Short-Pulse Laser Spectroscopy for Combustion Diagnostics - Laser-Induced Fluorescence of Polyatomic Molecules and Developments in Measurement and Evaluation
Sammanfattning: Combustion is the most important source of energy worldwide. A better understanding of it is essential if pollutant emissions are to be reduced and combustion devices be made more efficient. This requires measurement of physical and chemical combustion parameters. Laser-induced fluorescence (LIF) is widely employed as a laser-diagnostic technique for the detection of combustion species but it suffers from 'quenching' processes, which can obstruct the determination of species concentrations. Correcting for quenching effects requires knowledge of fluorescence quantum yields, which can be determined from fluorescence lifetimes as obtained in time-resolved LIF experiments. In this work, fluorescence lifetimes of species relevant for combustion were measured in the gas phase under varying conditions of temperature, pressure and oxygen content. The thesis also concerns the development of laser-spectroscopic measurement and evaluation techniques, namely two-dimensional measurement of fluorescence lifetimes using a streak camera, and proper curve fitting of data from experiments with pulsed lasers subject to intensity fluctuations. The work on PAH involved the substances fluorene, naphthalene, anthracene and pyrene. These were excited in a gas cell at atmospheric pressure and at temperatures of 400 to 1200 K, using a picosecond laser system providing radiation at 266 nm. Emission spectra were observed that showed broadening and red-shifts as the temperature was increased. The fluorescence lifetimes decreased considerably with an increase in temperature and oxygen concentration. The observations were explained in terms of sequence bands and of the density of vibrational states. Naphthalene and pyrene, were also seeded into the post-flame zone of a premixed methane/air flame for studies at still higher temperatures and at different stages of soot formation. Formaldehyde was excited in a cell at temperatures up to 770 K and pressures up to 10 bar employing picosecond laser radiation at 355 nm. The resulting fluorescence was detected spectrally and temporally resolved. Temperature and pressure were found to shorten the fluorescence lifetimes. Absorption measurements on formaldehyde at wavelengths of about 355 nm and less were also performed. An increase in temperature resulted in broadening of the vibronic bands towards longer wavelengths and in particular features in the ro-vibronic structure being pronounced. The ro-vibronic spectra appeared also broadened as the pressure was increased. A technique for the two-dimensional visualisation of fluorescence lifetimes using a streak camera was tested numerically and was demonstrated on static fluorescing objects. The precision and accuracy of the technique were characterised. Lifetime ranges for which the technique is best suited were also determined. Finally, a curve-fitting algorithm applicable to data from experiments using pulsed lasers subject to pulse-to-pulse intensity fluctuations is presented, one which appears to be particularly useful in connection with non-linear spectroscopy. It is based on a measurement model representing the fluctuating laser intensities by random variables. Application of the maximum-likelihood method resulted in a very versatile fitting scheme, which could be of considerable use in other scientific fields as well. The scheme is simple since it takes advantage of existing least-squares algorithms and it is comparably fast without showing any appreciable loss in precision or accuracy.
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