Development of photofragmentation- based diagnostics

Development of photofragmentation-based diagnostic concepts for studying H2O2, H2O, HO2 and O3 in three different measurement environments, i.e. a flow of vaporized H2O2/H2O, a premixed laminar methane/air flame and a plasma, produced by a gliding arc discharge, are reported in this thesis project. In addition, quantitative NO2 laser-induced fluorescence, LIF, measurements in a plasma generated by electron beam in a sterilization rig at Tetra Pak are discussed and compared to electron dose simulations. Hydrogen peroxide and water vapor are simultaneously visualized in a flow of vaporized H2O2/H2O by using photofragmentation laser-induced fluorescence, PFLIF, and two-photon LIF, tp-LIF. It is found that OH fragments stemming from dissociation of H2O is minor compared to OH fragments stemming from photodissociation of H2O2. A concept solely based on PFLIF is also discussed for simultaneous imaging of H2O2 and H2O, which utilizes the spectral and temporal shape of the OH emission, stemming from photolysis of H2O2, to extract both waterand hydrogen peroxide mole fractions. Structured illumination, SI, is combined with PFLIF to address issues that arise when the pump beam generates fragments of species that also are naturally present in the probe volume. The concept is demonstrated for imaging of hydrogen peroxides in a premixed laminar methane/air flame, where the pump beam is spatially intensity modulated and thereby giving the OH fragments a recognizable signature that allows them to be separated from naturally present OH radicals in the post processing of the image. The concept provides, for the first time, single-shot measurements of hydrogen peroxides in premixed laminar methane/air flames. Further, picosecond laser pulses both allow studies of OH production in the reactionand product zone of the flame and at the same time minimizes the signal interference of chemically produced OH in the product zone, stemming from photochemistry of hot CO2. The experimental results are compared with a chemical model, from where it is found that about 20 % of the oxygen fragments are formed in the singlet state, upon photolysis with picosecond laser pulses. A detailed study of O3 imaging using PFLIF is carried out in an ozone flow. The concept is also demonstrated on ozone produced by the gliding arc discharge. From experiments in the gliding arc discharge, it is found that the lifetime of hot O2 is significantly shorter than the lifetime of O3 and therefore the discharge was turned off, several microseconds prior to the arrival of the pump pulse. Nevertheless, the amount of naturally present hot O2 produced by the discharge (for electrons with a kinetic energy of 0.8 eV) is estimated to be insignificant compared to the number of hot O2, produced from photodissociation of O3, and hence ozone imaging is also carried out when the discharge is operating.