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320 R. Saidur et al. / Renewable and Sustainable[(Fig._18)TD$FIG] Energy Reviews 15 (2011) 310–323 of CuO/H2O nanofluid flow inside micro-channels. The simulation results show that (i) at a given Reynolds number, compared to the host fluid the pressure gradient increase are less than 2% and 5% at nanoparticle volume fractions of 1% and 4%, respectively and (b) compared to the host fluid at a given mean velocity the pressure gradient enhancement are less than 5% and 15% at nanoparticle volume fractions of 1% and 4%, respectively. Researchers show that the pressure drop at solid–liquid phase of nanofluid is larger than that of the host fluid and the increase of the pressure drop is related to the nanoparticle concentration. Bartelt et al. [84] obtained insignificant effect on the phase change pressure drop of refrigerant/nanolubricant mixture (R134a/POE/CuO nanofluid) in flow boiling inside a horizontal tube. Significant pressure drop caused by lubricating oil overrules the detection of insignificant effect of nanoparticles in phase change nanofluid [85]. 10. Binary nanofluids in absorption system The binary mixture of NH3/H2O with nanoparticles of CNT or Al2O3 was used as a working fluid to investigate heat transfer performance along with the stability of nanorefrigerant by Ref. [86]. Authors reported that binary nanofluids are potential candidate for next generation working fluid of absorption systems [65]. Authors found that the heat transfer and absorption rate with 0.02 vol% CNT particles are about 17% and 16% higher than those without nanoparticles, respectively. Heat transfer and absorption rate with 0.02 vol% Al2O3 nanoparticles were 29% and 18% higher than those without nanoparticles, respectively. Authors recom- mended that the concentration of 0.02 vol% of Al2O3 nanoparticles be the best candidate for NH3/H2O absorption system. 11. Challenges of nanofluids Many interesting properties of nanofluids have been reported in the review. In the previous studies, thermal conductivity has received the maximum attention, but many researchers have recently initiated studies on other thermo-physical properties as well. The use of nanofluids in a wide variety of applications appears promising. But the development of the field is hindered by (i) lack of agreement of results obtained by different researchers; (ii) poor characterization of suspensions; (iii) lack of theoretical under- standing of the mechanisms responsible for changes in properties. Therefore, this paper highlighted several important issues that should receive greater attention in the near future 11.1. Long term stability of nanoparticles dispersion 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 [87]. On the other hand, if the heat exchanger operates under laminar conditions, the use of nanofluids seems advantageous, the only disadvantages so far being their high price and the potential instability of the suspension [84]. Generally, long term stability of nanoparticles dispersion is one of the basic requirements of nanofluids applications. Stability of nanofluids has good corresponding relationship with the enhance- ment of thermal conductivity where the better the dispersion behavior, the higher the thermal conductivity of nanofluids [88]. Fig. 18. The sedimentation of diamond nanoparticles at settling times of (a) 0 min, (b) 1 min, (c) 2min, (d) 3min, (e) 4min, (f) 5min, and (g) 6min [8]. However the dispersion behavior of the nanoparticles could be influenced by period of time as can be seen in Figs. 17 and 18. As a result, thermal conductivity of nanofluids is eventually affected. Eastman et al. [24] revealed that, thermal conductivity of ethylene glycol based nanofluids containing 0.3% copper nanoparticles is decreased with time. In their study, the thermal conductivity of nanofluids was measured twice: first was within 2 days and second was 2 months after the preparation. It was found that fresh nanofluids exhibited slightly higher thermal conductivities than nanofluids that were stored up to 2 months. This might be due to reduced dispersion stability of nanoparticles with respect to time. Nanoparticles may tend to agglomerate when kept for long period of time. Lee and Mudawar [77] compared the Al2O3 nanofluids stability visually over time span. It was found that nanofluids kept for 30 days exhibit some settlement and concentration gradient compared to fresh nanofluids. It indicated that long term degradation in thermal performance of nanofluids could be happened. Particles settling must be examined carefully since it may lead to clogging of coolant passages. Choi et al. [90] reported that the excess quantity of surfactant has a harmful effect on viscosity, thermal property, chemical stability, and thus it is strongly recommended to control the addition of the surfactant with great care. However, the addition of surfactant would make the particle surface coated, thereby resulting in the screening effect on the heat transfer performance of nanoparticles. Authors also mentioned that the surfactant may cause physical and/or chemical instability problems. In contrast to other common base fluids such as water or ethylene glycol, a remarkably rapid agglomeration and settling of common nanoparticles was observed in refrigerants [81]. 11.2. Higher viscosity The viscosity of nanoparticle–water suspensions increases in accordance with increasing particle concentration in the suspen- sion. Therefore, the particle mass fraction cannot be increased unlimitedly [91]. Jin et al. [65] concluded that in industrial heat exchangers, where large volumes of nanofluids are necessary and turbulent flow is usually developed, the substitution of conven- tional fluids by nanofluids seems inauspicious. Vassallo et al. [20] reported that the viscosity increased so rapidly with increasing particle loading that volume percentages of CNTs are limited to less than 0.2% in practical systems. 11.3. Lower specific heat From the literatures, it is found that specific heat of nanofluids is lower than basefluid. Praveen et al. 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