Experimental and numerical studies of the hydrogen ratio and initial temperature on the instability of spherically propagating NH3/O2/N2 premixed flames at elevated pressure

Ammonia gas is a renewable fuel with a high octane number and a better fossil fuel alternative. This study investigates the effects of hydrogen ratio (XH2) and initial temperature (Tu) on NH3/O2/N2 flame instability by the spherical expansion flame method under initial pressure (Pu) of 2 atm. Additionally, the experimentally obtained laminar burning velocity (LBV) is measured. The flame instability is evaluated through the dimensionless growth rate (DGR) of perturbation and equivalent cell radius. The findings indicate that the LBV can reach up to 35 cm/s at Tu = 500 K. At Φ = 1, the critical Karlovitz number (Kac) at Tu = 500 K is higher compared to that under XH2 = 0.4, with a value of 0.045. The linear stability theory indicates that at XH2 = 0.1, the DGR is −0.4, with the thermal diffusion effect stabilizing the flame. In contrast, at XH2 = 0.4, the DGR is 0.5, and the flame instability is primarily caused by hydrodynamic instability. A quantitative analysis of the cells indicates that the equivalent cell radius decreased from 4 mm at low hydrogen ratio to 2 mm at high hydrogen ratio. These results provide theoretical guidance for the practical application of ammonia combustion.