Flame and surface structure of laminate propellants with coarse and fine ammonium perchlorate

The combustion of two-dimensional laminate propellants of ammonium perchlorate (AP) and hydroxyl-terminated polybutadiene is investigated experimentally and theoretically. The experiments use UV emission and transmission imaging to obtain simultaneous information about flame structure and burning-surface profile for pressures ranging from 2 to 55 atm. Previous experimental work for laminates with pure binder has been extended to cases including laminates with oxygenated binder. The modeling uses numerical computations based on finite-rate chemistry with simplified kinetics and a free surface. Results show that flame-surface structure is a function of length scale (in this case, fuel-layer thickness), pressure, and equivalence-ratio disparity between the nonpremixed fuel and the oxidizer regions (binder-matrix equivalence ratio). Factors that promote split (diffusion) flame-surface structure are large length scale, high pressure, and large equivalence-ratio disparity. The opposite factors (including oxygenating the binder) promote merged (premixed) flame-surface structure. For oxygenated binder loaded to the monomodal-AP limit (fine-AP/binder 76/24) the transition thickness between split and merged structure is 5 to 10 times larger than that for pure binder. A correlation is shown between this transition and the optimal thickness that maximizes regression rate (at a given pressure). It has been determined both computationally and experimentally through the use of triple-layer laminates that the flame-surface structure at the center of the laminate is relatively uninfluenced by the outer boundary conditions. This provides firm justification for using the simpler, single-fuel-layer laminates to validate computational simulations through characterizing the effects of pressure, thickness, and binder equivalence ratio on flame and burning-surface structure.