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314 R. Saidur et al. / Renewable and Sustainable Energy Reviews 15 (2011) 310–323 Table 2 Summary of research on nanoparticles and refrigerants. Year Investigator Refrigerant Nanoparticles Size of % volume Performance nanoparticles concentrations 2007 [13] 2009 [8] 2009 [54] 2009 [61] 2009 [62] 2009 [63] 2010 [56] 2006 [64] R123, R134a Carbon 20nm nanotubes R141b TiO2 21nm R113 CuO 40nm R134a CuO 30nm R113 CuO 40 nm R134a CuO 30nm R113 Diamond 10 nm 134a TiO2 – R134a CuO 1mm 1.0% 0.01%, 0.03%, 0.05% 0.15–1.5% 0.5%, 1.0%, 2.0% 0, 0.1%, 0.2%, 0.5% 0.5% 0–0.05% – – 0.06%/0.08% Heat transfer coefficient enhancement up to 36.6% Nucleate pool boiling heat transfer deteriorated with increasing particle concentrations Maximum enhancement of heat transfer coefficient, 29.7% Frictional pressure drop increased by 20.8% Enhancement of heat transfer coefficient of between 50% and 275% Nucleate pool boiling heat transfer coefficient increased by 63.4% Reduction in energy consumption by 7.43% No significant pressure drop, Heat transfer coefficient increased by more than 100% Heat transfer rate was 20% higher than those without nanoparticles 2010 [59] [65] NH3/H2O Al2O3/CNT by Ref. [39]. Table 2 shows the thermal conductivity ratio (i.e. thermal conductivity of solid to liquids) of nanofluids. The ratios are found to be in the range of 3–17,100. This shows an indication that when solid particles are added in conventional liquids/ coolants, thermal conductivity can be increased tremendously. Research has shown that the thermal conductivity and the convection heat transfer coefficient of the fluid can be largely enhanced by suspended nanoparticles. Choi et al., Choi, Xuan and Roetzel, Choi et al. [1,27,41–43] observed that the thermal conductivity of this nanofluid was 150% greater than that of the oil alone. Table 1 shows the thermal performances of different types (metallic, non-metallic, MWCNT) and concentrations of nanofluids. The enhanced thermal conductivity of nanofluids offer several benefits such as higher cooling rates, decreased pumping power needs, smaller and lighter cooling systems, reduced inventory of heat transfer fluids, reduced friction coefficients, and improved wear resistance. Those benefits make nanofluids promising for applications like refrigerants, coolants, lubricants, hydraulic fluids, and metal cutting fluids. Fig. 6 shows the boundary of properties of nanofluids for different applications. 4. Pool boiling heat transfer performance The phase change heat transfer characteristics of the refriger- ant-based nanofluids in the heat exchangers, especially in the evaporator, is an important factor to consider. In order to investigate the overall performance of the heat exchangers of refrigeration systems using refrigerant-based nanofluids, the heat transfer characteristics of them must be known. It is reported that the concentration of nanoparticles in nanorefrigerant has influence on the boiling heat transfer coefficient as the thermo-physical properties, influence the boiling heat transfer coefficient such as thermal conductivity and viscosity and they change with the change of concentration of nanoparticles in the base fluid [1,6,14,15,20,45,54]. The researches on the boiling heat transfer characteristics of refrigerant-based nanofluids are focused on the pool boiling heat transfer [8,12,13,55] and there are no notable published researches on the flowing boiling heat transfer characteristics of refrigerant- based nanofluids. Park and Jung [12,13] investigated the pool boiling heat transfer of CNTs (carbon nanotubes)–R22, CNTs–R123 and CNTs–R134a nanofluids on a horizontal smooth tube and found that CNTs enhanced the pool boiling heat transfer coefficients of refrigerants. Authors also reported that the enhancement became more pronounced at lower heat flux and the maximum enhancement could reach 36.6%. Wu et al. [55] observed that the pool boiling heat transfer was enhanced at low nanoparticles concentration of TiO2 in R11 but deteriorated under the condition of high nanoparticles concentra- tion. Trisaksri and Wongwises [8] investigated TiO2 in HCFC 1416 in a cylindrical copper tube and found that the nucleate pool boiling heat transfer deteriorated with increasing nanoparticle concentrations especially at higher heat fluxes. Researches on the boiling heat transfer characteristics showed that the type of nanoparticles or fluid, the nanoparticles concentration, the heat flux and the type of heating surface have influences on the nucleate boiling heat transfer of nanofluids [14,15–21,23]. Das et al. [14] reported that pool boiling performance is deteriorated at all levels of nanoparticle concentrations because of change in surface characteristics due to deposition of nanoparticles. Bang and Chang [21] also reported that boiling performance of nanofluids deteriorates. Vassallo et al. [20] reported that no enhancement in nucleate boiling with silica–water nanofluid. Vassallo et al. [20] conducted an experiment with different concentration of alumina nanoparticles with water and found that pool boiling performance is deteriorated. The pool boiling heat transfer characteristics of refrigerant-based nanofluids is different from the flow boiling heat transfer characteristics and the pool boiling heat transfer characteristics only cannot represent the influence of nanoparticles on the heat transfer of refrigerant-based nanofluid inside heat exchanger tube in the evaporator. Thus it has become necessary to investigate flow boiling heat transfer characteristics using refrigerant-based nano- fluids as the working fluid. There are correlations existing to predict heat transfer coefficient of pure refrigerant flow boiling inside the horizontal smooth tube, but they are not developed yet to predict the heat transfer coefficient of refrigerant-based nanofluid flow boiling inside a horizontal smooth tube to design a heat exchanger using refrigerant-based nanofluid as the working fluid. Hao et al. [54] investigated the heat transfer characteristics of refrigerant-based nanofluids flow boiling inside a smooth tube at different nanoparticles concentration, mass fluxes, heat fluxes, and inlet vapor qualities to analyze the influence of nanoparticles on the heat transfer characteristics of refrigerant-based nanofluid flow boiling inside the smooth tube. Authors used average 40 nm diameter of CuO nanoparticles and Transmission Electron Micro- scope [33] was used to identify the images. Mass fractions being considered as 0.1%, 0.2% and 0.5% and ultrasonic vibration was used to stabilize the dispersion of nanoparticles. Surfactant was not being added to improve the dispersion and stability of nanofluid as it has influence on the heat transfer characteristics ofPDF Image | Renewable and Sustainable Energy Reviews
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