Experimental investigation of two phase flow boiling heat transfer of mixtures of refrigerant R410A and nanolubricants in a horizontal smooth copper tube

In air conditioning systems, a small amount of lubricant leaves the compressor and circulates through the system. The lubricant acts as a contaminant, affecting heat transfer and pressure losses in heat exchangers. The mixture of refrigerant and nanolubricants (NLs), that is, nanoparticles dispersed in the lubricant oil, have shown potential to augment the two-phase flow boiling heat transfer in the literature. However, the mechanisms of heat transfer enhancements due to the NLs are still not well understood. Published models of two-phase flow boiling lack experimental validation with refrigerant and NL mixture. This work addresses this gap by presenting new experimental heat transfer coefficient and pressure drop results for the saturated two-phase flow boiling of R410A with two NLs in a 9.5 mm I.D. smooth copper tube: a ZnO nanoparticle laden lubricant and a gamma-Al2O3 nanoparticle laden lubricant. In smooth tube, R410A-NL mixtures, which had higher thermal conductivity and kinematic viscosity than R410A-POE lubricant mixture, showed degradation in heat transfer coefficient compared to R410A-POE lubricant case; but they also had lower pressure drops. Relocation of the nanoparticles closer to the inner wall of the tube could lead to enhancement in heat transfer at an expense of increased frictional pressure drop.