Unsteady thermal and chemical response of a porous media burner to fuel supply interruption: application to carbon-free NH₃/H₂/air flames

Constructed from open-cell ceramic foams, porous media burners (PMBs) excel at stabilizing flames with poor combustion properties and low pollutant emissions, for applications such as jet engines, process heaters... PMBs rely on heat recirculation through the solid ceramic matrix to enhance thermal diffusion and preheat reactants. Another advantageous feature of PMBs is their ability to store heat in the ceramic matrix: this thermal inertia makes PMBs resilient to disturbances in the fuel supply. We study experimentally relight, the limit case of such disturbances: we interrupt for a few seconds the fuel supply to a PMB. The flame extinguishes, the burner cools down. Upon reintroduction of the fuel, the combustible mixture may reignite if the ceramic foam remains sufficiently hot, and the flame will return to its steady state after a short transient. The fuel of interest is a carbon-free blend of NH₃/H₂, whose ignition characteristics can be easily varied by adjusting the NH₃/H₂ ratio. We examine the impact of fuel composition, equivalence ratio and mass flux on the dynamical response of the PMB and propose a simple model to characterize both the thermal transient following extinction, and the subsequent propensity for re-ignition of the burner.