Session: 04-04: Nano/Microscale Boiling and Condensation Heat Transfer

Paper Number: 131733

131733 - Molecular Dynamic Study on the Nucleation Characteristics of Carbon Dioxide During Pressure Drop 

Abstract: 

Heterogeneous nucleation is the most effective mechanism for phase transformation. The presence of impurities can reduce the activation energy required for phase transformation and form a new thermodynamic phase, thus promoting nucleation. Based on the simulation results and classical nucleation theory, the heterogeneous nucleation process of CO₂ refrigerant step-down phase transformation in CO₂ injector was studied by molecular dynamics simulation method. The simulation system was divided into homogeneous CO₂ system and heterogeneous CO₂-PAG system which contained PAG lubricating oil. The inlet pressure was reduced from (8MPa-11MPa) to outlet pressure (2.7MPa-3.38MPa). The mass fraction of lubricating oil ranged from 0% to 3%. The simulation system adopted periodic boundary conditions, the thermostat and constant pressure device were set to control the molecular temperature and pressure of the system. The constant particle number, volume, and temperature ensemble were adopted for data acquisition, output, and calculation. For homogeneous CO₂ systems, the simulation results showed that the higher the system pressure, the smaller the Gibbs free energy fluctuation corresponding to different sizes of CO₂ clusters in the system, the more molecules of clusters were formed, and the nucleation rate of CO₂ molecules in the system was positively correlated with the pressure. For heterogeneous CO₂-PAG systems, the effect of lubricating oil on the nucleation rate of CO₂ clusters of different sizes was studied by changing the content of lubricating oil in the system. The results showed that when the mass fraction of PAG lubricating oil was in the range of 0% to 3%, PAG lubricating oil was miscible with CO₂ droplets. The addition of lubricating oil could increase the molecular weight of the clusters in the system, which had a direct effect on the nucleation rate of CO₂ clusters. In the simulation system of PAG lubricating oil containing 1% and 2% mass fraction, super large CO₂-PAG droplets would be formed. The smaller the cluster size, the greater the influence of lubricating oil on the nucleation rate of CO₂ clusters, and the nucleation rate of the smallest N3 clusters could be increased by 0.42×10³²m^-2s^-1 at most. When PAG lubricating oil mass fraction continued to rise to 3%, the influence of lubricating oil on the rate of CO₂ cluster nucleation in the system was minimal. However, compared with the homogeneous CO₂ system, the total amount of CO₂ cluster molecules in the system was still increased by nearly 10%. The calculated system density was in good agreement with the NIST standard reference data.

Presenting Author: Fang Liu Shanghai University of Electric Power

Presenting Author Biography: Dr. Fang Liu is a full professor at Shanghai University of Electric Power

Authors:

Xiang Wang Shanghai University of Electric Power
Fang Liu Shanghai University of Electric Power