A review on the performance of nanoparticles suspended with refrigerants and lubricating oils in refrigeration systems
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 nanoparticles 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 nanotechnological 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]:
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High specific surface area and therefore more heat transfer surface between particles and fluids.
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High dispersion stability with predominant Brownian motion of particles.
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Reduced pumping power as compared to pure liquid to achieve equivalent heat transfer intensification.
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Reduced particle clogging as compared to conventional slurries, thus promoting system miniaturization.
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Adjustable properties, including thermal conductivity and surface wettability, by varying particle concentrations to suit different applications.
Recently scientists used nanoparticles in refrigeration systems 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 30 kW/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], [8], [9], [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.
Section snippets
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 [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. Thermal conductivity enhancement of some refrigerants with nanoparticles is shown in Fig. 2, Fig. 3, Fig. 4.
The nanofluid is a new type of heat transfer fluid by suspending nano-scale materials in a conventional host fluid and has
Thermal conductivities of nanofluids
Thermal conductivity of nanofluids found to be an attracting characteristic for many applications including refrigeration and air conditioning. It represents the ability of material to conduct or transmit heat. Considerable researches have been carried out on this topic. It may be mentioned that it is a driving factor that leads to an idea of considering nanofluids as refrigerant. Eastman et al. [24] found that thermal conductivity of 0.3% copper nanoparticles of ethylene glycol nanofluids is
Pool boiling heat transfer performance
The phase change heat transfer characteristics of the refrigerant-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
Lubricity and material compatibility
A few investigations were carried out with nanoparticles in refrigeration systems to use advantageous properties of nanoparticles to enhance the efficiency and reliability of refrigerators. For example, Wang and Xie [68] found that TiO2 nanoparticles can be used as additives to enhance the solubility of the mineral oil in the hydrofluorocarbon (HFC) refrigerant. In addition, refrigeration systems using a mixture of HFC134a and mineral oil with TiO2 nanoparticles appear to give better
Surface roughness
Table 4 shows the surface roughness of fixed plate operated at the orbiting speed of 1000 rpm and the normal force up to 1000 N for 100-min test period. The surface roughness of the fixed plate for raw mineral oil was distinctively high, 0.106 μm in depth at the scratched circle, while those of nano-oil I, II, III, and IV were 0.077, 0.067, 0.052, and 0.048 μm, respectively [72].
Energy performance
The refrigerator performance with the nanoparticles was investigated using energy consumption tests and freezer capacity tests by Ref. [69]. Authors reported that refrigerator's performance was better with 26.1% less energy consumption with 0.1% mass fraction of TiO2 nanoparticles compared to the HFC134a and POE oil system. The same tests with Al2O3 nanoparticles showed that the different nanoparticles properties have little effect on the refrigerator energy performance. Thus, nanoparticles can
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 °C. 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
Pressure drop performance of nanorefrigerant
In the modern avenue of research, refrigerant-based nanofluids formed by suspension of nanoparticles in pure refrigerants have been used as a new kind of working fluid to improve the 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
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
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)
Conclusions
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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.
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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
Recommendations for future work
The heat transfer results show that nanofluids have significant potential for improving the flow boiling heat transfer of refrigerant/lubricant mixtures. However, the reasons behind this marked improvement with nanoparticle volume fractions at different concentrations are not clearly understood. It is unclear why a large increase in heat transfer is observed with an insignificant increase in pressure. Moreover, obvious challenges with particle circulation and unknown effects on the compressor
Acknowledgements
The authors would like to acknowledge the financial support from the Vice Chancellor, University of Malaya. This research was carried under the High Impact Research Grant (HIRG) scheme.
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