Development and application of photofragmentation laser-induced fluorescence for visualization of hydrogen peroxides

Abstract: The work presented in this thesis is mainly motivated by the need for an optical diagnostic technique which can be used to visualize hydrogen peroxide (H2O2) in its gas phase. Due to the lack of bound electronic states, H2O2 cannot be detected using laser-induced fluorescence based on electronic excitation. Absorption in the ultraviolet leads to photodissociation. Thus a technique called photofragmentation laser-induced fluorescence (PF-LIF) has been developed and applied. It is a pump-probe technique that can be used for indirect detection of some non-fluorescing species. The species of interest is first dissociated using the pump-laser pulse, and the generated fragments are then probed via laser-induced fluorescence (LIF). Photofragmentation laser-induced fluorescence can be used for imaging, line measurements as well as for point measurements, and it can provide high temporal and spatial resolution. Experimental investigations have been carried out in free flows of gas phase H2O2/H2O/N2/O2 mixtures, in an industrial test rig as well as in premixed, laminar H2/O2, CH4/O2 and CH4/air flames. Qualitative single-shot imaging during injection of H2O2/H2O/air mixtures into a bottle has been applied to study H2O2 concentration buildup. Quantitative H2O2 concentration measurements have been performed by studying the chemical consumption of OH photofragments following dissociation. In order to be calibration free, the measurements should be carried out with low pump fluence. The chemical consumption of OH photofragments will then be governed by the reaction OH+H2O2→HO2+H2O. This method has been demonstrated for H2O2 number densities in the range 1.16•1016 cm-3 to 3.0•1017 cm-3. Photofragmentation laser-induced fluorescence is generally subject to the usual difficulties of making LIF quantitative. The main issue is often spatial differences in fluorescence quantum yield. In order to be able to measure the radiative lifetime distribution, which is directly linked to the fluorescence quantum yield, a separate campaign was directed towards development of a new wide-field method for fluorescence lifetime imaging. Extensive measurement campaigns have been directed towards PF-LIF measurements in laminar, premixed flames. It turned out that the majority of the OH photofragments in the studied flames were stemming from HO2, with smaller contributions from H2O2 and CH3O2. A major challenge with the flame measurements was that OH is naturally present in flames. This signal contribution, which generally was much stronger than the signal from OH photofragments, was subtracted by recording a separate image with the pump laser blocked. This image was then subtracted from the image obtained when both the pump and the probe lasers were used.