Infrared Laser Absorption Spectroscopy of Nitric Oxide for Sensing in High-enthalpy Air PDF Download

Author: Christopher Andrew Almodovar
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Languages : en
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Book Description
Motivated by thermometry in high-enthalpy air, advancements towards the measurement and modeling of high-pressure laser absorption spectroscopy (LAS) of nitric oxide (NO) are presented. The primary application of this thermometer is to characterize the stagnation conditions (T = 1000--2500 K and P = 10--130 atm) in a clean-air hypersonic wind tunnel facility. By characterizing the thermodynamic conditions upstream of the expansion nozzle, the flow conditions of the expanding air can be determined via enthalpy matching. At high temperatures, the Zeldovich mechanism describes increasing NO formation in air with increasing temperature, making NO an attractive species for LAS-based temperature measurements in air. Two optical transitions in the R-branch of the fundamental rovibrational band of NO are selected and their fundamental spectroscopic parameters are characterized at high temperatures. The temperature sensor design is demonstrated in reflected shock wave experiments in a large diameter shock tube at pressures up to 5 atm. Although the target application's operating pressure range is well outside the demonstration range, the fundamental concept of two-wavelength absorption is still valid. However, at high pressures, the selected optical transitions begin to blend with their neighboring transitions. Thus, accurate knowledge of the high-temperature and high-pressure absorption at the selected wavelengths requires knowledge of the spectroscopic parameters defining the neighboring transitions. To measure the spectroscopic parameters of the many neighboring transitions, a high-pressure, high-temperature (HPHT) optical cell (up to 800 K and over 30 atm) is designed and demonstrated for mid-infrared spectroscopy with usable transmission up to approximately 8 microns. With a functional HPHT optical cell, a detailed, temperature-dependent study (up to 800 K) of the optical transitions in the NO R-branch near 5.3 microns is performed. To extend the study to temperatures relevant for the target sensing application, shock tube measurements from 1000 to 2500 K supplement the detailed study. Finally, the spectrum is studied at high pressures. Static cell measurements reveal deviations from the classical line shape models used accurately at low pressures. The deviations are attributed to collisional line mixing that emerges when the line widths of the optical transitions are of similar or greater magnitude than the separation of optical transitions. A temperature-dependent line mixing model is built using statistically-based energy gap fitting laws and the full relaxation matrix expression. A comparison with measured data reveals good agreement in the regions where inter-branch coupling can be neglected. In the end, a thorough treatment of the NO spectrum has provided a temperature- and pressure-dependent model that can be used to predict the absorption spectra of NO in the R-branch of the fundamental rovibrational band.