The objectives of the project are to develop and demonstrate a high-fidelity simulation tool to predict and optimize high-pressure heavy fuel injectors. During Phase I research, Cascade Technologies developed a multiscale breakup model that resolves the continuous liquid phase using the geometric Volume-of-Fluid method, and models subgrid interfacial structures (ligaments, sheets and drops) by the discrete Lagrangian model. The numerical method has been validated for academic problems, successfully predicting droplet distributions even for coarse gird resolutions, which leads to a significant reduction in computational cost.In the proposed phase II project, focus will be placed on further development and demonstration of the tool for realistic liquid fuel injectors used for both a dual fuel gas turbine and an I.C engine. Simulations of both systems will be performed in realistic geometries and operating conditions and validated against available experimental data. Finally, we will perform an ensemble of simulations for the Bosch injector (CRIN3) and map a parametric design space to guide injector optimization and quantify performance improvements.