Visualization and understanding of combustion processes using spatially and temporally resolved laser diagnostic techniques

Laser diagnostic techniques have for more than 30 years added very valuable input for a deepened understanding of combustion processes. The present paper will focus on techniques developed for visualization of important parameters with the ability to get detailed information in space and time. The paper is not meant to be a complete review of the entire research field but rather a survey with a majority of the contributions from the authors’ laboratory. After a short introduction and background, essentially three techniques will be highlighted. Planar Laser-Induced Fluorescence, PLIF, will be described in terms of further development directed towards the use of a high-power Alexandrite laser, in some cases yielding a substantially increased sensitivity as compared to the more conventional Nd:YAG and Excimer lasers. There will also be a description of possibilities for high-speed visualization, 3D imaging and the potential to visualize species requiring a two-photon excitation process. Regarding thermometry, the paper will describe and exemplify the use of Thermographic Phosphors, mainly for surface measurements but also in droplets and gas flows. The third area to be described is the use of Polarization Spectroscopy, PS, for spatially and temporally resolved measurements in the IR spectral region, e.g. CO2 and H2O, various hydrocarbons, HCl and HCN. The last chapter focuses on present challenges and future applications. A major challenge is diagnostics in environments with limited optical access. Two methods that are applicable under such conditions are discussed, namely endoscopic approaches and picosecond LIDAR. Another challenging area addressed is application of diagnostics in optically dense environments, e.g. sprays, where a new technique, Structured Laser Illumination Planar Imaging, SLIPI, has been developed for suppressing the influence of multiple scattering. Finally, approaches for measurements of “new” species, challenges associated with LIF quantification, as well as measurements in environments possessing strong background radiation are discussed.