Ultrafast Laser-Absorption Spectroscopy in the Mid-Infrared for Spatiotemporally Resolved Measurements of Gas Properties

Ryan J Tancin, Purdue University

Abstract

Laser-absorption spectroscopy (LAS) is widely used for providing non-intrusive and quantitative measurements of gas properties (such as temperature and absorbing species mole fraction) in combustion environments. However, challenges may arise from the line-of-sight nature of LAS diagnostics, which can limit their spatial resolution and can complicate accurate interpretation of LAS measurements. Further, time-resolution of such techniques as scanned direct-absorption or wavelength-modulation spectroscopy is limited by the scanning speed of the laser and the optical bandwidth is often limited by a combination of a laser’s intrinsic tunability and its scanning speed. The work presented in this dissertation investigated how recent advancements in mid-IR camera technology and lasers can be leveraged to expand the spatial, temporal, and spectral measurement capabilities of LAS diagnostics. In addition, the high-pressure combustion chamber (HPCC) and high-pressure shock tube (HPST) were designed and built to enable the study of, among others, energetic material combustion, spectroscopy, non-equilibrium and chemistry using optical diagnostics. A brief overview of laser absorption spectroscopy is provided. The fundamental principles of absorption spectroscopy and statistical mechanics are described. Various physical and instrumental broadening mechanisms which are relevant to this dissertation are discussed. The design and initial application of the HPCC is also presented in this work. The HPCC exhibits several unique design attributes which mitigate some of the challenges which complicate optical characterization of flames at high-pressure. The HPCC employs a flangeless and weldless design to provide a compact, easy to access, and relatively light weight (for its size and pressure capability) test chamber. It is capable of operating at pressures from vacuum to 206 bar, and it enables laser or wire ignition of propellants and energetic materials to be preformed. Some of the HPCC’s testing capabilities are demonstrated via optical characterization of laser-ignited HMX flames. Laser-absorption-spectroscopy measurements of temperature and CO at 2 bar and high-speed IR imaging and at pressures from 2 to 25 bar are presented. The design and initial application of a mid-infrared laser-absorption-imaging (LAI) technique for two-dimensional (2D) measurements and tomographic reconstruction of gas temperature and CO in laminar flames is presented. In this technique, the output beam from a quantum-cascade laser (QCL) is expanded, passed through the test gas, and imaged in 2D using a high-speed mid-infrared camera. The wavelength of the QCL is scanned across the P(0,20) and P(1,14) transitions of CO near 4.8 µm at 50 Hz to provide 2D measurements of path-integrated gas temperature and CO column density across over 3,300 lines-of-sight simultaneously. This enabled the first sub-second (0.1 s), high-resolution (140 µm), 2D laser-absorption measurements and tomographic reconstruction of flame temperature and CO mole fraction using mid-infrared wavelengths. Prior to entering the test gas, the beam was reflected off two diffusers spinning at 90,000 RPM (≈9400 rad/s) to break the laser’s coherence and prevent diffraction-induced image artifacts. This technique was validated with measurements of CO in an isothermal jet and then demonstrated in laminar, partially premixed, oxygen-ethylene flames despite large background emission from soot and combustion products. Next, the development of an ultrafast (i.e., femtosecond), mid-infrared, laser-absorption diagnostic and its initial application to measuring temperature, CO, and CH4 in flames is presented. This diagnostic offers several unique advantages: (1) ultrafast (sub-nanosecond) time resolution, (2) access to strong fundamental absorption bands located throughout the mid-IR using a single light source and camera, and (3) potential for single-shot, spatially resolved (1D) thermometry and species measurements at 5 kHz. The diagnostic employs a Ti:sapphire oscillator emitting 55-fs pulses near 800 nm which were amplified and converted into the mid-infrared though optical parametric amplification (OPA) at a repetition rate of 5 kHz. The pulses were directed through the test gas and focused into an imaging spectrometer where they were dispersed and recorded using a high-speed, mid-infrared camera.

Degree

Ph.D.

Advisors

Goldenstein, Purdue University.

Subject Area

Analytical chemistry|Chemistry|Medical imaging|Optics

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