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Text | Xi Zhi Xu A
Editor|Xizhi Xu A
preface
Nanorefrigerants are defined to improve thermal performance, utilizing as few nanoparticles as possible in the base refrigerant. An improved refrigerant term called "nanorefrigerant" allows nanoparticles to be added to the refrigerant to improve system performance, including improved flow and pool boiling heat transfer characteristics and condensation heat transfer in flow cells.
Today, nano refrigerants and nano lubricants have become important choices for enhancing cooling cycle performance, improving tribological properties, heat transfer quality characteristics and refrigerant/oil mixture relationships. Specifically, nano refrigerants can improve the heat transfer coefficient of evaporation and condensation, while nano lubricants can improve tribological properties and thus compressor performance.
In this chapter, we will explore the mechanisms that use nanolubricants and nanorefrigerants to improve overall VCR cycle performance. We hope that this paper will help define gaps in research and understand the contribution of nanorefrigerants and nanolubricants in the refrigeration cycle.
Research related to Al2O3
To date, researchers have developed various methods to improve the heat transfer properties of refrigerants in vertical and horizontal pipes. Adding nanoparticles to the base refrigerant is one of the most effective ways to enhance the thermal characteristics of the refrigerant.
A team dispersed 0.1% mass fraction of Al2O3 particles in polyester oil and reported a 2.4% reduction in compressor work. A significant increase in thermal conductivity and viscosity was then reported in R141b dispersed with Al2O3 nanoparticles. Later, it was observed that at a fixed mass flux of 100 kg/m2 s, the particle volume fraction was between 1% and 5%, and the heat transfer coefficient and pressure drop increased by 383% and 181%, respectively.
Another team reported that R134a/Al2O3 nanorefrigerant produced a greater frictional pressure drop due to the higher steam quality when R134a/Al2O3 nanorefrigerant flowed in a smooth horizontal pipe.
The flow boiling heat transfer effect of Alumina-R141b, Cu-R141b, Al-R141b, and CuO-R141b in a horizontal tube with 0.1% to 0.3% mass fraction in a computer-aided test device has been studied, and the Cu-R141b nanorefrigerant has the highest heat transfer coefficient compared to other mixtures.
Al2O3 of 1.6% by volume fraction was dispersed on the R134a/POE mixture flowing on a horizontal and rough flat surface, and it was found that higher volume fraction and lower average diameter nanoparticles had a greater positive effect on the heat transfer characteristics of the base refrigerant. In addition, the effect of Al2O3 nanoparticles on the boiling heat transfer characteristics of R134a/POE mixture on the surface of rectangular fins was also studied, and it was reported that the volume fraction of 3.6% nanoparticles improved the boiling heat transfer performance by 113%.
If Al2O3 is dispersed in R141b refrigerant, thermal conductivity and viscosity are studied. The authors report that the viscosity and thermal conductivity of R141b/Al2O3 nanorefrigerants with 2% volume fractions are 179 times higher and 1.626 times higher than pure refrigerants, respectively. Experiments were conducted with R134a and R12 using 0.05% to 2% Al2O3 particle mass fraction, and it was reported that R134a refrigerant replaced R12 and polyester oil replaced mineral oil.
In addition, the 0.1% mass fraction nanoparticles in R134a refrigerant reduce energy consumption by 2.4%, significantly increasing the COP of the refrigerant. Kumar and Elansezhian experimentally studied the effect of R134a/Al2O3/PAG mixture on the overall performance of VCR cycles and observed a 10.32% reduction in energy consumption. The authors note that the use of nanoparticles in refrigeration systems is a cost-effective method that can increase their COP and reduce capillary tube length.
Subramani and Prakash observed a 25% reduction in VCR cycle energy consumption using Al2O3 nanorefrigerants and a 33% increase in overall COP. The freezing capacity of the cycle has also been improved. Yusof et al. dispersed 0.2% Al2O3 particles in polyester lubricants and reported a 7% increase in system COP and a 2.1% reduction in compressor energy consumption. Cremaschi et al. found that alumina nanoparticles did not improve the solubility between refrigerant and lubricant, while the addition of nanoparticles slightly reduced the solubility of R410a/POE.
Research related to CuO
Kedzierski and Gong observed that 50% to 275% improvement in heat transfer could be achieved using 0.5% CuO particle mass fraction with R134a/RL68H and R134a/POE mixtures. In addition, the R134a/RL68H mixture showed higher heat transfer enhancement compared to the R134a/POE mixture.
In a follow-up study, a 2% CuO particle volume fraction was used with R134a refrigerant, and it was reported that nano refrigerants had higher heat flux. Bartelt et al. dispersed 0.5-1% CuO nanoparticle mass fractions in the R134a/polyester mixture under horizontal flow boiling conditions and found heat transfer enhancement effects of 42-82% and 50-101%, corresponding to 1% and 2% mass fractions.
Peng et al. dispersed 0.1% and 0.5% CuO nanoparticle mass fractions in R113 refrigerant, studied the heat transfer performance in horizontal rough tubes, and reported a heat transfer coefficient of 29.7% achieved in the base refrigerant using nanoparticles.
Henderson et al. reported that 0.5% and 0.05% CuO and SiO2 nanoparticle volume fractions were dispersed in the R134a and R134a/POE mixtures under boiling flow conditions in horizontal tubes, with lower heat transfer performance. In addition, using 0.02%, 0.04%, and 0.08% CuO nanoparticle volume fractions with R134a/POE mixtures, the authors observed that 0.04% and 0.08% nanoparticles improved heat transfer performance, improving heat transfer performance by 52% and 76%, respectively.
Kedzierski and Gong dispersed 0.5% CuO nanoparticle mass fraction in polyester oil and observed a 275% improvement in heat transfer performance compared to the base refrigerant R134a. Bartelt et al. then extended the Kedzierski and Gong experiments and observed that a 2% concentration of CuO nanoparticles provided an improvement of up to 101%.
Abdel-Hadi et al. experimentally found that CuO nanoparticles with an average size of 25 nanometers and a concentration of 0.55% were the best values to significantly improve the heat transfer coefficient of evaporation. Kumar et al. observed that dispersing 0.2-1% CuO nanoparticles mass fraction in compressor lubricating oil reduced compressor energy consumption by 7% and increased COP by 46%. In addition, the authors report the use of nanoparticles to reduce friction and wear in the base lubricant.
Peng et al. used copper nanoparticles in the R113/VG68 mixture. Compared to other 50nm and 80nm particle sizes, the use of copper nanoparticles with an average size of 20nm significantly improved heat transfer performance by 23.8%. Akhavan-Behabadi et al. found that using 1.5% CuO nanoparticle mass fraction dispersed in R600a/polyester oil, the heat transfer rate of the condensation process inside the horizontal tube increased by 83%.
titanium dioxide
To date, researchers have carried out various methods to improve the heat transfer properties of refrigerants in vertical and horizontal pipes. Among them, adding nanoparticles to the base refrigerant is one of the most effective ways to improve the thermal characteristics of the refrigerant.
If the 0.1% Al2O3 mass fraction is dispersed in polyester oil, a 2.4% reduction in compressor power consumption is reported. Mahbubul et al. dispersed Al2O3 nanoparticles in R141b and observed a significant increase in thermal conductivity and viscosity.
Since then, it has been observed that at a fixed mass flux of 100 kg/m2 s, the heat transfer coefficient and pressure drop increase by 383% and 181%, respectively, at particle volume fractions between 1% and 5%. Mahbubul et al. reported that the frictional pressure drop of R134a/Al2O3 nanorefrigerant is greater than that of R113/CuO nanorefrigerant when flowing in a horizontal smooth tube, due to its high vaporization quality.
The effects of Alumina-R141b, Cu-R141b, Al-R141b and CuO-R141b were studied, and it was found that Cu-R141b nanorefrigerant had the highest heat transfer coefficient compared to other mixtures.
Kedzierski dispersed 1.6% Al2O3 by volume in the R134a/POE mixture and found that nanoparticles with higher volume fractions and smaller average diameters had a more pronounced positive effect on the heat transfer characteristics of the base refrigerant. In addition, Kedzierski studied the pool boiling properties of the R134a/POE mixture on the surface of rectangular fins and found that a 3.6% nanoparticle volume fraction improved boiling heat transfer performance by 113%. Tang et al. reported that the use of δ-Al2O3 with R141b significantly improved the pool boiling heat transfer coefficient in the system, but the addition of surfactants at higher concentrations impaired the heat transfer process.
Mahbubul et al. dispersed Al2O3 in R141b refrigerant to study thermal conductivity and viscosity. The authors report that at 2% volume fraction, the viscosity and thermal conductivity of R141b/Al2O3 nanorefrigerants are 179 times and 1.626 times higher than pure refrigerants, respectively.
Jwo et al. used 0.05% to 2% mass fraction of Al2O3 particles dispersed in R134a and R12, and reported that R134a replaced R12 and polyester oil replaced mineral oil, reducing energy consumption by 2.4%, significantly improving the performance of refrigerators. Kumar and Elansezhian experimentally studied the effect of R134a/Al2O3/PAG mixture on the overall performance of VCR cycles and observed a 10.32% reduction in energy consumption. The authors note that the use of nanoparticles in refrigeration systems is a cost-effective method that can improve COP and reduce capillary tube length.
Subramani and Prakash observed a 25% reduction in VCR cycle energy consumption using Al2O3 nanorefrigerants and a 33% increase in overall COP. The freezing capacity of the cycle has also been improved. Yusof et al. dispersed 0.2% Al2O3 particles in polyester lubricant
Research related to carbon nanotubes (CNTs) and other nanoparticles
The researchers have extensively studied the applications of carbon nanotubes and other nanoparticles in the field of refrigeration. Experimental studies of thermal conductivity using CNTs of different concentrations and diameters dispersed in R113 refrigerant were carried out and it was found that thermal conductivity can be increased by 50% to 104%. The results show that CNTs with 1.0% volume fraction are the best choice.
Park and Jung performed a nuclear boiling heat transfer analysis of CNTs using R123 and R134a refrigerants. The results show that the heat transfer rate increases at low heat flux densities and begins to decrease at high heat flux densities.
Peng et al. dispersed CNTs in the R113/oil mixture and improved the heat transfer coefficient by 61%. In addition, the study also found that longer CNTs and smaller outer diameters can increase the heat transfer coefficient. Jiang et al. conducted experimental studies on nanofluids based on carbon nanotubes (CNTs) and proposed an improved Yu-Choi model with a deviation of about 5.5% from the experimental results. Henderson et al. used SiO2 particles dispersed in polyester oil, and the flow boiling performance was reduced by 55% due to the difficulty of dispersion and stability of nano-lubricants. However, the use of Al2O3/POE nanolubricants can significantly improve heat transfer properties.
In addition, there are studies that explore the application of other nanoparticles. Kumar and Singh used 1.0% by weight fraction of ZnO nanoparticles dispersed in the R290/R600a/MO mixture and found a 7.48% reduction in energy consumption and a 48% increase in COP. Peng et al. dispersed diamond nanoparticles in the R113/VG68 mixture to study the nuclear boiling heat transfer coefficient and found that the heat transfer coefficient increased by 63.4% under nanoparticles of 0.05% to 0.5% by weight.
Using 0.5%, 1% and 2% mass fractions of diamond nanoparticles dispersed in R134a, Kedzierski found a 98% improvement in boiling heat transfer performance. Nathon et al. studied the effect of titanium nanoparticles on the efficiency of copper heat pipes, using R11 as the base refrigerant, and found that a nanoparticle fraction of 0.01% had the highest efficiency. Wang et al. achieved a 6% improvement in overall COP using a novel nanooil made by mixing NiFe2O4 nanoparticles into naphthyl oil B32 as an alternative to polyester VG32.
Therefore, the application of carbon nanotubes and other nanoparticles in the field of refrigeration has the potential to improve the heat transfer performance of refrigerants and the efficiency of the system. Further research will contribute to a deeper understanding of the mechanism of action of nanoparticles in refrigeration systems and promote the development and improvement of refrigeration technology.
conclusion
In this chapter, previous research on heat transfer characteristics, solubility, and system performance of vapor compression refrigeration cycles is presented. The literature related to pool boiling heat transfer enhancement of refrigerants and rheological behavior of lubricants is mainly reviewed. The use of nano-refrigerants has improved the heat transfer performance of VCR systems, especially in nuclear boiling and pool boiling heat transfer.
Compared to other nanoparticles, carbon nanotubes can be considered the best choice for improving the heat transfer performance of the base refrigerant. As the size of the nanoparticles decreases, the heat transfer rate increases while the pressure drop decreases. Nano lubricants have good lubricating properties in reducing wear rates and friction compared to base lubricants.
TiO2 and CuO are the best nanoparticles in improving the friction characteristics of compressors. It has been reported that the application of nanoparticles in basic refrigerants can improve the COP and freezing speed of the refrigeration cycle. The study found that the improvement of VCR system parameters depends largely on the concentration of nanoparticles. Therefore, the optimal particle score must be defined for better and stable results.
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