Laser-Based Investigations of Combustion Phenomena in Gas Turbine Related Burners

Sammanfattning: Experiments were performed on various combustion devices to investigate the flames employing different optical measurement techniques under atmospheric pressure conditions. Non-intrusive laser-based techniques provided in-situ information concerning different parameters of the combustion process, with a high degree of temporal and spatial resolution. In the thesis work, different parameters of interest in combustion research, such as visualization of different intermediate combustion species and measurement of the flow velocity field, were studied by use of laser-induced fluorescence (LIF) and particle image velocimetry (PIV). The aim was to better understand the reaction and post-flame zone, flame stabilization, flame-anchoring position, as well as turbulent-flow interactions under different operating conditions, while varying the equivalence ratio, Reynolds number, burner geometry, and composition of fuel employed. Also, high-speed chemiluminescence imaging was used to investigate the dynamics of the flame.A large part of the thesis work involves experimental studies of the flame produced in a downscaled prototype 4th generation dry low emission (DLE) burner in connection with the development of the Siemens SGT-750 gas turbine burner. The burner consists of three concentrically arranged sections: a) an outer Main section, b) an intermediate section (Pilot) and c) a central pilot body termed the RPL (Rich-Pilot-Lean) section. Each of these sections is designed to be premixed and to enable the equivalence ratio to be varied for achieving optimal combustion. There is a Quarl, a diverging conical section located at the burner exit, used for expanding the flow area and holding the flame. Planar laser-induced fluorescence (PLIF) of OH and chemiluminescence imaging were employed for studying the flame at the burner exit. The primary combustion occurred inside the RPL section, whereas the main combustion was stabilized downstream of the burner throat. The main flame was anchored inside the Quarl and was elongated within the combustor. Both the main and the primary flames were stabilized by the swirling motion of the flow. Vortex breakdown and recirculation zones assisted the steady combustion process. Investigations were carried out to study how these three sections of the burner interact with one other and affect the main combustion process for altering the operating conditions in each section separately, using methane (CH4) as fuel. It was found that the RPL-Pilot flame was mostly confined within the Quarl and played an essential role in flame stabilization and in locating the flame-anchoring position. The flame stabilization region moved downstream when the RPL equivalence ratio (φ) was changed from lean to rich conditions, whereas it moved upstream when the global φ and the Pilot φ was increased. The flame was investigated for the change in geometry that occurred by removing the Quarl to understand the effect of the Quarl on the flame stabilization. Significant changes in the flame characteristics were observed for sudden expansion of the flow at the burner exit when the Quarl was not present. Since without the Quarl the flame can expand radially without any constraints, the flame shortened at the burner exit under such conditions. For the global φ ≥ 0.52, outer recirculation zones in the flame were observed when the Quarl was absent, but they were not observed when the Quarl was present. Experiments were also performed to investigate the flame for the enrichment of hydrogen (H2) in the methane (CH4) fuel. Three H2/CH4 fuel mixtures, having ratios (volumetrically) of 0/100, 25/75, and 50/50, respectively, were studied. The characteristics of the flame changed with the addition of H2 due to the greater diffusivity, higher reaction rate, and higher laminar burning velocity of H2. The H2 enrichment shortened the flame, and the flame stabilization zone moved further downstream. At a constant global φ for alternation of the RPL φ and the Pilot φ, the trends of the changes in the flame position and stabilization that occurred in 0/100 and 25/75 (expressed in the vol. fuel ratio) of the H2/CH4 fuel mixtures were similar.In addition, experiments were performed on two laboratory-scaled burners: a Triple Annular Research Swirler (TARS) and a CECOST swirl burner. These are generic burners that can simulate the characteristics of a gas turbine burner, regarding both the fuel injection and the flame stabilization. The hysteretic behavior of flashback and flash-forward were investigated on the swirl-stabilized flame of the TARS burner, using OH-PLIF, CH2O-PLIF, high-speed OH' chemiluminescence and particle image velocimetry (PIV). Two hysteresis loops were identified both for flashback and flash-forward, one for lean mixtures and the other for rich mixtures. The impacts of the relevant parameters including the Reynolds number, equivalence ratio, fuel type, combustion chamber geometry, preheating and mixing tube protrusions were also investigated. It was found that the equivalence ratios at which rich and lean flashbacks occurred, approached stoichiometric condition with an increase in the Reynolds number. On the average, flashback events under lean conditions were significantly slower than not only the same events under rich conditions but also the flash-forward events. Confinement effects were found to be less critical in both the cases of flashback and flash-forward events. Finally, the stabilization, lean blowout limit, and flashback of the flame were investigated for the CECOST swirl burner, employing simultaneous OH-PLIF and CH2O-PLIF as well as high-speed chemiluminescence imaging. The lean blowout limit was found to be nearly invariant for all measured Reynolds numbers. On the other hand, the flashback was found to approach around equivalence ratio (φ = 0.6) at lower Reynolds numbers and at higher equivalence ratios when the Reynolds number was increased up to Re = 17000. With a further increase in the Reynolds number, no flashback was observed, even when the equivalence ratio was increased close to stoichiometry. In the combustion chamber, the flame was stabilized by creating a central recirculation and outer recirculation zones. At these higher Reynolds number (Re > 17000), the flame size was reduced, and the central recirculation zone became stronger inside the combustion chamber with an increase in equivalence ratio.