Analysis of CuOwater nanofluid aplication on heat pipe.
Applied Mechanics and Materials Vol. 590 (2014) pp 234-238
Online available since 2014/Jun/30 at www.scientific.net
© (2014) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/AMM.590.234
Analysis of CuO-Water Nanofluid Application on Heat Pipe
Nandy Putra1, a*, Wayan Nata Septiadi1, b and Ranggi Sahmura1, c
1
Heat Pipe Technology Research Cluster, Department of Mechanical Engineering University of
Indonesia
a
nandyputra@eng.ui.ac.id, bwayan.nata@gmail.com, cranggi.sahmura@ui.ac.id
*
Corresponding author
Keywords: Heat pipe, CuO, Nanofluid, Biomaterial.
Abstract. Since their first introduction to the world, both heat pipe and nanofluid have caught the
interest of many researchers. Heat pipe with its unique and exceptional capability in transferring
heat passively and effectively, was studied intensively and developed extensively for many
applications. While nanofluid with its higher thermal conductivity and some other upgraded
properties compared to conventional fluid rose as appealing research subject especially on fluid and
thermal research area. This study analyzes the utilization of CuO-water nanofluid on biomaterial
wick heat pipe. Laboratory-developed CuO-water nanofluid was used as working fluid for vertically
straight-shaped biomaterial wick heat pipe. From the experiment, it was shown that the application
of CuO-water nanofluid reduced the heat pipe thermal resistance up to 83%. It was figured out that
this enhancement is due to the combination of higher thermal conductivity and better wettability of
the fluid. It was also found that the heat pipe with nanofluid did not show significant degradation
though being inactivated for several weeks. However, it was figured out that unlike the application
of low concentration nanofluid, application of high concentration nanofluid was insignificant in
improving thermal performance of the heat pipe.
Introduction
Heat pipe and nanofluid are among breakthrough in applied thermal and fluid area of research.
Since their debut in the nineteenth century, both have caught the attention of many researchers.
Heat pipe, as a heat exchanger, has extraordinary ability to transfer heat effectively – via phase
change of working fluid – and efficiently due to the natural circulation of working fluid through
gravity or capillary wick structure, make it highly potential in many application [1]. In order to
develop a performer heat pipe, researchers have been investigating the effect of deciding factors
like wick, working fluid, and material as well as geometry. Grooved-structure, screen mesh,
sintered-powder, and metal foam has been tested as capillary wick structure, meanwhile water,
acetone, ethanol, refrigerant and nanofluid has been utilized as working fluid on both cylindrical
and flat-plate heat pipe [2-5].
Nanofluid, introduced by Choi [6] in 1995, arises as an interesting subject of research. It is
basically a suspension of nano-sized particle inside the base fluid. Having good thermal ability, this
fluid could be used for heat exchanger or other cooling system [7]. For heat pipe, nanofluid of many
types like Silver (Ag), Gold (Au), Cooper (Cu) and other oxides like Aluminum oxide (Al2O3),
Titanium dioxide (TiO2), and Zinc oxide (ZnO) was tested by Putra et al., by Saleh et al. and also
by other researchers as overviewed by Buschmann [2, 8-10]. Most of the studies stated that
nanofluid utilization delivered noticeable enhancement on thermal performance of heat pipe.
However, long term stability of nanofluids and long term operating condition of heat pipe operated
with nanofluids has not been yet investigated intensively [10].
This study closely analyzed the application of Copper-oxide-water (CuO-water) nanofluid in
straight, vertical heat pipe with biomaterial wick. The CuO-water nanofluid at both low and high
concentration to obtain the optimum value of concentration were tested. The long term stability of
nanofluid as well as the effect of nano-particle to the wettability of the fluid were also investigated
in this research.
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,
www.ttp.net. (ID: 152.118.24.10-17/07/14,06:13:39)
Applied Mechanics and Materials Vol. 590
235
Methods
Nanofluid Preparation
The nanofluid used in this study was CuO-water. Both the CuO nano-particle and the CuO-water
nanofluid was synthesized and made at the laboratory. The CuO nano-particle was synthesized
using the same method used previously [11] that is through sol-gel method, which is extracting CuO
particle from dissolution reaction of some amount of CuSO4.5H2O with NaOH. The chemical
reaction of nanoparticle synthesizing shown by Eq. (1).
CuSO4.5H2O + 2NaOH Cu(OH)2 + Na2SO4 + 5H2O
(1.a)
Cu(OH)2 CuO + H2O
(1.b)
To ensure the size and the content of the particles, Scanning Electron Microscope (SEM) image
and Energy Dispersive X-ray (EDX) was obtained, and the result showed that the particles are nano
in size and the particle consisted of 67.73% Cu and 32.27% O. The SEM image and the EDX result
are shown in Fig. 1. The nano-particle then dispersed to water as its base fluid using ultrasonic
processor at 60 Hz for 30 minutes. The nanofluid volume fraction used the same formulation as the
author did previously [2].
Figure 1. (a) SEM Image and
(b) EDX result of CuO nano-particle
Experimental Setup
The heat pipe being used in this study is a copper-based with biomaterial wick. It has the length of 5
cm and diameter of an inch, with the wick structure wrapped the inner diameter of the pipe, from
evaporator to condenser. The heat pipe is shown in Fig 2(a). The experiment conducted mainly
consisted of three parts. The first part is to assess the quality of the nanofluids before utilization.
The second one is to investigate the thermal performance of heat pipe, charged by CuO-water
nanofluids with various concentrations. Electric heater was used to simulate the heat given by
computer microprocessor at minimum and maximum load. The experimental setup for the second
part of experiment is shown in Fig. 2(b). The third part is to figure out the long term stability of the
nanofluid. The heat pipe containing nanofluid was tested twice, with each test separated by a period
of 4 weeks.
Figure 2 (a) The heat pipe used in this study
(b) Experimental setup of this study
236
Innovative Solutions in the Field of Engineering Sciences
Result and Discussion
The Assessment of CuO-Water Nanofluid
The measurement of nanofluid thermal conductivity was done using KD2 Pro Thermal Properties
Analyzer Decagon and compared with other studies [11]. From Fig. 3(a), it can be seen that the
relative thermal conductivity of nanofluid increased as the volume concentration increased. The
higher thermal conductivity compared to its base fluid could be addressed to the existence of high
thermal conductivity particle dissolved inside the fluid. The Brownian effect as well as the layer of
liquid-solid interface of nano-particle and a more even molecular structure at the interface of solid
nano-particle also have role in enhancing the thermal properties of the fluid [12]. The more particles
dissolved at higher concentration of nanofluid leads to the magnification of those effects, thus,
higher thermal conductivity. Though the result obtain from this study are slightly different with the
model by Xue and the model by Maxwell [13], the trend of the results are quite similar. The
presence of some amount of nano-particle dispersed inside the base fluid also has other effect,
which is an improvement in wettability [14] Wettability is the ability of fluid to wet the surface, and
usually measured by contact angle. From Fig. 3(b), it can be seen that the addition of CuO
nano-particle decreased the contact angle of water from 43o to 32o, hence better wettability.
Figure 3. (a) Relative thermal conductivity
of CuO-water used in this study, and
comparison with some other studies,
(b) Comparison of wettability
between water and CuO-water
nanofluid
Thermal Performance of Heat Pipe with CuO-Water Working Fluid
The experiment conducted in this study utilized both low and high concentration nanofluids to heat
pipe. The low volume concentration nanofluids are ones with volume fraction below 1%, while the
high concentration nanofluids are those with volume fraction higher than 1%. As can be seen in Fig.
4, the increment of volume fraction up to 7% (from 0.1% to 7%) yielded lower heater surface
temperature. But then the surface temperature rose again as the volume fraction increased from 7%
to 9% and 10%. The occurrence of this phenomenon explained as follows.
As previously explained, nanofluid with higher concentration has higher thermal conductivity.
The increment of nanofluid’s thermal conductivity being used affects the overall effective thermal
conductivity of the heat pipe. Hence up to some extent, increment of nanofluid’s volume fraction –
also means increment of nanofluid’s thermal conductivity – would also leads to the increment of the
overall effective thermal conductivity of the heat pipe. Thus, more heat dissipated and lower heater
surface temperature [15, 16]. But then there is viscous limit [17]. It explained that there is a
condition when the fluid becomes too viscous, and unable to flow inside the heat pipe well. This
hampered the heat transfer capability of the heat pipe. The theory explained why the surface
temperature of the heater rose at the application of nanofluids with very high volume fraction. At
this very high volume fraction, the fluid becomes too viscous that it exceeds the viscous limit of the
heat pipe design. To be specific, the addition of 0.711 gram of CuO nano-particles on the 0.9%
CuO-water nanofluid reduced the heater surface temperature as much as 6.5%, and as shown in Fig.
5(a) reduced the thermal resistance as much as 70%. Meanwhile the addition of 5.530 gram of CuO
nano-particles on the 7% CuO-water nanofluid reduced the heater surface temperature as much as
Applied Mechanics and Materials Vol. 590
237
14.3 % and reduced the thermal resistance as much as 83%. These facts showed that the more
significant improvement found at the addition of small amount of nano-particles, not at the large
one. To be precise, the 7% CuO-water nanofluid added 4,819 gram of CuO nano-particles to the
0.9% CuO-water nanofluids, but only reduce the surface temperature as much as 8.3 % and reduce
the thermal resistance by 43.3 %. This again could be attributed to the viscous limit, that is already
been closed down by the 7% CuO-water nanofluid.
Long Term Stability of Nanofluid
To test the long term stability of the nanofluid, heat pipe charged with low concentration, 0.9%
CuO-water nanofluid and one with high concentration, 5% CuO nanofluid tested twice. Each test
separated over period of 4 weeks, and in that period the heat pipe was not being operated. It can be
concluded that by the use of low concentration nanofluid, the heat pipe did not show any significant
reduction in performance (not shown in figure). Unlikely, the use of high concentration nanofluid
clearly show degradation in performance, indicated by the surface temperature that is higher at the
second operation as shown in Fig. 5(b).
Figure 4. Thermal performance of heat pipe with various CuO-water nanofluid concentration
Figure 5. (a) Thermal resistance of heat
pipe at application of 0.9% and 7%
CuO-water nanofluid
(b) Thermal performance of heat pipe with
nanofluid after not being operated for 4
weeks
In both case, the nanofluids left deposition at the evaporator. However, at lower nanofluid
concentration, the deposition form thin layer of nano-particle and enhancing wettability and
intensifying bubble generation, thus decreasing thermal resistance [9]. While at higher nanofluid
concentration, the deposition was so thick that it obstructed the heat transfer. The capillary pores of
the wick were also covered by the thick deposition, increasing the thermal resistance.
Summary
The analysis of CuO-water nanofluid as working fluid for heat pipe has been conducted. It was
figured out that due to the existence of both positive and negative effect of nanofluid application,
there is an optimum value of nanofluid concentration that yielded the best heat pipe performance. It
238
Innovative Solutions in the Field of Engineering Sciences
was also found that the addition of high amount of nano-particles proven to be insignificant to the
heat pipe thermal performance. The best improvement both in thermal performance and long term
stability found on the application of low concentration nanofluid, that is in this case CuO-water
nanofluid with 0.9% volume fraction.
Reference
[1] Chen, Y.-S., Chien, K.-H., Hung, T.-C., Wang, C.-C., Ferng, Y.-M., Pei, B.-S., T Numerical
simulation of a heat sink embedded with a vapor chamber and calculation of effective thermal
conductivity of a vapor chamber. Applied Thermal Engineering. 29(13) (2009) 2655-2664
[2] Septiadi, W.N., Putra, N., Juarsa, M., Putra, I.P.A., Sahmura, R., Characteristics of screen mesh
wick heat pipe with nano-fluid as passive cooling system, Atom Indonesia. 39 (2013)
[3] Ji, X, J. Xu, and A.M Abanda, Copper foam based vapor chamber for high heat flux
dissipation, Experimental Thermal and Fluid Science. 40 (2012) 93-102
[4] Naphon, P. and S.Wiriyasart, Study on the vapor chamber with refrigerant R-141b as working
fluid for HDD cooling, Int. Communications in Heat and Mass Transfer. 39(9) (2012) 1449-1452
[5] Wong, S.-C and C.-W Chen, Visualization and evaporator resistance measurement for a
groove-wicked flat-plate heat pipe, Int. J. of Heat and Mass Transfer. 55(9-10) (2012) 2229-2234
[6] S.U. Choi and J. Eastman, Enhancing thermal conductivity of fluids with nanoparticles,
Argonne national Lab., IL, United States. (1995)
[7] Saleh, R., Putra, N., Wibowo, R.E., Septiadi, W.N., Prakoso, S.P., Titanium dioxide
nanofluids for heat transfer applications, Experimental Thermal and Fluid Science. 52 (2014) 12-29
[8] Putra, N., Septiadi, W.N., Rahman, H., Irwansyah, R., Thermal performance of screen mesh
wick heat pipes with nanofluids, Experimental Thermal and Fluid Science. 40 (2012) 10-17
[9] Saleh, R., Putra, N., Prakoso, S.P., Septiadi, W.N., Experimental investigation of thermal
conductivity and heat pipe thermal performance of ZnO nanofluids, International Journal of
Thermal Sciences. 62 (2013) 125-132
[10] M.H. Buschmann, Nanofluids in thermosyphons and heat pipes: Overview of recent
experiments and modelling approaches, International Journal of Thermal Sciences. 72 (2013) 1- 17
[11] N. Putra, W.N. Septiadi, R. Saleh, R.A. Koestoer, S.P. Prakoso, The effect of Cu-water
nanofluid and biomaterial wick on loop heat pipe performance, Advanced Material Research.
875-877 (2014) 356-361
[12] P. Keblinski, R. Prasher, and J. Eapen, Thermal conductance of nanofluids: is the controversy
over?, Journal of Nanoparticle research. 10 (2008) 1089-1097
[13] Q.-Z. Xue, Model for effective thermal conductivity of nanofluids, Physics Letters A. 307
(2003) 313-317
[14] S. Khandekar, Y.M. Joshi and B. Mehta, Thermal performance of closed two-phase
thermosyphon using nanofluids, International Journal of Thermal Sciences. 47 (2008) 659-667
[15] C. Li and G. Peterson, The effective thermal conductivity of wire screen, International Journal
of Heat and Mass Transfer. 49 (2006) 4095-4105
[16] D.A.R.P.A. Kew, Heat Pipe: Theory, Design and Applications. USA: Butterworth-Heinemann
is an imprint of Elsevier (2006)
[17] P. Nemec, A. Caja, and M. Malcho, Mathematical model for heat transfer limitations of heat
pipe, Mathematical and Computer Modelling. 57 (2013) 126-136
Innovative Solutions in the Field of Engineering Sciences
10.4028/www.scientific.net/AMM.590
Analysis of CuO-Water Nanofluid Application on Heat Pipe
10.4028/www.scientific.net/AMM.590.234
DOI References
[3] Ji, X, J. Xu, and A. M Abanda, Copper foam based vapor chamber for high heat flux dissipation,
Experimental Thermal and Fluid Science. 40 (2012) 93-102.
http://dx.doi.org/10.1016/j.expthermflusci.2012.02.004
[7] Saleh, R., Putra, N., Wibowo, R.E., Septiadi, W.N., Prakoso, S.P., Titanium dioxide nanofluids for heat
transfer applications, Experimental Thermal and Fluid Science. 52 (2014) 12-29.
http://dx.doi.org/10.1016/j.expthermflusci.2013.08.018
[8] Putra, N., Septiadi, W.N., Rahman, H., Irwansyah, R., Thermal performance of screen mesh wick heat
pipes with nanofluids, Experimental Thermal and Fluid Science. 40 (2012) 10-17.
http://dx.doi.org/10.1016/j.expthermflusci.2012.01.007
[9] Saleh, R., Putra, N., Prakoso, S.P., Septiadi, W.N., Experimental investigation of thermal conductivity and
heat pipe thermal performance of ZnO nanofluids, International Journal of Thermal Sciences. 62 (2013) 125132.
http://dx.doi.org/10.1016/j.ijthermalsci.2012.07.011
[11] N. Putra, W.N. Septiadi, R. Saleh, R.A. Koestoer, S.P. Prakoso, The effect of Cu-water nanofluid and
biomaterial wick on loop heat pipe performance, Advanced Material Research. 875-877 (2014) 356-361.
http://dx.doi.org/10.4028/www.scientific.net/AMR.875-877.356
[12] P. Keblinski, R. Prasher, and J. Eapen, Thermal conductance of nanofluids: is the controversy over?,
Journal of Nanoparticle research. 10 (2008) 1089-1097.
http://dx.doi.org/10.1007/s11051-007-9352-1
[14] S. Khandekar, Y.M. Joshi and B. Mehta, Thermal performance of closed two-phase thermosyphon using
nanofluids, International Journal of Thermal Sciences. 47 (2008) 659-667.
http://dx.doi.org/10.1016/j.ijthermalsci.2007.06.005
[17] P. Nemec, A. Caja, and M. Malcho, Mathematical model for heat transfer limitations of heat pipe,
Mathematical and Computer Modelling. 57 (2013) 126-136.
http://dx.doi.org/10.1016/j.mcm.2011.06.047
Online available since 2014/Jun/30 at www.scientific.net
© (2014) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/AMM.590.234
Analysis of CuO-Water Nanofluid Application on Heat Pipe
Nandy Putra1, a*, Wayan Nata Septiadi1, b and Ranggi Sahmura1, c
1
Heat Pipe Technology Research Cluster, Department of Mechanical Engineering University of
Indonesia
a
nandyputra@eng.ui.ac.id, bwayan.nata@gmail.com, cranggi.sahmura@ui.ac.id
*
Corresponding author
Keywords: Heat pipe, CuO, Nanofluid, Biomaterial.
Abstract. Since their first introduction to the world, both heat pipe and nanofluid have caught the
interest of many researchers. Heat pipe with its unique and exceptional capability in transferring
heat passively and effectively, was studied intensively and developed extensively for many
applications. While nanofluid with its higher thermal conductivity and some other upgraded
properties compared to conventional fluid rose as appealing research subject especially on fluid and
thermal research area. This study analyzes the utilization of CuO-water nanofluid on biomaterial
wick heat pipe. Laboratory-developed CuO-water nanofluid was used as working fluid for vertically
straight-shaped biomaterial wick heat pipe. From the experiment, it was shown that the application
of CuO-water nanofluid reduced the heat pipe thermal resistance up to 83%. It was figured out that
this enhancement is due to the combination of higher thermal conductivity and better wettability of
the fluid. It was also found that the heat pipe with nanofluid did not show significant degradation
though being inactivated for several weeks. However, it was figured out that unlike the application
of low concentration nanofluid, application of high concentration nanofluid was insignificant in
improving thermal performance of the heat pipe.
Introduction
Heat pipe and nanofluid are among breakthrough in applied thermal and fluid area of research.
Since their debut in the nineteenth century, both have caught the attention of many researchers.
Heat pipe, as a heat exchanger, has extraordinary ability to transfer heat effectively – via phase
change of working fluid – and efficiently due to the natural circulation of working fluid through
gravity or capillary wick structure, make it highly potential in many application [1]. In order to
develop a performer heat pipe, researchers have been investigating the effect of deciding factors
like wick, working fluid, and material as well as geometry. Grooved-structure, screen mesh,
sintered-powder, and metal foam has been tested as capillary wick structure, meanwhile water,
acetone, ethanol, refrigerant and nanofluid has been utilized as working fluid on both cylindrical
and flat-plate heat pipe [2-5].
Nanofluid, introduced by Choi [6] in 1995, arises as an interesting subject of research. It is
basically a suspension of nano-sized particle inside the base fluid. Having good thermal ability, this
fluid could be used for heat exchanger or other cooling system [7]. For heat pipe, nanofluid of many
types like Silver (Ag), Gold (Au), Cooper (Cu) and other oxides like Aluminum oxide (Al2O3),
Titanium dioxide (TiO2), and Zinc oxide (ZnO) was tested by Putra et al., by Saleh et al. and also
by other researchers as overviewed by Buschmann [2, 8-10]. Most of the studies stated that
nanofluid utilization delivered noticeable enhancement on thermal performance of heat pipe.
However, long term stability of nanofluids and long term operating condition of heat pipe operated
with nanofluids has not been yet investigated intensively [10].
This study closely analyzed the application of Copper-oxide-water (CuO-water) nanofluid in
straight, vertical heat pipe with biomaterial wick. The CuO-water nanofluid at both low and high
concentration to obtain the optimum value of concentration were tested. The long term stability of
nanofluid as well as the effect of nano-particle to the wettability of the fluid were also investigated
in this research.
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,
www.ttp.net. (ID: 152.118.24.10-17/07/14,06:13:39)
Applied Mechanics and Materials Vol. 590
235
Methods
Nanofluid Preparation
The nanofluid used in this study was CuO-water. Both the CuO nano-particle and the CuO-water
nanofluid was synthesized and made at the laboratory. The CuO nano-particle was synthesized
using the same method used previously [11] that is through sol-gel method, which is extracting CuO
particle from dissolution reaction of some amount of CuSO4.5H2O with NaOH. The chemical
reaction of nanoparticle synthesizing shown by Eq. (1).
CuSO4.5H2O + 2NaOH Cu(OH)2 + Na2SO4 + 5H2O
(1.a)
Cu(OH)2 CuO + H2O
(1.b)
To ensure the size and the content of the particles, Scanning Electron Microscope (SEM) image
and Energy Dispersive X-ray (EDX) was obtained, and the result showed that the particles are nano
in size and the particle consisted of 67.73% Cu and 32.27% O. The SEM image and the EDX result
are shown in Fig. 1. The nano-particle then dispersed to water as its base fluid using ultrasonic
processor at 60 Hz for 30 minutes. The nanofluid volume fraction used the same formulation as the
author did previously [2].
Figure 1. (a) SEM Image and
(b) EDX result of CuO nano-particle
Experimental Setup
The heat pipe being used in this study is a copper-based with biomaterial wick. It has the length of 5
cm and diameter of an inch, with the wick structure wrapped the inner diameter of the pipe, from
evaporator to condenser. The heat pipe is shown in Fig 2(a). The experiment conducted mainly
consisted of three parts. The first part is to assess the quality of the nanofluids before utilization.
The second one is to investigate the thermal performance of heat pipe, charged by CuO-water
nanofluids with various concentrations. Electric heater was used to simulate the heat given by
computer microprocessor at minimum and maximum load. The experimental setup for the second
part of experiment is shown in Fig. 2(b). The third part is to figure out the long term stability of the
nanofluid. The heat pipe containing nanofluid was tested twice, with each test separated by a period
of 4 weeks.
Figure 2 (a) The heat pipe used in this study
(b) Experimental setup of this study
236
Innovative Solutions in the Field of Engineering Sciences
Result and Discussion
The Assessment of CuO-Water Nanofluid
The measurement of nanofluid thermal conductivity was done using KD2 Pro Thermal Properties
Analyzer Decagon and compared with other studies [11]. From Fig. 3(a), it can be seen that the
relative thermal conductivity of nanofluid increased as the volume concentration increased. The
higher thermal conductivity compared to its base fluid could be addressed to the existence of high
thermal conductivity particle dissolved inside the fluid. The Brownian effect as well as the layer of
liquid-solid interface of nano-particle and a more even molecular structure at the interface of solid
nano-particle also have role in enhancing the thermal properties of the fluid [12]. The more particles
dissolved at higher concentration of nanofluid leads to the magnification of those effects, thus,
higher thermal conductivity. Though the result obtain from this study are slightly different with the
model by Xue and the model by Maxwell [13], the trend of the results are quite similar. The
presence of some amount of nano-particle dispersed inside the base fluid also has other effect,
which is an improvement in wettability [14] Wettability is the ability of fluid to wet the surface, and
usually measured by contact angle. From Fig. 3(b), it can be seen that the addition of CuO
nano-particle decreased the contact angle of water from 43o to 32o, hence better wettability.
Figure 3. (a) Relative thermal conductivity
of CuO-water used in this study, and
comparison with some other studies,
(b) Comparison of wettability
between water and CuO-water
nanofluid
Thermal Performance of Heat Pipe with CuO-Water Working Fluid
The experiment conducted in this study utilized both low and high concentration nanofluids to heat
pipe. The low volume concentration nanofluids are ones with volume fraction below 1%, while the
high concentration nanofluids are those with volume fraction higher than 1%. As can be seen in Fig.
4, the increment of volume fraction up to 7% (from 0.1% to 7%) yielded lower heater surface
temperature. But then the surface temperature rose again as the volume fraction increased from 7%
to 9% and 10%. The occurrence of this phenomenon explained as follows.
As previously explained, nanofluid with higher concentration has higher thermal conductivity.
The increment of nanofluid’s thermal conductivity being used affects the overall effective thermal
conductivity of the heat pipe. Hence up to some extent, increment of nanofluid’s volume fraction –
also means increment of nanofluid’s thermal conductivity – would also leads to the increment of the
overall effective thermal conductivity of the heat pipe. Thus, more heat dissipated and lower heater
surface temperature [15, 16]. But then there is viscous limit [17]. It explained that there is a
condition when the fluid becomes too viscous, and unable to flow inside the heat pipe well. This
hampered the heat transfer capability of the heat pipe. The theory explained why the surface
temperature of the heater rose at the application of nanofluids with very high volume fraction. At
this very high volume fraction, the fluid becomes too viscous that it exceeds the viscous limit of the
heat pipe design. To be specific, the addition of 0.711 gram of CuO nano-particles on the 0.9%
CuO-water nanofluid reduced the heater surface temperature as much as 6.5%, and as shown in Fig.
5(a) reduced the thermal resistance as much as 70%. Meanwhile the addition of 5.530 gram of CuO
nano-particles on the 7% CuO-water nanofluid reduced the heater surface temperature as much as
Applied Mechanics and Materials Vol. 590
237
14.3 % and reduced the thermal resistance as much as 83%. These facts showed that the more
significant improvement found at the addition of small amount of nano-particles, not at the large
one. To be precise, the 7% CuO-water nanofluid added 4,819 gram of CuO nano-particles to the
0.9% CuO-water nanofluids, but only reduce the surface temperature as much as 8.3 % and reduce
the thermal resistance by 43.3 %. This again could be attributed to the viscous limit, that is already
been closed down by the 7% CuO-water nanofluid.
Long Term Stability of Nanofluid
To test the long term stability of the nanofluid, heat pipe charged with low concentration, 0.9%
CuO-water nanofluid and one with high concentration, 5% CuO nanofluid tested twice. Each test
separated over period of 4 weeks, and in that period the heat pipe was not being operated. It can be
concluded that by the use of low concentration nanofluid, the heat pipe did not show any significant
reduction in performance (not shown in figure). Unlikely, the use of high concentration nanofluid
clearly show degradation in performance, indicated by the surface temperature that is higher at the
second operation as shown in Fig. 5(b).
Figure 4. Thermal performance of heat pipe with various CuO-water nanofluid concentration
Figure 5. (a) Thermal resistance of heat
pipe at application of 0.9% and 7%
CuO-water nanofluid
(b) Thermal performance of heat pipe with
nanofluid after not being operated for 4
weeks
In both case, the nanofluids left deposition at the evaporator. However, at lower nanofluid
concentration, the deposition form thin layer of nano-particle and enhancing wettability and
intensifying bubble generation, thus decreasing thermal resistance [9]. While at higher nanofluid
concentration, the deposition was so thick that it obstructed the heat transfer. The capillary pores of
the wick were also covered by the thick deposition, increasing the thermal resistance.
Summary
The analysis of CuO-water nanofluid as working fluid for heat pipe has been conducted. It was
figured out that due to the existence of both positive and negative effect of nanofluid application,
there is an optimum value of nanofluid concentration that yielded the best heat pipe performance. It
238
Innovative Solutions in the Field of Engineering Sciences
was also found that the addition of high amount of nano-particles proven to be insignificant to the
heat pipe thermal performance. The best improvement both in thermal performance and long term
stability found on the application of low concentration nanofluid, that is in this case CuO-water
nanofluid with 0.9% volume fraction.
Reference
[1] Chen, Y.-S., Chien, K.-H., Hung, T.-C., Wang, C.-C., Ferng, Y.-M., Pei, B.-S., T Numerical
simulation of a heat sink embedded with a vapor chamber and calculation of effective thermal
conductivity of a vapor chamber. Applied Thermal Engineering. 29(13) (2009) 2655-2664
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