Structure and Propagation Characteristics of Turbulent Premixed Ammonia-Air Flames

To obtain fundamental insights into the propagating behaviour of turbulent ammonia-air premixed flames, direct numerical simulations are carried out with complex chemistry for the flame-in-a-box configuration. The study compares the turbulent flame speeds of different mixtures at the same location in the Borghi–Peters diagram, including lean (equivalence ratio, ϕ= 0.81) and rich (ϕ= 1.2) NH 3 -air, lean (ϕ= 0.81) NH 3 -H 2 -N 2 -air and lean (ϕ= 0.41) H 2 -air, as well as their respective equidiffusive counterparts. It is found that the lean NH 3 -H 2 -N 2 -air and H 2 -air mixtures have a higher level of turbulent flame speed enhancement than the lean and rich NH 3 -air flames. While the effect of the diffusive-thermal instability on mean turbulent flame speed is minor for NH 3 -air flames, it is notable for the hydrogen-containing ones. The location of the heat release peak and effective root-mean-square turbulent velocity (u′) at the flame front are also found to influence the different normalized flame speeds for different fuels with similar nominal turbulence parameters. For the NH 3 -air flames the burning rate of the lean one is higher than that of the rich one, mainly because the effective u′ is larger for the lean flame, leading to greater flame front wrinkling. The impact of different turbulence conditions (Karlovitz number 30–557 and turbulent Reynolds number 36–386) on the behaviour of rich ammonia-air flames is also investigated, finding that the level of turbulent flame speed enhancement is closely linked to the size of the most energetic turbulent eddies. Additionally, the flame structure and the effect of the Lewis number are also examined, concluding that the latter is more pronounced in flames subjected to turbulence with a larger integral scale. The size of the integral length scale is a key factor in determining the level of flame wrinkling and distortion of the preheated zone, although the preheated zone is also affected in flames with a high Karlovitz number.