Effect of Flame Stabilization Regime on the NOX Emissions of a Hydrogen Multi Jet in Swirled Crossflow Burner

A multi-jet-in-crossflow burner operating with 100% hydrogen was investigated to understand how crossflow swirl intensity, equivalence ratio, and thermal power influence flame stabilization and NOx emissions.Using simultaneous OH- and acetone-PLIF diagnostics, three flame stabilization modes were identified. At high equivalence ratios, anchored jet flames form, which transition into anchored M-shaped flames as swirl increases or equivalence ratio decreases. Both anchored modes are associated with high thermal stress and are therefore undesirable. At sufficiently high airflow velocities, a detached, partially premixed M-shaped flame stabilizes, reducing thermal loads on the injector and offering more favorable operating conditions. NOx emissions and combustor exit temperatures were measured using FTIR spectrometry and thermocouples across thermal powers of 3-17 kW, equivalence ratios from 0.1 to 0.9, and swirl numbers up to 1.79. Results show that lower equivalence ratios and higher swirl intensities significantly reduce NOx emissions. A marked decrease in NOx occurs during the transition from jet flames to M-shaped flames, likely due to the formation of an internal recirculation zone (IRZ), while flame lifting itself has little effect. An effective swirl number, introduced in prior work, successfully characterizes the combined influence of swirl and equivalence ratio on NOx formation, enabling identification of an optimal operating window with low emissions and stable lifted flames. Additionally, a scaling law based on the residence time within the flame volume was assessed, showing a good agreement with the experimental dataset.