Viscous effects on liquid sheet breakup and interfacial transport scaling in planar atomizing flows

The disintegration of planar liquid sheets is central to atomization efficiency in agricultural spraying, yet the fundamental physics governing the transition from bulk mass to droplets remains complex. While viscous adjuvants are routinely employed to modify droplet size, the coupled influence of fluid rheology and internal nozzle geometry on the primary breakup region remains poorly quantified compared to the far-field spray. To address this gap, this study experimentally investigates the near-field dynamics and spatial evolution of flat-fan sprays using high-speed shadowgraphy and laser diffraction. The experiments mapped the effects of fluid viscosity (1-15 mPa·s) and injection pressure (1–6 bar) on industry-standard single-orifice and pre-orifice nozzle designs. Results indicate that viscosity stabilizes the liquid sheet by suppressing Kelvin-Helmholtz instabilities and thickening the rim, whereas injection pressure enhances aerodynamic shear to accelerate atomization. Hereafter, this stabilizing effect of viscosity and destabilizing role of pressure are referred to as viscous damping and inertial forcing, respectively. A critical comparison reveals that pre-orifice geometries significantly dissipate kinetic energy, resulting in coarser droplets with distinct breakup morphologies. Furthermore, the analysis captures the spatial transition of the droplet size distribution from a bimodal profile in the ligament-dominated near-field to a unimodal form downstream. Crucially, when normalized by the volume mean diameter (D₄₃), these distributions collapse onto a universal self-similar Gamma curve. These findings bridge the gap between idealized theory and industrial application, providing a robust quantitative framework for optimizing spray transport in viscous atomizing flows.