Experimental Investigation of Two-Phase Flow Heat Transfer of R410A and ZnO Nanolubricant Mixture in a Smooth Tube

In air conditioning and refrigeration systems, a small amount of the compressor lubricating oil is carried with the refrigerant and it circulates to the system components. In heat exchangers, oil can act as a contaminant affecting heat transfer and pressure losses. Literature on nanolubricants, that is, nanoparticles dispersed in the non-volatile component of a refrigerant and oil mixture, have shown potential to augment the performance of heat exchangers. However, the mechanisms of heat transfer enhancement due to the nanolubricants are still not well understood. Several published models for two-phase flow boiling postulates heat transfer enhancements for nanofluids, but these models lack experimental validation. This work aims to address this gap by presenting new experimental heat transfer coefficient and pressure drop data for the saturated two-phase flow boiling of pair of R410A and ZnO based nanolubricant mixture in a smooth copper tube of 9.5 mm (0.375 in.) ID. Non-spherical ZnO nanoparticles (20 to 40 nm) chosen for the study were dispersed in RL32-3MAF Polyolester (POE) lubricant at a mass concentration of 20 wt.%. In smooth tube, R410A-ZnO nanolubricant mixture with higher thermal conductivity and kinematic viscosity than R410A-POE mixture showed degradation in heat transfer coefficient and had lower pressure drops, in addition the ZnO nanoparticles showed sedimentation on inner wall of the smooth tube.