Limit phenomena and kinetic modeling of ammonia fuels in an unstrained diffusion flame burner

A unique experimental facility that can produce nominally unstrained one-dimensional (1D) diffusion flames is used to study the limit phenomena of flame stability and extinction of ammonia (NH₃) blended with varying levels of hydrogen (H₃), methane (CH₄), methanol (CH₃OH). Diffusive–thermal instabilities, including cellular-pulsating flames, are observed prior to extinction. The absence of significant parasitic hydrodynamic effects, omnipresent in common research burners, enables the characterization of the fundamental properties of the instabilities across a wide range of experimental conditions. Extinction is attained by increasing the fraction of inert species (CO₂ or N₃) in the fuel. For all fuel blends, increasing the ammonia fraction is shown to favor extinction, and insights are provided via sensitivity and reaction pathway analyses. The finite-rate chemistry is leveraged to compare 11 thermochemical reaction mechanisms through measured extinction limits. Noticeable discrepancies with the experimental data and among the predictions from the mechanisms are observed.

Novelty and significance statement: Existing experimental data on the limit phenomena of ammonia flames are largely focused on premixed systems, and limited in fuel composition. In this paper, an unstrained diffusion flame burner is used to study more than 70 mixtures of ammonia blended with hydrogen, methane, methanol and ethanol, spanning ammonia fractions from 0 to 100%. Extinction limits are presented, which are of direct relevance to the ongoing efforts to design efficient low-carbon engines, given the inherently low reactivity of ammonia. These measurements also establish a new experimental benchmark for improving ammonia reaction mechanisms, as significant discrepancies are observed among 11 commonly used mechanisms when evaluated in this flame configuration for the first time. Diffusive–thermal flame instabilities, which are typically obscured by parasitic hydrodynamic effects in conventional research burners, are observed prior to extinction. These measurements provide rare experimental access to flame stability behavior in a configuration directly compatible with theoretical flame stability models.