Multi-Dimensional Quantitative Laser-based Diagnostics - Development and Practial Applications

University dissertation from Division of Combustion Physics, Department of Physics, Lund University

Abstract: Laser based optical diagnostic methods are routinely used in combustion research. Many of the more common approaches are based on illuminating a cross-section of the sample with a thin laser sheet. For example, by targeting an electronic transition in a molecule, its concentration within the plane illuminated by the laser sheet can be deduced. By probing the relative occurrence of an atom or molecule in different rotational or vibrational states or by probing the Doppler shift in Rayleigh scattering, it is possible to extract the temperature. The flow field can be measured by seeding particles into the measurement volume and following them through multiple exposures. The work reported in the thesis concerns the development, improvement and applications of measurement techniques based on laser sheet illumination. The aforementioned techniques are most often employed on a single shot basis, providing independent snapshots of two-dimensional (2D) data. In some examples, the measurement techniques are extended to the third spatial dimension, and in recent years, studies employing high repetition rate measurements capable of resolving the dynamics in time have become more frequent. In the thesis, a method for simultaneously extending the measurements to the third spatial dimension and to the time dimension, is presented. A high repetition rate laser and detection system is combined with oscillating mirrors, the laser sheet being scanned back and forth throughout the measurement volume. The deflections from two mirrors operated at different frequencies are combined to obtain equidistant laser sheets in the measurement region. The method is demonstrated on the Mie-scattering from a flow of droplets and is used to probe the planar laser induced fluorescence (PLIF) from the OH in a flame. Post processing methods to calculate concentrations and flame-fronts from large data sets are demonstrated. Measurements of droplet concentration and size distribution in sprays, based on recording the light scattered from a laser sheet, suffer from uncertainties due to multiple scattering (MS) and attenuation of the illuminating and scattered light. A method is demonstrated here, that takes advantage of the ability to suppress the MS light by means of structured illumination. After MS suppression, the attenuation of the laser and signal light can be compensated for by comparing the transmission through the spray with the side-scattered signal. In the process, the local extinction coefficient is calculated from the Beer-Lambert law. Laser based optical diagnostic techniques are in general developed for atmospheric flames under ideal laboratory conditions. In the application of the same techniques in more realistic situations, such as internal combustion (IC) engines, the harsh conditions involving vibrations, varying pressure, moving parts, limited optical access and a sooty environment have to be taken into account. Several of the measurement campaigns reported in the thesis were conducted in IC engines. Although the main goals of these campaigns were to answer combustion or engine related questions, time has also been invested in improving and adopting the measurement techniques to the existing conditions. By following the spray propagation in a light duty-diesel engine over time, knowledge was gained regarding how early spray injections should be conducted to avoid wall wetting. From high speed laser induce incandescence (LII) measurements in a heavy-duty Diesel engine, conclusions regarding soot formation and oxidation were drawn. The implementation of LII at high repetition rates in IC engines was investigated here. Challenges associated with attenuation of the laser and signal light were also addressed. Visualization of the flame jet propagation in a large-bore gas engine was made possible by means of fuel tracer LIF. Apart from the combustion related conclusions, it was shown that the image quality could be improved substantially by the use of correction optics.