In situ probe-enabled tunable diode laser absorption measurements in flames, shock tubes and shock tunnels
Keywords: Mechanical Engineering.

Spectroscopic measurement strategies enabled by in situ laser-based probes were implemented in low-pressure flames, shock tubes, and a hypersonic reflected shock tunnel. In particular, tunable diode laser absorption spectroscopy (TDLAS) was leveraged to accurately infer quantities of interest such as temperature, species concentration, and pressure. Targeted environments spanned both flowing and static samples, reacting and inert compositions, and varying degrees of thermal equilibrium. Two bodies of work falling under this general theme of research are presented. First, two applications of a single-ended optical probe that incorporates a mid-infrared (MIR) CO2 TDLAS diagnostic are summarized. The rovibrational CO2 diagnostic, with transitions centered near 4.2 um, was developed for sensitive two-line thermometry in combustion environments, optimal in the temperature range of 1200 -- 2100 K and the pressure range of 0 - 2 atm. The selected transitions correspond to the strong v3 asymmetric stretch mode of CO2, whose fundamental band boasts the strongest MIR lines among common combustion products (i.e. CO2, CO, H2O, OH, NO). In the first application of this diagnostic, an interband quantum cascade laser (ICL) was directed across a low-pressure burner-stabilized flame using a single-ended optical probe composed of two thin sapphire rod waveguides. Probe-based measurements of temperature and CO2 mole fraction were collected in flames of 25 torr and 60 torr total pressure, and at distances from the burner surface in the range of 3 - 23 mm. In another study, this CO2 diagnostic with a similar single-ended probe implementation was applied to shock tube experiments. The shock tube endcap probe developed enables measurements of relevant reflected-shock region (region 5) quantities (e.g. temperature and CO2) at variable distances from the shock tube endwall, and offers an alternative path length option to traditional sidewall optical portholes. TDLAS measurements of temperature and CO2 mole fraction made with the endwall probe were typically subject to uncertainties of 1% and 5%, respectively. The sensor performance was validated in inert shocked mixtures with 1 - 7% CO2 diluted in argon or nitrogen, and spanning the temperature range of 1200 - 2000 K and pressure range of 0.7 - 1.2 atm. Probe-based measurements were compared directly with traditional sidewall window measurements (i.e. full tube diameter path length) and empirically supported simulations. Finally, perturbation of region 5 conditions by the probe was assessed with a series of tests. The main body of work discussed in this thesis concerns a series of studies conducted at the T5 reflected shock tunnel located at the California Institute of Technology. The focus of these experiments was to conduct spectroscopic measurements of various species in hypersonic nonequilibrium air flows generated at the facility, in support of freestream and flow-model investigations. Freestream characterization of T5 was conducted through two iterative efforts, first involving quasi-quantitative path-averaged measurements of nitric oxide (NO) across the entire (nonuniform) test section. In a subsequent effort, a custom flow-cutting optical probe was used to measure absorbing rovibrational NO transitions in the (uniform) core flow of the freestream. Measured quantities included NO rotational and vibrational temperature, partial pressures of NO, CO, H2O, K, and flow velocity. During this set of experiments, the uniformity of the measured quantities across the core and beyond was assessed by repeating the experiment with distinct probe lengths (i.e. different optical path lengths). Finally, NO, CO and electronically excited oxygen absorption were measured at spatially-precise locations in the post-shock flow generated around a cylindrical model. The path-averaged measurements were processed to infer post-shock quantities of interest, using simple models of the pathwise condition distribution. Insights are drawn by comparing these preliminary measurements with existing 3D CFD simulations of the cylinder post-shock flowfield