Development and applications of laser spectroscopic techniques related to combustion diagnostics

Thanks to features as non-intrusiveness combined with high spatial and temporal resolution, various laser diagnostic techniques have during the last decades become of utmost importance for characterization of combustion related phenomena. In the following presentation some further development of the techniques will be highlighted aiming at a) surface temperatures using Thermographic Phosphors, TP, b) species specific, spatially and temporally resolved detection of species absorbing in the IR spectral region using polarization spectroscopy and Laser-induced fluorescence, and finally c) high speed visualization using a special designed laser system in combination with a framing camera. In terms of surface thermometry, Thermographic Phosphors have been used for many years for temperature measurements on solid surfaces. We have during the last years further developed and applied this technique for temperature measurements on burning surfaces and on materials going through phase shifts, e.g. pyrolysis and droplets. The basic principle behind this technique is to apply micron size particles to the surface of interest. By exciting the TP with a short pulse UV laser (ns), the phosphorescence will exhibit a behaviour where the spectral emission as well as the temporal decay are dependent on the temperature. It is thus possible to measure the temperature both in one and two dimensions. The presentation will include basic description of the technique as well as various applications, e.g in fire science, IC engines and gasturbines. Several of the species of interest for combustion/flow diagnostics exhibit a molecular structure which inhibits the use of conventional laser-induced fluorescence for spatially and spectrally resolved measurements. We have during the last years investigated the use of excitation and detection in the infrared region of the spectrum. Here, it is possible to detect both carbonmono/dioxide, water as well as species specific hydrocarbons. The techniques which have been investigated are laser-induced fluorescence and polarization spectroscopy. In the latter case several species have been detected both in free flows as well as flames, e.g in addition to water and carbondioxide also the hydroxyl radical and different hydrocarbons including methane, acetylene, ethane and the methyl radical. One parameter of utmost importance for visualization combustion/flow phenomena is the possibility to make this visualization in real time with adequate time resolution. We have for several years developed and applied a special designed laser/detector system for high repetition rate measurements which has been applied for several real-world applications, e.g internal combustion engines. The system can also be used for three dimensional visualization by sweeping the laser pulses through the region of interest and consequent detection with the framing camera. Also in this case experiment from industrial applications will be exemplified