The due to the addition of nanoparticles

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The majority of researchers observed an increase in effective thermal conductivity and heat transferproperties of conventional heat transfer fluids due to the addition of nanoparticles (Wang et al.,1999; Yoo et al., 2007) and, later, this concept has been extended to refrigerants and give birth to theconcept of nanorefrigerants. As expected, same trend is observed in nanorefrigerants as well, forinstance, the thermal conductivity increased by as much as 104% by addition of CNT nanoparticlesin R113refrigerant (W. Jiang et al., 2009a). The nanorefrigerant research fraternity is evenly dividedinto two groups. The first group of researchers relies on suspending nanoparticles directly to thebase refrigerant, whereas the other group focuses on suspending nanoparticles to the lubricant itselfto analyze the system performance. Both observed favorable system performance. The study ofnanorefrigerants is not just limited to the thermophysical properties such as thermal conductivity,viscosity etc. The phase change heat transfer is a key area of research when it comes to refrigerants.The study of boiling heat transfer involves complexities and these complexities are only going toincrease due to the addition of nanoparticles in the refrigerants. Researchers observed mixed resultsfor boiling heat transfer in nanorefrigerants. Controversies are still exist related to the existence andnature of mechanisms involved in nanofluids and nanorefrigerants, and their role in enhancing theheat transfer characteristics of colloidal suspensions. However that should not disparage researchersby any means.In literature, a few reviews on thermophysical properties, different modes of heat transfer andapplication of nanorefrigerants in heat transfer devices have been reported (Celen et al., 2014, Alawiet al., 2015a, Azmi et al., 2016). However, the studies on particle degradation during continuousalternation processes of condensation and evaporation, aggregation characteristics and migrationbehaviour of nanoparticles and applications in specific areas such as automotive air-conditioningsystems, heat pumps etc. have not been addressed, to the best of author’s knowledge.The focus of the present research is to provide a comprehensive novel review ofexperimental studies on thermo physical properties, phase change heat transfer characteristics,pressure drop characteristics, aggregation, degradation and migration behaviour of nanoparticles in refrigerants. Besides, it provides insights to the application of nanorefrigerants in refrigeration andair-conditioning systems, heat pipes and heat pumps. Future scopes of research work and challengeshave been suggested on the basis of this review. It is expected that it could be a quick referenceguide to have an overview of the recent developments and applications of refrigerants appendedwith nanoparticles and the most essential parameters that influence the exceptional thermalperformance of it. Adding nanoparticles to a base fluid can change the effective thermophysical properties.Determination of thermophysical properties of nanorefrigerants is important to meter theeffectiveness of it. The thermal conductivity, viscosity, specific heat, latent heat, density and surfacetension are the most important thermophysical properties of a fluid. Investigations on thermalconductivity of nanorefrigerants were the first target of researchers in the past. But later, theyconcentrated in studies of viscosity, density, specific heat, capillary constant, surface tension andsolubility. The studies on viscosity have an impact on flow applications of nanorefrigerants(Bashirnezhad et al., 2016). The increase in viscosity may eventually results drop in pressure. Eventhough the enhancement in thermal conductivity overhangs the rise in viscosity, moreexperimentation is required to arrive at an ample conclusion. It is important to extend this researchto other thermophysical properties such as latent heat, since it will give a better idea of the heattransfer performance of nanorefrigerants. Table 1 shows the summary of studies on thermophysicalproperties of nanorefrigerants. W. Jiang et al. (2009a) experimentally determined the thermal conductivity of R113/CNTnanorefrigerant and proposed a model to predict the thermal conductivity of CNT basednanorefrigerant. Transient plane source (TPS) method is used to measure the thermal conductivity.Four kinds of CNTs having different aspect ratios 100,667.7, 18.8, and 125 were used. Experimentalresults show that smaller the diameter of CNT or larger the aspect ratio, the larger the thermalconductivity enhancement and established that the diameter and aspect ratio of CNT can influencethe thermal conductivity of CNT nanorefrigerants. The proposed model predicts the thermalconductivity with a mean deviation of 5.5%.Jiang et al.,(2009b) experimentally investigated thermal conductivity of nanorefrigerants andproposed a model to predict the thermal conductivity in particularly for nanorefrigerants. R-113 wasused as the host refrigerant. The nanoparticles include copper, aluminum, nickel, copper oxide, andaluminum oxide. The electron micrographs showed that nanoparticles were spherical and their meandiameters were 25, 18, 20, 40, and 20 nm respectively. The principle of measurement of thermalconductivity was transient plane source method. The volume fraction of the nanoparticles has beenranges between 0.1% and 1.2%. From the experimental results, they concluded that the thermalconductivity of a nanorefrigerant increases with increase in nanoparticle volume fraction. When thenanoparticle volume fraction, increased to 1.0%, there was an increase about 20% in thermalconductivity was observed. The experimental results show that the thermal conductivities ofdifferent nanorefrigerants are close to one another if the volume fractions were the same. Theexperimental results were compared with the calculated results of the classical models. Fig.1 showsthe comparison of thermal conductivity between the experimental data model predictions. None ofthese models can predict the thermal conductivities of all five nanorefrigerants in a mean deviationof less than 5% or a maximum deviation of less than 10%. A new model has been developed basedon particle aggregation theory and observed that the new model was better than the existing models in the prediction of the thermal conductivity of nanorefrigerants. The deviations of the new modelprediction from the experimental data for nanorefrigerants were –5% to +5%. The effect of shear rate on the viscosity of nanofluids and nanorefrigerants cannot be sidelined.Kumar et al., (2016) summarized various studies on the Newtonian and non- Newtonian behaviourof nanofluids in their review. According to their study, it was realized that, as the particle dosinglevel was increased, Newtonian behaviour of nanofluids converted into non Newtonian andnanofluids based on spherical nanoparticles are more likely to show the trend. Studies on rheologicalproperties of nanorefrigerants have great significance before practical implementation in variousheat transfer systems. To predict the pressure drop and pumping power through various conduitsexperimental studies on the rheological properties of nanorefrigerants are essential.Systematic experimental investigations were conducted by Mahbubul et al., (2014) to explore therheological behavior of Al2O3/R141b nanorefrigerant. The studies were conducted for volumeconcentration from 0.05 to 0.15% and at temperature from 4 to 16 0C. The shear rate was up to305.75 s-1. A mechanical shaker (orbital incubator type) was used to homogenize the nanoparticlesin the refrigerant. In this study, LVDV-III ultra-programmable rheometer was used to measure theviscosity and shear stress at desired shear rate. For each sample, the temperature of nanorefrigerantwas varied at an interval of 4 0C to examine the effect of temperature on the rheological behavior.The popular Brinkman model was used to compare the experimental values of viscosities. Thedependence of shear stress with shear rate for 0.10 vol% concentration of Al2O3/R141b with fourdifferent temperatures was shown in Fig.2(a) and(b).Results indicate that, viscosity increases with increase of shear rates. At low shear rates, theseincreasing trends were found to be non-Newtonian behavior up to the yield stress point. Beyond theyield stress, the rheograms showed almost Newtonian trend. This is an important observation as theshear thickening behavior exhibited by the nanorefrigerant is disappeared with the increase of shearrate. The sedimentation formed because of the particle agglomeration easily broken to form adispersed suspension with the increase of shear rate, which exhibits near Newtonian behavior. Itcould be a favorable benefit for practical application of nanorefrigerants in refrigeration cycles withcompressors, as with the force provided by the compressor de-agglomerates to form a dispersedsolution. Viscosity decreases with the increase of temperature. Moreover, this decreasing trend wasmore significant for higher particle concentrations and shear rates. The experimental viscosity found

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