High Repetition Rate Laser Diagnostics for Combustion Applications

Abstract: High repetition rate laser diagnostics has proven to be a useful tool for studying combustion phenomena. Laser techniques are, in general, relatively non-intrusive and have the potential to measure specific species at very low concentrations and with extremely high temporal resolution. However, most laser systems in this field of research are limited to repetition rates below 100 Hz, and are thus incapable of resolving the fastest time scales in turbulent combustion. For this purpose, the multi-YAG laser systems at Lund University have been used. These consist of four double-pulsed Nd:YAG lasers combined into one unit with a single optical output. This allows eight high-energy laser pulses to be fired in very rapid succession. In order to match the repetition rate of the multi-YAG systems, framing cameras consisting of eight intensified CCD detectors with a single optical input were used. Several of the measurements presented in this thesis were made in optically accessible engines. These engines were based on production line engines that were modified for optical measurements through the addition of a Bowditch piston extension. This gives optical access to the combustion chamber from below via a 45 degree mirror and in some cases from the sides as well. The combustion process in the engines has been studied using a number of laser techniques including laser-induced fluorescence (LIF), Raman scattering and laser-induced incandescence (LII). In the work presented in this thesis, multi-YAG laser systems were used to study mixture preparation through the application of high-speed fuel tracer LIF in several different optical engines, both light- and heavy-duty, and various combustion concepts. High-speed LII and Raman spectroscopy were used to study combustion in a heavy-duty diesel engine. This thesis also presents fuel LIF measurements on a fuel tracer using structured laser illumination planar imaging (SLIPI) in the same heavy-duty diesel engine. Various laser techniques were further developed, including high-speed LII and the adaptation of the SLIPI technique for engine measurements. Development of simultaneous measurements of four species in a turbulent flame, and comparisons of different schemes for carbon monoxide LIF were also made. Finally, an optical parametric oscillator (OPO) unit was tested and evaluated together with the multi-YAG system.