Numerical methods for liquid atomization and application in detailed simulations of a diesel jet

In the context of rising environmental concerns and record-high oil prices, the need for cleaner and more efficient combustion devices becomes pressing. Because of the steady progress in computer power and computational fluid dynamics (CFD) methods, computational modeling of combustion systems emerges as a promising tool that can drive the design of future devices. In these systems, fuel is usually injected in liquid form. Atomization of the liquid fuel, or the process by which a coherent liquid flow disintegrates into droplets, represents one of the key challenges that remain to be tackled to make predictive simulations possible. Because atomization governs the size of the fuel droplets, and therefore their subsequent evaporation rate, it will have far-reaching repercussions on many aspects of the combustion process, for example pollutant formation. However, the inherent multi-physics and multi-scale nature of this process limits both experimental and numerical investigations.;For this problem to become numerically tractable, this work combines several key ingredients. Since the Reynolds number associated with atomization is generally high, numerical schemes have to be tailored for the simulation of turbulence. For this purpose, an arbitrarily high order conservative finite difference scheme for variable density, low Mach number flows is developed. Combining the second order form of this scheme with the Ghost Fluid Method (GFM), the discontinuous material properties encountered in multiphase flows can be robustly handled. Also, the surface tension force, which is singular in nature, is considered in a more accurate way. In order to represent the phase-interface geometry, two new level set schemes are developed. First, the accurate conservative level set approach (ACLS) combines a hyperbolic tangent level set function which is transported and re-initialized using conservative schemes with a standard distance level set function for improved accuracy and robustness. This method combines good mass conservation properties with the simplicity and ease-of-use of standard level set schemes. Then, spectral refinement of a level set function is introduced. Thanks to a sub-cell polynomial reconstruction of the level set function, this approach provides great accuracy even for poorly resolved interface structures, while the use of a narrow-band formulation and semi-Lagrangian transport leads to a limited cost. The latter method is found more appropriate for simulating primary atomization because of the excellent resolution it provides.;The present approach is applied in a detailed study of the turbulent atomization process characteristic of Diesel injection. First, a temporally evolving turbulent planar jet is simulated for various Reynolds and Weber numbers. Direct visualization of the flow structures allows to lay out a clear picture of the atomization process. Early interface deformation is caused by turbulent eddies that carry enough kinetic energy to overcome surface tension forces. Then, liquid protrusions are stretched out into ligaments that rupture following Rayleigh's theory. Results suggest that aerodynamic effects contribute to both the early disruption of the liquid core and the rupture of liquid ligaments. Finally, a spatially evolving round liquid jet is simulated and qualitatively compared to recent experimental visualizations of Diesel injection.