Turbulent partially cracked ammonia/air premixed spherical flames

Abstract

The combustion of ammonia requires, for most energy conversion systems, a combustion promoter such as hydrogen to guarantee the start-up, stability and combustion efficiency. Partially cracked ammonia (PCA) can provide sufficient hydrogen concentrations to enhance the burning velocity in comparison with pure ammonia. However, little work exists on the use of PCA blends operating under relevant turbulent conditions. To that end the outwardly propagating spherical flame configuration was employed to determine the laminar and turbulent flame propagation characteristics of PCA (NH3/(H2+N2)) and corresponding binary (NH3/H2) mixtures across various turbulent combustion regimes. First, PCA and ammonia-hydrogen blends exhibit similar flame propagation rates under various turbulent intensities, even for the laminar case. The highest turbulent burning velocity was observed at leanest conditions, as opposed to laminar flames which exhibited highest flame speed at conditions above stoichiometry. Under rich conditions, no substantial flame enhancement due to turbulence was measured irrespective of the hydrogen content. This lack of flame enhancement under turbulent conditions is attributed to the effect of preferential diffusion with good agreement observed with trends in measured Markstein numbers. The normalized turbulent flame speed is dominated by the enhanced molecular diffusivity afforded by the presence of hydrogen up to 15 % enrichment, prior to decreasing upon further hydrogen addition under lean and stoichiometric conditions. This ‘bending’ phenomenon may be the contribution of several factors including; the transitioning between combustion regimes associated with low Damköhler numbers (Da) and flame thickening; merging of flamelets due to the presence of ammonia enhancing wrinkling; and combined changes in laminar burning velocity and preferential diffusional behavior. Furthermore, good agreement for turbulent flame speed is observed with a correlation that includes the influence of turbulent stretch (Ka) and non-equidiffusion (Le), with the agreement reducing with decreasing chemical to turbulent time scale ratios (Da << 1).