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1. Introduction Nanofluids are a relatively new class of fluids which consist of a base fluid with nano-sized particles (1–100 nm) suspended within them. These particles, generally a metal or metal oxide, increase conduction and convection coefficients, allowing for more heat transfer out of the coolant [1]. Serrano et al. [2] provided excellent examples of nanometer in comparison with millimeter and micrometer to understand clearly as can be seen in Fig. 1. In the past few decades, rapid advances in nanotechnology have lead to emerging of new generation of heat transfer fluids called ‘‘nanofluids’’. Nanofluids are defined as suspension of nanoparti- cles in a basefluid. Some typical nanofluids are ethylene glycol based copper nanofluids, water based copper oxide nanofluids, etc. Nanofluids are dilute suspensions of functionalized nanoparticles composite materials developed about a decade ago with the specific aim of increasing the thermal conductivity of heat transfer fluids, which have now evolved into a promising nanotechnolo- gical area. Such thermal nanofluids for heat transfer applications represent a class of its own difference from conventional colloids for other applications. Compared to conventional solid–liquid suspensions for heat transfer intensifications, nanofluids possess the following advantages [1]: High specific surface area and therefore more heat transfer surface between particles and fluids. High dispersion stability with predominant Brownian motion of particles. Reduced pumping power as compared to pure liquid to achieve equivalent heat transfer intensification. Reduced particle clogging as compared to conventional slurries, thus promoting system miniaturization. Adjustable properties, including thermal conductivity and surface wettability, by varying particle concentrations to suit different applications. Recently scientists used nanoparticles in refrigeration systems F[(ig._1)TD$FIG]because of its remarkable improvement in thermo-physical, and heat transfer capabilities to enhance the efficiency and reliability of refrigeration and air conditioning system. Elcock [3] found that TiO2 nanoparticles can be used as additives to enhance the solubility of the mineral oil with the hydrofluorocarbon (HFC) refrigerant. Authors also reported that refrigeration systems using a mixture of HFC134a and mineral oil with TiO2 nanoparticles appear to give better performance by returning more lubricant oil to the compressor with similar performance to systems using HFC134a and POE oil. Hindawi [4] carried out an experimental study on the boiling heat transfer characteristics of R22 refrigerant with Al2O3 nanoparticles and found that the nanoparticles enhanced the refrigerant heat transfer characteristics with reduced bubble sizes. Eastman et al. [5] investigated the pool boiling heat transfer characteristics of R11 refrigerant with TiO2 nanoparticles and showed that the heat transfer enhancement reached 20% at a particle loading of 0.01 g/L. Liu et al. [6] investigated the effects of carbon nanotubes (CNTs) on the nucleate boiling heat transfer of R123 and HFC134a refrigerants. Authors reported that CNTs increase the nucleate boiling heat transfer coefficients for these refrigerants. Authors noticed large enhancements of up to 36.6% at low heat fluxes of less than 30kW/m2. Thus, the use of nanoparticles in refrigeration systems is a new, innovative way to enhance the efficiency and reliability in the refrigeration system. In the literatures a number of reviews on thermal and rheological properties, different modes of heat transfer of nanofluids have been reported by many researchers [7–10]. However, to the best of authors’ knowledge, there is no comprehensive literature on the nanoparticles as additives with conventional refrigerants and oils used in refrigeration system. It is authors’ hope that this review will be useful to fill identified research gaps and to overcome the challenges of nanorefrigerants. 2. Thermal conductivity of nanoparticles used in refrigerants Different concentrations of nanoparticles of CuO, Al2O3, SiO2 diamond, CNT, TiO2 were used in base refrigerants such as R11, R113, R123, R134a, and 141b as found in the available literatures R. Saidur et al. / Renewable and Sustainable Energy Reviews 15 (2011) 310–323 311 11.2. Higherviscosity ............................................................................................ 320 11.3. Lowerspecificheat.......................................................................................... 320 11.4. Thermalconductivity........................................................................................ 321 11.5. Highcostofnanofluids....................................................................................... 321 11.6. Difficultiesinproductionprocess .............................................................................. 321 11.7. Fouling.................................................................................................... 321 12. Conclusions..................................................................................................... 321 13. Recommendationsforfuturework................................................................................... 322 Acknowledgements............................................................................................... 322 References...................................................................................................... 322 Fig. 1. Length scale and some examples related [2].PDF Image | Renewable and Sustainable Energy Reviews
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