SPATIOTEMPORALLY RESOLVED MID-INFRAREDEMISSION AND ABSORPTION SPECTROSCOPYDIAGNOSTICS FOR PROPELLANT FLAMES
Creators: name: McDonald, Austin J
Publisher: name: Purdue University Graduate School
Year: 2024
Subjects: classificationCode: 400199
subject: Aerospace engineering not elsewhere classified
classificationCode: 409902
subject: Engineering instrumentation
classificationCode: 401799
subject: Mechanical engineering not elsewhere classified

description: Emission and absorption spectroscopy diagnostics are useful for providing non-invasive,<br>quantitative measurements of various gas properties in combustion environments, including<br>temperature and species concentrations. These measurements become even more useful<br>when they are applied with high spatial and temporal resolution. This dissertation describes<br>several ways that both emission and absorption diagnostics were advanced through leveraging<br>improvements in mid-IR camera and laser technology and through refining the use of existing<br>techniques.<br>A literature review is provided for both laser absorption and emission spectroscopy. Previous advancements in spatially resolved techniques are explained. The fundamental equations<br>of spectroscopic diagnostics are reviewed, starting from statistical mechanics.<br>A spectrally-resolved emission imaging diagnostic is presented. This diagnostic provided<br>1-dimensional measurements of gas temperature and relative mole fraction of CO<sub>2</sub> and HCl<br>in flames. An imaging spectrometer and a high-speed mid-infrared camera were used to<br>provide 1D measurements of CO<sub>2</sub><sub> </sub>and HCl emission spectra with a spectral resolution of<br>0.46 cm<sup>-1</sup> at rates up to 2 kHz. Measurements were acquired in HMX and AP-HTPB flames<br>burning in air at 1 atm. This diagnostic was applied to characterize how the path-integrated<br>gas temperature of HMX flames varies in time and with distance above the burning surface.<br>Additionally, Abel inversion with Tikhonov regularization was applied to determine the radial<br>distribution of temperature and relative concentration of CO<sub>2</sub> and HCl within the core of<br>AP-HTPB flames.<br>Next, a similar emission imaging diagnostic is presented which uses spectrally-resolved<br>measurements of emission spectra at visible wavelengths, unlike the mid-infrared measure-<br>ments in the rest of this dissertation. This diagnostic provided 1D temperature measure-<br>ments of aluminum oxide (AlO), an intermediate product of aluminum combustion. While<br>this author created the AlO diagnostic, these measurements were performed alongside a CO<br>absorption diagnostic used by a different researcher to compare the flame bath gas (via CO)<br>and the region immediately around aluminum particles (via AlO) when varying forms of<br>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<br>the kinetically controlled combustion regime, while micron-aluminum particles burn in the<br>diffusion-controlled regime.<br>Multi-spectral emission imaging of hypergolic ignition of ammonia borane (AB) is then<br>presented. Three high-speed cameras with multiple optical filters were used to capture<br>infrared and visible wavelength videos of four individual species during AB ignition: BO,<br>BO<sub>2</sub>, HBO<sub>2</sub>, and the B-H stretch mode of AB were imaged. The ignition process was<br>observed to act in two steps: gas evolution and then propagation of a premixed flame. The<br>evolution of the species and flame front revealed that boranes may continue to complete<br>combustion to a further degree than other boron fuels. This author performed the infrared<br>camera imaging and also ran infrared spectrograph measurements to confirm which species<br>were viewed through the optical filters.<br>Next, a scanned-wavelength direct-absorption diagnostic for directly measuring NH<sub>3</sub> in<br>high-temperature combustion environments is presented. A quantum cascade laser (QCL)<br>was scanned at 5 kHz over multiple NH<sub>3</sub> transitions between 959.9 cm<sup>−</sup><sup>1</sup> and 960.3 cm<sup>−</sup><sup>1</sup> to<br>measure path-integrated NH<sub>3</sub> temperature and mole fraction. Many NH<sub>3</sub> transitions overlap<br>with high-temperature water lines at commonly used diagnostic frequencies, severely limiting<br>those diagnostics’ capabilities in water-rich, high-temperature environments that are typical<br>of combustion applications. The optical frequencies used in this diagnostic are insensitive<br>to water absorption and thus remedy this issue. This diagnostic was demonstrated within<br>the flame of ammonia borane. AB-based fuels were burned in ambient air and translated<br>vertically to effectively scan the measurement line-of-sight vertically through the flame. Ad-<br>ditionally, flames of these fuels were characterized at a stationary height in an opposed-flow<br>burner (OFB) under O<sub>2</sub> flow.<br>The final chapter presents scanned-wavelength direct-absorption measurements of path-<br>integrated temperature and CO mole fraction in opposed-flow diffusion flames of hydroxyl-<br>terminated polybutadiene (HTPB). HTPB strands were held in an opposed-flow burner<br>under an opposed flow of O2 or 50/50 O<sub>2</sub>/N<sub>2</sub> to create quasi-steady and quasi-1D diffusion<br>flames above the fuel strand. The opposed-flow burner was translated vertically to effectively<br>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<br>fundamental vibration bands near 2008 cm<sup>−</sup><sup>1</sup> at 10 kHz to determine the path-integrated<br>temperature and CO mole fraction. The laser beam was passed through sapphire rods<br>held close to the flame edge to bypass the flame boundary and provide a well defined path<br>length for mole fraction measurements. The measured profiles and fuel regression rates<br>were compared to predictions produced by a steady opposed-flow 1D diffusion flame model<br>produced by researchers at the Army Research Lab. The model was generated with chemical<br>kinetics mechanisms employing two different assumptions for the nascent gaseous product of<br>HTPB pyrolysis: C<sub>4</sub>H<sub>6</sub> or C<sub>20</sub>H<sub>32</sub>. It was found that the C<sub>20</sub>H<sub>32</sub> model produced temperature<br>and CO profiles along with regression rates that agreed more closely with the measured<br>results.<br>