Nanolubricants flow boiling heat transfer enhancement in a microfin tube evaporator—IRG0021

Abstract

The current article experimentally documents the effects of the nanoparticles thermal conductivity and aspect ratio on the two-phase flow heat transfer coefficient and pressure drop of nanolubricants in mixture with refrigerant R410A. Two nanoparticles types were dispersed in the polyolester lubricant: spherical Alumina (γ-Al2O3) nanoparticles with 40 nm nominal particles diameter and ZnO nanoparticles with 20 to 40 nm nominal particles diameter and with an elongated shape. While the two nanolubricants had different nanoparticle aspect ratio, they shared similar thermal conductivity. In refrigerant R410A and polyolester mixtures, they led to measurably different heat transfer coefficient for two-phase flow boiling inside a horizontal 9.5 mm micro-fin evaporator tube. Depending on mass flux, concentration, and heat flux, nanolubricants provided either an enhancement or a degradation, supporting the hypothesis that the nanolubricants thermal conductivity was not the main property responsible for the heat transfer coefficient intensification during flow boiling. The current article presents an analysis of the nanolubricants heat transfer models that adopted homogeneous liquid phase modeling approaches during the two-phase flow process. A sensitivity analysis of the thermodynamic properties highlighted that these type of models were not able to satisfactorily predict the heat transfer coefficient and pressure drop of refrigerant-nanolubricants mixtures.