Spectral Gas Absorption Coefficient Model Effects on Radiative Source Term in a 2D Axisymmetric Diffusion Flame

A calculation of the radiative source term in combustion processes is an important part of the simulation process, because high temperatures are involved and the coupling of radiation to chemistry affects the overall flame characteristics. While relatively simple gas absorption coefficient models have been used in the recent past, it is becoming clearer that more accurate gas models alter the distribution of radiative sources in the flame. To accurately evaluate the radiative losses, it is necessary to use gas models in which the gas absorption coefficient is wavelength dependent. Such analyses can be computationally expensive depending on the particular treatment of the spectral dependence. It is important to understand the relative costs and benefits of different treatment of these effects. In this work, the divergence of the radiative heat flux is calculated for a two-dimensional cylindrical axisymmetric chamber using four different models: a simple gray gas model, the weighted-sum-of-gray-gases (WSGG) model, the spectral line-based weighted-sum-of-gray-gases (SLW) model, and the cumulative wavenumber (CW) model. The gray gas model and the WSGG model are widely used in recent studies and in most commercial software, because they are simple to implement and provide fast results. In general, however, they are not able to accurately predict the radiative losses. On the other hand, the SLW and CW models detail the variations of the absorption coefficient with the wavelength, and can give more accurate answers for the radiative source term, but require bigger computational effort. The divergence of the radiative flux predictions are compared with these four models, using temperature and concentration fields obtained from previous numerical simulations. The overall differences in radiation properties and in the overall cost of computations are detailed.