This paper reports a numerical investigation of diffusion-convective mass transfer in the CO₂–Si–N₂ gas–aerosol system at a low dispersed-phase volume fraction of φ = 0.5% under isothermal and isobaric conditions (T=300 K, p=0.1 MPa). Computations were carried out in ANSYS Fluent, combining the Species Transport formulation with the Eulerian Multiphase Flow framework, which together capture the coupled motion of gas components and solid microparticles with a diameter of 10 μm.

The results demonstrate that even at such a low particle loading, gravitational settling of Si particles generates a local density inversion near the bottom wall of the upper vessel — sufficient to trigger convective instability. The mixing evolves through a sequence of well-defined stages: initial particle sedimentation, formation and subsequent breakup of a dense particle cluster, sustained downward transport through the connecting channel, and the emergence of a characteristic teardrop-shaped solid-phase structure in the lower vessel. Throughout this process, the two phases exhibit markedly different transport kinetics: CO₂ accumulates gradually and continuously, whereas Si migrates in discrete pulses, producing pronounced oscillations in the dispersed-phase volume fraction.

The principal novelty of this work lies in demonstrating that Rayleigh–Taylor-type convective instability develops at φ = 0.5% — a loading level at which any solid-phase influence on the mass transfer mechanism would not be expected a priori. This finding points to the existence of a lower concentration threshold below which the system transitions from gas-like diffusive behavior to a qualitatively different convection-driven regime.

The combined Species Transport and Eulerian Multiphase approach proves capable of reproducing diffusion-convective dynamics in dilute gas–solid systems without recourse to empirical fitting parameters. The outcomes of this study are relevant to modeling aerosol particle behavior under atmospheric and industrial conditions, and may inform the design of experiments aimed at measuring mutual diffusion coefficients in gas–aerosol systems.