Liquid-Film Breakup and Viscosity-Dominated Transition from Hollow-Cone to Spiral-Blade Morphology in a Pressure-Swirl Nozzle

Efficient atomization of liquid fuels over a broad range of physical properties is important for practical utilization. However, the response of pressure-swirl atomization to substantial variations in liquid viscosity remains insufficiently understood, particularly the transition from a hollow-cone atomization to a degraded atomization regime. A volume-of-fluid numerical method is employed to investigate liquid-film formation and primary breakup in a pressure-swirl nozzle over a range of viscosities and surface tensions of waste-derived pyrolysis oils in this study. The results show that increasing viscosity substantially alters the liquid-film morphology, cone angle, and primary breakup behavior. At high Reynolds numbers, a hollow-cone liquid film develops and undergoes interfacial destabilization and breakup. As the Reynolds number decreases, the hollow-cone morphology is replaced by a spiral-blade morphology. The high-viscosity condition is accompanied by substantially weakened internal swirl and circumferential mixing, allowing the liquid streams originating from individual tangential slots to retain distinct spiral paths. Consequently, centrifugal expansion is suppressed and continuous liquid persists in the central region. These results clarify the flow characteristics associated with the viscosity-dominated transition from hollow-cone to spiral-blade atomization and provide insight into the pressure-swirl atomization of liquids with a broad range of viscosities.