In-depth radiation absorption is one of the main mechanisms for liquid fuels evaporation in fires. The common approach in modeling of thermal radiation for liquids is applying the constant absorption coefficient (i.e., gray method) in solving the radiative transfer equation (RTE) and same reflectivity values at different sides of the interfaces. This approach neglects the spectral nature of the absorption coefficient and redistribution of the penetrated thermal radiation at the interfaces. To consider the spectral nature, we previously presented a novel gray modeling that accounts the absorption spectra. This physics-based gray method applies different values for the absorption coefficient based on the flame temperature and depth from the sample surface. For the interface effect, equivalent reflectivity is introduced at different sides of the interface applying the Fresnel relation. The new method was implemented into Fire Dynamics Simulator (FDS) that is an opensource CFD software for fire-driven flows. Simulations were done using 1040 cores on a supercomputer applying around 5.8 million grids for a 10×7×5 m³ room with two sizes of the heptane pools. Results showed that new approach could predict the measured burning rates for heptane pools. At the initial stage of the burning, the common approach underestimates the burning rate, while at the last stage it overestimates the burning rate compared to the new approach. The new approach brings the detailed radiation physics into the account and solves the issue of applying unrealistic values for the absorption coefficient and reflectivity in fire simulations.