Effects of fuel-air unmixedness on flame stability and emissions in rapidly mixed hydrogen-enriched swirling flames

Abstract

The transition to hydrogen-enriched combustion is fundamentally constrained by the challenge of simultaneously ensuring safety, flame stability, and low NO x emissions. This is because hydrogen’s high burning velocity increases flashback susceptibility, while its high reactivity and diffusivity make combustion more sensitive to local fuel–air mixing variations, which can generate localized high-temperature regions and promote thermal NO formation. Rapidly mixed combustion offers a promising strategy for addressing these competing constraints by distributing the reaction zone and heat release. However, the effects of fuel–air unmixedness on pollutant formation and combustion stability in hydrogen-enriched rapidly mixed flames remain insufficiently understood. To fill this gap, this study experimentally and numerically investigates how fuel–air mixing quality influences flame stability and NO x formation in hydrogen-rich rapidly mixed flames. Flame structure, lean blow-off limits, and emissions (NO x , CO) are measured and complemented by chemical reactor network modeling with a Beta-PDF representation of fuel–air unmixedness (μ). The roles of equivalence ratio (Φ), residence time, and inlet temperature are analyzed to elucidate mixing-chemical interactions. Results show that rapidly mixed flames improve lean blow-off stability by maintaining distributed combustion through local fuel-rich regions under globally lean conditions, while reducing the intensity of localized hot spots observed in diffusion flames. NO x emissions increase nonlinearly with μ, exhibiting weak sensitivity at low Φ but becoming strongly pronounced at Φ ≥ 0.6 due to intensified coupling between mixture stratification and thermal NO formation. Chemical-mixing timescale analysis indicates that the sensitivity of NO x to μ increases as the system transitions into a chemistry-dominated regime, i.e., Damköhler number ( Da ) > 1. Under such conditions, mixture inhomogeneity can persist into the reaction zone, promoting localized high-temperature regions and consequently enhancing NO formation. This explains the greater sensitivity of hydrogen-rich flames to μ. Residence time and inlet temperature dominate NO x formation by controlling reaction exposure and thermal intensity, respectively, while Φ sets the instantaneous formation level. Unmixedness acts as a regime-dependent amplifier of thermal NO x formation, whose influence becomes significant beyond the Da ≈1 transition regime.