OH and Soot Optical Diagnostics for Combustion Applications for Combustion Applications

Abstract: Optical diagnostics are remote non-intrusive sensing techniques. The thesis work concerns the use of OH and soot optical diagnostics for combustion research.Internal combustion (IC) engines are widely used for the generation of power and for transportation purposes. Soot emission, involving carbon particles that emanate from the combustion process, is one of major sources of pollutants in engine exhaust. Such particles can be inhaled into human lungs and have been found to be harmful to public health. For this reason, soot emissions from IC engines are strictly regulated. The flame lift-off length (LOL) of a diesel jet, which is the distance between the nozzle of the injector and the base of the flame, affects both diesel combustion and emission formation. The presence of OH radicals has been used commonly for determining LOL. Both the 2D imaging of OH' chemiluminescence and OH-laser-induced fluorescence (LIF) were employed here for determining the LOL of diesel spray flame. Laser extinction measurements (LEM), together with measurements of the natural luminosity (NL) of sooty flames and of laser-induced incandescence (LII) were made use of for in-cylinder soot detection in the thesis. The main goal in the use of optical diagnostics in engines was to answer various engine-related questions. The optical techniques and the data processing methods employed in the thesis work were also improved parallel to one another. The online/offline OH' chemiluminescence method that was used was able to successfully reduce part of the soot luminosity of the OH' chemiluminescence image obtained. A comparison of LOL results obtained on the basis of simultaneous OH' chemiluminescence and OH-LIF images was carried out. The OH-LIF resulted statistically in longer LOL than the OH'chemiluminescence results did. This can be partially explained by the difference between the two methods in the probing volumes and the flame asymmetry. A data correction for the LEM results was developed, one that helped to reduce the effects of the fluctuations in the probing laser intensities and the soot deposits on the optical window. Simultaneous OH-PLIF and soot-LII was performed for studying the soot oxidation process in the recirculation zone of a diesel optical engine. In addition to the optical diagnostics applied to the optical engines that were studied, simultaneous dual species PLIF techniques were developed and made use of in the thesis work. Splitting the beam from the multi-YAG laser into two, the one used to pump OPO and the other used directly for formaldehyde (CH2O) excitation, made the simultaneous probing of two species at a high repetition rate possible. Simultaneous OH and CH2O-PLIF was performed for demonstrations at a repetition rate of 50 kHz. The Frequency Recognition Algorithm for Multiple Exposure (FRAME) approach was also introduced. Through the use of structured illumination, FRAME permits several laser-induced signals to be superimposed upon a single detector.