A theoretical analysis of the influence of turbulence on radiative emission in turbulent diffusion flames of methane

A theoretical analysis is reported to quantify the increase of radiative emission due to turbulence for methane diffusion flames burning in air. The instantaneous thermochemical state of the reactive mixture is described by a flamelet model and a detailed chemical mechanism. Mean values of the absorption coefficient, blackbody radiation intensity and radiative emission are evaluated for different turbulence levels by assuming the pdf shape of mixture fraction. The results show that turbulent fluctuations generally contribute to reduce the Planck mean absorption coefficient of the medium, in contrast with the blackbody radiation intensity, which is significantly increased by turbulence. If the turbulence level is relatively small, the influence of turbulence on the absorption coefficient is marginal. Otherwise, fluctuations of the absorption coefficient of the medium should be taken into account. The mean radiative emission is underestimated if turbulent fluctuations are fully ignored and overestimated if only temperature fluctuations are considered, while neglecting fluctuations of the absorption coefficient of the medium, the error being generally higher in the latter case. The effects of turbulence on radiative emission are stronger in the fuel-lean region and close to stoichiometric conditions than in the fuel-rich region.