As industrial and technological products demand higher standards of function and capacity, the problem of heat dissipation from electrical appliances becomes a significant issue. To ameliorate this problem, there are four approaches commonly taken: (1) enlarge the heat exchanger area and structure, (2) fabricate the heat exchanger using materials with higher thermal conductivity, (3) increase the working fluid flow rate to the heat exchanger, and (4) improve the heat transfer performance of the heat exchange working fluid. Of these methods, enlargement of the heat exchanger area has reached a physical limit. Increasing the flow rate of heat exchange would create problems of volume, power consumption, and noise from the fan and pump. The thermal conductivity of copper and aluminum heat exchangers are quite high, and the addition of precious metal to improve thermal conductivity further would incur a tremendous increase in the heat exchanger cost. Therefore, we consider that in order to increase heat dissipation, the most feasible approach is to improve the heat transfer performance of the heat exchange working fluid.
The use of nanofluids to improve the heat-transfer performance of heat exchange working fluids deserves consideration. In 1995, Choi [1
] became the first person to use the term "nanofluid" to describe a fluid containing nanoparticles. Nanofluid manufacture involves dispersing metallic and non-metallic nanomaterials with high thermal conductivity, into a suitable "working fluid" such as engine oil, water, ethylene glycol, etc
., to enhance the heat transfer performance of traditional fluids [2
]. According to literature reports, the thermal conductivity of a nanofluid is strongly dependent on the volume fraction and properties of the added nanoparticles [3
]. In addition, for the addition of a given volume of particles, the solid-liquid surface contact area between nano-scale particles and the suspension fluid is greater than that for micro-scale particles. Hence, the size and shape of the particles added will have a significant effect on thermal conductivity and heat transfer characteristics [1
Nanofluids preparation generally follows one of two methods: a one-step and a two-step synthesis. The so-called "one-step synthesis" produces nanofluids by synthesizing the nanoparticles directly into a suspending fluid, while the two-step process produces the nanoparticles and then disperses them in a bulk liquid to form a stable suspension, as separate processes.
Many variations on the one-step synthesis of nanofluids exist. Akoh et al
] used the VEROS method to prepare nanofluids in a one-step by applying vacuum evaporation to a running oil substrate. Wagener et al
] adopted magnetron sputtering to improve the VEROS technique, and succeeded in developing an effective preparation of Ag, Fe nanofluids. Zhu et al
] employed a new chemical method to prepare Cu-ethylene glycol nanofluids from reaction under microwave irradiation. Eastman et al
] also improved on the VEROS technique, by using low-temperature and low-pressure conditions, and letting Cu vapor directly contact and flow with low-vapor-pressure ethylene glycol fluid, causing the Cu vapor to condense directly in the fluid to form Cu nanofluid. Lo et al
] used a submerged arc nanoparticle-synthesis system to prepare Cu-based nanofluids. Lo et al
. let Cu vapor, formed by electric arc discharge, directly condense in low-temperature and low-pressure deionized water, or ethylene glycol, to form CuO and Cu nanofluids. These researchers also used this method to produce Ni nanomagnetic fluids [18
], and achieved good results. Chang et al
] synthesized an Al2
nanofluid, with high suspension stability, using a modified plasma arc system. The vaporized metallic gas mixed thoroughly with the pre-condensed, deionized water, to form an Al2
/water nanofluid. The average particle size was in the range 25-75 nm. Hwang et al
] employed a modified magnetron sputtering system to produce Ag/silicon oil nanofluids. The Ag nanoparticles were relatively uniform with primary size less than 5 nm. Kumar et al
] fabricated copper nanofluids, of metallic copper dispersed in ethylene glycol, using sodium hypophosphite as reducing agent and conventional heating. Wei et al
] applied chemical solution methods to synthesize cuprous-oxide (Cu2
O) nanoparticles in water, to form Cu2
O nanofluids. Abareshi et al
] produced magnetite Fe3
nanoparticles by a co-precipitation method at various pH values. The concentration was around 0.25-3.0 vol.%. Generally, the one-step synthesis has the advantage that nanoparticles form directly in the bulk liquid. Normally, this method contains an intrinsic sorting mechanism, in which excessively large particles settle by static placement, and the supernatant, containing finer nano-sized particles as the dispersion, simply collected. This approach provides nanofluids with good suspension properties. Unless required by the preparation process, there is no need to add any dispersant or surfactant to improve the dispersion, and thus, not interference will arise from the addition of such additives. However, a disadvantage of the one-step method is that preparation conditions influence the size, shape and concentration of nanoparticles, the range of particle size distribution is broad, and an accurate control of the concentration is difficult.
Considering reports of two-step nanofluid formation, there are many accounts of Al2
nanofluid preparation using ultrasonic dispersion [16
]. Murshed et al
] employed ultrasonic dispersion to prepare TiO2
/water nanofluid, and applied the same method to prepare Au, Ag, SiC, and carbon nanotube nanofluid. In general, two-step syntheses are more suitable for the preparation of oxide nanofluids, but are less appropriate for the preparation of metallic nanofluids. Wen and Ding [27
] used a high shear homogenizer to solve an agglomeration problem with TiO2
nanoparticles. Operating the homogenizer at 24,000 rpm, with a shear rate of 40,000 s-1
disrupted nanoparticle agglomeration and provided an adequate dispersion of nanoparticles with narrow size distribution. Nevertheless, although this method improved on the agglomeration problem, it was still unavailable to acquire the particle size as observed by SEM and TEM. Choi et al
] used ZrO2
bead milling in a vertical, super-fine grinding mill, to mix Al2
and AlN with transformer oil at volume fractions up to 4%, and added n
-hexane to regard as dispersant in order to keep good suspension. Hwang et al
] treated carbon black (CB)/water, and Ag/silicon oil nanofluids, to various two-step procedures, using stirrer, ultrasonic bath, ultrasonic disrupter and high-pressure homogenizer methods in order to achieve small particle size, with good dispersion. The high-pressure homogenizer produced average CB and Ag particle diameters of 45 and 35 nm, respectively. Moosavi et al
] demonstrated a two-step synthesis of ZnO nanoparticles, by mixing ethylene glycol and glycerol with the aid of a magnetic stirrer. Moosavi et al
. added ammonium citrate to act as a dispersant, and enhance stability of the suspension. This method produced a mean ZnO particle size of 67.17 nm.
Generally, two-step methods are simpler than one-step methods, because the nanoparticles may either be self-made, or purchased, then added to a bulk liquid to form nanofluids. However, in the process of addition, agglomeration can occur easily, resulting in poor suspension, thus, two-step methods often require dispersion methods such as ultrasonic sonication, mechanical stirring, a homogenizer, or the addition of a surfactant or dispersant, to disrupt agglomeration and provide dispersion and stabilize the suspension. The advantages of two-step syntheses are facile and rapid preparation of large volume nanofluids, greater control over nanoparticle concentration and narrower particle size distribution is than that of single-step syntheses.
In this study, we employed a plasma arc system to produce a carbon/water nanofluid with stable suspension, in a one-step process, without addition of any dispersant or surfactant. We fully characterized the microstructure, particle size distribution, and fundamental properties by suitable instrumentation, in order to demonstrate the feasibility of the process described herein.