List of Tables
1.10. The concept of Nanofluid
1.10.5. Synthesis and Preparation of Nanofluid
Introduction
with increasing solid volume fraction for the two nanofluids. That means that the desirable heat transfer increase may be offset by the undesirable increase in pressure drop.
It was evaluated the viscosity of Al2O3-water nanofluids against shear rate [93].
Their results showed an increase of viscosity with increased particle concentrations.
There is a strong possibility that nanofluids may be non-Newtonian, even visco-elastic in some cases. Further experimental studies are needed to define the viscosity models of nanofluids so they can be used in simulation studies.
The viscosity of CNT-water nanofluids as a function of shear rate was measured [94]. It was observed that the viscosity of nanofluids increased with increasing CNT concentration and decreasing temperature. Also, the shear thinning behavior was found.
That means the nanofluids can provide better fluid flow performance due to the higher shear rate at the wall, which results in low viscosity there.
Introduction
1.10.5.1. The Single-step Process
Various methods have been tried to produce different kinds of nanoparticles and nano suspensions. It was provided a good overview of the synthesis methods [96]. The initial materials tried for nanofluids were oxide particles, primarily because they were easy to produce and chemically stable in solution. Various investigators have produced Al2O3 and CuO nano powder by an inert gas condensation process [82, 97] that produced 2–200 nm sized particles. The major problem with this method is its tendency to form agglomerates and its unsuitability to produce pure metallic nano powders. The problem of agglomeration can be reduced to a good extent by using a direct evaporation condensation method. The latter method is a modification of the inert gas condensation process that has been adopted at Argonne National Laboratory, USA [98]. Even though this method has limitations of low vapor-pressure fluids and oxidation of pure metals, it provides excellent control over particle size and produces particles for stable nanofluids without surfactant or electrostatic stabilizers.
The single-step direct evaporation approach was developed [99] and is called the Vacuum Evaporation onto a Running Oil Substrate technique. The original idea of this method was to produce nanoparticles, but it was difficult to subsequently separate the particles from the fluids to produce dry nanoparticles. It was developed amodified vacuum evaporation onto oil technique [100], in which Cu vapor is directly condensed into nanoparticles by contact with a flowing low-vapor-pressure liquid ethylene glycol.
It was presented a novel one-step chemical method for preparing copper nanofluids by reducing CuSO4・5H2O with NaH2PO2・H2O in ethylene glycol under microwave irradiation [101]. Results showed that addition of NaH2PO2・H2O and the adoption of microwave irradiation are two significant factors which affect the reaction rate and properties of Cu nanofluids. It was developed a vacuum-based submerged arc
Introduction
nanoparticle synthesis system to prepare CuO, Cu2O, and Cu based nanofluids with different dielectric liquids [102]. The morphologies of nanoparticles depended on the thermal conductivity of the dielectric liquids. An advantage of the one step technique is that nanoparticle agglomeration is minimized, while the disadvantage is that only low vapor pressure fluids are compatible with such a process.
1.10.5.2. The Two Step Process
The two-step method is extensively used in the synthesis of nanofluids considering the available commercial nano-powders supplied by several companies. In this method, nanoparticles were first produced and then dispersed in the base fluids.
Generally, ultrasonic equipment is used to intensively disperse the particles and reduce the agglomeration of particles. For example, many researchers used this method to produce Al2O3 nanofluids [92, 97, 100, 103]. Moreover, it was prepared TiO2
suspension in water using the two-step method [104]. Other nanoparticles reported in the literature are gold (Au) [105-108], silver (Ag) [105], silica and carbon nanotubes [109-112]. As compared to the single-step method, the two-step technique works well for oxide nanoparticles, while it is less successful with metallic particles. Except for the use of ultrasonic equipment, some other techniques such as control of pH or addition of surface active agents are also used to attain stability of the suspension of the nanofluids against sedimentation. These methods change the surface properties of the suspended particles and thus suppress the tendency to form particle clusters. It should be noted that the selection of surfactants should depend mainly on the properties of the solutions and particles. For instance, salt and oleic acid as dispersant are known to enhance the stability of transformer oil–Cu and water–Cu nanofluids, respectively [113]. Oleic acid and cetyltrimethylammoniumbromide (CTAB) surfactants were used [104] to ensure better stability and proper dispersion of TiO2–water nanofluids. Sodium dodecyl sulfate
Introduction
(SDS) was used during the preparation of water-based multi wall carbon nanotube dispersed nanofluids since the fibers are entangled in the aqueous suspension [114]. In general, methods such as change of pH value, addition of dispersant and ultrasonic vibration aim at changing the surface properties of suspended particles and suppressing formation of particles cluster to obtain stable suspensions. However, the addition of dispersants can affect the heat transfer performance of the nanofluids, especially at high temperature i.e. in the convective heat transfer and two-phase heat transfer regime.