EMBEDDED DNS CONCEPT FOR SIMULATING THE PRIMARY BREAKUP OF AN AIRBLAST ATOMIZER

The primary breakup represents the initial step of the liquid atomization process and is still not well understood. Prefilming airblast atomizers are utilized in aircraft engines to atomize the liquid fuel. The geometries of airblast atomizers are complex; the operating conditions are characterized by high Reynolds and Weber numbers. The investigation of airblasted sheets lack experimental data due to the limited accessibility of the prefilmer geometry. Numerical experiments represent an alternative. This paper introduces the embedded direct numerical simulation (DNS) concept that aims to fill this gap. The concept consists of three steps: a geometry simplification, the generation of inflow boundary conditions for the embedded domain, and the two-phase flow DNS of the breakup region. The annular airblast atomizer geometry is simplified to a planar configuration. A zonal large eddy simulation of the turbulent channel flow is performed prior to the DNS. The inflow paramters are mapped to the inlet of the embedded domain. The results from the turbulent channel flow computations illustrate a good agreement with DNS data. The primary breakup of an airblasted sheet is simulated by using the volume-of-fluid method. Two different grid resolutions are utilized. Qualitative studies show a good agreement of the liquid deformations and the dominant primary atomization mechanism with experimental results. Quantitative results discuss the resulting droplet distributions, the grid resolutions related to the representation of small structures and turbulence, and the breakup length and time scales. It is confirmed that the fine grid improves the resolution of small droplets due to the further breakup of already separated structures. The majority of the liquid mass instead is associated with the irregular appearing large scales, which are already resolved using the coarse grid. This paper proves the applicability of the embedded DNS approach for understanding the primary breakup of prefilming airblast atomizers.