In situ flame structure imaging of composite propellants using high-speed planar laser-induced fluorescence
Ammonium perchlorate (AP) is the most commonly used oxidizer in solid rocket propellants due to its availability, high oxygen balance, and combustion characteristics. Models of AP composite propellants have been made since the 1950s and have become highly advanced in recent years. However, experimental data have not kept pace, and the data required to validate models has lagged behind the models themselves. Recently, high-speed OH planar laser-induced fluorescence (PLIF) imaging has been applied to AP composite propellants to determine how microscale propellant flame structure varies with propellant formulation and pressure. Propellants with monomodal AP particle size distributions, changing coarse-to-fine AP particle size ratios, and different sizes and locations of burning rate catalysts have been investigated to determine the effect of propellant formulation on burning rate. It is found that AP particle size, propellant formulation, and pressure have a definite effect on propellant flame structure and burning rate. All propellants with AP particles below about 150 μm display similar flame structures for the pressures investigated (0.1-0.7 MPa). For propellants with AP particles larger than about 150 μm, all propellants burning at 1 atm display jet-like flames above individual coarse AP crystals. If the coarse AP concentration is high enough, group diffusion flames are seen where many coarse AP particles burn with one diffusion flame. At elevated pressures lifted arched diffusion flames are often seen; however, the circumstances under which the lifted flames develop depend on the propellant formulation. Burning rate was seen to increase as the average AP particle size decreased, and vice-versa. ^ Flame structures were also investigated for some propellants where the coarse AP was replaced with different oxidizers: ammonium dinitramide (ADN) or ammonium nitrate (AN). Though the flame structures above the AN-based propellants shared some similarities with AP-based propellants, diffusion flames were not in general seen close to the propellant surface at 1 atm. Instead, particularly for the ADN-based propellants, diffusion flames were lifted above the surface with a markedly different flame structure than those seen above the AP composite propellants. For the AN- and ADN-based propellants it is observed that the flame structure does not have as large of an effect on burning rate as AP-based propellants. This appears to be due to the lifted nature of the flames that have a wide dark zone immediately above the propellant surface, and exothermic condensed phase reactions in the case of the ADN-based propellants. ^ It is hoped that the data from these experiments will prove to be valuable in the validations of current computer models. Modelers desire to create high-fidelity computer models to simulate burning rocket propellants, and much progress has been made in recent years; however, relatively little is known about the actual flame structure in composite propellants: an area which has had limited advances. Knowledge of the variation of flame structure with pressure and propellant formulation will not only assist in the validation of these high-fidelity computer models but will also provide insight to propellant formulators as they seek to use alternate ingredients and methods.^
Steven F. Son, Purdue University, Robert P. Lucht, Purdue University.
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