Chemiluminescence of burner-stabilized premixed laminar flames

Abstract

The OH*, CH* and CO2* chemiluminescence signals of methane/air pre- mixed laminar flames stabilized over a nonadiabatic porous plug burner are compared to the signals measured from a nearly adiabatic conical flame in a series of experiments. The impact of reactant stream tem- perature is also characterized. A numerical study based on 1-D flame models then follows to support the experimental results. It is found both in experiments and in simulations that the linear relationship between the mixture flowrate and the chemiluminescence intensities is no longer valid when flames are closely attached to the burner surface due to the heat transfer between the flame and the burner. The transition between the linear and the nonlinear regimes is identified as the gas flow velocity drops below the adiabatic laminar burning velocity calculated at the bulk temperature of the flow leaving the burner. When the mass flow- rate is kept constant, preheating of the reactant stream increases the chemiluminescence intensity for a freely propagating flame, but has almost no impact for a burner-stabilized flame. It is finally found that the OH* and CH* chemiluminescence intensities correlate with the burnt gas temperature for the adiabatic but also the nonadiabatic flames. The underlying physical mechanisms are discussed. Finally, the evolution of the CH*/OH* ratio with the inlet gas velocity is discussed.