Ultraviolet laser absorption studies of the vibrational relaxation and reaction kinetics of shock-heated air species
Keywords: Mechanical Engineering.

This series of experimental studies leverages ultraviolet (UV) laser absorption diagnostics to probe quantum-state-specific time-histories, which are subsequently used to infer vibrational relaxation and reaction kinetics rate parameters for shock-heated air species. Validation of high-fidelity models for high-temperature hypersonic flows requires high-accuracy kinetics data, motivating these time-history and rate parameter measurements. Experiments were performed in a pressure-driven shock tube, with measurements made behind reflected shocks. Additionally, three new UV laser absorption diagnostics were developed to sensitively probe absorbance, temperature, and number density time-histories. Specific wavelengths were chosen to probe oxygen (O2) in multiple vibrational levels and nitric oxide (NO) in multiple rotational levels, enabling the inference of multiple temperatures and number densities. These time-histories enabled inferences of rate parameters for vibrational relaxation, coupled vibration-dissociation, and NO decomposition processes with low uncertainty. The combined use of reflected shocks and UV laser absorption diagnostics resulted in time-histories and rate parameters that address the need for low-uncertainty, quantum-state-specific data for validation of high-fidelity computational models of high-temperature air. Reflected shock experiments benefit time-history and rate parameter measurements by accessing a wide range of post-shock conditions and mixtures, by observing a well-defined time-zero for inferring rate parameters, and by nearly stagnating the test gas, thereby avoiding impacts of flow effects on rate measurements. Experimental temperatures ranged from 2,000 - 14,000 K, pressures from 0.022 - 1.524 atm, and test gases containing pure O2, mixtures of 2%, 5%, 20%, and 50% O2 diluted in argon (Ar), mixtures of 10% and 21% O2 diluted in nitrogen (N2), and mixtures of 2%, 1%, and 0.4% NO diluted in either pure Ar, pure N2, or equal parts N2/Ar. For all of the mixtures considered, measurements were made at higher temperatures than any past studies, and measurements at 14,000 K represent a significant advancement in the use of reflected-shock experiments to probe high-temperature nonequilibrium phenomena. Besides extending data to high temperatures, the range of mixture compositions also extended the available rate data for the O2 and NO interactions with N2 that strongly influence high-temperature air. Three UV lasers -- two continuous-wave (CW) lasers and one pulsed laser -- measured absorbance time-histories from the fourth, fifth, and sixth vibrational levels of the electronic ground state of O2 and multiple rotational states in the ground vibrational and electronic state of NO. The absorbance time-histories subsequently yielded post-shock time-histories for vibrational temperature, translational/rotational temperature, total number density, and vibrational-state-specific number density. These state-specific temperature and number density time-histories demonstrated low uncertainty, as needed for high-temperature model validation, while also providing data to higher temperature than previous experiments. Five studies were completed to investigate high-temperature vibrational relaxation and reaction kinetics -- three for O2 and two for NO. For each study, the absorbance sensitivity to each of the reaction rate parameters was quantified, thereby informing the fitting windows and demonstrating the changing reaction sensitivity over the experimental test time. The sensitivity and time-histories were used to infer multiple rate parameters, and the results were compared to a range of models and literature studies. In the first study, the three vibrational relaxation times that influence O2 in high-temperature air were inferred from vibrational temperature time-histories. These measurements were sufficiently sensitive to distinguish between vibration-translation (VT) and vibration-vibration (VV) relaxation processes. Results for O2-O2 VT times agree with the Millikan and White correlation at temperatures below 4,000 K, while high-temperature data deviate from the Millikan and White correlation exhibiting a reduced temperature dependence. In contrast, data for O2-N2 VT times exceed the Millikan and White correlation by 70% at all temperatures but show reasonable agreement with previous data below 5,000 K. High-temperature results again show a reduced temperature dependence, but this study shows longer relaxation times than previous work. Finally, the data for O2-N2 VV times exceed the semi-empirical relation developed by Berend et al. by 70%, but overlap with previous measurements. Due to insensitivity of the chemical system to VV transfer at high temperatures, results for O2-N2 VV times were only inferred below 6,000 K. The full set of rate data thereby clarify three of the five important relaxation processes in air, while extending available data to nearly 9,000 K. In the second and third studies, shock-tube experiments with UV laser absorption were performed to measure quantum-state-specific time-histories and coupled vibration-dissociation (CVDV) rate parameters in shock-heated, nondilute O2 and oxygen-argon (O2/Ar) mixtures. The second study established the methodology in highly dilute O2/Ar mixtures, with the third study extending the method to investigate O2-O2 and O2-O interactions. The analysis of the temperature and number density time-histories allowed inference of rate parameters in the Marrone and Treanor CVDV model, including vibrational relaxation time, average vibrational energy loss, vibrational coupling factor, and dissociation rate coefficients. Results for each of these seven parameters show reasonable consistency across the range of temperatures, pressures, and mixtures and generally agree with a modified Marrone and Treanor (MMT) model by Chaudhry et al. Finally, results for these rate parameters exhibit much lower scatter than previous experimental studies and extend measurements to higher temperatures than previous experiments. These CVDV studies demonstrated the inference of coupling parameters in O2 experiments and extended rate data to nearly 14,000 K. In the fourth and fifth studies, the shock tube and UV laser absorption diagnostics were extended to probe NO vibrational relaxation and decomposition. The fourth study leveraged pure Ar dilution to investigate the autocatalytic decomposition of NO, while the fifth study extended time-history and rate measurements to dilution in N2 and equal parts N2/Ar. For these experiments, the absorbance, temperature, and number-density time-histories yielded four vibrational relaxation times and five rate coefficients for multiple NO decomposition reactions with reduced scatter and uncertainty. Few studies have directly inferred the NO-N2 rates from experiments, emphasizing the novelty of these data. Generally, these rate data are consistent with data from the literature, although NO VT times are observed to differ strongly from both the Millikan and White correlation and Park two-temperature model. Additionally, the NO-N2 VV time is four times slower than an inference from Taylor et al. The reaction rate coefficients are consistent with previous work by Wray and Teare, although the dissociation reaction rate coefficients notably differ from the Park two-temperature model. Together, the available data for NO vibrational relaxation and decomposition were extended to nearly 9,000 K. Overall, these experiments determined eight vibrational relaxation times, eight reaction rate coefficients, and two coupling parameters, providing low-uncertainty rate data and extending the available data to high temperatures. The rate parameters were directly compared to available model and literature values, but the full set of data broadly enable model validation. Zero-dimensional model results can be compared to the particle-time-adjusted time-history measurements, whereas models that include the one-dimensional incident and reflected shock processes can be compared to the lab-time measurements. Measuring 5 mm from the endwall ensured the difference between particle time and lab time was small. Similarly, the available data provide many options for the wide variety of nonequilibrium model frameworks, and details of the frameworks and comparisons to the current data are included throughout the various studies. As a specific example using the current time-histories for model validation, the O2/Ar time-histories were compared against state-to state modeling, and the temperature and state-specific number density time-histories agree reasonably well with the state-to-state modeling at low temperatures but deviate significantly at high temperatures. Additional validation studies are anticipated, enabling many more opportunities for model refinement and validation