Effects of radiative emission and absorption on the propagation and extinction of premixed gas flames

Premixed gas flames in mixtures of CH₄, O₂, N ₂ , and CO₂ were studied numerically using detailed chemical and radiative emission-absorption models to establish the conditions for which radiatively induced extinction limits may exist independent of the system dimensions. It was found that reabsorption of emitted radiation led to substantially higher burning velocities and wider extinction limits than calculations using optically thin radiation models, particularly when CO ₂, a strong absorber, is present in the unburned gas. Two heat loss mechanisms that lead to flammability limits even with reabsorption were identified. One is that for dry hydrocarbon-air mixtures, because of the differences in the absorption spectra of H₂O and CO₂, most of the radiation from product H₂O that is emitted in the upstream direction cannot be absorbed by the reactants. The second is that the emission spectrum of CO₂ is broader at flame temperatures than ambient temperature; thus, some radiation emitted near the flame front cannot be absorbed by the reactants even when they are seeded with CO₂. Via both mechanisms, some net upstream heat loss due to radiation will always occur, leading to extinction of sufficiently weak mixtures. Downstream loss has practically no influence. Comparison with experiment demonstrates the importance of reabsorption in CO₂-diluted mixtures. It is concluded that fundamental flammability limits can exist due to radiative heat loss, but these limits are strongly dependent on the emission-absorption spectra of the reactant and product gases and their temperature dependence and cannot be predicted using gray-gas or optically thin model parameters. Applications to practical flames at high pressure, in large combustion chambers, and with exhaust-gas or flue-gas recirculation are discussed.