Infrared imaging of AP/HTPB laminate propellant flames

The combustion of ammonium perchlorate (AP), hydroxyl-terminated polybutadiene (HTPB) two-dimensional laminate propellants is investigated using simultaneous infrared (IR) and ultraviolet (UV) emission imaging at 1–50 atm. The IR (3300–3900 nm) measurements sense flame presence nominally via HCl rovibrational excitation and the UV (310 nm) measurements via OH electronic excitation, although continuum emission is also present. The following findings are made. (1) The new IR results generally confirm the flame-structure regime map (split vs merged flames) obtained previously from UV observations for both pure and oxygenated fuel-binders. Here “merged” refers to lateral merging of the two diffusion flame branches (one for each fuel–AP interface) as pressure and fuel-layer thickness are reduced. (2) A new finding (not seen previously in the UV) is that for pure binder systems at relatively low pressures (<15 atm)(<15 atm), IR imaging clearly shows two stream-wise distinct diffusion flames, which are possibly the primary diffusion flame (PDF) and final diffusion flame (FDF) structures postulated in the BDP model. (3) As pressure increases the PDF and FDF flames observed for pure binder merge stream-wise and become indistinguishable. (4) Oxygenated fuel-binder (75:25 fine-AP/HTPB-binder) laminates do not show distinct PDF and FDF flames. (5) The IR imaging technique improves on the UV in terms of surface definition because of the differences in IR versus UV absorption/emission properties of fuel-rich gaseous species near the burning surface: oxygenated binder surfaces that appear protruding in UV in the split-flame regime are less so in IR. (6) Computational results using just the AP-premixed and final diffusion flames capture the flame-surface structure regime map qualitatively but not quantitatively, further underscoring the importance of the primary diffusion flame.