Impact of Soot Radiative Properties, Pressure and Soot Volume Fraction on Radiative Heat Transfer in Turbulent Sooty Flames

            The effect of soot radiation modelling, pressure and level of soot volume fraction are investigated in ethylene-air turbulent fames. A jet flame at atmospheric pressure is first studied before considering a confined pressurized flame, experimentally studied at DLR, at 3 bars. Both cases have previously been computed with large-eddy simulations coupled to thermal radiation. A tabulated chemistry formalism based on radiative Flamelet Progress Variable model for the gaseous phase description was retained with a lumped PAH transport equation and a relaxation model. A sectional method was employed to predict soot formation, growth and oxidation while describing the soot particle size distribution. The present study aims at determining and analyzing the thermal radiation field for different models from these numerical results.

             A Monte-Carlo solver based on the Emission Reciprocity Method is used to solve the radiative transfer equation with detailed gas and soot properties in both configurations.  The participating gases properties are described by an accurate narrow-band ck model. Emission, absorption and scattering from soot particles are accounted for. Two formulations of the soot refractive index are considered: a constant value and a wavelength formulation dependency. This is combined with three models for soot radiative properties: Rayleigh theory, Rayleigh-Debye-Gans theory or a state-of-the-art extension for fractal aggregates.

            Sensitivity of emission and absorption contributions to the soot properties model are analyzed while comparing gas and soot radiation. For both cases, the effect of scattering is observed to be small on the predicted thermal heat transfer which is consistent with the number of soot particles and size of the aggregates. The scattering effect is the lowest with the Rayleigh model, while with the RDG-FA model, the increase of optical thickness slightly enhances absorption by soot particles. 

            The impact of soot radiation is finally amplified by artificially increasing the soot-volume fraction values and/or pressure up to 30 bars to estimate the change in thermal radiation in various industrial conditions. The impact is characterized in terms of net, emission and absorption radiative power and spectral analysis to highlight soot-only and soot-gas radiative heat transfer interaction.