Effect of Non-Condensable Gas on the Thermophysical Properties of Bubbly Water and on Bubble Collapse Dynamics Probed by Molecular Simulations
Abstract
Numerical results and visualizations for equilibrium properties of bubbly water and for bubble collapse dynamics obtained from Monte Carlo and molecular dynamics simulations for coarse-grained water and nitrogen models are presented and discussed to elucidate the effects of nitrogen and to provide insights for improving the underlying physical assumptions used in computational fluid dynamics studies of multi-phase flow. With regard to equilibrium properties, our simulations indicate that, at room temperature, the solubility of nitrogen increases significantly as water is placed under tension (i.e., homogeneously stretched water), but that the effects of nitrogen on bubble volume fraction and bubble stability are relatively small. Molecular dynamics simulations for the collapse of a vapor bubble indicate that very large system sizes (> 10^7 molecules) are needed to reach convergent behavior. Here, supersonic speeds for the decrease of the bubble radius are reached preceding the initial bubble collapse. Concomitantly, extreme local heating with temperatures exceeding 2000 K for more than 1% of the molecules at the center of the collapsing bubble is observed. After initial collapse, the bubble rebounds and a high-density wave propagates outward at supersonic speeds. The presence of nitrogen (at a mole fraction of 1.2 × 10^−5 corresponding to the amount of nitrogen in aqueous solution at 298 K and a vapor pressure of 1 bar) leads to a slight increase in the bubble collapse time and reduction in the speed for the decrease of the bubble radius before collapse, but the rebound is more pronounced.
