Temperature distribution at different section of the thermosyphon against varying heat input, inclination angle and different concentration of graphene nanofluid (a) Evaporator section, ([r]
(1)Original Article
Role of graphene nanofluids on heat transfer enhancement in
thermosyphon
Sidhartha Dasa,*, Asis Giria, Sutanu Samantaa, S Kanagarajb
aDepartment of Mechanical Engineering, North Eastern Regional Institute of Science and Technology, Nirjuli, Arunachal Pradesh, 791109, India bDepartment of Mechanical Engineering, Indian Institute of Technology, Guwahati, Assam, 781039, India
a r t i c l e i n f o
Article history:
Received 28 October 2018 Received in revised form 21 January 2019 Accepted 22 January 2019 Available online 30 January 2019 Keywords:
Graphene platelet nanoparticle Thermosyphon
Thermal conductivity Viscosity
Thermal resistance
a b s t r a c t
The thermophysical properties of graphene nanofluids in thermosyphon have been studied at different power inputs, temperatures and angles of inclination The thermal conductivity of the graphene nano-fluid is found to be 29% higher than that of the deionized water at 45C The viscosity of the graphene
nanofluid increased with the concentration of graphene nanoparticles and decreased with increasing the temperature It is observed that the wall temperature distribution of graphene nanofluid is found to be decreased in comparison to that of deionised water The thermal resistance of thermosyphon is reduced with increasing the power input and irrespective of the inclination angle
© 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)
1 Introduction
Tremendous demands for higher heat transfer devices exist due to the advancement of microelectronics, which necessitate better thermal management solutions A thermosyphon is a device, which uses the phase transformation of a workingfluid to transport heat and therefore heat transport by this device is fundamentally higher than any highly conducting material having same cross section Thus, the boiling characteristics, vapour pressure, thermal con-ductivity and surface tension of the workingfluid play an important role in the performance of thermosyphon Nanofluid, solideliquid suspension is produced by dispersing nanoparticles with the workingfluid A lot of research has been carried out to enhance the thermal performance of the thermosyphon using nanofluids
Noie et al.[1] conducted an experimental study on the two-phase closed thermosyphon (TPCT) using Al2O3/water nanofluids The efficiency of TPCT was found to enhance up to 14.7% for a concentration ranging from to vol.% TPCTfilled with water-based Al2O3 and TiSiO2 nanofluids was investigated by Kamyar et al [2] for a nanoparticle loading of 0.01%, 0.02%, 0.05% and
0.075% involving a thermal load ranging from 40 W to 210 W They observed a decrease in the thermal resistance of the heat pipe up to 65% for 0.05 vol.% of Al2O3and 57% for 0.075 vol.% TiSiO4 Kole and Dey[3]examined surfactant free water based copper nanofluids and observed a thermal conductivity enhancement of 15% for 0.5 wt.% at 30C Further, the nanofluid was used in the wicked heat pipe, which indicated a thermal resistance as low as 27% at higher thermal load Al2O3, CuO and laponite in water caused the decrease of the performance of heat pipe This was reported by Khandekar et al [4] It was predicted that nanoparticles entrapment in the grooves of the rough surface was the reason for such behaviour The oscillating heat pipe (OHP) was examined by Qu and Wu [5], by using Al2O3/water and SiO2/water nanofluids, where a reduction in thermal resistance was found for both the workingfluids
Using water-based TiO2 and Au nanofluids, Buschmann and Franzke [6] investigated thermal performance of heat pipe A maximum reduction in the thermal resistance of 24% was observed from the experiment The performance of refrigerant based Ti nanofluid was observed in a heat pipe by Naphon et al.[7] An optimum condition was revealed for a heat pipe with 0.1% nano-particle concentration, which provided 1.4 times higher efficiency than the pure refrigerant The experimental study on the Al2O3/ water nanofluid performed by Ho et al.[8]showed an improved heat transfer Moraveji and Razvarz[9]studied the heat transfer rate in the heat pipe with 90bend using Al2O3/water nanofluid
* Corresponding author
E-mail addresses:sidhartha_me15@nerist.ac.in(S Das),measisgiri@yahoo.com
(A Giri), suta_sama@yahoo.co.in (S Samanta), kanagaraj@iitg.ernet.in
(S Kanagaraj)
Peer review under responsibility of Vietnam National University, Hanoi
Contents lists available atScienceDirect
Journal of Science: Advanced Materials and Devices
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d
https://doi.org/10.1016/j.jsamd.2019.01.005
(2)They observed a decrease in thermal resistance and wall temper-ature difference in a heat pipe using nanofluid compared to pure water
Solomon et al.[10]studied the performance of an anodized TPCT with refrigerant as workingfluid and found that TPCT performed better at the 45 inclination Ghanbarpour et al.[11]used silver based nanofluid in the heat pipe with two layers of screen mesh It was found that the 60inclination of the heat pipe was superior to other inclinations Torii et al.[12]experimentally studied the heat transfer performance in a circular pipe containing aqueous sus-pensions of nanoparticles, i.e., diamond, Al2O3and CuO They found an increase in relative viscosity and better performance compared to that of pure water
The effect of graphene oxide concentration in water was re-ported by Hajjar et al.[13] An enhancement of 33.9% in thermal conductivity was observed with the addition of 0.25 wt.% at 20C Ghozatloo et al.[14] examined the thermal performance of gra-phene nanofluid in shell and tube heat exchanger, where convec-tive heat transfer coefficient increased by 35.6% at 38C for 0.1 wt.% of graphene nanofluid Shadeghinezhad et al [15] observed the performance of heat pipe with graphene nanoparticles and found a maximum reduction of 48.14% compared to that of deionized (DI) water using sintered wick heat pipe It was also found that maximum effective thermal conductivity enhancements for the heat pipe with GNP concentration is found to be significant at 60 inclination
Cited literature reveals that there exist a handful of literature, which uses Al2O3, CuO, TiO2 nanofluid as a working medium in thermosyphon with forced convection However, compared to other nanofluids (i.e., Al2O3, CuO, TiO2nanofluids), the effect of graphene nanofluid in thermosyphon is nominal in the literature Moreover, it is noticed that very few experimental investigation has been carried out in a smaller sized circularfinned thermosyphon Graphene is particularly interesting since it enhances the thermal properties of basefluid significantly In the present report, an attempt is made to measure the thermal conductivity and viscosity of graphene nano-fluid at low concentration along with its thermal performance in thermosyphon at different heat input and inclinations
2 Materials and methods 2.1 Nanofluid preparation
Graphene platelet nanopowder is procured from Sisco Research Laboratories Pvt Ltd (GPN Type 1, 55093), which is having 99.5% purity To prepare the graphene nanofluids, graphene nanoparticles are mixed with DI water in the required concentration and mag-netic steering is done with the help of a magmag-netic stirrer for 10 h at 750 rpm at a temperature of 28C After that Gum Acacia (Fisher Scientific, CAS No - 9000-01-5) is mixed with nanoparticles in weight percentage ratio of 0.5:1 The sample thus prepared is then sonicated for h to form colloid of graphene particles with DI water To see the sedimentation of the particles, a visualization method is followed for a period of 30 days and minimal sedimen-tation is noticed (Fig in the Supplementary File) In the process of sonication, liquid sample gets heated up and therefore liquid evaporates The evaporated liquid will escape if sonication bath is open to atmosphere and hence appropriate cover for the sonication bath is needed to avoid the escape of evaporated liquid A small volume of gum acacia is helpful in retaining the nanoparticles in colloidal form Scanning electron microscope (SEM) picture of particles is depicted inFig 1, which is made by drying dilute so-lution over the glass slide SEM picture indicates that particles are platelet type Concentrations of graphene nanoparticles of 0.02, 0.04, 0.06, 0.08 and 0.10 wt.% are prepared for the study The
advantage of these concentrations is that particle remains in colloid form for days with nominal sedimentation
2.2 Measurement of thermal conductivity and viscosity of graphene nanofluid
For measuring the thermal conductivity of the nanofluids, the KD2 Probe was used (Decagon Devices, Inc.) with a single needle (KS1) which has a size of 1.3 mm diameter and cm long KD2 probe measures the thermal conductivity by the transient hot wire method in which, a thin metallic conducting wire is used for both as a line heat source and a temperature sensor Thermal conductivity of liquid is measured by submerging the metallic wire in the liquid Current is passed through the wire and the temperature is moni-tored over time, which is used for measuring the conductivity This is the basic principle used for the measurement of the thermal conductivity in the KD2 probe To prevent free convection in the fluid, the temperature of fluid was maintained lower than 50C as suggested in the KD2 Pro Manual Moreover, time duration for taking measurement is also reduced to 60s to avoid any further convection in thefluid
The viscosity of DI water and graphene nanofluid is measured by Rheometer (Physica, MCR 101, Anton Paar) The rheometer consists of a stationary cylindrical surface and a moving cylindrical bob which are parallel to each other with a small gap and the liquid is kept between them The cylindrical bob is connected to driver motor, which rotates at different speeds and the stationary cylin-drical surface connects to the torque measuring device in order to evaluate the resistance of the sample to the motion
2.3 Thermosyphon and experimental setup
The thermosyphon used presently and the experimental setup is sketched inFig 2aeb A 120 mm long copper tube with an outer diameter of mm and inner diameter of mm is made to form the device The device consists of three sections: (i) 50 mm long evaporator section, (ii) 20 mm long adiabatic section, (iii) 50 mm long condenser section Evaporator section is covered with a heating unit to apply constant heat input Adiabatic section is covered with glass wool placed over the evaporator and it is 20 mm long A 50 mm long condenser section is positioned above the adiabatic section, wherein 23 equally spaced radialfins are placed to assist natural convection cooling Eachfin has the dimensions of 26 mm outer diameter, mm inner diameter and mm thickness To measure the thermal performance, T-type thermocouples (i.e., copper-constantan) are positioned on the thermosyphon at the locations of 10 mm, 20 mm, 45 mm, 60 mm, 72 mm, 95 mm, 115 mm and 119 mm from the evaporator end At the location of 72 mm, 95 mm, and 115 mm, thermocouples are positioned on the surface of thefins The remaining five thermocouples are placed on the surface of the thermosyphon Data acquisition system (Unilog Pro Plus, PPI) is used to collect the temperature of different loca-tions in the thermosyphon The uncertainty in the measurement of temperature is calculated by calibrating it against a standardfluke made digital thermometer (Fluke 17B) having a resolution of 0.1C at a temperature range from 25C to 100C The maximum vari-ation in the measurement of temperature is found to be±0.5C.
(3)type thermocouples Each experiment is repeated three times for its repeatability Experiments are conducted for W, W and 12 W Heat losses from the evaporator section by radiation and free convection are neglected Thermal performance of thermosyphon is tested for vertical as well as for inclined position Ambient temperature during the experiment remains 25C The uncertainty in resistance between the evaporator and the condenser is calcu-lated[3]by
DR
R ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi D
Qin Qin
2
ỵ D
ðDTÞ
DT 2
s
(1)
The maximum uncertainty in the measurement of the heat input (Qin) and total resistance is around 0.79% and 1.80% which are less than 1% and 2%, respectively
3 Results and discussion 3.1 Thermal conductivity
Water based graphene nanofluid is prepared for different weight concentration of graphene particles and its conductivity is measured Measured conductivity variation of graphene nanofluid with temperature is exemplified inFig 3a From thefigure, it may be noted that conductivity increases with temperature for all weight concentration of graphene nanofluid Further, it is identified that as weight concentration increases, the thermal conductivity of nanofluid increases at a fixed temperature Moreover, thermal conductivity of graphene nanofluid is always higher than DI water Thermal conductivity enhancement at the highest concentration (i.e., 0.1 wt.%) is about 17% of water conductivity at a temperature of 25C, while the same enhancement at 45C is around 29% of water conductivity It is also noticed from the published data of Ahammed et al [16] that similar enhancement of 37.2% was noted for 0.15 vol.% of graphene nanofluid at a temperature of 50 C
Fig SEM image of Graphene platelet nanoparticle
(4)compared to that of DI water at the same temperature The con-ductivity of graphene nanofluids is enhanced by 7% by changing the temperature from 25 to 45C for 0.02 wt.% Thermal conductivity of 0.64 W/mK is observed for 0.04 wt.% of graphene nanofluid at 25C, which is 4.7% higher compared to that of 0.05 vol.% at 20C[16] In addition, the thermal conductivity of graphene nanofluid having 0.10 wt.% is noted to be 0.81 W/mK at 45C and it is 8.3% less in comparison to the thermal conductivity of 0.15 vol.% of graphene nanofluid at 50C[16].
As the concentration of weight of graphene particle increases, the random motion of the graphene particles is enhanced in the basefluid It is expected that the movement of such particles in-duces the collision between nanoparticles At higher temperature, these collisional effects might be more and thus, the thermal con-ductivity of nanofluid is found to be improved In addition, when the concentration increases, the conduction electron (i.e., free electrons available in the atoms, such as metal atoms, which are primarily accountable for thermal conductivity) is enhanced since the distance between atoms in afixed volume of graphene nano-fluid decreases Whenever concentration of nanoparticle is increased, the common surface areas between atoms of nano-particles and the base liquid are enhanced This leads to an enhancement in thermal conductivity As the temperature is enhanced, the thermal conductivity is also enhanced This is possibly due to two reasons, (1) basefluid thermal conductivity is enhanced due to increased Brownian motion and (2) conduction electrons will be positioned at a high energy level causing electron to move faster and thus heat will be transported at a faster rate which leads to higher thermal conductivity As the molecules in the liquid are closely spaced they yield stronger intermolecular force
The heat conduction in liquid occurs due to the molecular collision and diffusion In general, the thermal conductivity of liquid de-creases with temperature However, water is an exceptional case, as thermal conductivity gets enhanced with temperature In nano-fluid, solid particle thermal conductivity must also be taken into account The thermal energy is being transferred by phonons in non-metallic compound and free electrons in metallic compound Since graphene has both phonons and free conduction electron, both phonons and free electron influence the thermal conductivity of graphene nanofluid Hence, three factors influence the enhancement of thermal conductivity of graphene nanofluid: (i) phonons (vibrations), (ii) free electron, (iii) rapid molecular colli-sion and diffucolli-sion A similar observation is also made by Ahammed et al.[16] Presently measured thermal conductivity of graphene nanofluid is compared with Nan's model[17]and it is expressed in Eq.(2)as
knf ¼ kbf
3ỵ 4ẵ2b111 L11ị ỵb331 L33ị
3 42b11L11ỵb33L33ị
(2)
where Liiand ỉ are the geometrical factors and the volume fraction of particles, respectively, andbiiis dened as:
biiẳ
kp kbf
kbf ỵ Lii
kp kbf
(3)
Maximum deviation between the theoretical and experimental conductivity is noted to be 8.5% for 0.10 wt.% of graphene nanofluid Overall agreement between theoretical model (Nan's) with the measured result is reasonably good and is presented inFig 3b 3.2 Viscosity
Viscosity of water increases with the addition of graphene particles which is depicted inFig 4a In the present study, the viscosity enhancement of graphene nanofluid for the highest concentration of 0.10 wt.% is around 175% higher in comparison to DI water at 20C InFig 4a, viscosity of graphene nanofluid is also found to decrease with temperature and this decrease is as high as 25% for a concentration of 0.10 wt.% of nanofluids Rheological study is made to characterize the graphene nanofluids.Fig 4bec depicts such behaviour in the form of shear rate deformation at different temperature and it is found to be linear Linear deforma-tion rate only indicates that graphene nanofluid considered pres-ently is Newtonian in behaviour for all the temperature attempted in the present study
3.3 Temperature distribution
Fig 5aec represents the temperature distribution of the ther-mosyphon at a distance of 45, 60 and 119 mm for the evaporator, adiabatic and condenser section respectively at different heat input and inclination angle It can be observed from thefigures that the wall temperature distribution of DI water is higher compared to that of graphene nanofluid and as the concentration increases, wall temperature is decreased further
It is noted fromFig 5a, the wall temperature of DI water is 42.5C and with the addition of 0.10 wt.% of graphene nanofluid, there is a decrease of 13.9% in the evaporator wall temperature for a heat input of W, 60 inclination In addition, it can also be observed from the results of Kamyar et al.[2] that a maximum of 24.53% decrease in wall temperature is found at 0.05 vol.% of Al2O3 nanofluid compared to DI water at 40 W heat input Moreover, as the inclination angle increases from 30to 60, the average wall temperature of the evaporator section decreases at 60inclination
(5)After 60 inclination, average evaporator temperature increases again Similar trend is observed for adiabatic and condenser section of the thermosyphon (Fig 5bec) Gravitational effect on conden-sate return to the evaporator is the primary reason behind this Gravitational effect of condensate return enhances with the in-crease in inclination angle, which causes the enhancement of liquid return Hence, at the inclination angle 60, evaporator temperature is low Gravitational effect is maximum at 90, which causes the presence of more liquid in the evaporator section creating a flooding condition This causes an increase in evaporator temper-ature A similar observation is also made by Moraveji and Razvarz
[9] The wall temperature distribution in thermosyphon for 12 W heat input also follows the same trend Therefore, better thermal
performance may also be observed with graphene nanofluid. Evaporator wall temperature distribution for 60 inclination is lower compared to any other inclination angle Therefore, ther-mosyphon is expected to perform better at 60 inclination It is observed fromFig 5d that the evaporator temperature of graphene nanofluid is lower than DI water wall temperature and there is a decreasing temperature gradient from the evaporator section to the condenser section Moreover, there is a reduction in temperature difference between the evaporator and condenser section of the thermosyphon with the increase in concentration of graphene nanofluid At a heat input of 12 W, 60 inclination, temperature difference between the evaporator and condenser section for DI water is 10.9% whereas with the addition of 0.10 wt.% of graphene nanofluid the temperature difference is reduced to 6.4% This is possibly due to porous layer formation on the surface of thermo-syphon This creates more nucleation site The increase in number of nucleation site enhances the boiling characteristics by intro-ducing significantly large number of small nucleation bubbles Formation of small nucleation bubble introduces lower thermal resistance due to continuous rewetting of evaporator, while on the other hand, large size bubble causes a high thermal resistance to heatflow A similar enhancement is noted by Singh et al.[18]in connection with anodized thermosyphon
3.4 Thermal resistance
The thermosyphon performance may be relatively estimated by the thermal resistance[19](R) defined as follows:
R¼Te Tc
Q (4)
where Teand Tcare the evaporator and condenser temperatures, respectively Q in Eq.(4)represents heat input Variation of thermal resistance with heat input is shown in Fig 6aed for different inclination of the thermosyphon It is understood from thefigures that thermal resistance is decreased sharply with the increased heat input for all cases of nanofluids and DI water Around 72% decrease in thermal resistance is observed by increasing heat input from to 12 W for the highest concentration of nanofluid and at all inclinations of TPCT Thermal resistance is decreased by around 25% compared to DI water for a heat input of W, at a concentration of 0.10 wt.% of graphene nanaofluid at 30inclination of TPCT The thermal resistance of TPCTfilled with graphene nanofluid reduces considerably due to the reduction of evaporator temperature and simultaneous increase of condenser wall temperature Singh et al
[18], Shukla et al.[19]and Riehl and Santos[20]made similar types of observation in their studies Further, it is noted that the thermal resistance of nanofluid filled TPCT is lower than the DI water filled TPCT Moreover, deposition of graphene nanoparticles on the evaporator surface causes nucleation site to increase and this im-proves the regime of nucleate boiling Further, due to the deposi-tion of nanoparticles, there occurs a change in surface wettability In addition, turbulence is being generated at higher heat input in the graphene nanofluid due to the movement of nanoparticle in the fluid A similar observation is being made by Shukla et al.[19]in their study of heat pipe using CuO nanofluid More nucleation sites are created as the nanoparticle deposits on the surface of the evaporator The performance of evaporator with the deposition of nanoparticles highly depends on bubble departure diameter, nucleation site density, frequency of bubble departure and ther-mophysical properties of the working medium The performance of the evaporator of TPCTfilled with graphene nanoparticle may be
Fig Variation of viscosity (a) Viscosity of Graphene nanofluid at different temper-ature; (b) Rheological behaviour of nanofluid at different weight percentage for 20C;
(6)described through the correlation proposed by Mikic-Rohsenow
[10]as under:
Re¼h1 eAe¼
2 NaAeDb
ffiffiffi f
p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
pklrlCl
p ; (5)
where heis the coefficient of heat transfer, Aeis the evaporator area, Nais the nucleation site density, Dbis the departure diameter of the
bubble, f is the frequency of bubble departure kl,rland Clare the conductivity, density and specific heat, respectively Because of nanoparticle deposition, nucleation site density increases manifold due to the formation of porous layer Bubble departure diameter may decrease due to decrease in surface tension at higher temperature, but bubble departure frequency and effective surface area will in-crease Overall effects cause an improvement in thermal perfor-mance It is found that inclination angle has nominal influence on
Fig Temperature distribution at different section of the thermosyphon against varying heat input, inclination angle and different concentration of graphene nanofluid (a) Evaporator section, (b) Adiabatic section, (c) Condenser section, (d) Wall temperature distribution of the thermosyphon against different heat input, inclination angle and different concentration of graphene nanofluid
(7)the performance of thermosyphon although evaporator, adiabatic and condenser section temperature is lower at 60⁰ inclination This is in contrary to other studies available in the literature in which higher performance is noted with 45and 60inclinations of thermosy-phon Naphon et al.[7]and Ghanbarpour et al.[11]observe 60 inclination performs better However, Singh et al.[18], and Solomon et al.[10]observe better performance with 45inclination of TPCT It may be noted that Khandekar et al.[4]observe a decrease in thermal performance using nanofluid Almost same thermal resistance is observed irrespective of inclination angles
4 Conclusions
Water based graphene nanofluid has been characterized through SEM, thermal conductivity and viscosity measurement SEM image reveals that the graphene particles are platelet type Following conclusions may be drawn from the present investigation:
An enhancement in thermal conductivity of around 17% is noticed compared to that of DI water for 0.10 wt.% of graphene nanofluid at a temperature of 25C, while the same is around 29% at 45C Therefore, thermal conductivity enhancement is temperature dependent for the involved operating range The viscosity of the graphene nanofluid is noted to be enhanced
by 175% at 0.10 wt % for 20C However, viscosity is decreased by 25% when temperature increases from 20C to 50C Gra-phene nanofluid shows a Newtonian behaviour.
Use of graphene nanofluid in thermosyphon indicates a reduc-tion in the wall temperature distribureduc-tion and consequently thermal resistance decreases At a heat input of W, 60 incli-nation of the thermosyphon, maximum wall temperature re-ported is 42.5 C for DI water and with the application of 0.10 wt.% of graphene nanofluid there is a reduction of 13.9% in the evaporator wall temperature
Moreover, increasing the heat input from to 12 W, a reduction resistance of around 72% is noted for the highest concentration of nanoparticle The thermal resistance of thermosyphon is observed to be almost same irrespective of inclination angles
Conflict of interest None
Appendix A Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jsamd.2019.01.005
References
[1] S.H Noie, S.Z Heris, M Kahani, S.M Nowee, Heat transfer enhancement using Al₂O3/water nanofluid in a two-phase closed thermosyphon, Int J Heat Fluid Flow 30 (2009) 700e705
[2] A Kamyar, K.S Ong, R Saidur, Effects of nanofluids on heat transfer charac-teristics of a two-phase closed thermosyphon, Int J Heat Mass Tran 65 (2013) 610e618
[3] M Kole, T.K Dey, Thermal performance of screen mesh wick heat pipes using water-based copper nanofluids, Appl Therm Eng 50 (2013) 763e770
[4] S Khandekar, Y.M Joshi, B Mehta, Thermal performance of closed two-phase thermosyphon using nanofluids, Int J Therm Sci 47 (2008) 659e667 [5] J Qu, H Wu, Thermal performance comparison of oscillating heat pipes with
SiO2/water and Al2O3/water nanofluids, Int J Therm Sci 50 (2011) 1954e1962
[6] M.H Buschmann, U Franzke, Improvement of thermosyphon performance by employing nanofluid, Int J Refrig 40 (2014) 416e428
[7] P Naphon, D Thongkum, P Assadamongkol, Heat pipe efficiency enhance-ment with refrigerant nanoparticles mixtures, Energy Convers Manag 50 (2009) 772e776
[8] C.J Ho, Y.N Chung, Chi-Ming Lai, Thermal performance of Al2O3/water nanofluid in a natural circulation loop with a mini-channel heat sink and heat source, Energy Convers Manag 87 (2014) 848e858
[9] M.K Moraveji, S Razvarz, Experimental investigation of aluminium oxide nanofluid on heat pipe thermal performance, Int Commun Heat Mass Tran 39 (2012) 1444e1448
[10] A.B Solomon, R Roshan, W Vincent, V.K Karthikeyan, L.G Asirvatha, Heat transfer performance of an anodized two-phase closed thermosy-phon with refrigerant as workingfluid, Int J Heat Mass Tran 82 (2015) 521e529
[11] M Ghanbarpour, N Nikkamb, R Khodabandeh, M.S Toprak, Thermal per-formance of inclined screen mesh heat pipes using silver nanofluids, Int Commun Heat Mass Tran 67 (2015) 14e20
[12] S Torii, Y Satou, Y Koito, Experimental study on convective thermal-fluid flow transport phenomena in circular tube using nanofluids, Int J Green Energy (2010) 289e299
[13] Z Hajjar, A.M Rashidi, A Ghozatloo, Enhanced thermal conductivities of graphene oxide nanofluids, Int Commun Heat Mass Tran 57 (2014) 128e131
[14] A Ghozatloo, A Rashidi, M Shariaty-Niassar, Convective heat transfer enhancement of graphene nanofluids in shell and tube heat exchanger, Exp Therm Fluid Sci 53 (2014) 136e141
[15] E Sadeghinezhad, M Mehrali, M.A Rosen, A.R Akhiani, S.T Latibari, M Mehrali, C.S.H Metselaar, Experimental investigation of the effect of gra-phene nanofluids on heat pipe thermal performance, Appl Therm Eng 100 (2016) 775e787
[16] N Ahammed, L.G Asirvatham, J Titus, J.B Bose, S Wongwises, Mea-surement of thermal conductivity of graphene water nanofluid at below and above ambient temperatures, Int Commun Heat Mass Tran 70 (2016) 66e74
[17] C.W Nan, Z Shi, L Lin, A simple model for thermal conductivity of carbon nanotube-based composites, Chem Phys Lett 375 (2003) 666e669 [18] R.R Singh, V Selladurai, P.K Ponkarthik, A.B Solomon, Effect of anodization
on the heat transfer performance offlat thermosyphon, Exp Therm Fluid Sci 68 (2015) 574e581
[19] K.N Shukla, A.B Solomon, B.C Pillai, B.J.R Singh, S.S Kumar, Thermal per-formance of Heat pipe with suspended nanoparticles, Heat Mass Tran 48 (11) (2012) 1913e1920