Role of graphene nanofluids on heat transfer enhancement in thermosyphon

Journal of Science: Advanced Materials and Devices (2019) 163e169 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Role of graphene nanofluids on heat transfer enhancement in thermosyphon Sidhartha Das a, *, Asis Giri a, Sutanu Samanta a, S Kanagaraj b a b Department of Mechanical Engineering, North Eastern Regional Institute of Science and Technology, Nirjuli, Arunachal Pradesh, 791109, India Department of Mechanical Engineering, Indian Institute of Technology, Guwahati, Assam, 781039, India a r t i c l e i n f o a b s t r a c t Article history: Received 28 October 2018 Received in revised form 21 January 2019 Accepted 22 January 2019 Available online 30 January 2019 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 nanofluid is found to be 29% higher than that of the deionized water at 45  C 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/) Keywords: Graphene platelet nanoparticle Thermosyphon Thermal conductivity Viscosity Thermal resistance 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 working fluid 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 conductivity and surface tension of the working fluid play an important role in the performance of thermosyphon Nanofluid, solideliquid suspension is produced by dispersing nanoparticles with the working fluid 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 twophase 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.% TPCT filled with waterbased Al2O3 and TiSiO2 nanofluids was investigated by Kamyar et al [2] for a nanoparticle loading of 0.01%, 0.02%, 0.05% and * 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 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 Al2O3 and 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 30  C 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 working fluids 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% nanoparticle 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 90 bend using Al2O3/water nanofluid https://doi.org/10.1016/j.jsamd.2019.01.005 2468-2179/© 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/) 164 S Das et al / Journal of Science: Advanced Materials and Devices (2019) 163e169 They observed a decrease in thermal resistance and wall temperature 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 working fluid 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 60 inclination 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 suspensions of nanoparticles, i.e., diamond, Al2O3 and 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 reported by Hajjar et al [13] An enhancement of 33.9% in thermal conductivity was observed with the addition of 0.25 wt.% at 20  C Ghozatloo et al [14] examined the thermal performance of graphene nanofluid in shell and tube heat exchanger, where convective heat transfer coefficient increased by 35.6% at 38  C 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, TiO2 nanofluids), 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 circular finned thermosyphon Graphene is particularly interesting since it enhances the thermal properties of base fluid significantly In the present report, an attempt is made to measure the thermal conductivity and viscosity of graphene nanofluid at low concentration along with its thermal performance in thermosyphon at different heat input and inclinations 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 magnetic steering is done with the help of a magnetic stirrer for 10 h at 750 rpm at a temperature of 28  C 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 sedimentation 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 in Fig 1, which is made by drying dilute solution 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 monitored 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 50  C as suggested in the KD2 Pro Manual Moreover, time duration for taking measurement is also reduced to 60s to avoid any further convection in the fluid 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 cylindrical 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 in Fig 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 radial fins are placed to assist natural convection cooling Each fin 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 the fins 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 locations in the thermosyphon The uncertainty in the measurement of temperature is calculated by calibrating it against a standard fluke made digital thermometer (Fluke 17B) having a resolution of 0.1  C at a temperature range from 25  C to 100  C The maximum variation in the measurement of temperature is found to be ±0.5  C The experiment is being conducted with graphene nanofluid at different weight percentage The input heat to thermosyphon is being made to the desired level by the use of an autotransformer and measured with wattmeter (Multi-Span) The setup is operated for 45 before any measurement during which time steady state is attained which means the temperature does not change more than ±0.1  C at a given heat input and the temperatures of the thermosyphon are recorded using data logger connected with ‘T’ S Das et al / Journal of Science: Advanced Materials and Devices (2019) 163e169 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi     DQin DDTị ẳ ỵ R Qin DT DR 165 (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 Results and discussion 3.1 Thermal conductivity Fig SEM image of Graphene platelet nanoparticle 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 25  C The uncertainty in resistance between the evaporator and the condenser is calculated [3] by 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 in Fig 3a From the figure, 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 25  C, while the same enhancement at 45  C 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 (a) Sectional front view and location of the thermocouple in circular finned thermosyphon; (b) Experimental setup 166 S Das et al / Journal of Science: Advanced Materials and Devices (2019) 163e169 The heat conduction in liquid occurs due to the molecular collision and diffusion In general, the thermal conductivity of liquid decreases with temperature However, water is an exceptional case, as thermal conductivity gets enhanced with temperature In nanofluid, 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 collision and diffusion 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 ẳ kbf ỵ 4ẵ2b11 L11 ị ỵ b33 L33 ị 42b11 L11 ỵ b33 L33 ị (2) where Lii and ỉ are the geometrical factors and the volume fraction of particles, respectively, and bii is defined as: bii ¼ kbf kp 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 in Fig 3b Fig (a) Thermal conductivity variation of nanofluid with temperature for different weight percentages, (b) Comparison of thermal conductivity variation of graphene nanofluid with different temperature and concentration (Nans Model) compared to that of DI water at the same temperature The conductivity of graphene nanofluids is enhanced by 7% by changing the temperature from 25 to 45  C for 0.02 wt.% Thermal conductivity of 0.64 W/mK is observed for 0.04 wt.% of graphene nanofluid at 25  C, which is 4.7% higher compared to that of 0.05 vol.% at 20  C [16] In addition, the thermal conductivity of graphene nanofluid having 0.10 wt.% is noted to be 0.81 W/mK at 45  C and it is 8.3% less in comparison to the thermal conductivity of 0.15 vol.% of graphene nanofluid at 50  C [16] As the concentration of weight of graphene particle increases, the random motion of the graphene particles is enhanced in the base fluid It is expected that the movement of such particles induces the collision between nanoparticles At higher temperature, these collisional effects might be more and thus, the thermal conductivity 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 a fixed volume of graphene nanofluid decreases Whenever concentration of nanoparticle is increased, the common surface areas between atoms of nanoparticles 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) base fluid 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 3.2 Viscosity Viscosity of water increases with the addition of graphene particles which is depicted in Fig 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 20  C In Fig 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 deformation rate only indicates that graphene nanofluid considered presently 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 thermosyphon 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 the figures 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 from Fig 5a, the wall temperature of DI water is 42.5  C 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 30 to 60 , the average wall temperature of the evaporator section decreases at 60 inclination S Das et al / Journal of Science: Advanced Materials and Devices (2019) 163e169 167 performance may also be observed with graphene nanofluid Evaporator wall temperature distribution for 60 inclination is lower compared to any other inclination angle Therefore, thermosyphon is expected to perform better at 60 inclination It is observed from Fig 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 thermosyphon This creates more nucleation site The increase in number of nucleation site enhances the boiling characteristics by introducing 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 heat flow 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: R¼ Fig Variation of viscosity (a) Viscosity of Graphene nanofluid at different temperature; (b) Rheological behaviour of nanofluid at different weight percentage for 20  C; (c) Rheological behaviour of nanofluid at different weight percentage for 50  C 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 condensate return to the evaporator is the primary reason behind this Gravitational effect of condensate return enhances with the increase 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 temperature 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 Te À Tc Q (4) where Te and Tc are 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 the figures 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 30 inclination of TPCT The thermal resistance of TPCT filled 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 improves the regime of nucleate boiling Further, due to the deposition 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 thermophysical properties of the working medium The performance of the evaporator of TPCT filled with graphene nanoparticle may be 168 S Das et al / Journal of Science: Advanced Materials and Devices (2019) 163e169 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 described through the correlation proposed by Mikic-Rohsenow [10] as under: Re ¼ pffiffiffipffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi; ¼ he Ae Na Ae Db f pkl rl Cl (5) where he is the coefficient of heat transfer, Ae is the evaporator area, Na is the nucleation site density, Db is the departure diameter of the bubble, f is the frequency of bubble departure kl, rl and Cl are 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 increase Overall effects cause an improvement in thermal performance It is found that inclination angle has nominal influence on Fig Thermal resistance variation of thermosyphon with heat load for different nanoparticles concentrations at (a) 30 angle of thermosyphon; (b) 45 angle of thermosyphon; (c) 60 angle of thermosyphon; (d) 90 angle of thermosyphon S Das et al / Journal of Science: Advanced Materials and Devices (2019) 163e169 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 45 and 60 inclinations of thermosyphon 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 45 inclination 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 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 25  C, while the same is around 29% at 45  C 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 20  C However, viscosity is decreased by 25% when temperature increases from 20  C to 50  C Graphene nanofluid shows a Newtonian behaviour  Use of graphene nanofluid in thermosyphon indicates a reduction in the wall temperature distribution and consequently thermal resistance decreases At a heat input of W, 60 inclination of the thermosyphon, maximum wall temperature reported 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.2019.01.005 169 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 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Thermal performance of Heat pipe with suspended nanoparticles, Heat Mass Tran 48 (11) (2012) 1913e1920 [20] R.R Riehl, N.D Santos, Water-copper nanofluid application in an open loop pulsating heat pipe, Appl Therm Eng 42 (2012) 6e10  ... 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,... 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... 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 30 inclination of TPCT