A Moment-based, Scale-bridging Algorithm for Neutral Particle Transport Problems

20 The Boltzmann transport equation describes the statistical behavior of particles that are interacting with surrounding media. The transport equation is used to simulate neutral particle physics in many physical systems–for example, combustion systems, nuclear power reactors, and internal confinement fusion. Unfortunately, many iterative schemes, based on an explicit scattering and re-emission source iteration, suffer from slow convergence in an optically thick region, where particles undergo a large number of scattering and/or absorption re-emission events. In such regions, a continuum description of the transport equation with an approximate closure (e.g., Fick's law of diffusion) can adequately represent the particle behaviors. On the other hand, diffusion and other approximated closure equations become invalid in an optically thin region due to the presence of significant transport effects. For computational efficiency it is natural to seek a (geometric) hybrid transport-diffusion algorithm [1,2].