Effect of CO-to-H2 ratio on syngas flame behaviors in a micro flow reactor with controlled wall temperature gradient

Abstract

Flame characteristics of CO/H2/O2/N2 mixture (O2:N2 = 1:7) with different CO/H2 ratios in a micro-channel with a controlled wall temperature gradient were investigated through numerical simulation. Five flame regimes were identified and their global diagram was drawn. The influence of CO/H2 ratio on two typical flame dynamics (one is stable and the other is unstable), i.e., “coexisting upstream normal flame (UNF) and downstream weak flame (DWF)” and “uninterrupted flame with repetitive extinction and ignition (UFREI)”, were discussed. It is revealed that in the UNF-DWF regime, a higher H2 proportion can strengthen OH generation in the UNF through H2-OH and H-HO2-OH reaction paths, which causes the UNF to propagate upstream. However, the upstream lower wall temperature results in weaker near-wall reaction intensity for the UNF, which brings about more CO residual and thus enhances the DWF. For the UFREI regime, both the period and the propagation distance of flame dynamics are reduced with the augmentation of CO/H2 ratio because of the weakened flame intensity. Moreover, the characteristic flame positions are also affected by the CO/H2 ratio. In details, the farthest position of UFREI moves upstream while the transition position slightly shifts downstream as the CO/H2 ratio is increased, which is substantially affected by the transport of residual fuel after the extinction of UFREI. In summary, the present investigation unraveled the complicated influence of CO/H2 ratio on the syngas flame dynamics in a micro-channel with a controlled wall temperature gradient.