Towards Fast Dual Frequency Comb Spectroscopy in Dynamic High Pressure Systems

Draper, Anthony Diego

Graduate Thesis Or Dissertation

Towards Fast Dual Frequency Comb Spectroscopy in Dynamic High Pressure Systems Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/q524jp206

Abstract

Laser absorption spectroscopy is a non-intrusive diagnostic tool particularly well-suited to investigate the dynamic and harsh conditions commonly found within combustion systems. By measuring the amount of light absorbed at specific wavelengths that are resonant with rotational-vibrational transitions in molecules, absorption spectroscopy gives a measure of the molecular population in particular quantum states. Experimental spectra are fit with a simulation generated from spectral line shape models combined with a spectroscopic database to infer species concentrations, temperature, and pressure. Dual frequency comb spectroscopy (DCS) with mode-locked frequency comb lasers is an emerging form of absorption spectroscopy that yields both high resolution (<1 GHz) and broad bandwidth spectra (>10 THz) on rapid timescales (< 2 ms). There are two key challenges facing DCS in dynamic combustion environments. First, obtaining high signal-to-noise-ratio (SNR) spectra has traditionally involved coherently averaging hundreds of individual spectra over seconds to minutes before fitting. Second, at the high temperatures and pressures commonly found within combustion systems, the existing line shape models and spectroscopic databases are known to not capture all of the key molecular physics, thereby requiring empirical extension and validation. This work presents techniques to enable rapid DCS measurements of thermodynamic properties in dynamic high-pressure, high-temperature, environments through power optimization and apodization to improve the short-term SNR. A rapid compression machine at Colorado State University is instrumented with a portable DCS spectrometer and temperature is recovered at 704 µs resolution from 1-21 bar and 294-566 K. This demonstrates the ability of DCS to be applied to combustion-relevant timescales for both broad bandwidth and high resolution non-intrusive measurements of harsh systems. The design development of an optical testbed that creates a well-known, high-temperature, and high-pressure environment is additionally discussed. This subsequently will enable determination of the accuracy limitations of existing molecular absorption models, as well as allow for model expansion. Together these abilities enable laser measurements to better evaluate and optimize combustion systems, including improved understanding of the underlying molecular processes. Proper understanding of the molecular dynamics will allow for instrumentation and quantification of more extreme environments such as inside rocket engines or the atmospheres of distant planets.

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