REVIEW

J. Cent. South Univ. (2019) 26: 1021-1041

DOI: https://doi.org/10.1007/s11771-019-4068-9

A review on application of nanofluid in various types of heat pipes

Mohammad Alhuyi NAZARI¹, Mohammad H. AHMADI², Milad SADEGHZADEH¹,Mohammad Behshad SHAFII³, Marjan GOODARZI⁴

1. Department of Renewable Energy and Environmental Engineering, University of Tehran,Tehran 1439957131, Iran;

2. Faculty of Mechanical Engineering, Shahrood University of Technology, Shahrood 3619995161, Iran;

3. Faculty of Mechanical Engineering, Sharif University of Technology, Tehran 111559567, Iran;

4. Sustainable Management of Natural Resources and Environment Research Group, Faculty of Environment and Labour Safety, Ton Duc Thang University, Ho Chi Minh City 700000, Vietnam

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019

Abstract: Nanotechnology is widely used in heat transfer devices to improve thermal performance. Nanofluids can be applied in heat pipes to decrease thermal resistance and achieve a higher heat transfer capability. In the present article, a comprehensive literature review is performed on the nanofluids’ applications in heat pipes. Based on reviewed studies, nanofluids have a high capacity to boost the thermal behavior of various types of heat pipes such as conventional heat pipes, pulsating heat pipes, and thermosyphons. Besides, it is observed that there must be a selected amount of concentration for the high-performance utilization of nanoparticles; high concentration of nanoparticles causes a higher thermal resistance which is mainly attributed to increment in the dynamic viscosity and the higher possibility of particles’ agglomeration. Enhancement in heat transfer performance is the result of increasing in nucleation sites and the intrinsically greater nanofluids’ thermal conductivity.

Key words: heat pipe; nanofluid; thermal resistance; thermal performance

Cite this article as: Mohammad Alhuyi NAZARI, Mohammad H. AHMADI, Milad SADEGHZADEH, Mohammad Behshad SHAFII, Marjan GOODARZI. A review on application of nanofluid in various types of heat pipes [J]. Journal of Central South University, 2019, 26(5): 1021–1041. DOI: https://doi.org/10.1007/s11771-019-4068-9.

1 Introduction

Many studies have focused on efficiency-enhancing of energy systems and obtaining better working conditions [1–5]. Among various methods applied in energy efficiency enhancement such as heat recovery, optimizing working condition and utilizing more appropriate materials [6–13], nanotechnology is a promising approach.It has been applied in several devices and systems in order to decrease the size of the system and improve its efficiencies [14–19]. By applying nanotechnology, it is possible to achieve higher efficiency and better performance in energy systems [20, 21]. Nanotechnology has been widely investigated in solar-based thermal systems in order to boost up the system efficiency [22, 23]. Nanofluids are obtained by dispersion of nanoparticles (or other structures such as nanotubes, nanosheets) in a base fluid [24–28]. Due to the higher surface to volume ratio of nanoparticles [29], the nanofluids have more appropriate thermal properties, compared with pure fluids or micro-fluids [24, 26, 30, 31]. Nanofluids’ thermal properties make it a viable solution for performance enhancement of condensers and evaporators [32–35].

Heat pipes are thermal devices which work based on two-phase heat transfer [36, 37]. The concept of heat pipes was started in 1831 by Angier March Perkins and then developed by Jacob Perkins and F.W. Gay, in 1936 and 1942, respectively [38]. Heat pipes can be employed for various purposes as cooling electronic devices, desalination systems, renewable energy systems and etc. [39–44]. Heat pipes are mainly made of a tube, in some cases a flat plate with channels, which are partially filled with a working fluid [45, 46]. Basically, heat pipes have two main heat transfer sections: the evaporator and the condenser [47, 48]. An arbitrary adiabatic unit can exist where a gap between a heat sink and a heat source exists. The working fluid inside heat pipes receives heat from the heat source and evaporates and liquefies due to heat transfer in heat sink [49, 50]. In the following, the liquid returns to the evaporator, so, there is a continuous heat transfer between the heat sink and the source [51–56]. Structure of the heat pipe controls the mechanism of liquid returning from the condenser to the evaporator.

Conventional heat pipes employ capillary force, to deliver the liquid from the condenser to the evaporator [57]. The capillary structure can be provided by a wick or by grooving on the inner surface of the heat pipes [58]. This type of heat pipes is less sensitive to the orientation and can work efficiently in the absence of the gravity force. Another type of heat pipe, more compact in size, is pulsating heat pipes (PHPs) [59, 60]. PHPs are made of a capillary tube and have several bends [61–65]. Pressure instabilities inside the tube, which is due to heat input to various turns, are the main reason for fluid motion [66, 67]. Thermosyphons are other kinds of heat pipes. In thermosyphons, the condensed liquid in the condenser comes back to the evaporator by the gravity assistance [68–70]. Thermosyphons are operating efficiently in heat transfer; however, their operations are very sensitive to gravity and orientation [71–73]. Different classes of heat pipes are illustrated in Figure 1 [74–76].

Several parameters affect heat transfer and thermal behavior of heat pipes [48, 77, 78]. Orientation, the structure of heat pipes, filling ratio, the tube’s material, heat input, and the working fluid are among the most influential factors in the heat pipes, thermal performance [79–82].

Figure 1 Schematic of wick heat pipe (a), thermosyphon (b) and pulsating heat pipe (c) (It is reprinted with permission from Elsevier (publisher) [74–76])

Thermophysical properties of working fluids noticeably have an influence on the thermal performance of various types of heat pipes. Working fluid selection depends on the applications and working conditions. For instance, for cryogenic heat transfer purposes, it is necessary to use working fluids with very low boiling temperature [83–85]; while for high-temperature applications such as solar thermal collectors, concentrated solar power plants(CSP), and high-temperature thermal storage, fluids with high boiling point are more appropriate [86] since the possibility of dry-out exists for fluids which do not have high boiling temperature.

Since the dominant heat transfer mechanism in heat pipes is boiling heat transfer, enhancement in the boiling will cause an improvement in thermal performance. Several studies have shown that nanofluids are able to improve the boiling heat transfer. In addition, the thermal conductivity of nanofluids is greater than pure fluids which is favorable [87–90]. Fluids with a higher thermal conductivity are more appropriate for heat transfer enhancement. Adding nanoparticles to the base fluid can increase the thermal conductivity of the obtained fluid [24, 91–95], which makes them applicable to be used in heat pipes for heat transfer improvement.

Since nanofluids have a high potential for heat pipes’ thermal performance improvement [96, 97], numerous studies have been carried out on the utilization of nanofluids in the heat pipes [98–102]. In this investigation, a review is performed to monitor the effects of using nanofluid in different classes of heat pipes including conventional, pulsating, and thermosyphon heat pipes. In addition, the published articles and the studied parameters in each class of heat pipe including the type of used nanofluid, the concentration of the nanofluid in the base fluid, shape, size, filling ratio, and the inclination angle are summarized, including conventional heat pipes, thermosyphon heat pipes, and pulsating heat pipes, respectively.

2 Nanofluids in conventional heat pipes

Nanofluids with various characteristics including nanoparticle type and concentration are utilized in heat pipes as working fluids [103]. KANG et al [104] utilized Ag/water nanofluid in a sintered wick heat pipe to assess the impact of adding nanoparticles on the thermal performance and heat pipe temperature difference. The diameters of the nanoparticles were 10 and 35 nm, with the concentrations of 1×10^–6, 10×10^–6 and 100×10^–6. Their obtained results showed that at an input work of 30–50 W, adding nanoparticles can decrease temperature differences up to 0.56–0.65 °C in comparison with deionized water. In addition, it was concluded that the best concentration among the tested ones was 10×10^–6. Moreover, nanoparticles with 35 nm in diameter had a better heat transfer capability in the heat pipe due to the boosted thermal conductivity.

The material of nanoparticles also influenced the thermal performance of heat pipes [105]. VIJAYAKUMAR et al [106] investigated the impacts of adding two different metal-oxide nanoparticles (CuO and Al₂O₃) to deionized water (DI water) in a sintered wick heat pipe. Several concentrations of nanoparticles (0.5 wt.%, 1 wt.% and 1.5 wt.%) were evaluated in the study. Their results revealed that the optimum concentration for the nanofluid used in a heat pipe can be dependent on the material of particles. The best concentrations for CuO and Al₂O₃ were 1 wt.% and 1.5 wt.%, respectively. The highest reduction in heat pipe surface temperature for CuO nanofluid with 1 wt.% concentration was 5.9 °C, while it was 5.3 °C for Al₂O₃ nanofluid with 1.5 wt.% concentration [106]. WAN et al [107] investigated the impact of copper/ water nanofluid on the thermal characteristics of a loop heat pipe. The average size of nanoparticles was 50 nm with 1 wt.%, 1.5 wt.% and 2 wt.% concentrations. They found that the optimal concentration of nanoparticles for utilization in the heat pipe was 1.5 wt.%. The results showed that utilizing nanoparticles improved the heat transfer coefficient up to 19.5% at 100 W heat input [107]. The enhancement in heat transfer was yielded due to the increased wettability and increase in nucleation sites. GUNNASEGARAN et al [108] utilized diamond/water nanofluid in a loop heat pipe containing sintered nickel as a wick. The mass concentration of the nanofluids in this study was in the range of 0–3%. Results revealed that increment in the concentration of nanoparticles led to the lower thermal resistance of loop heat pipe. Employing nanofluids reduced the thermal resistance of the loop heat pipe in the range of 5.7%–10.8% in comparison with the water as a working fluid. In this work, enhancement in heat transfer was the result of increasing the nucleation sites by using nanofluid [108].

In addition to heat pipes with a sintered wick, nanofluids are applied in heat pipes with other capillary structures. GHANBARPOUR et al [109] investigated a heat pipe contained two layers of screen mesh. SiC/water was employed as working fluid with various mass concentrations. By using nanofluid, thermal resistance decreased. Reductions in thermal resistances were 11%, 21% and 30% for 0.35%, 0.7% and 1% concentrations, respectively. The reduction in thermal resistance occurred in all tested orientations. SENTHIL et al [110] utilized Al₂O₃/water with 1 vol. % concentration as carrying fluid to evaluate the thermal behavior of a wire mesh heat pipe. Their results showed that using the mentioned nanofluid increased thermal efficiency of the heat pipe and the maximum reduction in thermal resistance was 52%. In another study, ALY et al [111] used Al₂O₃/water as a working fluid in a helically-micro-grooved heat pipe. The concentration of nanoparticles was 3 vol.%. Results indicated that using nanofluid as working fluid led to up to 18.2% reduction in thermal resistance of the heat pipe [111]. Al₂O₃/water was utilized in another study as working fluid by MOUSA [112] in a heat pipe utilized two layers of copper meshes as a wick structure. Three concentrations including 0.25 wt.%, 0.5 wt.% and 1.2 wt.% were tested in the experimental procedure. It was observed that adding nanoparticles decrease thermal resistance of heat pipe. Based on a study conducted by GHANBARPOUR et al [113], an optimal concentration of Al₂O₃/water nanofluid is used in the heat pipes, which can be seen in other reported studies [114, 115]. High concentrations can deteriorate the thermal behavior of screen mesh heat pipe [113]. Other metal-oxide nanoparticles such as CuO, TiO₂ and MgO nanoparticles were also utilized in heat pipes [116, 117]. MOHANRAJ et al [118] applied CuO/water in a flat plate heat pipe. It was reported that by using the nanofluid, the temperature difference between evaporator and condenser decreased by 2 to 4 °C, compared with utilizing pure fluid. Reduction in thermal resistances of heat pipe by using Cu nanofluid was observed in other studies by VENKATACHALAPATHY [119, 120] and WANG et al [93, 94].

Other metallic nanoparticles such as silver have been tested in screen mesh wick heat pipe. HAJIAN et al [121] utilized silver nanoparticles in DI water as a base fluid in three concentrations including 50×10^–6, 200×10^–6 and 600×10^–6. Based on obtained data, heat pipe filled with nanofluid with 50×10^–6 concentration had the lowest thermal resistance in comparison with other working fluids. Higher thermal resistance in high concentrations was attributed to the more possibility of particles agglomeration and sedimentation which have unfavorable results on the thermal behavior [121]. Silver/water nanofluid was used in an inclined screen mesh heat pipe by GHANBARPOUR et al [122]. The tested concentrations were 0.25 wt.%, 0.5 wt.% and 0.75 wt.%. They found that higher concentration resulted in a lower range of thermal resistance. Thermal resistances of the aforementioned heat pipe for various concentrations in horizontal and vertical orientations are shown in Figure 2. Other metal-oxide nanoparticles such as ZnO nanoparticles were added to a base fluid in order to be applied in a screen mesh heat pipe. SALEH et al [123] utilized ethylene glycol as the base fluid and added ZnO nanoparticles at a concentration of (0.025–0.5) vol.%. It was observed that utilizing nanofluid led to a decrease in wall temperature of the heat pipe, compared with the heat pipe filled with pure fluid. Obtained wall temperature of the heat pipe in a case of ZnO nanofluid is shown in Figure 3.

In addition to metal and metal-oxide nanoparticles, carbon nanotubes have been used in a flat plate heat pipe. ARYA et al [124] applied carbon nanotubes (CNTs) in a heat pipe which used screen mesh as wick structure. Their results indicated that by applying CNTs, it is possible to improve the heat transfer of the heat pipe up to 40% over the pure fluid. Utilizing CNTs improved thermal performance of wick structure owing to fouling layer creation on the mesh. Creation of the fouling layer changed the contact angle between the working fluid and the surface; as a consequence, the capillary forced increased [124]. In another study, MEHRALI et al [125] investigated the utilization of nitrogen-doped graphene (NDG) on the heat transfer of a grooved heat pipe. The maximum observed decrease in thermal resistance was 58.6% in the case of using the nanofluid with 0.06 wt.% concentration. The enhancement in heat transfer was the mainly due to the creation of a porous hydrophilic layer on the evaporator, due to the existence of NDG nanosheets [125].

Figure 2 Thermal resistance vs heat input (It is reprinted with permission from Elsevier (publisher) [122]):

Figure 3 Heat pipe wall temperature at different heat inputs for ZnO nanoparticles (It is reprinted with permission from Elsevier (publisher) [123]):

Similar to wick heat pipes, various shapes of nanostructures can be used in screen mesh wick heat pipes [126]. For instance, graphene sheets were dispersed in water in order to be used in a miniature loop heat pipe (with screen mesh wick) [127]. The thickness of graphene nanosheets was in the range of 1–5 nm and the concentrations were 0.003 vol.%, 0.006 vol.% and 0.009 vol.%. Among the tested concentrations, 0.006 vol. % showed the best heat transfer improvement. The lowest achieved thermal resistance was 21.6% lower than that using distilled water. Various shapes of nanoparticles are also utilized in heat pipes [128]. For instance, graphene nanoplatelets with 0.025 wt.%, 0.05 wt.%,0.075 wt.% and 0.1 wt.% concentrations were dispersed in water and utilized in a heat pipe to evaluate thermal performance by SADEGHINEZHAD et al [129]. Based on the obtained results, the largest decrease in the thermal resistance of the sintered wick heat pipe at 0.1 wt.% nanofluid was 48.4% [129].

The shape of nanoparticles for a specific particle material affects thermophysical properties of nanofluid. Therefore, their performance can be changed in a heat pipe. KIM et al [130] experimented on the effect of Al₂O₃ nanoparticles dispersed in acetone on the thermal performance of a flat plate heat pipe. Sphere-, brick- and cylinder-shaped nanoparticles were investigated in this study. They realized that reductions in thermal resistance of the heat pipe, compared with using simple acetone, were 33%, 29% and 16% for sphere-shaped, brick- shaped and cylinder-shaped nanoparticles, respectively. Differences in thermal resistance were attributed to the effect of nanoparticles shape on the porosity of nanoparticles’ layer, which influenced thermal performance of the heat pipe. The size of nanoparticles is another parameter affecting thermophysical properties of nanofluids [139, 140]. As a consequence, nanofluids with various sizes of particles can show different behavior in heat pipes. HASSAN et al [141] investigated the effect of size and concentration of nanoparticles, on the thermal performance of a flat heat pipe subjected to periodic

heat input. Results showed that heat pipe temperature reduced by increasing concentration and decreasing the diameter size of nanoparticles.

Table 1 summarizes various studies conducted on conventional heat pipes by using nanofluid as working fluid.

Table 1 Summary of conducted investigations on application of nanofluid in conventional heat pipes

3 Nanofluids in thermosyphons

Thermosyphons are types of heat pipes which highly depend on orientation since the gravity forced the fluid in the condenser to return into the evaporator. There are several applications for thermosyphons in literature such as using them in heat exchangers, solar collectors, and other energy systems [142, 143]. To obtain better efficiency in the systems utilizing thermosyphons, it is necessary to enhance their thermal performance. Thermophysical properties of working fluid affect thermal performance of these devices. In recent years, several studies have focused on using nanofluids in the thermosyphons, to enhance heat transfer [144–146]. Nanofluids are able to improve thermal behavior of thermosyphons; however, there are some related issues such as nanofluid fouling which may worsen their performance for long-term usage [147]. The studies which are conducted in this field are presented here to review the various influential parameters of nanofluids and impact of using them on thermosyphons.

HUMINIC et al [148] utilized iron oxide/water nanofluid in a two-phase closed thermosyphon. The mean diameter of applied nanoparticles was 4–5 nm. The thermosyphon was tested in several inclination angles including 30°, 45°, 60° and 90° with two nanoparticles concentrations (2 vol.% and 5 vol.%). Their results revealed that adding nanoparticles to carrying fluid enhances the thermal performance of thermosyphon, in all mentioned inclination angles and concentration. In addition, it was observed that higher concentration led to better heat transfer which was similar to the results of their future numerical study [149]. The improvement in heat transfer was made due to the larger thermal conductivity and lower solid/liquid contact angle by utilizing nanofluids [148]. Concentration augmentation leads to higher thermal conductivity which is appropriate for heat transfer. However, there is an optimum concentration due to the unfavorable effect of high concentrations on other properties such as dynamic viscosity and the possibility of agglomeration consolidation [150]. In another study, Fe₂O₃/water was used in a loop thermosyphon with concentration of 0.3 wt.%, 0.8 wt.%, 1 wt.%, 1.5 wt.% and 2 wt.%. Results showed that the heat transfer coefficient can be increased up to 20% by using the nanofluid, compared to base fluid [151].

BUSCHMANN et al [152] investigated a thermosyphon with gold/water and titanium oxide/ water nanofluids with various concentrations. Up to 24% drop in thermal resistance was achieved by applying nanofluid. Moreover, the optimum concentration was reached between 0.2 vol.% and 0.3 vol.%. GRAB et al [153] utilized water contained dispersed titanium oxide and gold nanoparticles in a thermosyphon. They observed that nanofluids can improve the thermal performance of a thermosyphon. However, the improvement depended on several parameters such as heat input, size, and type of nanoparticles. Thermal performance enhancement was yielded to the change in evaporator surface which affected evaporation behavior [153]. HERIS et al [154] applied CuO/water and Al₂O₃/water as working fluid in a thermosyphon. In addition to studying the effect of nanoparticles type, the influence of the electrical field on the thermal performance was investigated. They found that using Al₂O₃/water as working fluid led to higher thermal performance, compared with CuO/water which was ascribed to the smaller size of alumina nanoparticle and its properties. The thermal performance improvement was also attributed to the higher thermal conductivity of nanofluids and the role of nanoparticles in breaking vapor bubbles which led to lower bubble resistance. In addition, it was observed that applying electrical field can enhance heat transfer [154]. In another study, KAMYAR et al [155] experimented the effect of using Al₂O₃/ water and TiSiO₄/water on the thermal behavior of a thermosyphon. Their findings indicated that using nanofluids as working fluid causes thermal resistance reduction, in comparison with water. Their obtained thermal resistances under various working conditions are shown in Figure 4. HOSEINZADEH et al [156] utilized Al₂O₃/water and SiC/water nanofluids in a thermosyphon. It was observed that filling thermosyphon with SiC/water nanofluid with 3 vol.% concentration, under input power equal to 300 W led to the highest efficiency. In addition to conventional nanofluids, some other innovative working fluid like fly ash nanofluid has been examined in thermosyphons. Fly ash nanofluid composed of various metal-oxide particles including silica, titanium oxide, alumina and etc.

Figure 4 Thermal resistance vs heat inputs water (It is reprinted with permission from Elsevier (publisher) [155]):

SZEN et al [157] used fly ash nanofluid at various concentrations in a thermosyphon. Results declared that using the nanofluid enhanced heat transfer. For instance, 26.39% increment was observed in thermosyphon efficiency by employing 4 wt.% nanofluid [157].

CuO/water nanofluid is commonly employed in heat transfer mediums [15, 158, 159] since its thermophysical properties are desirable [160–162]. LIU et al [163] utilized CuO/water nanofluid with an average particle size of 30 nm in a miniature thermosyphon. Their results showed that CuO/water nanofluid improves boiling in the evaporator section at sub-atmospheric pressure. For instance, using the nanofluid at 1.0 wt.%, heat transfer coefficient increased by 160% at 7.45 kPa, in comparison with using pure water under the same working conditions [163]. YANG et al [164] evaluated the thermal behavior of a loop thermosyphon with water and CuO/water nanofluid. Obtained results showed that using nanofluid instead of pure fluid could lead to enhancement in heat transfer coefficient and critical heat flux of flow boiling in the evaporator section. Improvement in thermal performance was the result of changes in thermophysical properties and coating layer existence. Obtained results for the heat transfer coefficient improvement at the mass concentration of 1.0% are shown in Figure 5.

Figure 5 Enhancement in heat transfer coefficient for various pressures (It is reprinted with permission from Elsevier (publisher) [164])

In addition to experimental studies, numerical simulations also confirm that using CuO/water nanofluid will boost the thermal performance of thermosyphons [165].

The base fluid affects thermophysical properties of a nanofluid; as a consequence, the performance of thermosyphon filled with nanofluids depends on the base fluid, too. In a study, SARAFRAZ et al [166] assessed the thermal performance of a thermosyphon heat pipe made of copper which was filled with Al₂O₃/water-ethylene glycol (water–EG) and Al₂O₃/water-diethylene glycol (water–DEG). The thermophysical properties of nanofluid in various concentrations were measured and results indicated that water-EG based nanofluids have better thermal specifications.

Various shapes of nanoparticles can be utilized in thermosyphons [167, 168]. AMIRI et al [169] analyzed thermal behavior of a thermosyphon which was filled with graphene nanoplatelet/water. Two types of nanofluids were produced by dispersion of graphene nanoplatelets (GNP) in carrying fluid including covalent nanofluid (GNP-COOH/water) and non-covalent nanofluid (GNP-SDBS/water). It was observed that at a constant concentration, the thermophysical properties, i.e. thermal conductivity, of GNP- COOH/water has more improvement, compared with GNP-SDBS/water. In addition, results revealed that the filling thermosyphon with GNP-COOH/ water causes higher thermal performance, compared to its filling with GNP-SDBS/water. Figure 6 illustrates the efficiency of the thermosyphons filled with various working fluids. In another work, ASIRVATHAM et al [170] utilized graphene/ acetone nanofluid in a thermosyphon made of glass. The concentrations of nanofluid were 0.05 vol%, 0.07 vol% and 0.09 vol%. Their results indicated that by utilizing nanofluid, the thermal resistance could be reduced by more than 70% [170].

Figure 6 Efficiency of thermosyphon filled with various working fluids (It is reprinted with permission from Elsevier (publisher) [169])

Performed previous studies revealed that dispersion of multi-walled carbon nanotubes (MWCTs) improves the thermal conductivity and makes them an appropriate choice for using in thermosyphons [171]. In a study, MWCNT/water and MWCNT-Ag/water nanofluids were used in a two-phase thermosyphon by SHANBEDI et al [172]. As shown in Figure 7, lower temperature difference is monitored between evaporator and condenser (which means lower thermal resistance).

Although most of the investigated nanofluids enhanced the thermal performance of thermosyphons, some of the nanofluids can deteriorate thermosyphons heat transfer [173]. Reduction in heat transfer coefficient by using some nanofluids such as water/SiO₂ functionalized was related to their thermophysical properties [174]. As a consequence, in order to achieve the highest thermal efficiency, choosing an appropriate nanofluid for the required application is crucial.

The summary of literature in the fields of thermosyphon heat pipes is represented in Table 2.

Figure 7 Temperature difference vs heat input for various concentrations (It is reprinted with permission from Elsevier (publisher) [172])

4 Nanofluids in pulsating heat pipes

Pulsating heat pipes (PHPs) are made more compact in comparison to other classes of heat pipes. The sensitivity of PHPs to the orientation is lower, in comparison with thermosyphons [175]. There are several applications for PHPs including renewable energy utilities, cooling electronic chipsets, space applications and etc. [176–179]. Similar to other types of heat pipes, PHPs thermal performance mainly depends on the used working fluid [180–184]. Utilizing nanofluids in PHPs can enhance heat transfer, which is investigated in several studies [185, 186]. Studies focused on the utilization of nanofluids in PHPs are discussed in this section.

JIA et al [187] used SiO₂/water nanofluid with various concentrations including 0.05 wt.%,0.1 wt.%, 0.3 wt.%, 0.4 wt.% and 0.5 wt.% in a closed-loop PHP. Results indicated that an optimal concentration of nanoparticles is necessary to achieve the highest thermal performance. Mentioned results are shown in Figure 8. Based on a numerical study by RUDRESHA et al [188], both SiO₂/water and Al₂O₃/water have the capability to improve heat transfer in a PHP. WANG et al [189] utilized Al₂O₃/water nanofluid as working fluids in a PHP.

The concentrations of the nanofluid in that study were 0.1 wt.% and 0.5 wt.%. It was found out that adding nanofluid could enhance heat transfer performance of the PHP. In addition, it was observed that the best concentration depended on the orientation of the PHP as shown in Figure 9.

Table 2 Summary of studies conducted on application of nanofluid in thermosyphons

Figure 8 Thermal resistance vs heat input (It is reprinted with permission from Elsevier (publisher) [187])

MOHAMMADI et al [192] used Fe₃O₄/water nanofluid with 2.5 g/L concentration as a magnetic fluid in an open loop PHP. After doing experiments, they realized that using nanofluid can enhance thermal performance. In addition, applying the magnetic field would increase the thermal performance augmentation due to the changes in the roughness of evaporator which resulted in boiling improvement [192]. In addition to open-loop PHP, Fe₃O₄/water nanofluid is able to enhance thermal performance in closed-loop PHP [45]. GOSHAYESHI et al [193] used a kerosene-based ferrofluid (Fe₂O₃/kerosene) in a closed-loop OHP which was under magnetic field. The obtained data showed that using magnetic field enhances heat transfer performance of the PHP.

Figure 9 Thermal resistance vs heat input (It is reprinted with permission from Elsevier (publisher) [189]):

In addition to metal-oxide nanofluids, metallic nanoparticles have been dispersed in base fluids and used in the PHPs. WANNAPAKHE et al [194] assessed the influence of silver nanofluid on the heat transfer rate of a closed-loop PHP. Based on observed results, using the nanofluid, it is possible to increase the heat transfer rate by more than 10%. Heat transfer improvement of the PHP was credited to the existence of nanoparticles which increased surface area and the heat capacity of the working fluid. Silver/water with 0.1 vol.% concentration was used as nanofluid in a fabricated closed-loop PHP setup by GONZALEZ et al [195] and thermal resistance was reduced in compared to water. Based on another study conducted on silver/water nanofluid by LIN et al [196], there is an optimum concentration for this nanofluid and higher concentrations deteriorate thermal performance due to increase in dynamic viscosity which acts as a resistance force for bubble pulsation.

Other types of metallic nanoparticles also have been used in the PHPs. In a study, PARK et al [197] dispersed CuNi nanoparticles in a high-quality liquid chromatography water and the obtained fluid was utilized in a well-balanced oscillating heat pipe. Results stated that by using the nanofluid, achieving enhanced heat transfer rate may be possible in the cases that oscillation motion in the heat pipe exists. KARTHIKEYAN et al [198] used silver/water and copper/water nanofluids in a PHP. Obtained results revealed that both nanofluids were able to decrease the thermal resistance of the PHP as shown in Figure 10.

The hybrid nanofluids (mixing various types of nanofluids) are a novel idea which was used in a PHP. TANSHEN et al [199] utilized Al₂O₃ nanoparticles, MWCNTs and their hybrid in a PHP. Experimental data showed that using the hybrid nanofluid resulted in the best thermal performance. Obtained results are shown in Figure 11. In another study, TANSHEN et al [200] charged a PHP with functionalized MWCNTs/water and evaluated its thermal performance. Four concentrations (0.05 wt.%, 0.1 wt.%, 0.2 wt.% and 0.3 wt.%) were applied for the nanofluid. Results indicated that the nanofluid with 0.2 wt.% has the best thermal performance among the mentioned concentrations.

Figure 10 Thermal resistance vs heat input for different working fluids (It is reprinted with permission from Elsevier (publisher) [139])

Figure 11 Thermal resistance vs heat input for 60% filling ratio (It is reprinted with permission from Elsevier (publisher) [199])

Since the shape of particles affects thermophysical properties of nanofluids, their behavior in the PHP can be influenced. In this regard, JI et al [201] investigated the effect of particle shape on the thermal performance of a PHP. Alumina nanoparticles with various shapes including platelet, blade, cylinder, and brick-shaped were dispersed in a mixture of water and EG (50:50 by volume) in order to prepare nanofluids. The PHP was charged with the nanofluids and tested. Results showed that the nanofluid with cylinder-shaped nanoparticles has the best performance among the mentioned nanofluids. In addition to particle shape, size of nanoparticles has a considerable impact on the thermophysical properties of nanofluids [202–205]. JI et al [206] used alumina nanoparticles with various average sizes of 50 nm, 80 nm, 2.2 μm, and 20 μm and dispersed them in water as the base fluid. A PHP was filled with the obtained carrying fluids. Results indicated that nanofluids contained particles with 80 nm average size have the best thermal performance among the investigated sizes. Obtained results for 50% filling ratio are shown in Figure 12.

Carbon-based nanofluids such as graphene oxide have the ability to enhance the thermal behavior of pulsating heat pipes. NAZARI et al [207] employed graphene oxide/water nanofluid in four concentrations including 0.25, 0.50, 1.00 and 1.50 g/L. It was observed that using the nanofluid at 0.25 g/L led to the best thermal resistances.

The conducted studies investigated the effect of nanofluids on PHPs as summarized in Table 3.

5 Conclusions

In this study, various applications of nanofluids in different classes of heat pipes are reviewed. Based on the literature, nanofluids are able to meaningfully enhance the thermal behavior of the heat pipes. The thermal performance improvement depends on several parameters such as the type of nanoparticles, the base fluid,concentrations, shape, and size of nanoparticles and working conditions. The most important obtained results are listed as follows.

Figure 12 Effect of particle size on temperature difference and thermal resistance (It is reprinted with permission from Elsevier (publisher) [206]):

1) An optimum concentration exists for dispersed nanoparticles in base fluids. High concentrations cause an unfavorable effect on thermal performance.

2) The size and shape of nanoparticles are influential factors affecting thermal behavior of heat pipes, due to their effect on thermophysical specifications of nanofluids.

3) Thermal performance improvement of heat pipes by applying nanofluids is mainly credited to the higher thermal conductivity of nanofluids and growth in nucleation sites.

4) Using hybrid nanofluid can lead to higher improvement in thermal performance.

5) The improvement and development of thermal efficiency of heat pipes by employing nanofluids are contingent on operating condition including filling ratio, inclination angle, etc., along with the design of the heat pipes.

Table 3 Summary of studies conducted on application of nanofluid in PHPs

Further study is essential to specify more precise properties of nanofluids including density, viscosity, and surface tension. A multi-phase numerical analysis would be beneficial on estimating the effect of the irregular distribution of nanoparticles on the performance of heat pipes. This simulation might be time-consuming but specify the effects of vaporization and condensation, agglomeration of nanoparticles, and also determine the influence of Brownian motion and the thermophoresis occurrence. Moreover, in the case of using metal oxide based nanofluids, much more efforts should be done to result in nanofluids with a better and higher rate of stability. Deeper investigation is needed on the low viscosity nanofluids. Moreover, high-temperature applications of nanofluids are still immature and more researches are required for its development. The most important concern to commercialize the nanofluid is its high production cost. Fabrication of cheaper nanofluids or finding a cost-effective production approach is a case that should be focused on to overcome the major barrier of production cost which hinders fast development and utilization of nanofluids.

Conflicts of interest

The authors declare that there is no conflict of interest regarding the publication of this paper.

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(Edited by YANG Hua)

中文导读

纳米流体在各种热管中的应用综述

摘要：纳米技术被广泛应用于传热装置中，以提高热性能。纳米流体应用于热管中，可降低热管的热阻，提高热管的传热能力。文中，对纳米在流体热管中的应用进行了全面的综述。阐述了纳米流体对传统热管、脉动热管、热虹膜等各种类型热管的热性能的影响。此外，发现了纳米颗粒的高效利用必须有一定的浓度；因动力黏度的增加导致颗粒团聚的可能性越高，高浓度的纳米颗粒会产生较高的热阻。热传递性能的增强是晶核位置的增多和纳米流体本身具有的更大热导率的结果。

关键词：热管；纳米流体；热阻；热性能

Received date: 2018-11-03; Accepted date: 2019-03-09

Corresponding author: Marjan GOODARZI, Researcher; E-mail: marjan.goodarzi@tdtu.edu.vn; ORCID: 0000-0002-1285-0593; Mohammad H. AHMADI, Assistant Professor; E-mail: mhosein.ahmadi@shahroodut.ac.ir; ORCID: 0000- 0002-0097-2534