Extinction limits of premixed combustion assisted by catalytic reaction in a stagnation-point flow

Motivated by the potential use of catalytic materials in micro-combustor application, the primary scope of this study is to investigate the lean extinction characteristics of premixed flames in the presence of catalytic reaction. In particular, the effects of mixture dilution on the lean flammability limit are examined by adopting a stagnation-point flow system with a methane/air mixture over a platinum surface. For the reference fuel/air conditions, it was found that the lean flammability limit of homogeneous reaction is actually lower than that of the heterogeneous reaction. However, when sufficient amount of nitrogen is added to the reactant mixture, the surface reaction shows the benefit of extending the lean extinction limit to a lower value. It is also found that the magnitude of flammability extension by surface reaction is more pronounced in the presence of surface heat loss. Unsteady extinction behavior in response to equivalence ratio fluctuations shows an overall trend consistent with the steady results. These phenomena are explained as result of different characteristic time scale of homogeneous and heterogeneous reaction. Analytical calculation proved that under same reactant and thermal boundary condition, surface reaction has smaller time scale which is rather insensitive to dilution while gas phase reaction has larger one which changes more with different dilution magnitude.