Scale-resolving simulation of primary spray breakup in complex two-phase flow systems relevant to internal combustion engines

The objective of this work is to numerically investigate primary breakup phenomena under conditions relevant to automotive applications using Computational Fluid Dynamics (CFD) simulations. Primary breakup of a liquid jet is a complex process that is not yet fully understood. In this work, the high-fidelity, scale-resolving approach has been applied, namely Large Eddy Simulation (LES) and Partially Averaged Navier-Stokes (PANS) models have been used to directly resolve most of the turbulence spectrum and to capture the flow structure that could play an important role in jet break-up. The Coherent Structure Model (CSM) was used to represent the unresolved subgrid scales (SGS) within the LES framework, while the Scale Supplying Variable (SSV) variant of PANS was chosen for the PANS approach. The multiphase flow problem has been addressed using the Volume of Fluid (VOF) paradigm because of its robustness, mass conservativity and ability to maintain a sharp interface between the liquid and gas phases. The described simulation model, which is based on the numerical method of control volumes within the FIRE computational program of AVL List GmbH (Graz, Austria), has been applied to different flow configurations representative of industrial injector flows: 1) a circular liquid jet with a fully turbulent inlet profile, where the aim was to investigate the influence of inlet velocity and gas density on the liquid core length, 2) two different nozzle designs from the experiment of Balewski (PhD thesis, TU Darmstadt, 2009), where the aim was to get a deeper understanding of the correlation between turbulence and jet disintegration, 3) a low-pressure SCR injector operated at three different injection pressures, where the predictive power of the applied modes was confirmed; As part of this work, a tool for off-line coupling with Lagrangian Particle Tracking (LPT) simulations was developed, 4) a real GDI (gasoline direct injection) injector, which is particularly challenging due to the high injection velocity and the presence of cavitation. All simulations performed provided plausible results that were in qualitative and quantitative agreement with the measurement results, suggesting that turbulence plays a very important role in the primary breakup process and that the influence of the internal nozzle design cannot be neglected in the spray characterisation. Despite certain limitations, high-fidelity CFD simulations can provide a deeper insight into the complex flow patterns and their impact on jet breakup, which can be useful to engineers involved in nozzle design development, and the results could be used as a basis for improving the accuracy of classical Lagrangian spray calculations.