Diffusion flame studies of solid fuels with nitrous oxide

Counterflow diffusion flame experiments were conducted to investigate hydroxyl-terminated polybutadiene (HTPB) solid fuel combustion under varied pressure environments from 0.1 to 2.2 MPa using nitrous oxide (N₂ O) as the oxidizer. A numerical model was developed to analyze the flame structure and predict regression rates. Results show solid fuel regression rates to increase with pressure for a fixed oxidizer momentum flux. The flame structure thins due to faster kinetics and shifts toward the regressing fuel surface with increasing pressure. The diffusion flame is positioned on the oxidizer side of the stagnation plane, which also shifted toward the fuel surface with increasing pressure. Flame temperature increases with pressure as well, due to decreasing radical formation, increasing the surface temperature gradient, resulting in enhancement of solid fuel pyrolysis. Heat release from N₂ O decomposition and pyrolyzed fuel oxidation occurs in two distinct stages at atmospheric pressure, while at elevated pressure (1.83 MPa) the exothermic peak associated with oxidation is distributed over a spatial domain thinner than at 0.1 MPa, but contains many small regions of isolated exothermicities. The flame structure with N₂ O exhibits a similar structure as O₂-HTPB diffusion flames in the spatial regions where N₂ O was not present because of decomposition. Leakage of O₂ and NO into the fuel pyrolysis zone also decreases with increasing pressure. Predicted regression rates with N₂ O are approximately 34% lower than those with O₂. A comparison of counterflow fuel regression rates with subscale hybrid motor fuel regression rates are in good agreement when the rates are extrapolated based on oxidizer mass flux.