Investigations of vortex dynamics driven primary breakup in diesel spray based on multiphase lattice Boltzmann flux solver

During the fuel injection process, there is competition, merging, and entrainment between the Rayleigh-Taylor and Kelvin-Helmholtz instabilities, eventually leading to the formation of jet turbulence. The associated vortex dynamics is crucial for understanding the micro-evolution of surface waves on fuel spray. Considering the characteristics of fuel jet with large density ratios, a pressure-corrected multiphase lattice Boltzmann flux solver is proposed in this study, coupled with the Cahn-Hilliard model, to capture the micro-scale evolution of diesel spray. The spray evolution during the primary breakup, under simulated engine operating conditions, is systematically conducted by defining the parameters of vortex dynamics. According to the growth feature of vortex velocity, the microscopic evolution during the primary breakup can be divided into five stages: diffusion growth, exponential growth, potential flow growth, re-acceleration, and chaotic mixing (CM), providing a theoretical basis for the instability analysis of spray breakup. The growth of the Rayleigh-Taylor instability is determined by the competitive relationship between buoyancy and viscous dissipation forces. In cases of higher density ratios, the buoyancy-driven Kelvin-Helmholtz and the secondary growth Rayleigh-Taylor waves mutually advance, elongate, disintegrate, and finally breakup. Therefore, the influence of the density ratio on tail velocities shows a scenario of first promoting and then inhibiting. As the density ratio increases, the instability reaches the CM stage of asymmetric development more quickly by undergoing complex vortical motions. At this stage, there is a complicated phenomenology associated with the evolution of spray interface, including multiscale curling, severe deformation, vortex disintegration, and droplet breakup, eventually leading to turbulence.