Study on the influence of lubricating oil on the dynamic characteristics of CO₂ flow pattern evolution.

The introduction of lubricant significantly changes the heat transfer and flow characteristics of CO2 two-phase flow system. Due to the significant differences in physical parameters (viscosity, surface tension) between the lubricant and CO2, its mixing not only changes the dynamic behavior of the gas-liquid interface, but also affects the oil-phase distribution morphology in different flow patterns, which in turn has an important impact on the heat transfer efficiency and flow stability in the evaporator. To obtain the influence mechanism of lubricant on the evolution of CO2 flow pattern, a visualization system is designed to visualize the flow boiling pattern of CO2/ lubricant mixture in a 5 mm horizontal smooth tube under diabatic conditions by using a secondary fluid (water) heating method. The effects of important parameters on the flow pattern characteristics and flow pattern distribution are analyzed, and the experimental results are compared with the conventional flow pattern maps. The results show that there is a large error in describing the flow pattern evolution of the refrigerant/lubricant system using the flow pattern map of a single-component refrigerant. Based on the dynamic characteristics of the flow pattern evolution and the changes of thermophysical properties of the mixtures, the effects of gravity, surface tension, viscous force, inertial force, and local oil concentration on the flow pattern distribution are comprehensively considered, and four dimensionless numbers, including the liquid-phase Froude number Frl, the gasphase Reynolds number Rev, the local oil concentration ωlocal, and the liquid-phase Weber number Wel, are introduced, so as to establish a conversion criterion from the intermittent flow to the annular flow for the CO2/ lubricant system.