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CuO/ethylene glycol nanofluids, SiO2/ethylene glycol nanofluids and Al2O3/ethylene glycol nanofluids exhibit lower specific heat compared to basefluids. An ideal refrigerant should possess higher value of specific heat which enable the refrigerant to remove more heat. 11.4. Thermal conductivity The existing models for predicting thermal conductivities of CNT nanofluids, including Hamilton–Crosser model, Yu–Choi model and Xue model, cannot predict the thermal conductivities of CNT nanorefrigerants within a mean deviation of less than 15% [11]. 11.5. High cost of nanofluids Higher production cost of nanofluids is among the reasons that may hinder the application of nanofluids in industry. Nanofluids can be produced by either one-step or two-step methods. However both methods require advanced and sophisticated equipments. Lee and Mudamawar [77] and Pantzali et al. [77,79] stressed that high cost of nanofluids is among the drawback of nanofluids applica- tions. 11.6. Difficulties in production process Previous efforts to manufacture nanofluids have often employed either a single step that simultaneously makes and disperses the nanoparticles into base fluids, or a two-step approach that involves generating nanoparticles and subsequently dispers- ing them into a base fluid. Using either of these two approaches, nanoparticles are inherently produced from processes that involve reduction reactions or ion exchange. Furthermore, the base fluids contain other ions and reaction products that are difficult or impossible to separate from the fluids. Another difficulty encountered in nanofluid manufacture is nanoparticles’ tendency to agglomerate into larger particles, which limits the benefits of the high surface area nanoparticles. To counter this tendency, particle dispersion additives are often added to the base fluid with the nanoparticles. Unfortunately, this practice can change the surface properties of the particles, and nanofluids prepared in this way may contain unacceptable levels of impurities. Most studies to date have been limited to sample sizes less than a few hundred milliliters of nanofluids. This is problematic since larger samples are needed to test many properties of nanofluids and, in particular, to assess their potential for use in new applications [92]. Yet the fact that nanofluids have more points in favor of them than against, for usage as cooling fluid, has emerged as an undisputed view. This calls for a more intensified effort in the research on nanofluids. In contrast to the traditional unilateral approach, this research needs to examine closely a variety of issues, such as synthesis, characterization, thermo-physical properties, heat and mass transport, modeling, and device- as well as system-level applications. Hence, a multi-disciplinary approach comprising researchers such as thermal engineers, chemical technologists, material scientists, chemists, and physi- cists needs to be undertaken. Only such an approach can ensure a ‘‘cooler future’’ with nanofluids [93]. 11.7. Fouling Even though many nanoparticles were applied to the single phase heat transfer of water, actual heat transfer improvement was not yet reported. Furthermore, when these particles were applied to the boiling heat transfer, they even caused fouling on heat transfer surface and consequently HTCs were decreased [14,20,94]. 12. Conclusions Based on the literatures, it has been found that the thermal conductivities of nanorefrigerants are higher than traditional refrigerants. It was also observed that increased thermal conductivity of nanorefrigerants is comparable with the increased thermal conductivities of other nanofluids. Thermal conductivities of refrigerant with carbon CNT found to be higher than refrigerant without CNT. It was observed that maximum thermal conductivity enhancement was found to be about 46%. It was also observed that thermal conductivities of nanorefrigerants depend on concentrations and aspect ratio of CNT. It has been observed that heat transfer enhancement can be achieved from a minimum value of 21% to a maximum value of 275% using nanorefrigerants compared to traditional refriger- ants. However, many researchers Das et al., Bang and Chang and Sobhan and Peterson [14,21,95] found that pool boiling performance is deteriorated for different concentrations and types of nanofluids. The refrigerator’s performance was found 26.1% better with 0.1% mass fraction of TiO2 nanoparticles compared to a refrigerator’s performance with the HFC134a and POE oil system [56]. The mineral lubricant with Al2O3 nanoparticles (0.05, 0.1, and 0.2wt%) was used to investigate the lubrication and heat transfer performance. Results indicated that the 60% R134a and 0.1 wt% Al2O3 nanoparticles provided optimal performance. Under these conditions, the power consumption was reduced by about 2.4%, and the coefficient of performance was increased by 4.4%. The friction coefficient of nano-oil IV shows 0.02, which is the lowest friction coefficient among various nano-oils studied by Ref. [72]. Surface roughness of this oil with refrigerant oil was found to be a minimum value of 0.048 mm compared to other oils [72]. Several literatures have indicated that there is significant increase of nanofluids pressure drop compared to basefluid. Lee and Mudawar [77] revealed that single phase pressure drop of Al2O3 nanofluids in micro-channel heat sink increases with nanoparticles concentration. Vasu et al. [77,78] studied the thermal design of compact heat exchanger using nano- fluids and found that pressure drop of 4% Al2O3 + H2O nanofluids is almost double of the basefluid. Pantzali et al. [79] reported there was substantial increase of nanofluids pressure drop and pumping power in plate heat exchanger and found about 40% increase in pumping power for nanofluids compared to water. Peng et al. [80] reported that the frictional pressure drop of refrigerant-based nanofluids flow boiling inside the horizontal smooth tube is larger than that of pure refrigerant, and increases with the increase of the mass fraction of nanoparticles. The maximum increase of frictional pressure drop was found to be about 20.8% under the experimental conditions. It was also found that there are inconsistencies in the reported results published by many researchers. Few researchers reported the inconsistencies between model and experimental results of thermal conductivity of nanofluids. Exact mechanism of enhanced heat transfer for nanofluids is still unclear as reported by many researchers. However, it should be noted that many challenges need to be identified and overcome for different applications. Nanofluids stability and its production cost are major factors that hinder the commercialization of nanofluids. By solving these challenges, it is expected that nanofluids can make substantial impact as coolant in heat exchanging devices. R. Saidur et al. / Renewable and Sustainable Energy Reviews 15 (2011) 310–323 321PDF Image | Renewable and Sustainable Energy Reviews
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