(458i) Microwave Enhancement of Boron Combustion for Solid Fuel Ramjet Applications

Solid fuel ramjet (SFRJ) propulsion systems will enable the
next generation of supersonic and hypersonic transport. SFRJ systems provide a
high degree of range, specific impulse, safety, and simplicity and boron-based
fuels are poised to enable this technology with boron’s high volumetric
theoretical combustion enthalpy, 1.5 times that of aluminum and 3 times that of
hydrocarbon fuels. Unfortunately, difficulties created by the oxide layer
surrounding the boron particles, such as fuel particle agglomeration, oxygen
and boron species transport barriers, and evaporative cooling, make swift
ignition and combustion of boron particles difficult to achieve within the
timescales of ramjet combustors. Microwave radiation has been found to couple
with both gas-phase flame regions and condensed-phase energetic materials. Recently,
a 60% increase in the burning rate of an alkali-doped, aluminized, ammonium
perchlorate composite propellant was reported. This burning rate enhancement
was hypothesized to be, in part, the result of dielectric absorption by high
temperature aluminum oxide species present in the flame, as the loss tangent of
many metal oxides exhibit exponential temperature dependence. This suggests
that microwave coupling with other metal flame structures, such as those in boron-based
fuels and propellants, could significantly enhance combustion efficiencies in
solid fuel ramjet applications. This research explores the microwave
enhancement of boron and boron carbide combustion, both within a composite
propellant and within particle seeded burners. S-band, 2.46 GHz microwave
energy is applied within a single-mode, impedance-matched, resonant microwave
cavity during combustion. Time-resolved temperature measurements of propellant
and seeded burner flames are made using single-point and spatially-resolved
methods. Microwave irradiation effects on the burning rates of boron/boron
carbide propellant are measured using high-speed video techniques, while its
effects on the burning time of boron/boron carbide particles are determined
using long-exposure photography.