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Infrared Laser Diagnostics for Characterizing Combustion of Energetic Materials and Detonation Flows
thesisposted on 30.07.2021, 18:49 by Garrett Carver MathewsGarrett Carver Mathews
Laser-absorption spectroscopy (LAS) is an established technique providing noninvasive measurements of temperature, pressure, and species concentrations in combustion environments. Developments in both near-infrared (NIR) and mid-infrared (MIR) laser technology have enabled measurements of numerous molecules to characterize combustion of hydrocarbon fuels and energetic materials. However, the harsh environments presented by advanced combustion systems can pose challenges for LAS diagnostics due to high pressures, density gradients, particulates in the environment, and low absorption signals. Furthermore, many thermochemical processes occurring in these environments evolve on μs timescales necessitating high speed measurements.
This work presents the development and application of several novel LAS diagnostics designed to address the challenges associated with performing measurements in harsh combustion environments. These diagnostics provided high speed measurements of temperature, pressure, H2O, CO, and optical density and were demonstrated in deflagrating fireballs of energetic materials, post-detonation fireballs, and a rotating detonation rocket engine (RDRE).
A new wavelength-modulation spectroscopy technique (WMS) was developed to overcome the challenges associated with particulates in fireballs of energetic materials while achieving MHz measurement rates. While WMS is capable of overcoming particulate scattering and beamsteering and has been widely utilized in harsh environments, previous implementations were limited to measurement rates of ≤ 100 kHz. The two-color WMS technique developed here utilizes near-GHz modulation frequencies to provide enhanced noise rejection and achieve scanned-wavelength measurements at rates up to 1 MHz. This diagnostic utilized two tunable diode lasers (TDLs) near 1.4 μm modulated at 35 MHz and 45.5 MHz to simultaneously access two NIR H2O absorption transitions along a single line-of-sight (LOS). Measurements of temperature and H2O were obtained from the WMS-2f/1f signals measured at the linecenter of each transition. Several challenges associated with modulating TDLs at frequencies > 10 MHz were addressed and new laser characterization methods were developed. The diagnostic was validated in a high temperature gas cell and measured temperatures within 20 K of known values from 700 to 1200 K and H2O concentrations within 2 to 10% of known values. The near-GHz WMS diagnostic was applied to measure the path-integrated temperature and H2O column density in fireballs produced by igniting 0.75 g of grade 3, class B HMX with and without H-5 micro-aluminum powder (20% by mass). Temperature measurements were acquired in the fireballs with a 1-σ precision of 50 K, 30 K, and 15 K for measurement rates of 1 MHz, 250 kHz, and 25 kHz, respectively.
The WMS diagnostic was extended by frequency multiplexing a third TDL to measure atomic iodine near 1.3 μm. The three TDLs were modulated at 35, 45.5, and 61.5 MHz. The three-color WMS diagnostic was applied to study fireballs with various initial formulations of HMX, micro-aluminum, and iodine crystals. The results indicated that the addition of H-5 micro-aluminum improves combustion performance and yields higher peak fireball temperatures, while the addition of I2 crystals significantly hinders fireball combustion.
A MIR scanned-wavelength direct-absorption (scanned-DA) diagnostic was developed to overcome challenges associated with the short measurement path and high pressures in the annulus of a RDRE. A quantum cascade laser (QCL) was used to probe strong MIR CO absorption transitions thereby facilitating measurements across the short path length (1 cm) in the RDRE annulus. Scanned-DA was utilized to maximize the wavelength scan amplitude enabling measurements of multiple absorption transitions exhibiting significant collisional broadening due to the engine's high operating pressure. The gas temperature, pressure, and CO partial pressure were inferred from the best-fit absorbance spectra which were simulated with a model utilizing the HITEMP2010 linestrengths and lower-state energies, E", and a custom collisional-broadening model.
The MIR scanned-DA diagnostic and the two-color NIR WMS diagnostic were applied to perform high-bandwidth measurements of (1) temperature, pressure, and CO, and (2) temperature and H2O, respectively, in the annular combustion chamber of an RDRE. The NIR diagnostic technique was extended to utilize measurements of WMS-4f/2f and WMS-2f/1f signals. WMS-4f/2f measurements were used to infer the collisional-broadening full-width at half-maximum (FWHM) of the H2O transitions, and the WMS-2f/1f signals were then used to infer the gas temperature. The diagnostics were packaged in single-ended sensor assemblies to enable measurements in the annulus of a methane-oxygen RDRE, and results are presented for a test case with the RDRE operating at an equivalence ratio of 1.15 and a total propellant mass flow rate of 0.6 lb/s.
The MIR scanned-DA diagnostic was applied to measure the temperature, pressure, and CO column pressure at 1 MHz in post-detonation fireballs produced by RP-80 detonators. An RP-80 detonates a 123 mg pellet of RDX explosive producing a shock wave and a reacting gas-phase fireball. To characterize this environment, the QCL's output beam was passed through a blast chamber via wedged windows over the center of the detonator at a height of 91 mm or 51 mm above the detonator surface. Measured absorbance spectra were compared to simulated absorbance spectra using a nonlinear fitting routine to infer the gas temperature, pressure, and CO column pressure. A dual-zone absorption model was utilized to simultaneously infer two distinct temperature and pressure measurements when the measured spectra were clearly impacted by LOS nonuniformities. LAS measurements were compared to simulations of the RP-80 fireballs using a synthetic measurement technique.
Two infrared laser-induced fluorescence (IR-LIF) diagnostics were developed using tunable continuous-wave (CW) lasers to provide spatially resolved measurements of combustion gases. The first IR-LIF diagnostic utilized wavelength modulation and lock-in detection to reject strong IR background emission and measure LIF signals from CO in a flame. The second IR-LIF diagnostic utilized a CW optical parametric oscillator (OPO) to perform scanned-wavelength excitation of CO2 and provide temperature measurements in a heated jet.
The work presented here represents significant advancements in LAS diagnostic techniques, particularly in performing high-speed measurements in harsh, multi-phase combustion and detonation environments. Complete details pertaining to the development and application of these diagnostics are presented in this dissertation.