| Ammonium perchlorate (AP) is a common ingredient in rocket propellants. The burning rate of heterogeneous propellants which include particles of AP in a polymeric binder can be modified by changing the AP particle sizes. This has been postulated to be due to heat feedback from a complex flame structure which changes as a function of particle size.; A model has been developed which simulates combustion of an ammonium perchlorate (AP) particle surrounded by a homogeneous binder made up of fine particles of AP in HTPB (hydroxy-terminated polybutadiene). The model is spatially two dimensional, steady-state, and includes coupling between the liquid (condensed) and gas phases at the burning surface. The condensed phase is based on previously reported 1-D models and includes distributed decomposition and reaction kinetics. The gas phase includes detailed kinetics for the combustion of AP and HTPB. The large system of equations is solved using a non-linear Newton-GMRES method. The solution method involves parallelization by domain decomposition, and a steady-state solution is obtained via a time-marching method. A reduced mechanism was developed to speed the calculations.; The flame structure at 20 atm above a 400 mum AP particle was simulated. It was found to be qualitatively similar to the multi-flame structure proposed by Beckstead, Derr and Price (1970): an AP monopropellant flame exists above the particle; the area near the particle/binder interface had the highest heat feedback due to a hot primary diffusion flame; and the products from the near-surface flames react in a final diffusion flame. An additional premixed flame was seen above the homogenized AP/HTPB binder.; Changing the binder composition showed that the composite burning rate was controlled by the combustion of the fine AP in the binder. Decreasing the particle size increased the influence of the primary diffusion flame on the heat feedback to the surface. This increased the burning rate, consistent with experimental trends. The influence of increasing pressure was found to be consistent with observed trends. Reaction rate increased with pressure, the near-surface flames moved closer to the surface, and the primary diffusion flame's thickness decreased. |
