The radiation heat-transfer model assumes that the radiation transport is through "optically thin" media. In the optically thin limit, the local gas does not re-absorb radiation emitted from other parts of the gas, such that the radiation does not need to be considered as a separate source of energy in the energy balance. The radiation heat loss, then, is due to exchange between the gas and the surroundings and between the particles and the surroundings. The optically-thin model is computationally efficient and allows quick assessment of the effects of radiation heat loss on flame structure and emissions.

The radiation model is provided as an option to calculate radiation heat loss from gas and particulate matter in unconfined, lightly sooting flames, for all Ansys Chemkin Flame simulators, including flat-flame burner and opposed-jet flow configurations.

The optically-thin radiation heat loss from a mixture of gas and particulates is given as

(13–14)

where is the Stefan-Boltzmann constant, is the gas temperature, is the ambient temperature, is the gas pressure, is the mole fraction of species , is the Planck mean absorption coefficient for species , and is the mean absorption coefficient for the particles.

Equation 13–14 assumes that locally gas and particles locally have the same temperature, since the energy balance of the particulate phase is not solved. The mean absorption coefficients of gas species and particles are treated as thermodynamic properties and are therefore required inputs to the radiation model.

Detailed information on how gas-phase radiation absorption coefficient data should be provided to the model is also described in Gas Species Radiation Absorption Coefficients in the Chemkin Input Manual Input Manual. Handling of particulate absorption is described below. The input parameter controlling particulate absorption behavior is also described in the Chemkin Input Manual Input Manual.