5
0
50
100
150
200
Position of thermocouples (mm)
(b)
44.1
Pure Water
50W
5ppm
Figure 3 shows the distribution of wall
temperature according to the axial length of
200mm heat pipes having the diameter of 6mm
under the water-cooling. As Figure 3(a) shown
by findings, the distribution of wall temperature
of heat pipe containing pure water were 41.06
℃ ,40.96 ℃ , 40.92 ℃ , 40.89 ℃ , 40.81 ℃ ,
respectively. After adding extremely small
amount of silver nanoparticles in the pure water
illustrated a sizable decrease in the wall
temperature of heat pipe; from 40.72 ℃ to
40.58℃ (5 ppm).
It also shows the more the nanopartilces
were dispersed in working fluid, the smaller the
rise in wall temperature of heat pipe than pure
was filled in heat pipe under various heat load.
The 50 ppm nanofluid is the lowest rising wall
temperature of heat pipe.
1ppm
10ppm
43.7
100ppm
50ppm
T( ℃ )
4. Results and Discuss
43.9
43.5
43.3
43.1
42.9
0
50
100
(c)
150
Position of thermocouples (mm)
200
46
Pure Water
60W
5ppm
45.8
1ppm
45.6
10ppm
45.4
T( ℃ )
100ppm
50ppm
45.2
45
44.8
44.6
44.4
44.2
0
50
100
150
200
(d)
Position of thermocouples (mm)
Fig.3 Measured values of wall temperature of heat
pipe with pure water and nanofluid prepared by
different concentration.
(c)
(d)
(a)
(e)
(b)
Fig.4 Wall temperature versus concentration of
nanofluid at different heat load.
Figure 4 shows the wall temperature under
various concentrations of nanofluid and input
power. It can be seen that the wall temperature
of heat pipe containing pure water is the highest
than nanofluid under various heat load.
60W
60W
0.5
Rwater-Rnanofluid/Rwater
0.004
R(℃/W)
0.003
0.002
0.001
0.3
0.2
0.1
0
0
pure water
0.4
1 ppm
5 ppm
10 ppm
50 ppm
100 ppm
pure water
1 ppm
Concentration(micro gram/liter)
Fig.5 Measured values of thermal resistance of heat
pipe with pure water and nanofluid prepared by
different concentration
Figure 5 illustrates the thermal resistance
of grooved heat pipe containing silver nanofluid
and pure water as working fluid. The figure
shows the thermal resistance of heat pipes were
0.0036℃/W, 0.0029℃/W, 0.0023℃/W, 0.0022
℃/W, 0.0021℃/W, 0.002℃/W, respectively.
It can be seen that by increasing concentration
of nanofluid the thermal performance of heat
pipe can be decreased.
The effect of adding silver nanopartilces on
the thermal performance of the grooved heat
pipe is more evident if the data are expressed as
a plot of Rwater-Rnanofluid/Rwater versus nanofluid
concentration, as shown in Figure 6. This graph
shows the reducing rate of thermal resistance
were 0.28, 0.38, 0.4, 0.42, 0.44 under different
concentration, respectively.
However,
the
continuing
dramatically
increasing of thermal performance was not
observed at the highest concentration. As a
result, there may exist an optimum
concentration for a given grooved heat pipe at a
specific charge volume.
5 ppm
10 ppm
50 ppm
100 ppm
Concentration(micro gram/liter)
Figure 6 Reducing rate of thermal resistance at
different concentrations.
5. Conclusion
This paper deals with the thermal
enhancement of the heat pipe performance,
using silver-nanofluid as the working fluid. A
nanofluid is an innovative heat pipe working
fluid with metal nanoparticles dispersed on it. In
present case, the pure water with diluted 10 nm
silver particles, inside 211 microns wide
grooved circular heat pipes, was experimentally
tested, showing that the thermal performance of
the heat pipe was considerably increased.
The major reason for increasing the
thermal performance of heat pipe by using
nanofluid can be explained as follows. The
critical heat flux [11] and convective heat
coefficient [14] of nanofluid is higher than pure
water. Therefore, it is expected that the thermal
performance of heat pipe will be enhanced. As a
result, the higher thermal performances of the
nanofluid have proved its potential as substitute
for conventional pure water in the grooved heat
pipe.
Nomenclature
h fg =Latent heat of vaporization, J/kg
Pcap , max =The maximum capillary head
Pg =The pressure drop in the liquid due
to the effect of the gravitational
force in the direction of the heat
pipe, Pa
Pl =The liquid pressure drop, Pa
Pv =The vapor pressure drop, Pa
Pc , =The pressure drop due to the
Pe ,
condensation at liquid-vapor
interface, Pa
=The pressure drop due to the
evaporation at liquid and vapor
interface, Pa
Q = Heat load, W
R = Thermal resistance, ℃/W
R water = Thermal resistance of heat pipe
containing pure water, ℃/W
R water = Thermal resistance of heat pipe
containing nanofluid, ℃/W
Rb = Radius of vapor bubble, m
Tcondenser =Temperature of condenser section,℃
Tevaporator =Temperature of evaporator section
Tsat =Saturation temperature, ℃
Greek Alphabet
 =Liquid-vapor interface surface tension,
N/m
 =Liquid film thickness, m
 v, sat =Density of saturated vapor, kg/m3
Subscripts
b =bubble
cap =capillary
e =evaporator or evaporation
g =gravity
l =liquid phase
max=maximum
sat =saturation
v=vapor phase
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