Flame propagation characteristics and inherent instability in premixed carbon-free hydrogen/ammonia/oxygen combustibles

Abstract

Hydrogen-ammonia fuel is an ideal clean energy, and its use in the combustion field is important for achieving energy transformation. This study carried out a series of flame propagation experiments of hydrogen/ammonia/oxygen mixtures and analyzed the instability phenomenon and the competition mechanism of inherent instability during flame propagation at different fuel ratios (0.5–2.0), equivalence ratios (0.5–1.5), and initial pressures (0.1 atm − 1.0 atm). The results show that the hydrogen/ammonia combustion in oxygen is extremely susceptible to instability. Only when the P₀ = 0.1 atm, does the flame surface remain smooth in the observable region. Crack initiation and development are observed on the flame surface at P₀ = 0.5 atm, and the flame propagated to a late stage with a sudden increase in velocity, which is due to the destabilization of the flame. The increase in fuel ratio gradually shifts the Lewis value away from 1, but does not change the state of thermal-diffusional factor on flame stability. The flame thickness reaches the minimum at φ = 0.8. As the initial pressure increases from 0.1 atm to 1.0 atm, the Lewis number remains almost constant, while the flame thickness, Markstein length, and critical instability radius all decrease rapidly. At the theoretical equivalence ratio and fuel ratio of 1.0, the flame thickness decreased rapidly from 0.143 mm to 0.015 mm, with a decrease of 89.5 %. The flame thickness for hydrogen/ammonia combustion in air is approximately five times that in oxygen for the same operating conditions, implying that the oxygen atmosphere greatly increases the hydrodynamic instability of the hydrogen/ammonia premixed flame. The critical Peclet number (Pe_cr) of the hydrogen/ammonia/oxygen flames rises exponentially with increasing Markstein number (Ma), and the empirical correlation can be expressed as Pe_cr = 166.11 × exp (Ma/1.04) + 47.86 (P₀ = 0.5 atm) and Pe_cr = 538.29 × exp (Ma/0.53) + 142.78 (P₀ = 1.0 atm).