A Comparison Between Recent Experimental Results and Existing Correlations for Microfin Tubes for Refrigerant and Nanolubricants Mixtures Two Phase Flow Boiling

Driven by higher energy efficiency targets, there is critical need for major heat transfer enhancements in heat exchangers. Nanolubricants, that is, nanoparticles dispersed in the non-volatile component of a mixture, have the potential to increase the heat transfer coefficient by 20% or more for two-phase flow boiling with small or no penalization on the two-phase flow pressure drop. The present work builds upon these intriguing yet unexplained findings, which were documented in the experiments of the present study for one type of nanolubricant, but for which a theory still does not exist. This paper presents a comparison between existing models in the literature and recent new experimental data for two phase flow boiling in a microfin tube of refrigerant R410A and nanolubricants mixtures. Alumina Oxide (g-Al2O3) based nanolubricants with 40 nominal particle diameter of approximately spherical shape were investigated. The nanoparticles concentration in the lubricant varied from 10 to about 20 in mass percentage, and the lubricant concentration varied from 0 up to 3% in mass percentage. The models available in the open domain literature were not able to capture the effects of the nanoparticles on the two-phase flow heat transfer coefficient. The augmented thermal conductivity of the lubricant due to the addition of highly conductive nanoparticles was not the main mechanism responsible for the heat transfer enhancements. The discrepancy between the simulation results and the experimental data was postulated to be due to non-Newtonian behaviors due to the presence of nanoparticles and surfactants. The flow development of the liquid phase of the mixture and the localized thickening and thinning of the liquid film thickness around the inner walls of the tube can alter the film local convective thermal resistance.