Study of turbulence-radiation interaction in a methane flame

Abstract

Turbulence and radiation interacts to alter the flow field, temperature and species distributions, and hence, radiation intensity emitted from the flame. Especially, fluctuation of temperature and species mole fractions significantly alter the radiation from a turbulent flame, resulting in the decrease of flame temperature. To quantify the effects of turbulence-radiation interaction, radiative transfer equation can be time-averaged to isolate covariance terms derived from the emission and absorption terms, but the covariance term derived from the absorption term is not accurately modeled due to its need of multipoint correlations. Up to this day, stochastic method is considered the most accurate way to treat turbulence-radiation interaction, where mixture fraction fields along a radiation path are stochastically realized, followed by their conversion to temperature and species mass fraction fields using the state relationships. Then, radiative transfer equation is solved for each realization followed by its statistical analysis for the calculation of mean radiative intensity and TRI terms. In this research, 5 different radiation paths through a piloted $CH_4/air$ diffusion flame is stochastically simulated to accurately predict radiative intensity from a turbulent flame. Radiative intensity was calculated to be strongest from the diametric path and weaker as the path is moved away from the flame axis. However, the effect of turbulence-radiation interaction was stronger for paths away from the flame axis. Also, it was found that radiative intensity was enhanced mainly due to emission TRI which contributed to total mean radiative intensity by about 8 % in the diametric path and about 46 % in the chord-like path 32.4 mm away from the flame axis. On the other hand, the effect of absorption TRI was less than 4 % for all the radiation paths investigated. The radiative intensity and TRI effects are spectrally investigated, and it was found that TRI is strongest in $4.3 \mu m$ and $2.7 \mu m$ bands. Also, it was found that TRI effects being weakened near the flame axis was the reason for relatively weak TRI effect in the diametric path. In addition, the spectral investigation of $4.3 \mu m$ band revealed the spectrally non-uniform behavior of emission and absorption TRI terms, which is the reason for them being small or negative near the flame axis.