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Renewable and Sustainable Energy Reviews

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]Fig._5[)TD$FIG( R. Saidur et al. / Renewable and Sustainable6[(Fig._)TD$FIG] Energy Reviews 15 (2011) 310–323 313 Fig. 5. Comparison of the thermal conductivity of common liquids, polymers and solids [40]. higher thermal conductivity compared to SiO2 nanoparticles in the same basefluid. However, Yoo et al. [26] argued that surface to volume ratio of nanoparticles is a dominant factor that influences the nanofluids thermal conductivity rather than nanoparticles thermal conductivity. Surface to volume ratio is increased with smaller sizes of nanoparticles. Choi et al. [27] reported a 150% thermal conductivity enhancement of poly (a-olefin) oil with the addition of multi- walled carbon nanotubes (MWCNT) at 1% volume fraction. Similarly, Yang [28] reported a 200% thermal conductivity enhancement for poly (a-olefin) oil containing 0.35% (v/v) MWCNT. It is important to note that this thermal conductivity enhancement was accompanied by a three order of magnitude increase in viscosity. Eastman et al. [24] found a 40% thermal conductivity enhancement for ethylene glycol with 0.3% (v/v) copper nanoparticles (10nm diameter), although the authors added about 1% (v/v) thioglycolic acid to aid in the dispersion of the nanoparticles. The addition of this dispersant yielded a greater thermal conductivity than the same concentration of nanoparticles in the ethylene glycol without the dispersant. Jana et al. [29] measured the thermal conductivity of a similar copper containing nanofluid, except the base fluid was water and laurate salt was used as a dispersant. Authors observed a 70% thermal conductivity enhancement for 0.3% (v/v) cu nanoparticles in water. Kang et al. [30] reported a 75% thermal conductivity enhancement for ethylene glycol with 1.2% (v/v) diamond nanoparticles between 30 and 50 nm in diameter. Despite these remarkable results, some researchers have measured the thermal conductivity of nanofluids and have found no anomalous results. Also, those results can often Table 1 Summary of literature review for thermal conductivity of nanofluids [39]. Fig. 6. The data summary of oxide nanofluids and the boundary line of nanofluid effectiveness [20]. be predicted by conventional thermal conductivity models [31– 34]. Lee et al. [35] revealed thermal conductivity of nanofluids is affected by pH level and addition of surfactant during nanofluids preparation stage. Better dispersion of nanoparticles is achieved with addition of surfactant such as sodium dodecylbenzenesulfo- nate. Optimum combination of pH and surfactant leads to 10.7% thermal conductivity enhancement of 0.1% Cu/H2O nanofluids. Thermal conductivity of ethylene glycol based ZnO nanofluids measured by transient short hot wire technique is found to be increased non-linearly with nanoparticles volume fraction [36]. Jiang et al. [11] added that thermal conductivity of nanofluids also depend on the nanoparticles size and temperature. Vajjha and Das [37] also agreed that thermal conductivity is dependent not only on the nanoparticles concentration but also on the temperature. Authors concluded that, it will be more beneficial if nanofluids are used in high temperature applications. It has been noticed that most authors agreed that nanofluids provide higher thermal conductivity compared to basefluids. Its value increases with particles concentration. Temperature, parti- cles size, dispersion and stability do play important role in determining thermal conductivity of nanofluids [38]. Fig. 5 shows the comparison of thermal conductivity of heat transfer fluids and nanofluids. Fig. 6 shows the thermal conductivity of nanofluids at different temperatures. Table 1 also shows the enhanced thermal conductivities of metallic and non-metallic nanofluids as reported Particle Base fluid Average particle size Volume Thermal conductivity References fraction enhancement Metallic nanofluids Non-metallic nanofluids Cu Ethylene glycol Cu Water Fe Ethylene glycol Au Water Ag Water Al2O3 Water Al2O3 Water Al2O3 Water CuO Water CuO Water SiC Water TiO2 Water MWCNT Synthetic oil MWCNT Decene/ethylene glycol/water MWCNT Water 10 nm 100 nm 10 nm 10–20 60–80 13 nm 33 nm 68 nm 36 nm 50 nm 26 nm 15nm 25 nm 15 nm 100 nm in diameter 70 mm in length 0.3% 40% [42] 7.5% 78% [44] 0.55% 18% [45] 0.026% 21% [45] 0.001% 17% [46] 4.3% 30% [47] 4.3% 15% [48] 5% 21% [49] 3.4% 12% [48] 0.4% 17% [50] 4.2% 16% [49] 5% 30% [51] 1% 150% [27] 1% 20%/13%/7% [52] 0.6% 38% [53] nm nm in diameter 50 mm in length in diameter 30 mm in length

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