Sensitivity-Enhanced Raman Spectroscopy and High-Speed Fluorescence for In Situ Gas-Phase Diagnostics

Sammanfattning: In this work, combustion and thermal decomposition processes were studied employing laser-based diagnostic techniques, and the techniques were improved and demonstrated to achieve enhanced detection sensitivity. Raman spectroscopy was employed to study flames under elevated and atmospheric pressures with various fuels, such as methane, hydrogen-enriched methane, DME, ethylene, and ammonia. Raman spectra of major chemical species in flames were identified and converted into mole fractions. In addition, the Raman spectrum of nitrogen was utilized to obtain temperatures. The quantitative results are intended to serve for model development and validation. In this study, however, the ethylene flame was challenging to study with Raman spectroscopy due to soot particles emitting a laser-induced incandescence (LII) signal, much stronger than Raman scattering signal. Therefore, improving the diagnostic technique to solve this issue was motivated.In studies of thermal decomposition processes, emitted gas components by biomass were of interest. Raman spectroscopy served as a good technique, detecting hydrocarbons, carbon dioxide, and water released while heating. With Raman spectra, the quantity of the components was analyzed with the heating time and temperature. The limitation of the study was that the Raman signals are close to undetectable at high temperatures due to the strong laser-induced fluorescence (LIF) signal emitted from volatile hydrocarbons released during the decomposition process.Raman spectroscopy was thus improved to enhance detection sensitivity. A multi-pass concept was employed to optimize laser excitation, and filtering techniques were implemented to reject unwanted backgrounds and competing signals. The multi-pass concept enhanced the excitation by a factor of 45, enabling the detection of formaldehyde, a minor intermediate species in a DME flame. The so-called Periodic Shadowing filtering spatially rejected stray light and multiple scattering, offering narrower and clearer Raman peaks in spectra. A polarization lock-in filtering (PLF) technique was developed to reject competing laser-induced signals such as LII and LIF, solving the issues mentioned above while employing Raman spectroscopy. Therefore, with these filtering techniques, a sooting ethylene flame and biomass decomposition at high temperature could be studied by employing PLF-combined Raman spectroscopy.Another laser-based technique, planar laser-induced fluorescence (PLIF), was employed to study an actual-scale gas turbine burner and combustion characteristics. PLIF measurements at 2 kHz repetition enabled high-speed imaging of hydroxyl (OH) radical in turbulent flames for a fuel-flexibility study of the burner supplied with natural gas, hydrogen-enriched natural gas, and pure hydrogen as a fuel. The results provided useful information to understand flame characteristics when switching fuel from natural gas to hydrogen. In addition, PLIF imaging with higher laser repetition rates at 3 and 5 kHz was tested, and correlation between consecutive images was observed from the 5 kHz case, where flame image velocimetry technique can be employed for flow dynamics study.