Photo-physical characterization of aromatic compounds for laser-induced fluorescence based diagnostics of fuel concentration, temperature, and equivalence ratio in practical combustion processes

Abstract

This dissertation is based on experiments to determine photo-physical properties of the aromatics toluene, 1,2-xylene, 1,2,4-trimethylbenzene, anisole, 1,4-difluorobenzene, naphthalene and 1-methylnaphthalene under various environmental conditions in the gas phase. Most of these species are part of commercial fuels like kerosene, gasoline, or Diesel or have similar physical and chemical properties. Therefore, their photo-physical properties are used for diagnostics methods in combustion engines and gas turbines to either visualize the gas transport and mixing or to get information about the local thermo-dynamical state. To be able to realize the environmental conditions present in these combustion chambers in the laboratory, heated, optically-accessible highpressure cells were utilized. Flows of nitrogen, carbon dioxide, or an oxygen-nitrogen-mixture and a few vol% of the aromatics were investigated spectroscopically. Absorption spectra were measured with a deuterium lamp or Nd:YAG laser. Fluorescence information was determined by exciting the species with a frequency-converted picosecond Nd:YAG laser and the laser-induced fluorescence (LIF) was measured spectrally and temporally resolved by a fast photomultiplier or a spectrometer coupled with a streak camera. The investigated tracers differ in their photophysical properties and therefore vary in their applicability for specific measurement techniques. The core of this dissertation consists of four publications as well as some unpublished results. One of these publications results from a collaboration of three researchers of our department and is dealing with the principle of self-quenching, giving insight into origin and the interrelationship between the base deactivation pathways for excited states. The experiments of the other three publications have been planned, carried out, analyzed and published (mostly) by myself. They all provide fundamental photophysical properties of selected aromatic compounds for various environmental conditions and also give phenomenological fitting functions with which the estimated signal intensity per molecule for internal combustion engine relevant temperatures and pressures can be calculated. As unpublished results, the absorption cross-section of anisole and the fluorescence spectra and lifetime of 1-methylnapthalene in dependence on the environmental conditions fill gaps in the dataset of these tracers. A comparison of all fluorescence tracers investigated here and several that had been investigated before is made to give an overview, to enable tracer selection for specific applications, and to put the results of this dissertation into perspective.