Aggressive national goals for significantly reducing petroleum use and greenhouse gas emissions require major improvements in all aspects of our nation’s energy use. Combustion processes have historically dominated electrical power production and transportation systems. Despite significant advances in improving the efficiency and reducing the costs of alternative energy sources, combustion-based systems are projected to dominate the marketplace for decades, especially for hard-to-electrify sectors including aviation. Consequently, these systems must be optimized for energy efficiency and reduced emissions.
The motivating problem underlying this project is a sufficiently realistic simulation of the in-cylinder processes in an internal combustion engine using low-temperature combustion for which reactivity-controlled compression ignition (RCCI) is the exemplar. The enabled exascale-era simulations will address key scientific questions regarding fuel mixture effects, the multistage ignition of a diesel surrogate fuel, lifted flame stabilization, jet entrainment affected by cylinder-wall geometry, and emissions.
The simulation accounts for the isentropic compression, subsequent injection of the high-reactivity fuel, and combustion processes in a compression ignition engine. Necessary physics include gas compression and models of fuel injection process, spray vaporization (i.e., injecting liquid fuel sprays into high-pressure conditions), and mixing. Combustion processes include autoignition, flame propagation, and soot/thermal radiation, all in a nontrivial engine geometry. The scenario involves kinetically controlled processes in turbulent combustion, including ignition, extinction, and emissions. The application used for this project, Pele, implements a hybrid large-eddy simulation (LES)/direct numerical simulation (DNS) approach in both the compressible and low-Mach limits at which the project team will refine the mesh to the DNS limit where necessary to capture turbulence/chemistry interactions by using the machinery of adaptive mesh refinement while restricting grid resolution to that required for a high-fidelity LES model far from the flame.
This project is structured around providing a combination of first-principles DNS and near-first-principles DNS/LES hybrid simulations to advance the scientific community’s understanding of fundamental turbulence-chemistry interactions at device-relevant conditions. The exascale motivating problem is to perform high-fidelity simulations of the relevant processes in an RCCI internal combustion engine. The relevant processes include turbulence, mixing, spray vaporization, low-temperature ignition, flame propagation, and soot/radiation. RCCI is thermodynamically favorable relative to existing engines and thus holds the promise of groundbreaking efficiencies while operating in a regime that limits pollutant formation. The roadmap toward this exascale-era motivating problem includes simulations of a multi-injection low-temperature diesel jet into an open domain with a large alkane fuel undergoing two-stage ignition processes, dilute spray evaporation and mixing, and multi-injection with fuels of varying reactivity in a geometry that influences the mixing field. The multi-injection simulation forms the challenge problem to demonstrate new exascale capability.
The specific science-based challenge problem was derived from the roadmap toward the motivating exascale-era problem. Specifically, the challenge problem demonstrates the ability to simulate the interaction of two fuels with varying reactivity under a multi-pulse injection strategy into an engine-relevant geometry. It also serves as a baseline for future simulations utilizing a diverse range of sustainable aviation fuels in aero-engine applications where the impacts of spray vaporization on mixture formation, flame stabilization and fuel-lean flame blowout in complex swirl-stabilized confined flames can be studied with Pele. As with the RCCI problem, the widely differing reactivity of SAF fuels poses challenges during take-off and cruise conditions in aero-engines. High-fidelity simulations will be used to inform the development of predictive subgrid combustion and spray models used in CFD as part of the co-design of sustainable aviation fuels (SAF) with aero-engines. Besides reducing an aircraft’s carbon emissions, using a SAF blend can also result in much lower ice crystal contrails at cruising altitudes, further reducing aviation’s impact on the climate. Ice crystal formation occurs through its interaction with exhaust soot particles and supercooled water vapor, and soot particle size and morphology depend upon SAF properties. Exascale simulations with multi-physics and geometry capabilities will enable detailed analyses of aircraft ‘turbulence-chemistry’ interactions and characterization of exhaust soot particles, potentially offsetting the expense of ‘flying laboratory’ which currently trails behind aircraft, sampling and analyzing gases and particles in its wake.