Experimental investigation on the influence of lubricant oil on CO2 nucleate pool boiling heat transfer characteristics

As a natural working fluid, CO₂ is considered the most promising alternative refrigerant to hydrofluorocarbons. In the fields of automotive air conditioning and commercial heat pumps, transcritical CO₂ cycles show significant potential for performance enhancement. Although there are many studies on CO₂ flow boiling in the open literature, few studies involve CO₂ nucleate boiling heat transfer, which is the dominant mechanism of CO₂ flow boiling heat transfer process. This study is proposed to conduct experimental investigations of CO₂ nucleate boiling heat transfer. The influences of evaporation temperature, heat flux and lubricant oil addition on boiling heat transfer performance and bubble dynamic characteristics are discussed. The results show that in pure CO₂ nucleate boiling, heat flux increase leads to higher bubble density and bubble diameter in bulk liquid, which in turn enhances boiling heat transfer. The effect of evaporation temperature increases on bubble diameter is significant. The reduction in bubble diameter weakens the convective heat transfer caused by bubble motion, which leads to less variation in the CO₂ nucleate boiling heat transfer coefficient with evaporation temperature. Lubricant oil addition significantly changes the bubble dynamics of CO₂ nucleate boiling process, leading to larger bubble density and smaller bubble diameter. Moreover, the oil diffusion at the phase interface notably affects the heat transfer performance, resulting in greater differences in the boiling heat transfer characteristics of the mixtures compared to that of pure CO₂. The mixture boiling heat transfer coefficient is collectively influenced by evaporation temperature, heat flux and oil concentration. The experimental results suggest that the heat transfer coefficient of the mixture with an oil concentration of 0.5 % increases by an average of 25 % compared to pure CO₂ at an evaporation temperature of 0 °C. At higher evaporation temperatures and high oil concentrations (>1%), oil addition leads to heat transfer deterioration. Findings from this work can provide a better understanding of oil effect on refrigerant boiling heat transfer and a fundamental basis for heat exchanger design of CO₂ systems.