Multiple-parameter Visualization in Combustion using Laser Diagnostic Techniques
Division of Combustion Physics, Lund Institute of Technology, P.O Box 118, S-221 00 Lund Sweden, Email: Marcus.Alden@forbrf.lth.se
During the last decade various laser spectroscopic techniques have shown great potential for combustion diagnostics. The biggest advantages with these techniques are that they permit non- intrusive measurements with high spatial, temporal and spectral resolution. The most important parameters to be measured are: species concentration, temperature and velocity.
In the following presentation a brief review of past and present achievements will be given in the area of laser diagnostics, aiming at describing their application for multiple parameter visualization in a combustion environment. Of special interest is the possibility of visualizing the parameters above in one laser pulse, ~10 ns, by using a laser sheet and a two-dimensional detector, permitting instantaneous multiple point measurements. In the first case molecules, radicals and atomic species have been visualized. As a further step measurements of several parameters at the same time were achieved. Multiple species imaging can be achieved by multiple laser excitation, e.g. OH and formaldehyde as will be described in engine experiments, or by utilizing spectral coincidences using one laser system, e.g. OH, O and NO as described in flame experiments. Temperature visualization can be done by using two-line excitation of atoms or molecules seeded or naturally present in the flame environment. An alternative approach could be, when applicable, to use Rayleigh scattering. The main limitation with this technique is its problem with spectral disturbances by resonance scattering from particles/droplets or laser reflections. It has been demonstrated, however, that these problems to a large degree can be avoided by filtered Rayleigh scattering, which will be described in the presentation.
One feature of utmost importance for visualization of combustion/flow phenomena is the possibility of making this visualization in real time with adequate temporal resolution. Most pulsed lasers used for visualization are limited to 10-100 Hz, which is far too low for real turbulent applications. For several years, therefore, we have developed and applied a specially designed laser/detector system for high repetition rate measurements which has been used 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, using synchronous detection with the framing camera. Also in this case experiments from industrial (IC engine) applications will be exemplified.
The techniques which will be described are mainly Laser-Induced Fluorescence, LIF and Rayleigh scattering, but also new emerging techniques will be illustrated.
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