Numerical Investigation of a Hydrogen-Air Flame for NOx Prediction

Abstract Sustainable Aviation Fuels (SAF) are a major candidate to reduce pollutant emissions in future aeronautical engines. Recently, the use of hydrogen as a fuel has gained a high interest partly because its combustion is free from carbon dioxide, a greenhouse gas, and produces few pollutants, mainly nitrogen oxides (NOx). Over the last decades, efforts on numerical methods for combustion simulation in aero-engines have largely been focused on kerosene-air combustion. However, the current transition may have a significant impact on the computational methodologies for combustor design. Hydrogen defines novel modeling issues and challenges the current state of art on numerical methodologies. The current study presents a numerical investigation of a hydrogen-air burner using Large-Eddy Simulations (LES) with a focus on NOx prediction. The considered configuration is a two-staged combustor, similar to the well-known RQL (Rich-Quench-Lean) technology, supplied by a single coaxial injector characterized experimentally. Two combustion models are investigated: i) tabulated chemistry based on premixed flamelets ii) transported chemistry description by using a 21-species chemical scheme. Numerical results are compared with experimental data (NOx concentrations, temperature distributions, pressure losses). A focus on models predictions is carried out. Results show a good agreement to predict the main characteristics over different operating points and geometries for both frameworks. In contrast, NOx emissions are more sensitive: while the overall trend is well captured, the quantification is more scattered. Finally, an in-depth analysis is proposed to link NOx production with the non-premixed flame size.