Thermal conductivity and lubrication characteristics of nanofluids
Introduction
Nanofluid technology becomes a new challenge for the heat transfer fluid since it has been reported that the thermal conductivity of nanofluid is anomalously enhanced at a very low volume fraction [1], [2], [3], [4], [5], [10], [14]. As well as the thermal conductivity enhancement of nanofluids, the improved lubrication of nanoparticles-in-oil suspension has been reported by many researchers. In these researches, various kinds of nanoparticles are used such as CuO, Al2O3, Cu, multi-walled carbon nanotubes (MWCNTs), fullerene (C60) and so on. In some researches, MWCNT is the excellent media to enhance the thermal conductivity of base fluid in addition of very small volume concentration [2], [6]. Fullerene (C60) has great potentials to the anti-wear materials when C60 nanoparticles are suspended in the conventional oil [7]. However, preparation of homogeneous suspension remains a technical challenge since the nanoparticles always form aggregates due to very strong van der Waals interactions. To get stable nanofluids, physical or chemical treatment have been conducted such as an addition of surfactant, surface modification of the suspended particles or applying strong force on the clusters of the suspended particles. Dispersing agents, surface-active agents, have been used to disperse fine particles of hydrophobic materials in aqueous solution. Although the stability of nanofluid is very important for its application, there is a little study on estimating the stability of suspension. UV–vis spectrophotometric measurements have been used to quantitatively characterize colloidal stability of the dispersions [8]. It can be applied to all base fluid, while zeta potential analysis has a limitation of the viscosity of base fluid. Thermal conductivity of nanofluid has been studied by many researchers, recently. Its dependence on temperature of suspension and the thermal conductivity of base fluid and the suspended particles are reported [1]. Also, the mechanisms of anomalous thermal conductivity enhancement of nanofluids are suggested [5], [6], [9].
In this paper, the stability of nanofluid with sediment time is estimated with UV–vis spectrophotometer. To evaluate the properties of nanofluid, thermal conductivity and extreme pressure which is a limit load to maintain oil-to-solid contact in bearings, are measured. To measure the thermal conductivity of nanofluid, a transient hot-wire system has been used. FALEX EP tester has been used to measure the extreme pressure of nanofluids.
Section snippets
Materials
Table 1 shows the properties of materials for preparing nanofluids. The thermal conductivities of MWCNTs, CuO and SiO2 nanoparticles are 3000 W/mK [11], 76.5 W/mK and 1.38 W/mK, respectively. The thermal conductivities of base fluids, such as DI water, ethylene glycol and oil, are 0.613 W/mK, 0.252 W/mK and 0.107 W/mK, respectively.
Fig. 1 shows the photographs of the test particles. MWCNTs have fibrous morphologies and the average length and diameter are 10–50 μm and 10–30 nm, respectively. The average
Measuring thermal conductivity of nanofluids
Fig. 2 shows schematic diagram of transient hot-wire system for thermal conductivity measurement of carbon nanofluids. In this study, transient hot-wire method for measuring electrically conducting fluid because the particles, used in this experiment, are electrically conductive. Teflon coated platinum wire, which diameter is 76 μm and the thickness of Teflon insulation layer is 17 μm, is used for a hot wire in the measurement system. Initially, the platinum wire immersed in media is kept at
Colloidal stability of nanofluids
Recently, a new method which can be used to estimating the suspension concentration with increasing sediment time was introduced. Fig. 3 shows that the absorptions of MWCNT and fullerene in the refrigerant oil-based suspensions appear at 397 nm. The absorbance of MWCNTs and fullerenes in the refrigerant oil suspensions decreases with increasing sediment time. Fig. 4 shows that a linear relation is obtained between the supernatant concentration and the absorbance of suspended particles. From
Measurement of extreme pressure properties of nanofluids
Fig. 4 shows the test equipment for measuring extreme pressure of nanofluids. Test method consists of running a rotating steel journal at 290 ± 10 rpm against two stationary V-blocks immersed in the lubricant sample. Load is applied to the V-block by a ratchet mechanism. Increasing load is applied continuously after running 5-min at 1334 N for a run-in period. Load-fail value obtained is the criteria for level of load-carrying properties. Failure is indicated by breakage of shear pin or test pin,
Results and discussion
Fig. 5 shows the thermal conductivity enhancement of nanofluids. MWCNT nanofluid has the highest thermal conductivity enhancement among water-based nanofluid whereas SiO2 nanofluid has the lowest one. From this result, it is shown that the thermal conductivity enhancement of nanofluid depends on that of the suspended particles. Many previous researches show the similar results. Also, it is shown that MWCNT-in-oil nanofluid has higher thermal conductivity enhancement than MWCNT-in-water, and
Conclusions
To investigate the thermal conductivity enhancement of nanofluids, thermal conductivities of nanofluids are measured by the transient hot-wire method. From the results, thermal conductivity enhancement of nanofluid depends on the thermal conductivity and that of the suspended particles. Stability of nanofluid has been estimated by UV–vis spectrophotometric analysis. Fullerenes are dispersed very well in oil due to its non-polar characteristic. MWCNT nanofluids have poor stability because of its
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