Laser Spectroscopic Techniques for Combustion Diagnostics Directed Towards Industrial Applications

Sammanfattning: In the work presented in the thesis, different laser spectroscopic techniques were utilized for measurement of species concentrations, as well as for surface thermometry. Experiments were conducted in various combustion environments, ranging from laboratory burners to truck-sized internal combustion (IC) engines, gas turbine combustors and a full-size aircraft turbofan engine. A multiple Nd:YAG laser system generating rapid bursts of laser pulses, and a high-speed framing camera capable of recording sequences of up to eight images, were used to study flame species and fuel distributions with a high temporal resolution in flames, combustion cells and IC engines. The spatial and temporal evolution of the ignition process of a jet of hot gases impinging upon an initially quiescent hydrogen/air mixture was investigated by means of high-speed planar laser-induced fluorescence (PLIF) of the OH radical. In addition, high-speed OH PLIF was employed for tracking flame-front movements in a low-swirl methane/air flame. Simultaneous measurements of the velocity fields, as well as temperature-field imaging were also conducted. A multiple dye-laser system that was set up was employed for the OH PLIF experiments in the low-swirl flame. Each of the four dye lasers was pumped by an individual laser from the Nd:YAG cluster, producing tunable laser radiation without losses in laser-pulse energy or deterioration of the beam intensity profiles, which can occur when only a single dye laser is employed. The multiple Nd:YAG laser system and the high-speed framing camera were used for single-cycle-resolved studies of combustion processes in a homogeneous charge compression ignition (HCCI) engine. In-cylinder fuel distributions were investigated by means of fuel tracer PLIF in order to study the effects of combustion chamber geometry on combustion. Two different piston geometries were compared, clear differences between them in the onset and development of combustion being revealed. High-speed formaldehyde PLIF was also used to study the HCCI combustion. The low-temperature reactions and the early phase of the main combustion event were investigated by monitoring the formation and the consumption of formaldehyde, respectively. The experiments showed the possibilities of using formaldehyde as a naturally occurring fuel marker in HCCI combustion. In addition, studies of laser-spark- and spark-plug-assisted HCCI were carried out, the results being compared with those for conventional HCCI operation. Different optical and laser-based techniques were utilized for characterization of the afterburner of an aircraft turbofan engine. Fuel PLIF measurements were performed in the burning jet stream, close to the afterburner outlet, to investigate the extent to which unburned fuel exits from the engine. Laser-induced phosphorescence from thermographic phosphors was used for assessing surface temperatures at various locations in the afterburner. Since measurements were conducted over the entire load augmentation of the engine, temperature variations during full load transients could be studied. The laser spectroscopic techniques proved to be applicable in the extremely harsh environment prevailing inside and next to a jet engine operating at high afterburner loads. Additional work, related to gas turbine diagnostics included two-phase fuel visualization using simultaneous Mie scattering and LIF for characterizing a Jet-A fueled pilot burner. PLIF imaging for the visualization of flame-front regions (OH) and of fuel distribution in an industrial gas turbine burner operating on natural gas was also performed. The spatial resolution of a chemiluminescence sensor designed for monitoring spatial and temporal fluctuations in the equivalence ratio in industrial gas turbine combustors was investigated as well, the sensor being evaluated in terms of its capability, as compared with OH PLIF, of resolving flame fronts (OH').