Investigation of soot formation in pressurized swirl flames by laser measurements of temperature, flame structures and soot concentrations

Abstract Soot formation and oxidation were investigated in swirl flames operated with ethylene/air at elevated pressure in a gas turbine model combustor with optical access. Coherent anti-Stokes Raman scattering was used for temperature measurements, laser-induced incandescence for soot concentration and laser-induced fluorescence for the determination of OH radical distributions. A major focus of the experiments was the investigation of the influence of the injection of secondary oxidation air into the fuel-rich product gas of the primary combustion zone. Soot is mainly present in tiny filament-like regions left without OH signal. In the 3 bar flame with oxidation air injection these are found in a region separating the primary combustion zone, fed by combustion air and ethylene, and the secondary combustion induced by oxidation air and unburned hydrocarbons (UHC) that are transported into the inner recirculation zone. The different behavior of flames with and without oxidation air is most pronounced in the inner recirculation zone that is strongly influenced by the oxidation air admixture. This is reflected by changed OH distributions, mean temperatures and the shape of the temperature pdfs and results in significantly different soot distributions. The combined temperature statistics and correlated OH and soot distributions acquired at 3 and 5 bar are well suited to support the understanding of soot formation and oxidation and are expected to be a valuable input to soot model validation.