R.R. Dobbins, R.J. Hall, S. Cao, B.A.V. Bennett, M.B. Colket, and M.D. Smooke,

Combustion Science and Technology, 187, 230–248, 2015

Abstract

Thermal radiation from laminar, sooting, coflow diffusion flames at atmospheric pressure has been studied computationally as a function of ethylene fuel dilution by nitrogen. These flames had previously been investigated using laser diagnostics and thermocouple-gas sampling probe techniques, and the measured soot levels had been satisfactorily predicted by a sectional soot kinetics model. In this work, the discrete ordinates method for solution of the equation of radiative transfer in axisymmetric cylindrical coordinates has been coupled to the flow's energy conservation equation through the calculated divergence of the net radiative flux. Two self-consistent models for the absorption/emission of radiation by soot, CO₂, H₂O, CO, and ethylene were considered: the Planck mean model and one based on narrowband, wavelength-dependent absorption. The wavelength-dependent calculation was found to predict much more substantial reabsorption effects; we conclude that the Planck mean model inadequately characterizes reabsorption for these sooting flames. Reabsorption of radiation has been found to be important for accurate prediction of overall radiated power even in relatively small, atmospheric pressure flames with modest soot levels, and carbon dioxide was found to play a dominant role in both the emission and absorption spectra. In addition, the role of long wavelength radiation, absorption by the ethylene fuel, the spatial distribution of net radiation within these diffusion flames, and the impact of gridding upon the results are discussed.