Molecular Dynamic Study on the Nucleation Characteristics of Carbon Dioxide During Pressure Drop

Heterogeneous nucleation is one of the most effective mechanism for initiating phase transformation. Impurities serve as a catalyst in the formation of a new thermodynamic phase by lowering the activation energy necessary for a phase transition and promoting nucleation. This study utilizes molecular dynamics (MD) simulation to examine the heterogeneous nucleation process of CO2 refrigerant during pressure drop phase transformation in the motive flow section of the CO2 ejector. The study results indicate that elevated target pressures in CO2 systems can reduce the minimum Gibbs free energy required for the formation of CO2 clusters of varying sizes, thereby promoting refrigerant nucleation. At a target pressure of 2.70 MPa, the nucleation rate exhibits an increase of 0.34 × 1032 m−2s−1 at 2.86 MPa, 1.11 × 1032 m−2s−1 at 3.14 MPa, and 2.4 × 1032 m−2s−1 at 3.38 MPa. Furthermore, the introduction of PAG lubricating oil into the system decreases the minimum Gibbs free energy necessary for the formation of CO2 clusters of different sizes, while simultaneously enhancing intermolecular forces and facilitating the molecular nucleation of CO2 refrigerants. In comparison to an oil-free CO2 system operating at a target pressure of 3.14 MPa, elevating the mass fraction of PAG oil results in an increased nucleation rate of N3 clusters by 0.43 × 1032 m−2s−1 (2% of PAG), 0.37 × 1032 m−2s−1 (1 % of PAG), and 0.2 × 1032 m−2s−1 (3% of PAG) respectively. This acceleration contributes to the nucleation process of CO2 refrigerants. Nevertheless, it simultaneously leads to a decrease in the mass diffusion coefficient of CO2 molecules within the system. It is worth noting, however, that pressure continues to be the predominant factor influencing CO2 refrigerant nucleation.