The work reported here simplifies computing the local Planck-mean absorption coefficient in nonluminous flames as a function of the mixture fraction and fuel composition. Equilibrium is assumed for fuel/air mixtures up to the point where carbon is predicted to condense, beyond which the gaseous products are assumed to be frozen and to be diluted with cold fuel. The resulting algebraic expressions are suitable for inclusion in any turbulent or laminar diffusion flame model predicated on single-step chemistry. The method accounts for the nongray nature of the gaseous combustion products and their variation in concentration and temperature with mixture fraction, at a computational penalty little more than that for estimating variable fluid properties. Nonluminous flames (in air) of H2, CO, CH3OH, CH4 and lean regions of fuels with the general form CxHyOz can be modeled satisfactorily. The effects of pressure-pathlength and heat loss on the absorption coefficient are addressed.

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