Extended-NIR laser diagnostics for gas sensing applications [electronic resource]
Keywords: Mechanical Engineering

The development of diagnostics based on laser-absorption spectroscopy for combustion applications has been an important and active field of research over the past two decades due to the advantages of this non-intrusive optical sensing technique compared to traditional sampling-based sensing methods. Tunable diode laser (TDL) sensors, in particular, have shown the ability to provide in situ, time-resolved, line-of-sight measurements of temperature, gas species concentration, velocity, density, mass flux, and pressure in a variety of combustion environments. This thesis explores three new areas of TDL research: (a) extended near-infrared (NIR) diagnostics, (b) sensing under high-pressures, and (c) applications to chemical kinetics. Water vapor (H2O) and carbon dioxide (CO2) are attractive sensing targets for hydrocarbon-fueled systems as they are primary combustion products and their concentrations can be interpretrated to indicate combustion progress and efficiency. Both these gases have absorption spectra in the infrared (IR) region. Most previous TDL absorption sensors were designed to exploit robust telecommunications diode lasers and optical fiber technology in the 1.3-1.6 [mu]m (NIR) wavelength region. Recent developments in semiconductor diode-laser technology have extended the range of continuous wave (CW) room-temperature single-mode diode lasers to 2.9 [mu]m, allowing access to stronger vibrational bands of H2O and CO2 in the extended-NIR region. The first combustion diagnostics in the extended-NIR wavelength were demonstrated as part of this thesis work. The sensors were designed by selecting optimal transitions and then measuring the pertinent spectroscopic parameters in controlled laboratory environements. These sensors were then tested in the combustion environments of a flat flame and shock tube to validate their performance. These new sensors provide enhanced sensitivity and improved accuracy compared to previous TDL diagnostics. As part of this work, a novel diagnostic based on wavelength modulation spectroscopy (WMS) of CO2 was developed to make precise measurements of temperature behind reflected shock waves. This temperature diagnostic achieved an unprecedented uncertainty of < 0.3% under shock tube combustion conditions. Future engine concepts for efficient and clean combustion incorporate ultra-dilute mixtures at pressures considerably higher than are employed in current internal combustion engine technologies. Dilute mixtures keep peak temperatures of combustion and hence pollutant formation rates low, while high pressures minimize exergy loss during combustion. Despite the importance of high-pressure combustion measurements, little work can be found in the literature on the development of tunable diode laser (TDL) sensors for high-pressure environments. This work investigates how non-ideal behavior at high pressures affects direct absorption and WMS spectra. A new WMS-based diagnostic is developed for measuring CO2 concentration and temperatures at high gas densities (up to ~ 10 amagats). Line-mixing and finite collision duration effects are then studied at even higher densities (~ 30 amagats) for the CO2 spectra near 2.6 -- 2.9 [mu]m. Various high-pressure absorption line shape models are evaluated and although a new line-mixing model does a satisfactory job, it is recommended that further improvement is needed to aid the development of a high-pressure combustion diagnostic. The design and optimization of combustion systems relies heavily on accurate predictive modeling. These combustion models provide information regarding performance such as efficiency and pollutant emissions. An important component of any combustion model is the reaction mechanism that describes the chemistry of the combustion event. Such a reaction mechanism requires a database of accurate chemical reaction rate constants for the temperature range of interest. The test of the performance of a mechanism requires careful experiments to determine ignition times and species concentration time-histories in reacting or combusting flow with well-controlled temperature, pressure, and reaction time. In this work, the high- temperature decomposition of three simple methyl esters: methyl acetate, methyl propionate and methyl butanoate, were studied behind reflected shock waves using tunable diode laser absorption of CO2 near 2.7 [mu]m. The experiments provided the first laser-based time-history measurements of CO2 yields during pyrolysis of these bio-diesel surrogate fuels in a shock tube. Model predictions for CO2 yields during methyl butanoate pyrolysis at high temperatures are significantly lower than those measured in this study. These new findings imply that existing bio-diesel fuel models, which rely on the rapid formation of two oxygenate radicals from methyl esters (rather than a single non-reactive CO2 molecule) to account for the tendency for soot reduction, need further study.