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An improved understanding of the breakup processes of two-phase flows is essential to effectively control the fuel atomization for future aircraft engines. A detailed insight into the phenomena of primary breakup is a major limitation in gaining this knowledge. Aircraft engines use airblast atomizers to provide the fuel atomization. The geometries of airblast atomizers are complex, the operating conditions are characterized by high Reynolds-and Weber numbers. Direct Numerical Simulations (DNS) of liquid breakup under realistic conditions and geometries are hardly possible. The embedded DNS (eDNS) concept aims to fill this gap. The concept consists of three steps: a geometry simplification, the generation of realistic boundary conditions for the DNS and the DNS of the breakup region. The realistic annular airblast atomizer geometry is simplified to a planar geometry. Inside this domain the eDNS is located. The eDNS domain requires the generation of boundary conditions. A zonal Large Eddy Simulation (LES) of the turbulent channel flow is performed prior to the DNS. The parameters are stored transiently on the "virtual" DNS inlet planes. These variables are then mapped to the DNS. The Volume of fluid (VOF) method is used to solve for the two-phase flow. DNS are performed for a shear-driven liquid wall film and for a generic planar prefilming airblast atomizer. As the Reynolds and Weber number for the first operating point (OP) are low (Re air = 5,333/We film = 1.9), the liquid wall film as well as the liquid sheet show no surface waves. For the second case with Re air = 13,333 and We film = 11.9, the surface appears more wrinkled and streamwise waves are transported along the wall for the shear-driven wall film. Instantaneous snapshots in 2-D and 3-D illustrate the qualitative behavior of the liquid sheet in time. Leaving the prefilmer trailing edge, the liquid sheet starts to oscillate in a sinusoidal fashion. This oscillation appears crucial for the onset of primary breakup. The qualitative characterization of the breakup for OP 1 and OP 2, yield to the distinction of the stretched ligament breakup for the former, and the torn-sheet breakup for the latter OP. The breakup time for OP 1 is longer than for OP 2. This study proves the applicability of the eDNS concept for investigating breakup processes as the transient nature of the phase interface behavior can be captured. The approach offers the potential of simulating realistic annular highly-swirled airblast atomizer geometries under realistic conditions.

The primary breakup under high-altitude relight conditions is investigated in this study where ambient pressure is as low as 0.4 bar and air, fuel and engine parts are as cold as 265 K. The primary breakup is crucial for the fuel atomization. As of today, the phenomena dictating the primary breakup are not fully understood. Direct Numerical Simulations (DNS) of liquid breakup under realistic conditions and geometries are hardly possible. The embedded DNS (eDNS) approach represents a reliable numerical tool to fill this gap. The concept consists of three steps: a geometry simplification, the generation of realistic boundary conditions for the DNS and the DNS of the breakup region. The realistic annular airblast atomizer geometry is simplified to a Y-shaped channel representing a planar geometry. Inside this domain the eDNS is located. The eDNS domain requires the generation of boundary conditions. A Large Eddy Simulation (LES) of the entire Y-shaped channel and a Reynolds-Averaged Navier-Stokes Simulation (RANS) of the liquid wall film are performed prior to the DNS. All parameters are stored transiently on all virtual DNS planes. These variables are then mapped to the DNS. Thus, high-quality boundary conditions are generated. The Volume-of-Fluid (VOF) method is used to solve for the two-phase flow. The results provide a qualitative insight into the primary breakup under realistic high-altitude relight conditions. Instantaneous snapshots in time illustrate the behavior of the liquid wall film along the prefilmer lip and illustrate the breakup process. It is seen that a slight variation of the surface tension force has a strong impact on the appearance of the primary breakup. Case 1 with the surface tension corresponding to kerosene at 293 K indicates large flow structures that are separated from the liquid sheet. By lowering the surface tension related to kerosene at 363 K, the breakup is dominated by numerous small structures and droplets. This study proves the applicability of the eDNS concept for investigating breakup processes as the transient nature of the phase interface behavior can be captured. At this time, the authors only present a qualitative insight which can be explained by the lack of quantitative data. The approach offers the potential of simulating realistic annular highly-swirled airblast atomizer geometries under realistic conditions.