Ammonia/hydrogen spherically expanding flame: Propagation behavior and combustion instability

Experimental and theoretical investigations were performed on the propagation behavior of ammonia/hydrogen premixed flames, and an in-depth theoretical analysis of flame instability was carried out. The laminar burning velocity (LBV), burning flux, and net heat release rate (HRRnet) were calculated. The instability was measured by flame thickness, thermal expansion ratio, effective Lewis number, and perturbation dimensionless growth rate. The effects of equivalence ratios (0.8, 1.0, 1.2, and 1.4), initial pressures (1, 2, and 3 bar), and hydrogen additions (15, 20, 25, and 30%) on ammonia/hydrogen premixed flames were determined. The findings indicated that as the hydrogen addition rises, both the LBV and burning flux increase, and the peak HRRnet rises and shifts towards the low-temperature reaction zone, indicating that the diffusivity and reactivity of hydrogen can enhance the combustion intensity. The increment in the equivalence ratio improves the flame stability. Hydrodynamic instability effect always destabilizes the flame during the combustion process. In rich mixtures, the hydrogen addition improves thermal-diffusion stability, delaying the occurrence of flame instability whereas in lean mixtures, the hydrogen addition thermal-diffusionally destabilizes the flame.