Short-Pulse Photofragmentation and Fluorescence-based Diagnostics - Development and Applications

Sammanfattning: The work presented in the thesis covers the use of laser-induced fluorescence (LIF) and photofragmentation laser-induced fluorescence (PFLIF) with short laser pulses to determine species concentrations in different combustion environments. To attain quantitative species concentrations using LIF investigations of the influence of collisional quenching on the fluorescence signal strength is of vital importance, which can be done by measuring the fluorescence lifetime. A method for simultaneous measurements of fluorescence lifetimes of two species, present along a line, is described and discussed. The experimental setup is based on picosecond laser pulses tuned to excite two different species, whose fluorescence signals are detected with a streak camera. The concept is demonstrated for fluorescence lifetime measurements of CO and OH in laminar methane/air flames. The measured one-dimensional lifetime profiles generally agree well with lifetimes calculated from quenching cross sections found in literature and quencher concentrations predicted by the GRI 3.0 chemical mechanism. DIME, dual imaging with model evaluation, a method enabling fluorescence lifetime imaging of toluene is shortly discussed. The second technique, i.e. PFLIF, is used to study H2O2 and HO2, which both are molecules lacking accessible bound electronically excited states. Here, a pump laser pulse of 266 nm dissociates the molecules into OH fragments, which after a short time delay (nanosecond time scale), are probed with LIF using a second laser pulse tuned to an OH absorption line. The technique is investigated based on both nanosecond and picosecond pulses, in which the picosecond pulses offer the possibility to study the dissociation process in great detail. In the work presented, PFLIF is for the first time used for two-dimensional imaging of HO2 in laminar flames and for quantitative imaging of H2O2 in a homogenous charged compression (HCCI) engine. In methane/air flames an interfering OH signal contribution is observed in the product zone and found to originate from photolysis of hot CO2, whereas an interfering OH signal contribution is observed in the reaction zone arising from free oxygen atoms formed when HO2 is photodissociated and reacting with CH4 and/or H2O. In the HCCI experiments, one- and two-dimensional quantitative H2O2 concentrations at different piston positions are extracted via an on-line calibration procedure. In terms of mass fraction levels, the crank-angle resolved experimental data agree well with simulated H2O2 mass fractions. The minor deviations are mainly due to signal interference from HO2 not accounted for in the experiments, and inhomogeneities in the H2O2 spatial distributions not predicted by the models. Finally, PFLIF is employed for OH thermometry in different reacting flows, including the HCCI engine. The method creates OH molecules at a lower temperature range than where they naturally occur, i.e. providing OH thermometry at lower temperatures. In this temperature regime the technique also becomes more sensitive since the rotational population distribution has a stronger temperature dependence.