Laser Diagnostics and Chemical Modeling of Combustion and Catalytic Processes

Doctoral thesis, 2001

The H2+1/2O2<->H2O reaction on hot platinum surfaces has been studied. A stagnation flow geometry was used with a gas mixture of H2 and O2 at subatmospheric pressures. Experimental data of the OH concentration outside the surface were measured with Planar Laser Induced Fluorescence, PLIF, while Second Harmonic Generation, SHG, was used to determine the coverage on the surface. Detailed simulations of surface chemistry, mass-transport effects, and gas-phase chemistry, as well as the interaction between them, were performed with Chemkin. The formation energy of H2O and OH was found to be 67 kJ/mol, while the desorption energy of OH was 250 kJ/mol. It was also found that, for pressures up to 130 Pa, the OH molecules that desorb from the surface are not influenced by the gas-phase chemistry. At higher pressures the desorbed OH molecules are partially consumed in reactions with gas-phase species. Increasing the pressure even more will result in a reactive gas phase where water is produced. Comparison with an inert glass surface showed that the catalytic surface strongly inhibits the gas-phase ignition. An explanation of this behaviour is that gas-phase radicals adsorb onto the surface and form less reactive species, such as water. In this way the gas phase outside a catalytic surface becomes depleted of reactive radicals. Sensitivity analysis was also used to study the complex surface/gas interaction. In another study elastic light scattering from a fuel spray was studied. Image processing was used in an attempt to reproduce the internal structure of the spray, despite the problems with strong attenuation due to substantial scattering from the spray. The numerical implementation was based on the Beer-Lambert's law for an inhomogeneous medium. The method was partially successful, although, calibration studies of the scattered light dependence on spray structure and droplet density will need to be included in future work.