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 laserinduced 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.

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Visualization of soot inception in turbulent pressurized flames by simultaneous measurement of laserinduced fluorescence of polycyclic aromatic hydrocarbons and laser-induced incandescence, and correlation to OH distributions, Appl. Phys. B 119 (2015) 717-730.

This paper presents planar imaging of laser induced fluorescence (LIF) from key reactive species in the auto-ignition region of dilute turbulent spray flames of methanol. High-speed (5 kHz) LIF-OH imaging as well as low speed (10 Hz) imaging of joint LIF-OH-CH 2 O is performed. The product of the OH and CH 2 O signals is used as a qualitative indicator of local heat release. The burner is kept intentionally simple to facilitate computations and the spray is formed upstream of the jet exit plane and carried with air or nitrogen into a hot co-flowing stream of vitiated combustion products. The studied flames are all lifted but differ in the shape of their leading edge and heat release zones. Similarities with auto-ignition of gaseous fuels, as well as differences, are noted here. Formaldehyde is detected earlier than OH implying that the former is a key precursor in the initiation of auto-ignition. Growing kernels of OH that are advected from upstream, close in on the jet centreline and ignite the main flame. The existence of double reaction zones in some flames may be due to ignitable mixtures formed subsequent to local evaporation of droplets and subsequent mixing. When air is used as spray carrier, reaction zones broaden with distance, possibly due to increased partial premixing and regions of intense heat release occur near the flame centreline further downstream. With nitrogen as carrier, the flame maintains a nominal diffusion-like structure with reaction zones of uniform width and substantially less concentration of heat release on the flame centreline.

Alternative production pathways for liquid fuels provide the opportunity to adjust the chemical composition of the product in order to improve combustion performance. In this study, flame characteristics of selected single-component fuels were investigated to provide a basis for a better understanding of the influence of specific fuel components on the combustion behaviour. The measurements were performed in a redesigned gas turbine model combustor for swirl-stabilised spray flames under atmospheric pressure. The combustor features a dual-swirl geometry and a prefilming airblast atomiser. The combustion chamber provides good optical access and yields well-defined boundary conditions. As part of different projects in the field of alternative fuels, two liquid single-component fuels (n-hexane, n-dodecane) and kerosene Jet A-1 were investigated. Flow fields of the nonreacting and reacting flow were measured using stereo particle image velocimetry. The flame structure and spray distribution were derived from CH* chemiluminescence and Mie scattering respectively. Lean blowout limits were measured. Results show noticeable differences in combustion behaviour of the chosen fuels at comparable flow conditions. Furthermore, the results provide a detailed data base for the validation of numerical models.

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Two pattern synthesis procedures have been developed for a traveling wave linear array, fed at its center, with half the array fonvardfiring, the other half backward-firing, and with the total array required to produce a squinted main beam and controlled sidelobes. The procedure is illustrated by the case of a 36-element slot array, designed to create a sum pattern with a -30 dB sidelobe level and the main beam tilted 2.25 degrees off broadside. Center-fed traveling wave arrays offer the potential of increased bandwidth and a frequency-independent squint angle.