SPATIOTEMPORALLY RESOLVED MID-INFRAREDEMISSION AND ABSORPTION SPECTROSCOPYDIAGNOSTICS FOR PROPELLANT FLAMES

Emission and absorption spectroscopy diagnostics are useful for providing non-invasive,
quantitative measurements of various gas properties in combustion environments, including
temperature and species concentrations. These measurements become even more useful
when they are applied with high spatial and temporal resolution. This dissertation describes
several ways that both emission and absorption diagnostics were advanced through leveraging
improvements in mid-IR camera and laser technology and through refining the use of existing
techniques.
A literature review is provided for both laser absorption and emission spectroscopy. Previous advancements in spatially resolved techniques are explained. The fundamental equations
of spectroscopic diagnostics are reviewed, starting from statistical mechanics.
A spectrally-resolved emission imaging diagnostic is presented. This diagnostic provided
1-dimensional measurements of gas temperature and relative mole fraction of CO₂ and HCl
in flames. An imaging spectrometer and a high-speed mid-infrared camera were used to
provide 1D measurements of CO₂ and HCl emission spectra with a spectral resolution of
0.46 cm^-1 at rates up to 2 kHz. Measurements were acquired in HMX and AP-HTPB flames
burning in air at 1 atm. This diagnostic was applied to characterize how the path-integrated
gas temperature of HMX flames varies in time and with distance above the burning surface.
Additionally, Abel inversion with Tikhonov regularization was applied to determine the radial
distribution of temperature and relative concentration of CO₂ and HCl within the core of
AP-HTPB flames.
Next, a similar emission imaging diagnostic is presented which uses spectrally-resolved
measurements of emission spectra at visible wavelengths, unlike the mid-infrared measure-
ments in the rest of this dissertation. This diagnostic provided 1D temperature measure-
ments of aluminum oxide (AlO), an intermediate product of aluminum combustion. While
this author created the AlO diagnostic, these measurements were performed alongside a CO
absorption diagnostic used by a different researcher to compare the flame bath gas (via CO)
and the region immediately around aluminum particles (via AlO) when varying forms of
aluminum powder were used in a propellant. This comparison allows analysis of the burning regime of aluminum particles. Evidence was found that nano-aluminum particles burn in
the kinetically controlled combustion regime, while micron-aluminum particles burn in the
diffusion-controlled regime.
Multi-spectral emission imaging of hypergolic ignition of ammonia borane (AB) is then
presented. Three high-speed cameras with multiple optical filters were used to capture
infrared and visible wavelength videos of four individual species during AB ignition: BO,
BO₂, HBO₂, and the B-H stretch mode of AB were imaged. The ignition process was
observed to act in two steps: gas evolution and then propagation of a premixed flame. The
evolution of the species and flame front revealed that boranes may continue to complete
combustion to a further degree than other boron fuels. This author performed the infrared
camera imaging and also ran infrared spectrograph measurements to confirm which species
were viewed through the optical filters.
Next, a scanned-wavelength direct-absorption diagnostic for directly measuring NH₃ in
high-temperature combustion environments is presented. A quantum cascade laser (QCL)
was scanned at 5 kHz over multiple NH₃ transitions between 959.9 cm⁻¹ and 960.3 cm⁻¹ to
measure path-integrated NH₃ temperature and mole fraction. Many NH₃ transitions overlap
with high-temperature water lines at commonly used diagnostic frequencies, severely limiting
those diagnostics’ capabilities in water-rich, high-temperature environments that are typical
of combustion applications. The optical frequencies used in this diagnostic are insensitive
to water absorption and thus remedy this issue. This diagnostic was demonstrated within
the flame of ammonia borane. AB-based fuels were burned in ambient air and translated
vertically to effectively scan the measurement line-of-sight vertically through the flame. Ad-
ditionally, flames of these fuels were characterized at a stationary height in an opposed-flow
burner (OFB) under O₂ flow.
The final chapter presents scanned-wavelength direct-absorption measurements of path-
integrated temperature and CO mole fraction in opposed-flow diffusion flames of hydroxyl-
terminated polybutadiene (HTPB). HTPB strands were held in an opposed-flow burner
under an opposed flow of O2 or 50/50 O₂/N₂ to create quasi-steady and quasi-1D diffusion
flames above the fuel strand. The opposed-flow burner was translated vertically to effectively
scan the measurement line-of-sight vertically through the flame. A quantum-cascade laser (QCL) was scanned across the P(2,20), P(0,31), and P(3,14) absorption transitions in CO’s
fundamental vibration bands near 2008 cm⁻¹ at 10 kHz to determine the path-integrated
temperature and CO mole fraction. The laser beam was passed through sapphire rods
held close to the flame edge to bypass the flame boundary and provide a well defined path
length for mole fraction measurements. The measured profiles and fuel regression rates
were compared to predictions produced by a steady opposed-flow 1D diffusion flame model
produced by researchers at the Army Research Lab. The model was generated with chemical
kinetics mechanisms employing two different assumptions for the nascent gaseous product of
HTPB pyrolysis: C₄H₆ or C₂₀H₃₂. It was found that the C₂₀H₃₂ model produced temperature
and CO profiles along with regression rates that agreed more closely with the measured
results.