The revolution of fabrication technology allows the fabrication of materials at a nano-scale. Nanoparticles fabricated by different methods show various fancy characteristics in electronic, magnetic, optical, and catalytic applications. The concept of the nanofluid, which is the suspension of nanoparticles, was firstly proposed by Choi [1
]. Since then, a lot of researches have been carried out to study the heat transfer characteristics of nanofluids. The heat transfer characteristics of nanofluids started with the investigation of thermal conductivity [1
], then the single-phase flow heat transfer [4
], and now, the focus mainly is on the phase-changing heat transfer of nanofluids. Among the phase-changing heat transfer, the application of nanofluids in heat pipes gains increasing popularity [8
]. The involved heat pipes include the grooved heat pipe [8
], wicked heat pipe [10
], sintered heat pipe [12
], oscillated heat pipe [14
], and the thermosyphon [16
Xue et al.
] studied the heat transfer performance of carbon nanotube-water nanofluid in a thermosyphon. The mass concentration of nanoparticles is 1.3158 wt.%. The thermosyphon is a copper tube with an outer diameter (O.D.) of 20 mm. The filling ratio is 20%. Results show that the thermosyphon with carbon nanotube nanofluid has a higher evaporation section wall temperature, incipience temperature, and excursion, as well as thermal resistance. The carbon nanotube-water nanofluid deteriorates the heat transfer of the thermosyphon compared with the water case.
Khandekar et al.
] investigated the overall thermal resistance of a closed two-phase thermosyphon using water-based Al2
(40 to 47 nm), CuO (8.6 to 13.5 nm), and laponite clay (disks with a diameter of 25 nm and thickness of 1 nm) nanofluids. The length and the inner diameter of the thermosyphon are 720 and 16 mm, respectively. The nanoparticle mass concentration is 1.0 wt.%. Results show that all nanofluids have inferior thermal performance compared to pure water. A mechanism analysis guesses that the increase in wettability and entrapment of nanoparticles in the grooves of the surface cause a decrease of the Peclet number in the evaporator side and finally leads to poor thermal performance.
Naphon et al.
] studied the heat transfer performance of the TiO2
-water and TiO2
-alcohol nanofluids in a thermosyphon. The nanoparticle volume concentration is 0.01%, 0.05%, 0.10%, and 0.50%, respectively. The thermosyphon is made of a copper tube with an O.D. of 15 mm and a length of 600 mm. The authors investigated the effects of filling ratio, inclined angle, and volume concentration on the heat transfer performance. Results show that nanoparticles can enhance the heat transfer efficiency by 10.6%. Naphon et al.
] also studied the heat transfer of TiO2
-R11 nanofluid in a thermosyphon with the nanoparticle volume concentrations of 0.01%, 0.05%, 0.10%, 0.50%, and 1.0%. Results show that the thermosyphon efficiency can be enhanced by 40%.
Liu et al.
] investigated the effect of nanoparticle parameters on the thermal performance in a thermosyphon using CuO and carbon nanotube nanofluids without surfactants. The diameter, the evaporator, the adiabatic section, and the condenser of the thermosyphon have a length of 8, 100, 100, and 150 mm, respectively. The experimental results show that adding nanoparticles in the heat pipe could enhance both the heat transfer performance of evaporation section and the maximum heat flux (MHF). Different from other studies, their experiments were carried out at several steady operating pressures, and the experiments found that the operation pressure has a significant influence on the heat transfer enhancement.
Noie et al.
] studied the Al2
-water nanofluid in a thermosyphon. The thermosyphon is made of a copper tube with an inner diameter of 20 mm and a length of 1,000 mm. The length of the evaporator and the condenser is 350 and 400 mm, respectively. The nanoparticle volume concentration is 1% to 3%. Results show that the nanofluid can enhance the heat pipe efficiency by 14.7%, and the thermosyphon shows a more uniformly distributed temperature.
Paramatthanuwat et al.
] studied the heat transfer of Ag-water nanofluid in a thermosyphon. The effects of filling ratio (30%, 50%, 80%), the operating temperature (40°C, 50°C, 60°C), the ratio of length and diameter (5, 10, 20), and the diameter (7.5, 11.1, and 25.4 mm) on the heat transfer performance were investigated in detail. Results show that the heat transfer capacity can be enhanced by 70% by adding Ag nanoparticles.
Teng et al.
] studied the heat transfer performance of the Al2
-water nanofluid (mass concentrations of 0.5%, 1.0%, and 3.0%). The thermosyphon is made of a copper tube with an inner diameter of 8 mm and a length of 600 mm. The authors investigated the effects of inclination, filling ratio, and mass concentration on the heat transfer performance. The thermosyphon efficiency can be enhanced by 16.8% at the mass concentration of 1.0%.
Besides, the type and the preparation method of nanofluids can also lead to the difference of the heat transfer of a thermosyphon using nanofluids. Two ways are usually used to prepare nanofluids: the one-step method and the two-step method. The one-step method simultaneously makes and disperses nanoparticles into base fluids. The two-step method first produces the nanoparticles and then disperses nanoparticles in base fluids. The two-step method is more widely used because of its convenience, low cost, and large-amount producing capacity. Therefore, most of the literatures reported use the two-step method, but the stability of nanofluids prepared by the two-step method is a key issue preventing their commercial application. Nanoparticles tend to aggregate due to the van der Waals attraction. Nanoparticles will settle out of the base fluids if severe aggregation happens. The surface functionalization technique is a promising way to solve this problem. The current authors have reported a method to prepare a kind of functionalized nanofluid that have good stability for a long run [26
]. The nanoparticles used were functionalized silica nanoparticles by grafting silanes to the surface of silica nanoparticles. After the surface functionalization process, nanofluids were prepared by the two-step method using functionalized nanoparticles and deionized water. Functionalized nanoparticles were dispersed into deionized water, and the solution was kept standing for 12 h with an environmental temperature of 50°C. Then well-dispersed nanofluid can be prepared without any surfactant used. Functionalized nanoparticles can still keep dispersing well after the nanofluid has been standing for 12 months, and no sedimentation was observed. The covalent bonding "Si-O-Si" helps maintain the steric stabilization effect formed by the grafting silanes which contribute to the long-term stability of the nanofluids.
On the other hand, for traditional nanofluids (prepared with nanoparticles without functionalization), a deposition layer usually forms on the heated surface during the phase-changing heat transfer. However, for functionalized nanofluid, no deposition layer forms on the heated surface during the phase-changing heat transfer process, which guarantees the stability and the reliability of the operating equipment using nanofluids as working fluids [26
Based on the good stability and the no deposition feature of functionalized nanofluid, it is applied in a thermosyphon as the working fluid to improve the thermal performance of the thermosyphon in the present study. The main purpose is to investigate the sole effect of the thermophysical properties of nanofluids on the thermal performance of the thermosyphon under the condition that no coating layer exists on the smooth heated surface. The present work studied mainly the phase-change heat transfer characteristics including the evaporating and condensing heat transfer of functionalized nanofluid in a thermosyphon. The same work was also explored on traditional nanofluid for better understanding of the phase-change heat transfer mechanism of nanofluids in a thermosyphon. Nanoparticles used for traditional nanofluids are the same with those used for preparing functionalized nanoparticles. The experimental conditions are also the same. In addition, the surface characteristics of heated surfaces of functionalized nanofluid and traditional nanofluid after operating experiments are measured to judge the effect of heated surface on the thermal performance. The heat transfer mechanism of nanofluids is investigated and discussed in the present study.