Fuel Droplet Breakup in High-Pressure Propulsion Systems Using an Integrated Molecular-Dynamics and Direct Numerical Simulation Approach

The secondary breakup of fuel droplets into child droplets significantly impacts the mixing and combustion processes in liquid-fueled propulsion systems. In the pursuit of enhanced engine performance and reduced emissions, the design of propulsion systems is shifting towards pressures higher than the nominal critical pressure of the fuel and air. This leads to transcritical conditions, wherein the subcritical fuel is injected into the supercritical air, causing fuel to transition from a liquid-like to a gas-like behavior. Experimental analyses at low convective flow speeds have characterized the transcritical behavior as a transition from classical evaporation dominated by surface tension effects into a gas-like diffusion behavior at higher pressure and temperature. However, the transcritical droplet breakup mechanisms under high-speed conditions involving shockwaves have remained unresolved. In this study, the breakup behavior of a single subcritical n-dodecane droplet in a supercritical environment interacting with a shockwave is developed by a fully compressible multiphase Direct Numerical Simulation (DNS) approach with real-gas and surface effects. Molecular Dynamics simulations (MD) are utilized to study the interfacial behavior of an n-dodecane droplet to predict the surface tension coefficient under varying ambient conditions. This study integrates MD and continuum-based modeling by developing a data-driven model for breakup and evaporation at transcritical conditions.