Studies of radiation absorption on flame speed and flammability limit of CO₂ diluted methane flames at elevated pressures

The effects of spectral radiation absorption on the flame speed at normal and elevated pressures were experimentally and numerically investigated using the CO₂ diluted outwardly propagating CH₄-O₂-He flames. Experimentally, the laminar burning velocities of CH₄-O ₂-He-CO₂ mixtures at both normal and elevated pressures (up to 5 atm) were measured by using a pressure-release type spherical bomb. The results showed that radiation absorption with CO₂ addition increases the flame speed and extends the flammability limit. In addition, it was also shown that the increase of pressure augments the effect of radiation absorption. Computationally, a fitted statistical narrow-band correlated-k (FSNB-CK) model was developed and validated for accurate radiation prediction in spherical geometry. This new radiation scheme was integrated to the compressible flow solver developed to simulate outwardly propagating spherical flames. The comparison between experiment and computation showed a very good agreement. The results showed that the flame geometry have a significant impact on radiation absorption and that the one-dimensional planar radiation model was not valid for the computation of the flame speed of a spherical flame. An effective Boltzmann number is extracted from numerical simulation. Furthermore, the FSNB-CK model was compared with the grey band SNB model. It was shown that the grey band SNB model over-predicts the radiation absorption. It is concluded that quantitative prediction of flame speed and flammability limit of CO2 diluted flame requires accurate spectral dependent radiation model.