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

The use of Ammonia (NH 3 ) and blends with either Methane (CH 4 ) or Hydrogen (H 2 ) obtained by in-situ NH 3 cracking, seem to be promising solutions to partially or fully decarbonise our energy systems . To strengthen understanding of fundamental combustion characteristics of these NH 3 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 CH 4 or H 2 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 NH 3 /H 2 and NH 3 /CH 4 flame speed measurements. With respect to measured Markstein length, for a fixed equivalence ratio, addition of CH 4 to NH 3 resulted in a linear reduction in stretch sensitivity for the tested conditions. For lean NH 3 /H 2 flames, an initial decrease in Markstein length is observed up to 30–40% H 2 addition, at which point any further addition of H 2 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 NH 3 /CH 4 . Different theoretical relationships between the Markstein length and Lewis Number were explored alongside effective Lewis Number formulations. For lean NH 3 /H 2 mixtures, a diffusional based Lewis Number formulation yielded a favourable correlation, whilst a heat release model resulted in better agreement at richer conditions. For NH 3 /CH 4 mixtures, a volumetric based Lewis Number formulation displayed best agreement for all evaluated equivalence ratios. For NH 3 /H 2 , 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 CH 4 to NH 3 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 CH 4 and H 2 addition to NH 3 was undertaken. Augmented flame propagation of NH 3 /CH 4 and NH 3 /H 2 was demonstrated to be principally an Arrhenius effect, predominantly through the reduction of the associated activation energy .