Primary breakup of a jet coupled with vortex-induced string cavitation in a fuel injector nozzle

Fuel jet primary breakup strongly depends on the in-nozzle cavitation phenomena found in the high-pressure fuel injector nozzle. Nevertheless, limited attention has been paid to the mechanism of fuel jet primary breakup induced by in-nozzle vortex-induced string-type cavitation. This study involves simulations of in-nozzle string cavitating flow and simultaneously near-nozzle jet primary breakup process using large eddy simulation and volume of fluid, aiming at revealing the effects of string cavitation on jet primary breakup. The numerical results are in good agreement with experimental data in terms of string cavitation intensity, interfacial topology of jet, and spray spreading angle. The numerical investigations indicate that the external surface of the jet experiences Kelvin–Helmholtz instabilities, which results in the development of circumferential and axial surface waves at the fuel film surface. Subsequently, the fuel film surface undergoes progressive wrinkling, resulting in its breakup into multiple ligaments and large droplets. On the internal side of the jet, back-suction of air caused by negative pressure and its interaction with cavitation vapor at the core of the jet lead to the collapse of vapor bubbles. The resulting pressure waves and micro-jets facilitate the detachment of liquid sheets from the internal surface of the jet. Analysis of the enstrophy transport equation indicates that the driving mechanism behind string cavitation jet breakup further downstream is the baroclinic torque term, which is responsible for the generation of a cascade of smaller vortical structures. This effect dominates over vortex stretching and dilatation terms.