Onset of cellular instability and self-acceleration propagation of syngas spherically expanding flames at elevated pressures

Systematic study on the onset of cellular instability and self-acceleration propagation for syngas spherically expanding flames is investigated. Three onsets of crack branching, uniform cellularity and transition acceleration are obtained from Schlieren images and S_b-κcurves. Effective Lewis number Le_eff and thermal expansion ratio σ are controlled independently by adjusting equivalence ratio and oxygen-nitrogen ratio. Results show that the order of three onsets is crack branching < transition acceleration < uniform cellularity. The critical flame radius is increased with Le_eff but decreased with σ. Also, it is decreased with hydrogen volumetric fraction, leading to a much earlier onset at 85% hydrogen fraction. The theoretical result can give a qualitative prediction and closer to uniform cellularity onset quantitatively, although it overestimates Le_eff effects a little. The critical Peclet number of uniform cellularity onset is not only increased with Le_eff but also increased with σ, mainly due to the variation of flame thickness. Besides, there is a positive correlation between critical Peclet number and Markstein number, indicating a strengthened inhibition of stretch. The onset of similarity acceleration is also obtained through acceleration exponent and it performs a similar variation trend as that of cellular instability. All of these demonstrate that both cellular instability and similarity acceleration are the coupling results of diffusional-thermal instability, hydrodynamic instability and stretch.