The primary breakup mechanism in close-coupled gas atomization (CCGA) remains unclear due to strong gas recirculation and complex interfacial dynamics. We performed three-dimensional VOF-LES simulations with adaptive mesh refinement across Weber numbers ranging from 40.2 to 360, validated against digital inline holography experiments. Results show that liquid filming along the nozzle perimeter occurs at low to intermediate We, but only dominates at $\mathrm{We}=360$, where it leads to a complete disintegration of the liquid jet due to filming. The change in dominant liquid pathways correlates with a change in droplet size distributions from a power law at lower We to log-normal at the highest We. Distributions of normalized droplet velocities were independent of We, indicating that droplet velocities are dictated by drag. Droplet Weber numbers based on the instantaneous slip velocities only exceeded the critical value for secondary breakup at $\mathrm{We}=360$. Findings indicate that CCGA primary breakup is governed by the interplay between gas recirculation and filming behavior, rather than classical Kelvin–Helmholtz or Rayleigh–Taylor instabilities alone.

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• Atomization
• Interfacial flows
• Multiphase flows