Thermal performance of biomaterial wick loop heat pipes with water bases AL2O3 nanofluids.
International Journal of Thermal Sciences 76 (2014) 128e136
Contents lists available at ScienceDirect
International Journal of Thermal Sciences
journal homepage: www.elsevier.com/locate/ijts
Thermal performance of biomaterial wick loop heat pipes with
water-base Al2O3 nanofluids
Nandy Putra a, *, Rosari Saleh b, Wayan Nata Septiadi a, Ashar Okta a, Zein Hamid a
a
b
Heat Transfer Laboratory, Department of Mechanical Engineering University of Indonesia, Kampus UI, Depok 16424, Indonesia
Departemen Fisika, Fakultas MIPA-Universitas Indonesia, Kampus UI, Depok 16426, Indonesia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 September 2012
Received in revised form
29 August 2013
Accepted 30 August 2013
Available online 8 October 2013
Given the increase of heat flux generated by electronic equipment, particular components of a computer
(e.g., the CPU) should always be accompanied with good cooling to achieve optimal operating capability
and a high level of reliability. The use of loop heat pipes (LHPs) in the thermal management of electronic
cooling is a major alternative solution. Before LHPs are implemented as an alternative cooling method for
electronic devices, a quantity of reliability factors should be considered and evaluated, such as wick
structure, material, and the type of working fluid. In this case, the pore size distribution of a biomaterial
(Collar) that is smaller and more homogeneous than pore size distribution of sintered powder was
investigated. The purposes of this experimental study are to examine and analyze the application of a
biomaterial (Collar) as a wick on a loop heat pipe and the use of the nanofluid Al2O3ewater as a working
fluid. The performance of the biomaterial as a wick and nanofluid as a working fluid in LHPs was also
investigated in this experiment. The temperature differences between the evaporator and condenser
sections with the biomaterial wick were less than that using a sintered copper powder wick, and the use
of nanofluids over pure water also resulted in lower temperature differences; i.e., the thermal resistance
of the LHP was lowered when using the biomaterial wick and nanofluids. These results make the
biomaterial (Collar) and nanofluids attractive as wick and working fluid, respectively, in LHP technology.
Ó 2013 Elsevier Masson SAS. All rights reserved.
Keywords:
Loop heat pipe
Wick
Nanofluids
Biomaterial
1. Introduction
Loop heat pipes (LHPs) are widely used in thermal management
electronic cooling devices with high heat fluxes because of their
high efficiency in heat transfer performance, small temperature
differences, and because they contain no moving parts. An LHP is a
two-phase device that has a very high and effective thermal conductivity that uses capillary forces inside the wicked evaporator to
pump the working fluid inside a closed loop [1]. LHPs have several
excellent cooling capabilities and hold considerable promise for
electronic cooling applications. Maydanik et al. [2] summarized
some of the advantages of an LHP implementation in electronic
cooling applications. LHPs have a higher capacity and a lower
thermal resistance at comparable to other heatsinks, a flexibility in
packaging, high heat transfer loads over considerable distances,
and operate efficiently at any orientation in the gravitational field.
There are many examples of LHP experimental developments
and their implications in literature [1e6]. These studies indicate
* Corresponding author.
E-mail address: nandyputra@eng.ui.ac.id (N. Putra).
1290-0729/$ e see front matter Ó 2013 Elsevier Masson SAS. All rights reserved.
http://dx.doi.org/10.1016/j.ijthermalsci.2013.08.020
that LHPs are nearly ready for commercial application. However,
before LHPs are implemented as an alternative cooling method for
electronic devices, several reliability parameters should be
considered and investigated, such as wick structure and material
and the type of working fluid. The wick is the part of the heat pipe
that circulates fluid from the condenser to the evaporator. Hence,
the wick has an important role both in conventional heat pipes and
LHPs [7]. There are different types of capillary wick heat pipes, such
as the screen mesh, sintered powder, groove, and the wire, all of
which are made from metal, composites, and ceramics among other
materials [7e14].
Many studies of wick characteristics have been conducted. For
example, Li et al. [15] studied the ability of the wick to pump liquids
(i.e., the capillarity of wick) on a sintered powder wick capillary. In
their study, they found that the capillarity of the wick increases
with an increasing porosity value of the wick. They also explained
that in a typical real-time changing curve, there are three important
parameters: the capillary pumping amount, the capillary pumping
time and the capillary pumping rate. Leong and Liu [16] also
studied the heat pipes capillary wick, which is made of a sintered
powder concerning which the influence of sintering temperature
(800 C and 1000 C). They also investigated sintering time on wick
N. Putra et al. / International Journal of Thermal Sciences 76 (2014) 128e136
129
Fig. 3. Capillary based on mass amount transported.
Fig. 1. Samples of biomaterial and sintered powder capillary test.
porosity. They concluded that a sintering temperature of 800 C
gave a better porosity value than a sintering temperature of
1000 C. This was because the wick forms a solid stricter at 1000 C.
Consequently, less power capillarity will occur in the wick.
An assessment of wick sintered powder using nickel has been
performed by Mishra et al. [17]. Their results showed that a sintering temperature of 775 C produced the lowest porosity value,
while at 550 C produced the largest value. Zan et al. [18] analyzed
the parameters of a loop heat pipe with a sintered powder wick and
investigated the effect of wick porosity on the permeability of the
wick to determine the amount of heat that can be absorbed by an
LHP. An investigation on wick capillary action has also been performed by Tang et al. [19] using a thermal imaging method. In that
study, they examined a groove wick, a sintered powder wick and a
composite wick (sintered powder combined with the groove). The
results indicate that the capillarity of the composite wick is higher
than the capillarity of sintered powder wick and the groove wick.
Generally, sintered powder is used as a wick in heat pipe technology. The wick structure determines the thermal performance of
heat pipes because it provides the pumping force to return the
working fluid to the evaporator. The secondary purpose of the wick
is to distribute the fluid around the circumference of the tube to
maximize the evaporator surface area [20]. Sintered porous structures have attracted considerable attention over the last few decades because of their special properties, such as good
permeability, high porosity and fine pore size [21]. Although sintered powder is generally very good and has many advantages over
wire mesh and screen mesh, the manufacturing process is very
difficult and time-consuming.
Collaria is a porous biomaterial that has a relatively small
micropore size and a homogeneous pore size distribution. In its
porous media, deposits of calcium carbonate are mainly produced
by Collar in ordo scleractinia and sub-class Octocorallia. Collars
usually form a colony that becomes a porous media with various
dimensions. Given the homogeneous pore size distribution of its
porous media, Collaria is expected to have a high permeability,
porosity and capillary force, and the manufacturing process of using biomaterials as a wick heat pipe is easier.
Many studies have been performed on LHPs, but there have not
been any studies of LHP that use biomaterials as a wick. The purpose of this study was to investigate and characterize the application of biomaterials as a wick to improve the performance of LHP.
Fig. 2. The method of capillary test.
130
N. Putra et al. / International Journal of Thermal Sciences 76 (2014) 128e136
Fig. 4. SEM photograph and pore diameter distribution of biomaterial structure.
LHP wicks made from biomaterials and copper (Cu) metal powder
sintering were tested under the same operational conditions. All of
the wicks were of the same geometrical size, while the structural
parameters depended on their characteristic. The performance of
the nanofluid Al2O3ewater as a working fluid in LHP was also
investigated in this experiment.
2. Research methodology
2.1. Design of capillary sample and testing
The maximum amount of working fluid that can be pumped into
a porous wick is called the capillary pumping amount, and it is one
of the most important parameters of a porous wick. To know the
capillary pumping amount of Collaria, samples were prepared in
cylinder form, 13 mm in diameter and 100 mm in length. In this
experiment, various Collaria types, such as Branching, Massive,
Foliose and Tabulate, were tested. The sintered wick samples made
of Cu powder of 200 mm were prepared with varying of compaction
pressures (4.6 MPa, 6.75 MPa, 9.38 MPa and 12 MPa). The sintering
process was maintained at a temperature of 900 C and the sintering time was 30 min. All samples were then inserted in a glass
tube to ensure that the capillary action occurred only in axial direction, from the bottom to the top of the sample and easy to see
the motion of water in the samples. The use of difference tube
material is predicted has no affect to the results. Fig. 1 shows the
samples of Collaria and sintered powder in glass tube.
In this study, we used Li et al.’s [15] method of measuring the
capillary pumping parameter of a porous wick, the schematic diagram of which is shown in Fig. 2. The method was performed by
measuring the mass of liquid that can be transported in the porous
media at a given time. To do this, a beaker is placed on an analytical
balance and then pushed down the holder until the bottom of
sample touched the surface of the working fluid. When the porous
wick reaches the surface of the working fluid, it pumps working
fluid from the beaker due to capillarity. The reading of the electronic analytical balance, which is connected to the computer, decreases until the porous wick is saturated. The amount of working
fluid being pumped is equated to the reading reduction of the
electronic analytical balance. The accuracy of the electronic balance
used in this study is 0.1 mg. Capillarity measurement is carried out
at atmospheric pressure.
The capillary pumping amount of the Collaria and sintered
porous wicks are shown in Fig. 3. It can be seen from Fig. 3 that the
capillary pumping amount of the sample Tabulate was the best,
followed by Massive, Sintered, Foliose and Branching. Porous wicks
with good capillary pumping amount are the best candidates for
application in LHPs.
N. Putra et al. / International Journal of Thermal Sciences 76 (2014) 128e136
131
Fig. 5. SEM photograph and pore diameter distribution of sintered copper powder.
The micro-morphologies (SEM) of those samples, as shown in
Fig. 4 for types of coral samples and Fig. 5 for sintered wicks, were
further studied to determine the causes of the pumping performances. It was found that the mean pore diameters of the Massive,
Tabulate, Foliose and Branching were 83 mm, 56 mm, 170 mm and
124 mm, and the permeability of these samples was 5 10 15 m2,
4.40 10 15 m2, 6.8 10 15 m2 and 7.8 10 15 m2, respectively.
The mean pore diameters of the sintered wick samples with varying of compaction pressures 4.6 MPa, 6.75 MPa, 9.38 MPa and
12 MPa were 88 mm, 82 mm, 58.57 mm and 84.5 mm respectively.
The porosity was determined based on Archimedes’ principle as
follows: the weight of the wicks in a dry state was measured, and
then the pores were saturated with distilled water; finally, the
weights of the wicks saturated with water both in air and floated in
water were measured. The porosity (ε) of the wicks was calculated
by the following:
PorosityðεÞ ¼
volume of pore wick
volume of porous media
Table 1
Porosity of biomaterial wick and sintered powder.
No
Wick material
Porosity (ε) %
1
2
3
4
5
6
7
8
Tabulate
Massive
Foliose
Branching
S.P Cu 4.6 MPa; 900 C; 30 min
S.P Cu 6.75 MPa; 900 C; 30 min
S.P Cu 9.38 MPa; 900 C; 30 min
S.P Cu 12 MPa; 900 C; 30 min
30.15
35.43
49.75
63.32
47.49
44.47
39.95
15.82
Fig. 6. Loop heat pipe.
(1)
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N. Putra et al. / International Journal of Thermal Sciences 76 (2014) 128e136
Fig. 7. Experimental set-up.
The porosity of the biomaterials and the sintered copper powder
is shown in Table 1. The porosity of the wick depends on the pore
radius of the porous media. The porosity of Massive Collaria was
35.43%, Tabulate was 30.15%, sintered copper powder at 9.38 MPa
was 39.95% and sintered powder at 12 MPa was 15.82%. Based on
the characteristic of corral wick and sintered copper powder wick,
Tabulate and 9.38 MPa sintered copper powder were used as wick
in LHP in this experiment.
High capillary forces which is favorable for a wicking materials
are not only affected by their porous structure but also high surface
wettability or a hydrophilic wick system plays also necessary roll to
transport water in heat pipes. A water droplet will spreads on wick
surface immediately and is pulled into their porous structure due to
high capillary forces. But after exposure to room ambient air, they
will gradually lose their hydrophilic property and eventually turn
into hydrophobic surfaces. The reason for the loss of hydrophilicity
in the heat pipe is oxidation.
2.2. Design of LHP
The LHPs consist of a copper tube of 8 mm in diameter. The
evaporator side was manufactured using copper tube 20 mm in
diameter and 100 mm in length. The jacket used as the condenser
was 20 mm in diameter and 100 mm in length, with an annular gap
through which the cooling liquid flow entered, after which it exited
through two ends of the jacket, as shown in Fig. 6. The flow rates
were considered constant. Three types of LHPs were made and
tested: an LHP with a sintered Cu powder wick and biomaterial
wick with length of wick are 50 mm and an LHP without a wick. The
position of the wick can be seen in Fig. 6. The wicks used were made
of biomaterials of the Collaria Tabulate type, and the sintered
copper powder was subjected to a compaction pressure of 9.38 MPa
at sintering temperatures of 900 C for 30 min, because their pore
size distribution of biomaterial and the sintered powder in that
condition is the smallest and most homogeneous compared to each
type of wicks as shown in Figs. 4 and 5.
The working fluids charged in the LHP were distilled water and
nanofluids. In this investigation, aluminum oxide (Al2O3) nanoparticles 20 nm were dispersed in distilled water (H2O) as a base
fluid by ultrasonication at concentrations of 1%, 3%, and 5%. To
consider the effects of nanofluid volume fraction, the concentrations were calculated using the following equation:
Wnp
% volume fraction ¼
rnp
Wnp
W
(2)
bf
rnp þ rbf
3. Experimental set-up
The experimental set-up for the liquid cooled LHP is shown in
Fig. 7. The objective was to determine the thermal resistance of LHP
as well as the temperature distribution along the LHP for differing
heat loads, working fluids and wick materials. To reproduce electronic thermal activity, the evaporator heating source was powered
by electric heaters controlled by an adjustable DC power supply,
while heat removal was brought about by circulation of the cooling
water through a coaxial liquid cooling heat exchanger. A recirculating thermostatic bath was used to provide a continuous flow of
cooling water in the heat exchanger; the coolant tank’s temperature stability was controlled with an accuracy rate of 0.5 C and
was set at 25 C. The temperature distribution along the LHP was
monitored by six K-type thermocouples with an accuracy rate of
0.1 C. All the measured temperatures were transmitted to the
computer through a Data Acquisition System (NI) in real time and
were recorded once every second for further analysis. The entire
LHP remained thermally insulated for the duration of the tests.
Water and nanofluid were used as the working fluids.
The position of the thermocouples for every experiment can be
seen in Fig. 6. The distance of each point from the origin was
40 mm, 80 mm, 210 mm, 310 mm, 430 mm and 530 mm. From the
measurement of temperature distribution of LHP, the sum of the
thermal resistances (R) between the evaporator and the condenser
section are calculated and defined as
R ¼
Tc Þ
ðTe
Q
(3)
where Te and Tc are the evaporator temperature and condenser
temperature respectively [27]. Te is calculated from the average
value of T1 and T2, while Tc is calculated from the average value of
T4 and T5. The heating power input Q can be calculated by
measuring the input electric power given by
Q ¼ V$I
(4)
where V and I are the supplied voltage (in V) and the electric current (in A) respectively.
N. Putra et al. / International Journal of Thermal Sciences 76 (2014) 128e136
4. Result and discussion
4.1. The influence of wick type and working fluids on heat pipe
performance
Fig. 8 shows the temperature distribution of LHP with a
biomaterial wick and the variation of heat load on the evaporator.
All cases showed a decrease in temperature from the evaporator
section to the condenser section. The generated heat at the evaporator section can be better absorbed using a biomaterial wick LHP
charged with nanofluid Al2O3ewater at 5% volume fraction. The
temperature at the evaporator section using nanofluid 5% vol.
fraction decreases approximately 8 C, 25 C, and 35 C more than
the temperature decrease using only water for input power 10 W,
20 W and 30 W, respectively. The trend of temperature distribution
using a sintered powder wick in the LHP was the same as that of the
biomaterial wick. The temperature at the evaporator section also
increased with power enhancement.
Fig. 8. Temperature distribution of biomaterial wick LHP (a) with water as working
fluid, (b) nanofluids Al2O3ewater 5%.
133
Fig. 9 shows the temperature distribution of the sintered wick
and the biomaterial wick LHP charged with water and Al2O3ewater
nanofluids as the working fluids. As seen in the figure, the temperature range on the evaporator section was approximately 41e
51 C when the biomaterial wick is applied in LHP. If the sintered
wick is used, then the temperature range at the evaporator is between 46 C and 60 C. This indicated that the use of a biomaterial
can reduce temperature on the evaporator. One possible explanation for this is that the biomaterial wick, with its pore structure, can
provide more capillary force to circulate the working fluid.
The nanofluid with a 5% volume fraction could absorb more heat
in the evaporator LHP than distilled water. The presence of nanoparticles on the base fluids increases fluid momentum, and the
interfacial contact area of the particles becomes larger, increasing
the rate of heat transfer [22].
The comparison of LHPs with a sintered powder copper wick, a
biomaterial wick and without a wick or thermosyphons is depicted
Fig. 9. Temperature distribution of working fluids water and nanofluids with (a)
biomaterial wick and (b) sintered powder wick LHP.
134
N. Putra et al. / International Journal of Thermal Sciences 76 (2014) 128e136
Fig. 10. Distribution temperature of LHPs with sintered powder wick, biomaterial wick
and without wick.
Fig. 12. Wick condition before experiment.
Fig. 11. Thermal resistance of LHPs with biomaterial and sintered powder wick.
in Fig. 10. It can be seen that the temperature at the evaporator of
the LHP with a biomaterial wick was the lowest. The highest
temperature was in the LHP without a wick. The low temperature of
the biomaterial wick condition occurs because the pore size distribution of biomaterial is more homogeneous and smaller than
that of the sintered powder. The better performance of the LHP
with wick biomaterials is also due to the better capillarity and
permeability of biomaterials. For an LHP without a wick or thermosyphons with the absence of a capillary pump in the condensate
line, the condensate that flows from the condenser to the
Table 2
Thermal resistance of LHP with biomaterial, sintered powder wick and without
wick.
Wick material
Biomaterial
(tabulate)
S. Powder Cu
Without wick
Thermal Resistance
(Re-a) C/W
0.68
1.47
3.20
evaporator will be inhibited by steam flowing from the evaporator
to the condenser. Steam will flow not only in vapor line but also in
liquid line. This condition leads to the dry-out of evaporator near
the heated surface. The consequent reduction in temperature difference and the rise in the temperature of the evaporator make the
thermal resistance to increase.
Fig. 11 and Table 2 show the thermal resistance of an LHP of the
evaporator and the adiabatic section. A heat pipe charged with
Al2O3ewater nanofluid gave lower thermal resistance than that of
water as a working fluid for the same load level. These results were
similar to those of Kang et al. [23,24], Liu et al. [25,26], and Do et al.
[13]. The lowest thermal resistance of an LHP was produced by a
biomaterial wick and Al2O3ewater working fluid. A comparison of
thermal resistance between the LHPs with a biomaterial wick, a
sintered powder and no wick shows that the use of biomaterials
had the smallest value of thermal resistance. The application of the
biomaterial Tabulate as a wick can decrease the thermal resistance
by approximately 56.3% more than a sintered powder wick.
4.2. Wick condition after using nanofluids
To examine the condition of the various types of wicks after
charging Al2O3ewater nanofluids, the wicks were observed with
SEM and energy dispersive X-ray analysis (EDAX). The condition of
the wicks before and after the use of Al2O3ewater nanofluids is
shown in Figs. 12 and 13. The EDS test in Fig. 12 shows the condition
of the pore structure of biomaterials and sintered powder before
the experiments. The majority of the biomaterial wick structure
consists of the elements C (1.33%), O (69.51%) and Ca (29.26%),
N. Putra et al. / International Journal of Thermal Sciences 76 (2014) 128e136
135
Fig. 13. Wick condition after experiment.
while the wick sintered powder consists of the elements O (0.70%),
Si (0.05%) and Cu (99.25%). After the LHP is charged with Al2O3e
water nanofluid, the condition of each wick can be analyzed by the
thin layer of Al coating on the surface of biomaterials and the wick
sintered powder. This is shown in the SEM photograph and EDS test
in Fig. 13. The amount of Al was 11.38% and 13.43% on the biomaterial wick and the sintered powder wick, respectively.
5. Conclusion
An extensive experimental study was performed to investigate
the influence of a variety of parameters and configurations of LHPs.
The performance of the LHP was improved by using a biomaterial as
wick: the total heat resistance of the heat pipe was lowest when
using a biomaterial wick. The application of the biomaterial Tabulate wick can decrease the thermal resistance by 56.3% when
compared to a sintered powder wick. The use of nanofluids for any
concentrations showed better thermal performance than the use of
distilled water. The lowest thermal resistance of LHP was produced
by a biomaterial wick with Al2O3ewater as the working fluid.
Acknowledgments
The authors would like to thank Directorate of Higher Education
Minister of National Education Republic of Indonesia and the DRPM
University of Indonesia for funding this research.
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Contents lists available at ScienceDirect
International Journal of Thermal Sciences
journal homepage: www.elsevier.com/locate/ijts
Thermal performance of biomaterial wick loop heat pipes with
water-base Al2O3 nanofluids
Nandy Putra a, *, Rosari Saleh b, Wayan Nata Septiadi a, Ashar Okta a, Zein Hamid a
a
b
Heat Transfer Laboratory, Department of Mechanical Engineering University of Indonesia, Kampus UI, Depok 16424, Indonesia
Departemen Fisika, Fakultas MIPA-Universitas Indonesia, Kampus UI, Depok 16426, Indonesia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 September 2012
Received in revised form
29 August 2013
Accepted 30 August 2013
Available online 8 October 2013
Given the increase of heat flux generated by electronic equipment, particular components of a computer
(e.g., the CPU) should always be accompanied with good cooling to achieve optimal operating capability
and a high level of reliability. The use of loop heat pipes (LHPs) in the thermal management of electronic
cooling is a major alternative solution. Before LHPs are implemented as an alternative cooling method for
electronic devices, a quantity of reliability factors should be considered and evaluated, such as wick
structure, material, and the type of working fluid. In this case, the pore size distribution of a biomaterial
(Collar) that is smaller and more homogeneous than pore size distribution of sintered powder was
investigated. The purposes of this experimental study are to examine and analyze the application of a
biomaterial (Collar) as a wick on a loop heat pipe and the use of the nanofluid Al2O3ewater as a working
fluid. The performance of the biomaterial as a wick and nanofluid as a working fluid in LHPs was also
investigated in this experiment. The temperature differences between the evaporator and condenser
sections with the biomaterial wick were less than that using a sintered copper powder wick, and the use
of nanofluids over pure water also resulted in lower temperature differences; i.e., the thermal resistance
of the LHP was lowered when using the biomaterial wick and nanofluids. These results make the
biomaterial (Collar) and nanofluids attractive as wick and working fluid, respectively, in LHP technology.
Ó 2013 Elsevier Masson SAS. All rights reserved.
Keywords:
Loop heat pipe
Wick
Nanofluids
Biomaterial
1. Introduction
Loop heat pipes (LHPs) are widely used in thermal management
electronic cooling devices with high heat fluxes because of their
high efficiency in heat transfer performance, small temperature
differences, and because they contain no moving parts. An LHP is a
two-phase device that has a very high and effective thermal conductivity that uses capillary forces inside the wicked evaporator to
pump the working fluid inside a closed loop [1]. LHPs have several
excellent cooling capabilities and hold considerable promise for
electronic cooling applications. Maydanik et al. [2] summarized
some of the advantages of an LHP implementation in electronic
cooling applications. LHPs have a higher capacity and a lower
thermal resistance at comparable to other heatsinks, a flexibility in
packaging, high heat transfer loads over considerable distances,
and operate efficiently at any orientation in the gravitational field.
There are many examples of LHP experimental developments
and their implications in literature [1e6]. These studies indicate
* Corresponding author.
E-mail address: nandyputra@eng.ui.ac.id (N. Putra).
1290-0729/$ e see front matter Ó 2013 Elsevier Masson SAS. All rights reserved.
http://dx.doi.org/10.1016/j.ijthermalsci.2013.08.020
that LHPs are nearly ready for commercial application. However,
before LHPs are implemented as an alternative cooling method for
electronic devices, several reliability parameters should be
considered and investigated, such as wick structure and material
and the type of working fluid. The wick is the part of the heat pipe
that circulates fluid from the condenser to the evaporator. Hence,
the wick has an important role both in conventional heat pipes and
LHPs [7]. There are different types of capillary wick heat pipes, such
as the screen mesh, sintered powder, groove, and the wire, all of
which are made from metal, composites, and ceramics among other
materials [7e14].
Many studies of wick characteristics have been conducted. For
example, Li et al. [15] studied the ability of the wick to pump liquids
(i.e., the capillarity of wick) on a sintered powder wick capillary. In
their study, they found that the capillarity of the wick increases
with an increasing porosity value of the wick. They also explained
that in a typical real-time changing curve, there are three important
parameters: the capillary pumping amount, the capillary pumping
time and the capillary pumping rate. Leong and Liu [16] also
studied the heat pipes capillary wick, which is made of a sintered
powder concerning which the influence of sintering temperature
(800 C and 1000 C). They also investigated sintering time on wick
N. Putra et al. / International Journal of Thermal Sciences 76 (2014) 128e136
129
Fig. 3. Capillary based on mass amount transported.
Fig. 1. Samples of biomaterial and sintered powder capillary test.
porosity. They concluded that a sintering temperature of 800 C
gave a better porosity value than a sintering temperature of
1000 C. This was because the wick forms a solid stricter at 1000 C.
Consequently, less power capillarity will occur in the wick.
An assessment of wick sintered powder using nickel has been
performed by Mishra et al. [17]. Their results showed that a sintering temperature of 775 C produced the lowest porosity value,
while at 550 C produced the largest value. Zan et al. [18] analyzed
the parameters of a loop heat pipe with a sintered powder wick and
investigated the effect of wick porosity on the permeability of the
wick to determine the amount of heat that can be absorbed by an
LHP. An investigation on wick capillary action has also been performed by Tang et al. [19] using a thermal imaging method. In that
study, they examined a groove wick, a sintered powder wick and a
composite wick (sintered powder combined with the groove). The
results indicate that the capillarity of the composite wick is higher
than the capillarity of sintered powder wick and the groove wick.
Generally, sintered powder is used as a wick in heat pipe technology. The wick structure determines the thermal performance of
heat pipes because it provides the pumping force to return the
working fluid to the evaporator. The secondary purpose of the wick
is to distribute the fluid around the circumference of the tube to
maximize the evaporator surface area [20]. Sintered porous structures have attracted considerable attention over the last few decades because of their special properties, such as good
permeability, high porosity and fine pore size [21]. Although sintered powder is generally very good and has many advantages over
wire mesh and screen mesh, the manufacturing process is very
difficult and time-consuming.
Collaria is a porous biomaterial that has a relatively small
micropore size and a homogeneous pore size distribution. In its
porous media, deposits of calcium carbonate are mainly produced
by Collar in ordo scleractinia and sub-class Octocorallia. Collars
usually form a colony that becomes a porous media with various
dimensions. Given the homogeneous pore size distribution of its
porous media, Collaria is expected to have a high permeability,
porosity and capillary force, and the manufacturing process of using biomaterials as a wick heat pipe is easier.
Many studies have been performed on LHPs, but there have not
been any studies of LHP that use biomaterials as a wick. The purpose of this study was to investigate and characterize the application of biomaterials as a wick to improve the performance of LHP.
Fig. 2. The method of capillary test.
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N. Putra et al. / International Journal of Thermal Sciences 76 (2014) 128e136
Fig. 4. SEM photograph and pore diameter distribution of biomaterial structure.
LHP wicks made from biomaterials and copper (Cu) metal powder
sintering were tested under the same operational conditions. All of
the wicks were of the same geometrical size, while the structural
parameters depended on their characteristic. The performance of
the nanofluid Al2O3ewater as a working fluid in LHP was also
investigated in this experiment.
2. Research methodology
2.1. Design of capillary sample and testing
The maximum amount of working fluid that can be pumped into
a porous wick is called the capillary pumping amount, and it is one
of the most important parameters of a porous wick. To know the
capillary pumping amount of Collaria, samples were prepared in
cylinder form, 13 mm in diameter and 100 mm in length. In this
experiment, various Collaria types, such as Branching, Massive,
Foliose and Tabulate, were tested. The sintered wick samples made
of Cu powder of 200 mm were prepared with varying of compaction
pressures (4.6 MPa, 6.75 MPa, 9.38 MPa and 12 MPa). The sintering
process was maintained at a temperature of 900 C and the sintering time was 30 min. All samples were then inserted in a glass
tube to ensure that the capillary action occurred only in axial direction, from the bottom to the top of the sample and easy to see
the motion of water in the samples. The use of difference tube
material is predicted has no affect to the results. Fig. 1 shows the
samples of Collaria and sintered powder in glass tube.
In this study, we used Li et al.’s [15] method of measuring the
capillary pumping parameter of a porous wick, the schematic diagram of which is shown in Fig. 2. The method was performed by
measuring the mass of liquid that can be transported in the porous
media at a given time. To do this, a beaker is placed on an analytical
balance and then pushed down the holder until the bottom of
sample touched the surface of the working fluid. When the porous
wick reaches the surface of the working fluid, it pumps working
fluid from the beaker due to capillarity. The reading of the electronic analytical balance, which is connected to the computer, decreases until the porous wick is saturated. The amount of working
fluid being pumped is equated to the reading reduction of the
electronic analytical balance. The accuracy of the electronic balance
used in this study is 0.1 mg. Capillarity measurement is carried out
at atmospheric pressure.
The capillary pumping amount of the Collaria and sintered
porous wicks are shown in Fig. 3. It can be seen from Fig. 3 that the
capillary pumping amount of the sample Tabulate was the best,
followed by Massive, Sintered, Foliose and Branching. Porous wicks
with good capillary pumping amount are the best candidates for
application in LHPs.
N. Putra et al. / International Journal of Thermal Sciences 76 (2014) 128e136
131
Fig. 5. SEM photograph and pore diameter distribution of sintered copper powder.
The micro-morphologies (SEM) of those samples, as shown in
Fig. 4 for types of coral samples and Fig. 5 for sintered wicks, were
further studied to determine the causes of the pumping performances. It was found that the mean pore diameters of the Massive,
Tabulate, Foliose and Branching were 83 mm, 56 mm, 170 mm and
124 mm, and the permeability of these samples was 5 10 15 m2,
4.40 10 15 m2, 6.8 10 15 m2 and 7.8 10 15 m2, respectively.
The mean pore diameters of the sintered wick samples with varying of compaction pressures 4.6 MPa, 6.75 MPa, 9.38 MPa and
12 MPa were 88 mm, 82 mm, 58.57 mm and 84.5 mm respectively.
The porosity was determined based on Archimedes’ principle as
follows: the weight of the wicks in a dry state was measured, and
then the pores were saturated with distilled water; finally, the
weights of the wicks saturated with water both in air and floated in
water were measured. The porosity (ε) of the wicks was calculated
by the following:
PorosityðεÞ ¼
volume of pore wick
volume of porous media
Table 1
Porosity of biomaterial wick and sintered powder.
No
Wick material
Porosity (ε) %
1
2
3
4
5
6
7
8
Tabulate
Massive
Foliose
Branching
S.P Cu 4.6 MPa; 900 C; 30 min
S.P Cu 6.75 MPa; 900 C; 30 min
S.P Cu 9.38 MPa; 900 C; 30 min
S.P Cu 12 MPa; 900 C; 30 min
30.15
35.43
49.75
63.32
47.49
44.47
39.95
15.82
Fig. 6. Loop heat pipe.
(1)
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N. Putra et al. / International Journal of Thermal Sciences 76 (2014) 128e136
Fig. 7. Experimental set-up.
The porosity of the biomaterials and the sintered copper powder
is shown in Table 1. The porosity of the wick depends on the pore
radius of the porous media. The porosity of Massive Collaria was
35.43%, Tabulate was 30.15%, sintered copper powder at 9.38 MPa
was 39.95% and sintered powder at 12 MPa was 15.82%. Based on
the characteristic of corral wick and sintered copper powder wick,
Tabulate and 9.38 MPa sintered copper powder were used as wick
in LHP in this experiment.
High capillary forces which is favorable for a wicking materials
are not only affected by their porous structure but also high surface
wettability or a hydrophilic wick system plays also necessary roll to
transport water in heat pipes. A water droplet will spreads on wick
surface immediately and is pulled into their porous structure due to
high capillary forces. But after exposure to room ambient air, they
will gradually lose their hydrophilic property and eventually turn
into hydrophobic surfaces. The reason for the loss of hydrophilicity
in the heat pipe is oxidation.
2.2. Design of LHP
The LHPs consist of a copper tube of 8 mm in diameter. The
evaporator side was manufactured using copper tube 20 mm in
diameter and 100 mm in length. The jacket used as the condenser
was 20 mm in diameter and 100 mm in length, with an annular gap
through which the cooling liquid flow entered, after which it exited
through two ends of the jacket, as shown in Fig. 6. The flow rates
were considered constant. Three types of LHPs were made and
tested: an LHP with a sintered Cu powder wick and biomaterial
wick with length of wick are 50 mm and an LHP without a wick. The
position of the wick can be seen in Fig. 6. The wicks used were made
of biomaterials of the Collaria Tabulate type, and the sintered
copper powder was subjected to a compaction pressure of 9.38 MPa
at sintering temperatures of 900 C for 30 min, because their pore
size distribution of biomaterial and the sintered powder in that
condition is the smallest and most homogeneous compared to each
type of wicks as shown in Figs. 4 and 5.
The working fluids charged in the LHP were distilled water and
nanofluids. In this investigation, aluminum oxide (Al2O3) nanoparticles 20 nm were dispersed in distilled water (H2O) as a base
fluid by ultrasonication at concentrations of 1%, 3%, and 5%. To
consider the effects of nanofluid volume fraction, the concentrations were calculated using the following equation:
Wnp
% volume fraction ¼
rnp
Wnp
W
(2)
bf
rnp þ rbf
3. Experimental set-up
The experimental set-up for the liquid cooled LHP is shown in
Fig. 7. The objective was to determine the thermal resistance of LHP
as well as the temperature distribution along the LHP for differing
heat loads, working fluids and wick materials. To reproduce electronic thermal activity, the evaporator heating source was powered
by electric heaters controlled by an adjustable DC power supply,
while heat removal was brought about by circulation of the cooling
water through a coaxial liquid cooling heat exchanger. A recirculating thermostatic bath was used to provide a continuous flow of
cooling water in the heat exchanger; the coolant tank’s temperature stability was controlled with an accuracy rate of 0.5 C and
was set at 25 C. The temperature distribution along the LHP was
monitored by six K-type thermocouples with an accuracy rate of
0.1 C. All the measured temperatures were transmitted to the
computer through a Data Acquisition System (NI) in real time and
were recorded once every second for further analysis. The entire
LHP remained thermally insulated for the duration of the tests.
Water and nanofluid were used as the working fluids.
The position of the thermocouples for every experiment can be
seen in Fig. 6. The distance of each point from the origin was
40 mm, 80 mm, 210 mm, 310 mm, 430 mm and 530 mm. From the
measurement of temperature distribution of LHP, the sum of the
thermal resistances (R) between the evaporator and the condenser
section are calculated and defined as
R ¼
Tc Þ
ðTe
Q
(3)
where Te and Tc are the evaporator temperature and condenser
temperature respectively [27]. Te is calculated from the average
value of T1 and T2, while Tc is calculated from the average value of
T4 and T5. The heating power input Q can be calculated by
measuring the input electric power given by
Q ¼ V$I
(4)
where V and I are the supplied voltage (in V) and the electric current (in A) respectively.
N. Putra et al. / International Journal of Thermal Sciences 76 (2014) 128e136
4. Result and discussion
4.1. The influence of wick type and working fluids on heat pipe
performance
Fig. 8 shows the temperature distribution of LHP with a
biomaterial wick and the variation of heat load on the evaporator.
All cases showed a decrease in temperature from the evaporator
section to the condenser section. The generated heat at the evaporator section can be better absorbed using a biomaterial wick LHP
charged with nanofluid Al2O3ewater at 5% volume fraction. The
temperature at the evaporator section using nanofluid 5% vol.
fraction decreases approximately 8 C, 25 C, and 35 C more than
the temperature decrease using only water for input power 10 W,
20 W and 30 W, respectively. The trend of temperature distribution
using a sintered powder wick in the LHP was the same as that of the
biomaterial wick. The temperature at the evaporator section also
increased with power enhancement.
Fig. 8. Temperature distribution of biomaterial wick LHP (a) with water as working
fluid, (b) nanofluids Al2O3ewater 5%.
133
Fig. 9 shows the temperature distribution of the sintered wick
and the biomaterial wick LHP charged with water and Al2O3ewater
nanofluids as the working fluids. As seen in the figure, the temperature range on the evaporator section was approximately 41e
51 C when the biomaterial wick is applied in LHP. If the sintered
wick is used, then the temperature range at the evaporator is between 46 C and 60 C. This indicated that the use of a biomaterial
can reduce temperature on the evaporator. One possible explanation for this is that the biomaterial wick, with its pore structure, can
provide more capillary force to circulate the working fluid.
The nanofluid with a 5% volume fraction could absorb more heat
in the evaporator LHP than distilled water. The presence of nanoparticles on the base fluids increases fluid momentum, and the
interfacial contact area of the particles becomes larger, increasing
the rate of heat transfer [22].
The comparison of LHPs with a sintered powder copper wick, a
biomaterial wick and without a wick or thermosyphons is depicted
Fig. 9. Temperature distribution of working fluids water and nanofluids with (a)
biomaterial wick and (b) sintered powder wick LHP.
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N. Putra et al. / International Journal of Thermal Sciences 76 (2014) 128e136
Fig. 10. Distribution temperature of LHPs with sintered powder wick, biomaterial wick
and without wick.
Fig. 12. Wick condition before experiment.
Fig. 11. Thermal resistance of LHPs with biomaterial and sintered powder wick.
in Fig. 10. It can be seen that the temperature at the evaporator of
the LHP with a biomaterial wick was the lowest. The highest
temperature was in the LHP without a wick. The low temperature of
the biomaterial wick condition occurs because the pore size distribution of biomaterial is more homogeneous and smaller than
that of the sintered powder. The better performance of the LHP
with wick biomaterials is also due to the better capillarity and
permeability of biomaterials. For an LHP without a wick or thermosyphons with the absence of a capillary pump in the condensate
line, the condensate that flows from the condenser to the
Table 2
Thermal resistance of LHP with biomaterial, sintered powder wick and without
wick.
Wick material
Biomaterial
(tabulate)
S. Powder Cu
Without wick
Thermal Resistance
(Re-a) C/W
0.68
1.47
3.20
evaporator will be inhibited by steam flowing from the evaporator
to the condenser. Steam will flow not only in vapor line but also in
liquid line. This condition leads to the dry-out of evaporator near
the heated surface. The consequent reduction in temperature difference and the rise in the temperature of the evaporator make the
thermal resistance to increase.
Fig. 11 and Table 2 show the thermal resistance of an LHP of the
evaporator and the adiabatic section. A heat pipe charged with
Al2O3ewater nanofluid gave lower thermal resistance than that of
water as a working fluid for the same load level. These results were
similar to those of Kang et al. [23,24], Liu et al. [25,26], and Do et al.
[13]. The lowest thermal resistance of an LHP was produced by a
biomaterial wick and Al2O3ewater working fluid. A comparison of
thermal resistance between the LHPs with a biomaterial wick, a
sintered powder and no wick shows that the use of biomaterials
had the smallest value of thermal resistance. The application of the
biomaterial Tabulate as a wick can decrease the thermal resistance
by approximately 56.3% more than a sintered powder wick.
4.2. Wick condition after using nanofluids
To examine the condition of the various types of wicks after
charging Al2O3ewater nanofluids, the wicks were observed with
SEM and energy dispersive X-ray analysis (EDAX). The condition of
the wicks before and after the use of Al2O3ewater nanofluids is
shown in Figs. 12 and 13. The EDS test in Fig. 12 shows the condition
of the pore structure of biomaterials and sintered powder before
the experiments. The majority of the biomaterial wick structure
consists of the elements C (1.33%), O (69.51%) and Ca (29.26%),
N. Putra et al. / International Journal of Thermal Sciences 76 (2014) 128e136
135
Fig. 13. Wick condition after experiment.
while the wick sintered powder consists of the elements O (0.70%),
Si (0.05%) and Cu (99.25%). After the LHP is charged with Al2O3e
water nanofluid, the condition of each wick can be analyzed by the
thin layer of Al coating on the surface of biomaterials and the wick
sintered powder. This is shown in the SEM photograph and EDS test
in Fig. 13. The amount of Al was 11.38% and 13.43% on the biomaterial wick and the sintered powder wick, respectively.
5. Conclusion
An extensive experimental study was performed to investigate
the influence of a variety of parameters and configurations of LHPs.
The performance of the LHP was improved by using a biomaterial as
wick: the total heat resistance of the heat pipe was lowest when
using a biomaterial wick. The application of the biomaterial Tabulate wick can decrease the thermal resistance by 56.3% when
compared to a sintered powder wick. The use of nanofluids for any
concentrations showed better thermal performance than the use of
distilled water. The lowest thermal resistance of LHP was produced
by a biomaterial wick with Al2O3ewater as the working fluid.
Acknowledgments
The authors would like to thank Directorate of Higher Education
Minister of National Education Republic of Indonesia and the DRPM
University of Indonesia for funding this research.
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