Numerical Simulation and Characteristics Study of Near-Field Fan Jet Breakup in Airless Spraying

The breakup process of near-field jets in airless spraying directly determines the initial droplet size distribution and macroscopic spray pattern, making it a critical factor af-fecting coating quality. This study establishes a numerical model for near-field atomi-zation based on computational fluid dynamics, coupling the VOF interface capturing method with the SST k-ω turbulence model. The model’s accuracy is validated through mass flow rate experiments and high-speed imaging. By incorporating instability theory, expressions for the characteristic lengths of Kelvin-Helmholtz (KH) and Rayleigh-Taylor (RT) instabilities—including viscous corrections—are derived, revealing the competitive mechanisms among surface tension, viscous forces, and inertial forces during interface destabilization. Results show that the axial velocity along the center-line exhibits a segmented behavior characterized by “sharp acceleration—gradual decay—high-frequency oscillation,” while volume fraction and velocity display strong spatial phase coupling. Both core length and breakup length significantly decrease with increasing spray pressure but increase linearly with viscosity, leading to empirical formulas for characteristic lengths (with fitting errors < 15%). Power-law scaling analysis further indicates that the KH stage is dominated by surface peeling, with characteristic length sensitive to pressure, viscosity, and surface tension; whereas the RT stage is primarily governed by bulk breakup, where the characteristic length shows relative independence from pressure and is mainly controlled by viscosity and surface tension. The ratio of the two characteristic lengths ranges approximately between 4 and 6, providing theoretical guidance for optimizing airless spraying process parameters.