Conference Year

2016

Keywords

micro-fin tube, nanorefrigerant, nanolubricant, heat transfer enhancement, flow boiling

Abstract

In vapor compression cycles of air conditioning and refrigeration systems, a small amount of the compressor lubricating oil is carried with the refrigerant and it circulates in the system components. In the heat exchangers, oil is a contaminant and it decreases the heat transfer rate and increases the pressure losses: both are unavoidable challenges. Nanolubricants, that is, nanoparticles dispersed in the non-volatile component of a refrigerant and oil mixture and working as thermal energy transport vectors within the mixture, have the potential to overcome these challenges in a cost-neutral manner for both new and retrofitting applications. This papers documents the effects of the nanoparticles thermal conductivity and aspect ratio on the two-phase flow heat transfer coefficients and pressure drop of nanolubricants by experimentally studying two types of nanolubricants. The first type used spherical Alumina (γ-Al2O3) nanoparticles with 40 nm nominal particle diameter. The second type used ZnO nanoparticles with 20 to 40 nm nominal particle diameter and with an elongated hexagonal wurtzite shape. The experimental results indicated that the thermal conductivity of the nanolubricants increased linearly with the added amount of nanoparticles in POE oil, and γ-Al2O3 and ZnO based nanolubricants had similar thermal conductivity if the nanoparticle concentration was 10 wt. % and 20 wt. %. However, even when diluted in the liquid phase of the refrigerant R410A and POE oil mixture, these two types of nanoparticles provided heat transfer performance that were measurably different from each other during two phase flow boiling inside an horizontal 9.5 mm micro-fin evaporator tube. Depending on the mass flux, oil concentration, and heat flux, nanolubricants provided either an enhancement or a degradation of the heat transfer coefficients. These results supported the hypothesis that thermal conductivity of the nanolubricants was not the main property responsible for the heat transfer coefficient intensification during flow boiling. The experimental results also suggested that the magnitude of the enhancements of the two phase flow boiling heat transfer coefficient due to the nanoparticles dispersed in the POE oil were depended on the mass flux and shear rates within the liquid phase of the mixture. Interesting, even when high heat transfer coefficients were observed for the nanolubricants, the effect of the nanoparticles on the two phase flow pressure drop of the mixture was negligible.Â

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