Influence of hydrogen and methane addition in laminar ammonia premixed flame on burning velocity, Lewis number and Markstein length

Abstract

The use of Ammonia (NH3) and blends with either Methane (CH4) or Hydrogen (H2) obtained by in-situ NH3 cracking, seem to be promising solutions to partially or fully decarbonise our energy systems. To strengthen understanding of fundamental combustion characteristics of these NH3 blends, the outwardly propagating spherical flame configuration was employed to determine the flame speeds and Markstein lengths. The air/fuel mixtures were varied across a large range of compositions and equivalence ratios. In general addition of CH4 or H2 results in a linear and exponential increase in measured laminar burning velocity, respectively. Of the appraised mechanisms, Stagni and Okafor kinetics mechanisms yielded best agreement with NH3/H2 and NH3/CH4 flame speed measurements. With respect to measured Markstein length, for a fixed equivalence ratio, addition of CH4 to NH3 resulted in a linear reduction in stretch sensitivity for the tested conditions. For lean NH3/H2 flames, an initial decrease in Markstein length is observed up to 30–40% H2 addition, at which point any further addition of H2 results in an increase in Markstein Length, with a non-linear behaviour accentuated as conditions get leaner. Above stoichiometry similar stretch behaviour is observed to that of NH3/CH4. Different theoretical relationships between the Markstein length and Lewis Number were explored alongside effective Lewis Number formulations. For lean NH3/H2 mixtures, a diffusional based Lewis Number formulation yielded a favourable correlation, whilst a heat release model resulted in better agreement at richer conditions. For NH3/CH4 mixtures, a volumetric based Lewis Number formulation displayed best agreement for all evaluated equivalence ratios. For NH3/H2, changes in measured Markstein Length were demonstrated to potentially be the result of competing hydrodynamic and thermo-diffusive instabilities, with the influence of the thermo-diffusional instabilities reducing as the equivalence ratio increases. On the other hand, the addition of CH4 to NH3 results in the propensity of moderating hydrodynamic instabilities, resulting in a stabilising influence on the flame, reflected by increasing positive Markstein number values. Finally, a systematic analysis of the flame speed enhancements effects (kinetic, thermal, diffusive) of CH4 and H2 addition to NH3 was undertaken. Augmented flame propagation of NH3/CH4 and NH3/H2 was demonstrated to be principally an Arrhenius effect, predominantly through the reduction of the associated activation energy.