Synergistic Effects of Fuel Stratification and Preferential Diffusion for Ultra-Low NOx Formation in Hydrogen Flames

The unique transport properties of hydrogen give rise to fuel re-stratification and super-adiabatic flame temperature (SAFT) in premixed flames—phenomena that are generally not absent in fossil-fuel combustion. These effects undermine the effectiveness of conventional fuel/air fully premixed technology for achieving low NO x emissions in hydrogen energy and propulsion systems. In this study, a scaling law for SAFT, incorporating the Lewis number ( Le ) and Zel’dovich number ( Ze ), is revisited for fully premixed hydrogen laminar flames under flow straining conditions. The impact of SAFT on NO x formation, with particular emphasis on thermal NO x governed by the Zel’dovich mechanism, is systematically analyzed. A theoretical expression is derived to estimate the thermal NO x reaction rate as a function of SAFT. With these new insights, a novel concept based on the synergistic effects of fuel stratification and preferential diffusion is proposed to achieve ultra-low NO x emissions in hydrogen combustion. The effectiveness of this concept is demonstrated in multi-slot flames, where a designed nonuniform equivalence ratio ( ϕ ) at the inlet configuration reduces peak temperature by 53–236 K and NO emissions by 15%–54% over the typical gas-turbine operating range of 0.4 < ϕ < 0.7 , compared with a fully premixed inlet configuration. This counter-intuitive approach provides new insights into hydrogen flame control and offers a promising pathway to achieving ultra-low NO x emissions in hydrogen energy and propulsion systems.