On the diffusion approximation of the stationary radiative transfer equation with absorption and emission

TL;DR

This paper rigorously analyzes the diffusion approximation of the stationary radiative transfer equation with absorption and emission, showing convergence to an elliptic boundary value problem in the grey approximation without scattering.

Contribution

It provides a formal derivation and rigorous proof of the diffusion limit for the radiative transfer equation under the grey approximation, including boundary condition characterization.

Findings

The radiative energy converges to a solution of an elliptic PDE as the mean free path tends to zero.

The boundary conditions for the limit problem are explicitly derived using asymptotic expansions.

Maximum principle arguments are used to establish the convergence and properties of the solution.

Abstract

We study the situation in which the distribution of temperature a body is due to its interaction with radiation. We consider the boundary value problem for the stationary radiative transfer equation under the assumption of the local thermodynamic equilibrium. We study the diffusion equilibrium approximation in the absence of scattering. We consider absorption coefficient independent of the frequency $ \nu $ (the so-called Grey approximation) and the limit when the photons' mean free path tends to zero, i.e. the absorption coefficient tends to infinity. We show that the densitive of radiative energy $ u $, which is proportional to the fourth power of the temperature due to the Stefan-Boltzmann law, solves in the limit an elliptic equation where the boundary value can be determined uniquely in terms of the original boundary condition. We derive formally with the method of matched asymptotic expansions the boundary condition for the limit problem and we prove rigorously the convergence to the solution of the limit problem with a careful analysis of some non-local integral operators. The method developed here allows to prove all the results using only maximum principle arguments.