Hydrogen Combustor Simulations Identify Flame Bifurcation Threshold at Equivalence Ratio 0.895 with Power-Law NOx Scaling of 4.92

The study employed curvature-corrected SST turbulence modeling, hybrid finite-rate and eddy-dissipation chemistry, and discrete ordinates radiation on a workstation-scale mesh validated to ASME standards against laser diagnostics. Equivalence ratios from 0.55 to 1.00 were swept to map operability limits for neat hydrogen, revealing that advancing throttle triggers the bifurcation while maintaining flashback margins. This methodology enables rapid screening of zero-carbon combustor designs without full-scale rig testing.

Nitric oxide formation shifts from N2O-intermediate pathways at idle (28 ppm, EINOx 1.85 g/kg) to thermal Zeldovich dominance at takeoff (319 ppm, EINOx 35.4 g/kg), following a power-law exponent of 4.92. The exponent exceeds typical hydrocarbon values, highlighting hydrogen's steeper emission sensitivity to temperature. These results quantify the narrow window between lean stability and NOx compliance that engine designers must navigate.

Related experimental work on swirl-stabilized hydrogen flames (e.g., studies at DLR and Georgia Tech) shows similar M-to-V transitions but lacks the integrated flashback-NOx scaling reported here. The present simulations close that gap by coupling radiation and detailed kinetics, yet omit acoustic coupling that can alter flashback in real engines. Validation against full-annular tests will be required before certification pathways open.

Next steps include embedding these bifurcation maps into combustor design tools to accelerate certification of hydrogen turbines, potentially enabling 10-15% efficiency gains in narrow-body aircraft within three years if flashback margins hold under transient maneuvers.