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318 R. Saidur et al. / Renewable and Sustainable Energy Reviews 15 (2011) 310–323 Table 5 Energy consumption of HFC134a/POE oil and HFC134a/mineral oil/TiO2 nanoparticles systems. Mass fraction (%) POE 0.06 TiO2 0.1 Ti O2 0.1 (50 days later) Energy consumption (kWh/day) 1.077 0.849 0.796 0.800 Energy saving (%) 21.2 26.1 25.7 lubricant for chillers that incorporates a dispersion of nanometer- sized particles – has already been shown to improve the boiling heat flux by nearly 300% compared to the original nanoparticle- free refrigerant [60]. At the standard rating condition, the introduction of nanofluids gave rise to an increase in the COP by 5.15%, relative to a condition without nanofluids. Furthermore, the pressure drop penalty of the addition of nanofluids was almost negligible [73]. The data in Table 5 shows that the energy consumption of the system with nanoparticles was lower than that of the HFC134a and POE oil system. The energy consumption of 0.796 kWh/day was least at a nanoparticle mass fraction of 0.1%, which is 26.1% less than the POE oil system. In addition, the energy consumption was almost same after 50 days for the 0.1% mass fraction, which indicates that the refrigerator can work steadily for a long time. 8. Viscosity of nano-oil Fig. 13 shows the kinematic viscosity of nano-oils as a function of volume fraction of fullerene nanoparticles in suspension for temperature ranging from 40 to 80 8C. There was no considerable change in the kinematic viscosity of nano-oil at the various volume fractions of nanoparticles, indicating that the kinematic viscosity of nano-oils is a weak function of oil temperature considered [72]. Fig. 14 shows the change of kinetic viscosity as a function of volume fraction and temperature of the oil. When particles are added, the increase rate of viscosity of the nano-oil is within 1%. In the temperature range for a compressor with time, the viscosity of the nano-oil is about the same as for the mineral oil, but the viscosity of the nano-oil increases by 7% at 20 8C in comparison with the mineral oil. 9. Pressure drop performance of nanorefrigerant In the modern avenue of research, refrigerant-based nanofluids formed by suspension of nanoparticles in pure refrigerants have (Fig._]GIF$DT)31[been used as a new kind of working fluid to improve the Fig. 13. Kinematic viscosity of nano-oils as a function of fullerene nanoparticle concentration and oil temperature ranging from 40 to 80 8C [72]. performance of refrigeration systems [68,69,74]. Presence of nanoparticles in suspension form may change the pressure drop characteristics of the fluid, so this characteristic needed to be understood in selecting the refrigerant. Liquid solid phase pressure drop characteristics and liquid solid and vapor phase (phase change) pressure drop characteristics of nanofluids are studied by different researchers. Li and Kleinstreuer [75] studied by simula- tion of the pressure drop characteristics of solid and liquid phase of fluid. Pressure drop developed during the flow of coolant is one of the important parameters determining the efficiency of nanofluids application. Pressure drop and coolant pumping power are closely associated with each other. There are few properties which could influence the coolant pressure drop: density and viscosity. It is expected that coolants with higher density and viscosity experi- ence higher pressure drop. This has contributed to the disadvan- tages of nanofluids application as coolant liquids. Yu et al. [35,36] and Lee et al. [35] investigated viscosity of water based Al2O3 nanofluids and ethylene glycol based ZnO nanofluids. Results clearly show, viscosity of nanofluids is higher than basefluid. Praveen et al. [76] in their numerical study reviewed that density of nanofluids is greater than basefluid. Both properties are found proportional with nanoparticles volume fraction. Several litera- tures 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 concentra- tion. Vasu et al. [77,78] studied the thermal design of compact heat exchanger using nanofluids. In this study, it is 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. About 40% increase of pumping power was observed for nanofluids compared to water. Peng et al. [80] reported that the frictional pressure drop of refrigerant-based nanofluids flow boiling inside the horizontal F[(Fig._14)TD$IG]smooth tube is larger than that of pure refrigerant, and increases Fig. 14. Kinetic viscosity of fullerene-in-oil as a function of particle concentration and temperature [71].PDF Image | Renewable and Sustainable Energy Reviews
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