Numerical Investigation on Flame Behaviors of NH3/H2/air Mixtures in a Micro-Flow Reactor with a Controlled Wall Temperature Profile

Flame behaviors of stoichiometric NH3/H2/air mixtures (NH3:H2 = 1:1) in a micro-flow reactor (MFR) with a controlled wall temperature distribution were numerically examined. Three flame regimes were identified at high, medium and low inlet velocities, respectively, i.e., “coexistence of an upstream normal flame and a downstream weak flame (UNF+DWF)”, “flame with repetitive extinction and ignition (FREI) accompanied by triple bifurcations”, and “single weak flame (WF)”. It was revealed that the laminar burning velocity of NH3 is increased by H2 blending, and thus the UNF of NH3/H2 blends can be stabilized farther upstream with lower wall temperature compared to that of pure NH3 flame. This leads to incomplete consumptions of H2, NO, etc., and these species react downstream of the channel with a higher wall temperature to form the DWF. Another reason for the existence of DWF is that it is located sufficiently far from the UNF. The reasons for the triple bifurcations within one cycle of FREI are as follows. A normal flame is generated through the first bifurcation, which is due to the residual H2 and NO from the incomplete combustion of both current FREI and flames of the previous period. A weak flame is formed by the second bifurcation, which is attributed to leaked H2, NO, and HNO from the FREI. After the extinguishment of FREI, large amounts of H2, NH3 and O2 diffuse downstream and the third bifurcated flame (normal flame) is formed. In conclusion, the present study discovered and interpreted the complicated flame dynamics of NH3/H2 blends in an MFR with a wall temperature gradient.