Role of graphene nanofluids on heat transfer enhancement in thermosyphon

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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 0C.

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Journal of Science: Advanced Materials and Devices 4 (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 Department of Mechanical Engineering, North Eastern Regional Institute of Science and Technology, Nirjuli, Arunachal Pradesh, 791109, India b 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: The thermophysical properties of graphene nanoﬂuids in thermosyphon have been studied at different Received 28 October 2018 power inputs, temperatures and angles of inclination. The thermal conductivity of the graphene nano- Received in revised form ﬂuid is found to be 29% higher than that of the deionized water at 45 C. The viscosity of the graphene 21 January 2019 nanoﬂuid increased with the concentration of graphene nanoparticles and decreased with increasing the Accepted 22 January 2019 Available online 30 January 2019 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. Keywords: Graphene platelet nanoparticle © 2019 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. Thermosyphon This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Thermal conductivity Viscosity Thermal resistance 1. Introduction 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 Tremendous demands for higher heat transfer devices exist due 65% for 0.05 vol.% of Al2O3 and 57% for 0.075 vol.% TiSiO4. Kole and to the advancement of microelectronics, which necessitate better Dey [3] examined surfactant free water based copper nanoﬂuids thermal management solutions. A thermosyphon is a device, which and observed a thermal conductivity enhancement of 15% for uses the phase transformation of a working ﬂuid to transport heat 0.5 wt.% at 30 C. Further, the nanoﬂuid was used in the wicked heat and therefore heat transport by this device is fundamentally higher pipe, which indicated a thermal resistance as low as 27% at higher than any highly conducting material having same cross section. thermal load. Al2O3, CuO and laponite in water caused the decrease Thus, the boiling characteristics, vapour pressure, thermal con- of the performance of heat pipe. This was reported by Khandekar ductivity and surface tension of the working ﬂuid play an important et al. [4]. It was predicted that nanoparticles entrapment in the role in the performance of thermosyphon. Nanoﬂuid, solideliquid grooves of the rough surface was the reason for such behaviour. The suspension is produced by dispersing nanoparticles with the oscillating heat pipe (OHP) was examined by Qu and Wu [5], by working ﬂuid. A lot of research has been carried out to enhance the using Al2O3/water and SiO2/water nanoﬂuids, where a reduction in thermal performance of the thermosyphon using nanoﬂuids. thermal resistance was found for both the working ﬂuids. Noie et al. [1] conducted an experimental study on the two- Using water-based TiO2 and Au nanoﬂuids, Buschmann and phase closed thermosyphon (TPCT) using Al2O3/water nanoﬂuids. Franzke [6] investigated thermal performance of heat pipe. A The efﬁciency of TPCT was found to enhance up to 14.7% for a maximum reduction in the thermal resistance of 24% was observed concentration ranging from 1 to 3 vol.%. TPCT ﬁlled with water- from the experiment. The performance of refrigerant based Ti based Al2O3 and TiSiO2 nanoﬂuids was investigated by Kamyar nanoﬂuid was observed in a heat pipe by Naphon et al. [7]. An et al. [2] for a nanoparticle loading of 0.01%, 0.02%, 0.05% and optimum condition was revealed for a heat pipe with 0.1% nano- particle concentration, which provided 1.4 times higher efﬁciency * Corresponding author. than the pure refrigerant. The experimental study on the Al2O3/ E-mail addresses: sidhartha_me15@nerist.ac.in (S. Das), measisgiri@yahoo.com water nanoﬂuid performed by Ho et al. [8] showed an improved (A. Giri), suta_sama@yahoo.co.in (S. Samanta), kanagaraj@iitg.ernet.in heat transfer. Moraveji and Razvarz [9] studied the heat transfer (S. Kanagaraj). Peer review under responsibility of Vietnam National University, Hanoi. 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 4 (2019) 163e169 They observed a decrease in thermal resistance and wall temper- advantage of these concentrations is that particle remains in colloid ature difference in a heat pipe using nanoﬂuid compared to pure form for days with nominal sedimentation. water. Solomon et al. [10] studied the performance of an anodized TPCT 2.2. Measurement of thermal conductivity and viscosity of with refrigerant as working ﬂuid and found that TPCT performed graphene nanoﬂuid better at the 45 inclination. Ghanbarpour et al. [11] used silver based nanoﬂuid in the heat pipe with two layers of screen mesh. It For measuring the thermal conductivity of the nanoﬂuids, the was found that the 60 inclination of the heat pipe was superior to KD2 Probe was used (Decagon Devices, Inc.) with a single needle other inclinations. Torii et al. [12] experimentally studied the heat (KS1) which has a size of 1.3 mm diameter and 6 cm long. KD2 transfer performance in a circular pipe containing aqueous sus- probe measures the thermal conductivity by the transient hot wire pensions of nanoparticles, i.e., diamond, Al2O3 and CuO. They found method in which, a thin metallic conducting wire is used for both as an increase in relative viscosity and better performance compared a line heat source and a temperature sensor. Thermal conductivity to that of pure water. of liquid is measured by submerging the metallic wire in the liquid. The effect of graphene oxide concentration in water was re- Current is passed through the wire and the temperature is moni- ported by Hajjar et al. [13]. An enhancement of 33.9% in thermal tored over time, which is used for measuring the conductivity. This conductivity was observed with the addition of 0.25 wt.% at 20 C. is the basic principle used for the measurement of the thermal Ghozatloo et al. [14] examined the thermal performance of gra- conductivity in the KD2 probe. To prevent free convection in the phene nanoﬂuid in shell and tube heat exchanger, where convec- ﬂuid, the temperature of ﬂuid was maintained lower than 50 C as tive heat transfer coefﬁcient increased by 35.6% at 38 C for 0.1 wt.% suggested in the KD2 Pro Manual. Moreover, time duration for of graphene nanoﬂuid. Shadeghinezhad et al. [15] observed the taking measurement is also reduced to 60s to avoid any further performance of heat pipe with graphene nanoparticles and found a convection in the ﬂuid. maximum reduction of 48.14% compared to that of deionized (DI) The viscosity of DI water and graphene nanoﬂuid is measured by water using sintered wick heat pipe. It was also found that Rheometer (Physica, MCR 101, Anton Paar). The rheometer consists maximum effective thermal conductivity enhancements for the of a stationary cylindrical surface and a moving cylindrical bob heat pipe with GNP concentration is found to be signiﬁcant at 60 which are parallel to each other with a small gap and the liquid is inclination. kept between them. The cylindrical bob is connected to driver Cited literature reveals that there exist a handful of literature, motor, which rotates at different speeds and the stationary cylin- which uses Al2O3, CuO, TiO2 nanoﬂuid as a working medium in drical surface connects to the torque measuring device in order to thermosyphon with forced convection. However, compared to other evaluate the resistance of the sample to the motion. nanoﬂuids (i.e., Al2O3, CuO, TiO2 nanoﬂuids), the effect of graphene nanoﬂuid in thermosyphon is nominal in the literature. Moreover, it 2.3. Thermosyphon and experimental setup is noticed that very few experimental investigation has been carried out in a smaller sized circular ﬁnned thermosyphon. Graphene is The thermosyphon used presently and the experimental setup particularly interesting since it enhances the thermal properties of is sketched in Fig. 2aeb. A 120 mm long copper tube with an outer base ﬂuid signiﬁcantly. In the present report, an attempt is made to diameter of 8 mm and inner diameter of 6 mm is made to form the measure the thermal conductivity and viscosity of graphene nano- device. The device consists of three sections: (i) 50 mm long ﬂuid at low concentration along with its thermal performance in evaporator section, (ii) 20 mm long adiabatic section, (iii) 50 mm thermosyphon at different heat input and inclinations. long condenser section. Evaporator section is covered with a heating unit to apply constant heat input. Adiabatic section is 2. Materials and methods covered with glass wool placed over the evaporator and it is 20 mm long. A 50 mm long condenser section is positioned above the 2.1. Nanoﬂuid preparation adiabatic section, wherein 23 equally spaced radial ﬁns are placed to assist natural convection cooling. Each ﬁn has the dimensions of Graphene platelet nanopowder is procured from Sisco Research 26 mm outer diameter, 8 mm inner diameter and 1 mm thickness. Laboratories Pvt. Ltd (GPN Type 1, 55093), which is having 99.5% To measure the thermal performance, T-type thermocouples (i.e., purity. To prepare the graphene nanoﬂuids, graphene nanoparticles copper-constantan) are positioned on the thermosyphon at the are mixed with DI water in the required concentration and mag- locations of 10 mm, 20 mm, 45 mm, 60 mm, 72 mm, 95 mm, netic steering is done with the help of a magnetic stirrer for 10 h at 115 mm and 119 mm from the evaporator end. At the location of 750 rpm at a temperature of 28 C. After that Gum Acacia (Fisher 72 mm, 95 mm, and 115 mm, thermocouples are positioned on the Scientiﬁc, CAS No - 9000-01-5) is mixed with nanoparticles in surface of the ﬁns. The remaining ﬁve thermocouples are placed on weight percentage ratio of 0.5:1. The sample thus prepared is then the surface of the thermosyphon. Data acquisition system (Unilog sonicated for 5 h to form colloid of graphene particles with DI Pro Plus, PPI) is used to collect the temperature of different loca- water. To see the sedimentation of the particles, a visualization tions in the thermosyphon. The uncertainty in the measurement of method is followed for a period of 30 days and minimal sedimen- temperature is calculated by calibrating it against a standard ﬂuke tation is noticed (Fig. 2 in the Supplementary File). In the process of made digital thermometer (Fluke 17B) having a resolution of 0.1 C sonication, liquid sample gets heated up and therefore liquid at a temperature range from 25 C to 100 C. The maximum vari- evaporates. The evaporated liquid will escape if sonication bath is ation in the measurement of temperature is found to be ±0.5 C. open to atmosphere and hence appropriate cover for the sonication The experiment is being conducted with graphene nanoﬂuid at bath is needed to avoid the escape of evaporated liquid. A small different weight percentage. The input heat to thermosyphon is volume of gum acacia is helpful in retaining the nanoparticles in being made to the desired level by the use of an autotransformer colloidal form. Scanning electron microscope (SEM) picture of and measured with wattmeter (Multi-Span). The setup is operated particles is depicted in Fig. 1, which is made by drying dilute so- for 45 min before any measurement during which time steady state lution over the glass slide. SEM picture indicates that particles are is attained which means the temperature does not change more platelet type. Concentrations of graphene nanoparticles of 0.02, than ±0.1 C at a given heat input and the temperatures of the 0.04, 0.06, 0.08 and 0.10 wt.% are prepared for the study. The thermosyphon are recorded using data logger connected with ‘T’
S. Das et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 163e169 165 sﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ DR DQin 2 DðDTÞ 2 ¼ þ (1) R Qin DT 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 in Fig. 3a. From the ﬁgure, it may Fig. 1. SEM image of Graphene platelet nanoparticle. 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 type thermocouples. Each experiment is repeated three times for nanoﬂuid increases at a ﬁxed temperature. Moreover, thermal its repeatability. Experiments are conducted for 4 W, 8 W and 12 W. conductivity of graphene nanoﬂuid is always higher than DI water. Heat losses from the evaporator section by radiation and free Thermal conductivity enhancement at the highest concentration convection are neglected. Thermal performance of thermosyphon (i.e., 0.1 wt.%) is about 17% of water conductivity at a temperature of is tested for vertical as well as for inclined position. Ambient 25 C, while the same enhancement at 45 C is around 29% of water temperature during the experiment remains 25 C. The uncertainty conductivity. It is also noticed from the published data of Ahammed in resistance between the evaporator and the condenser is calcu- et al. [16] that similar enhancement of 37.2% was noted for lated [3] by 0.15 vol.% of graphene nanoﬂuid at a temperature of 50 C Fig. 2. (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 4 (2019) 163e169 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 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 3 þ 4½2b11 ð1 L11 Þ þ b33 ð1 L33 Þ knf ¼ kbf (2) 3 4ð2b11 L11 þ b33 L33 Þ where Lii and Ø are the geometrical factors and the volume fraction of particles, respectively, and bii is deﬁned as: kp kbf bii ¼ (3) kbf þ Lii kp kbf 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. 3. (a) Thermal conductivity variation of nanoﬂuid with temperature for different 3.2. Viscosity weight percentages, (b) Comparison of thermal conductivity variation of graphene nanoﬂuid with different temperature and concentration (Nans Model). Viscosity of water increases with the addition of graphene particles which is depicted in Fig. 4a. In the present study, the compared to that of DI water at the same temperature. The con- viscosity enhancement of graphene nanoﬂuid for the highest ductivity of graphene nanoﬂuids is enhanced by 7% by changing the concentration of 0.10 wt.% is around 175% higher in comparison to temperature from 25 to 45 C for 0.02 wt.%. Thermal conductivity of DI water at 20 C. In Fig. 4a, viscosity of graphene nanoﬂuid is also 0.64 W/mK is observed for 0.04 wt.% of graphene nanoﬂuid at 25 C, found to decrease with temperature and this decrease is as high as which is 4.7% higher compared to that of 0.05 vol.% at 20 C [16]. In 25% for a concentration of 0.10 wt.% of nanoﬂuids. Rheological addition, the thermal conductivity of graphene nanoﬂuid having study is made to characterize the graphene nanoﬂuids. Fig. 4bec 0.10 wt.% is noted to be 0.81 W/mK at 45 C and it is 8.3% less in depicts such behaviour in the form of shear rate deformation at comparison to the thermal conductivity of 0.15 vol.% of graphene different temperature and it is found to be linear. Linear deforma- nanoﬂuid at 50 C [16]. tion rate only indicates that graphene nanoﬂuid considered pres- As the concentration of weight of graphene particle increases, ently is Newtonian in behaviour for all the temperature attempted the random motion of the graphene particles is enhanced in the in the present study. base ﬂuid. It is expected that the movement of such particles in- duces the collision between nanoparticles. At higher temperature, 3.3. Temperature distribution these collisional effects might be more and thus, the thermal con- ductivity of nanoﬂuid is found to be improved. In addition, when Fig. 5aec represents the temperature distribution of the ther- the concentration increases, the conduction electron (i.e., free mosyphon at a distance of 45, 60 and 119 mm for the evaporator, electrons available in the atoms, such as metal atoms, which are adiabatic and condenser section respectively at different heat input primarily accountable for thermal conductivity) is enhanced since and inclination angle. It can be observed from the ﬁgures that the the distance between atoms in a ﬁxed volume of graphene nano- wall temperature distribution of DI water is higher compared to ﬂuid decreases. Whenever concentration of nanoparticle is that of graphene nanoﬂuid and as the concentration increases, wall increased, the common surface areas between atoms of nano- temperature is decreased further. particles and the base liquid are enhanced. This leads to an It is noted from Fig. 5a, the wall temperature of DI water is enhancement in thermal conductivity. As the temperature is 42.5 C and with the addition of 0.10 wt.% of graphene nanoﬂuid, enhanced, the thermal conductivity is also enhanced. This is there is a decrease of 13.9% in the evaporator wall temperature for a possibly due to two reasons, (1) base ﬂuid thermal conductivity is heat input of 4 W, 60 inclination. In addition, it can also be enhanced due to increased Brownian motion and (2) conduction observed from the results of Kamyar et al. [2]. that a maximum of electrons will be positioned at a high energy level causing electron 24.53% decrease in wall temperature is found at 0.05 vol.% of Al2O3 to move faster and thus heat will be transported at a faster rate nanoﬂuid compared to DI water at 40 W heat input. Moreover, as which leads to higher thermal conductivity. As the molecules in the the inclination angle increases from 30 to 60 , the average wall liquid are closely spaced they yield stronger intermolecular force. temperature of the evaporator section decreases at 60 inclination.
S. Das et al. / Journal of Science: Advanced Materials and Devices 4 (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, ther- mosyphon 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 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 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: Te Tc R¼ (4) Q 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 4 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 4 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. Fig. 4. Variation of viscosity. (a) Viscosity of Graphene nanoﬂuid at different temper- [18], Shukla et al. [19] and Riehl and Santos [20] made similar types ature; (b) Rheological behaviour of nanoﬂuid at different weight percentage for 20 C; (c) Rheological behaviour of nanoﬂuid at different weight percentage for 50 C. 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 After 60 inclination, average evaporator temperature increases evaporator surface causes nucleation site to increase and this im- again. Similar trend is observed for adiabatic and condenser section proves the regime of nucleate boiling. Further, due to the deposi- of the thermosyphon (Fig. 5bec). Gravitational effect on conden- tion of nanoparticles, there occurs a change in surface wettability. sate return to the evaporator is the primary reason behind this. In addition, turbulence is being generated at higher heat input in Gravitational effect of condensate return enhances with the in- the graphene nanoﬂuid due to the movement of nanoparticle in the crease in inclination angle, which causes the enhancement of liquid ﬂuid. A similar observation is being made by Shukla et al. [19] in return. Hence, at the inclination angle 60 , evaporator temperature their study of heat pipe using CuO nanoﬂuid. More nucleation sites is low. Gravitational effect is maximum at 90 , which causes the are created as the nanoparticle deposits on the surface of the presence of more liquid in the evaporator section creating a evaporator. The performance of evaporator with the deposition of ﬂooding condition. This causes an increase in evaporator temper- nanoparticles highly depends on bubble departure diameter, ature. A similar observation is also made by Moraveji and Razvarz nucleation site density, frequency of bubble departure and ther- [9]. The wall temperature distribution in thermosyphon for 12 W mophysical properties of the working medium. The performance of heat input also follows the same trend. Therefore, better thermal the evaporator of TPCT ﬁlled with graphene nanoparticle may be
168 S. Das et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 163e169 Fig. 5. 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 bubble, f is the frequency of bubble departure. kl, rl and Cl are the [10] as under: conductivity, density and speciﬁc heat, respectively. Because of nanoparticle deposition, nucleation site density increases manifold 1 2 due to the formation of porous layer. Bubble departure diameter may Re ¼ ¼ pﬃﬃﬃpﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ; (5) he Ae Na Ae Db f pkl rl Cl decrease due to decrease in surface tension at higher temperature, but bubble departure frequency and effective surface area will in- where he is the coefﬁcient of heat transfer, Ae is the evaporator area, crease. Overall effects cause an improvement in thermal perfor- Na is the nucleation site density, Db is the departure diameter of the mance. It is found that inclination angle has nominal inﬂuence on Fig. 6. 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.
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