Asymptotic PN Approximation in Radiative Transfer Problems

We study the validity of the time-dependent asymptotic P-N approximation in radiative transfer of photons. The time-dependent asymptotic P-N is an approximation which uses the standard P-N equations with a closure that is based on the asymptotic solution of the exact Boltzmann equation for a homogeneous problem, in space and time. The asymptotic P-N approximation for radiative transfer requires careful treatment regarding the closure equation. Specifically, the mean number of particles that are emitted per collision (omega(eff)) can be larger than one due to inner or outer radiation sources and the coefficients of the closure must be extended for these cases. Our approximation is tested against a well-known radiative transfer benchmark. It yields excellent results, with almost correct particle velocity that controls the radiative heat-wave fronts.