Radiative transfer calculations in fire simulations: An assessment of different gray gas models using the software FDS

Abstract

The accuracy of the gray gas (GG) assumption for modeling the radiative transfer in fire simulations is studied for a closed compartment, an open environment, and a compartment with an opening. The liquid fuels used for each case are ethanol, heptane and methanol, respectively. Three different GG formulations, implemented on the solver Fire Dynamics Simulator (FDS), are tested and compared to experimental data. Transient, large eddy simulations (LES) are carried out in FDS, where the radiative transfer is solved coupled to the fluid flow and combustion processes. For each fire configuration, the GG formulations are compared to each other and to experimental data. The mean radiative heat flux computed in the simulations showed a good correspondence to measurements, especially for the ethanol and heptane flames, with average deviations ranging between 7% and 21%, which indicates that the GG assumption is adequate even for these weakly-sooting fuels. Among the GG models tested, the default formulation employed by FDS and the one based on a weighted-sum-of-gray-gases estimation of the medium emittance performed similarly, both providing in general more accurate results than the approach using the Planck-mean absorption coefficient.