A novel constant pressure control method for nucleation boiling of CO2/lubricant in molecular dynamics simulation

Abstract

Conventional molecular dynamics simulations of nucleation boiling typically employ the fixed volume method, resulting in a non-constant pressure during the boiling process. In this paper, we propose an additional pressure control method, placing a cover plate at the top of the liquid phase to achieve liquid phase pressure control during boiling. The novel method yields a pressure control error within ±5 %. CO2 and CO2/PEC4 lubricating oil mixture are used to investigate the differences in heat transfer characteristics between non-constant and constant pressure nucleation boiling. Furthermore, the effect of PEC4 lubricating oil on CO2 nucleation boiling under a fixed pressure condition is explored. The results show that both the bubble nucleation and growth time of CO2 and the CO2/PEC4 mixture are prolonged under constant pressure. This causes a compression in CO2 molecular spacing, thereby reducing CO2 diffusion and mass transfer capabilities. Heat transfer from the solid to the liquid, and within the liquid itself, is diminished, while the distribution of PEC4 molecules within the liquid phase becomes more even. The migration rate of PEC4 molecules in the liquid phase slows down. Under the constant pressure model, PEC4 increases the mass transfer resistance of CO2. As the lubricant mass fraction rises, The nucleation and growth rate of CO2 bubbles decline. In comparison to CO2, PEC4 molecules exhibit a stronger interaction energy with the Cu wall surface. During boiling, some PEC4 molecules accumulate on the Cu surface, forming an oil film that impedes the direct contact between CO2 molecules and the Cu wall.