A priori Analysis and modelling of Dilution Equation for MILD Combustion Using DNS Data

Addressing climate change requires innovative concepts and technologies for energy and transport sectors. A promising approach is MILD (Moderate to Intense Low-oxygen Dilution) combustion, offering enhanced energy efficiency, reduced pollutant emissions, and increased operational flexibility, especially with zero-C fuels like Ammonia. MILD combustion relies on naturally occurring fuel-air mixture dilution through flow recirculation in the combustor, which varies in time and space, influencing chemical conversion. However, understanding the dilution's impact on fuel consumption, particularly in the context of LES (Large Eddy Simulation), remains limited. In this study, a transport equation for the dilution factor is derived from fundamental principles and analyzed using DNS (Direct Numerical Simulation) data of turbulent MILD combustion. The goal is to develop simple models, focusing on various source terms in the filtered dilution equation. These sources arise from overall fuel consumption progress, chemical reactions affecting the dilution variable, and turbulent mixing represented by scalar dissipation rates. By treating each LES grid cell as a perfectly stirred reactor (PSR), simple models are developed, effectively capturing the background physics observed in the DNS data. Results from LES with the dilution equation and the tested sub-grid models will be discussed in the presentation.