LEE- Uçak ve Uzay Mühendisliği Lisansüstü Programı
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ÖgeInvestigations on the effects of conical bluff body geometry on nonpremixed methane flames(Graduate Institute, 2021) Ata, Alper ; Özdemir, İlyas Bedii ; 675677 ; Department of Aeronautics and Astronautics EngineeringThis thesis is composed of three experimental studies, of which the first two are already published, and the third is under peer review. The first study investigates the effects of a stabilizer and the annular co-flow air speed on turbulent nonpremixed methane flames stabilized downstream of a conical bluff body. Four bluff body variants were designed by changing the outer diameter of a conically shaped object. The co-flow velocity was varied from zero to 7.4 m/s, while the fuel velocity was kept constant at 15 m/s. Radial distributions of temperature and velocity were measured in detail in the recirculation zone at vertical locations of 0.5D, 1D, and 1.5D. Measurements also included the CO2, CO, NOx, and O2 emissions at points downstream of the recirculation region. Flames were visualized under 20 different conditions, revealing various modes of combustion. The results evidenced that not only the co-flow velocity but also the bluff body diameter play important roles in the structure of the recirculation zone and, hence, the flame behavior. The second study analyzes the flow, thermal, and emission characteristics of turbulent nonpremixed CH4 flames for three burner heads of different cone heights. The fuel velocity was kept constant at 15 m/s, while the coflow air speed was varied between 0 – 7.4 m/s. Detailed radial profiles of the velocity and temperature were obtained in the bluff body wake at three vertical locations of 0.5D, 1D, and 1.5D. Emissions of CO2, CO, NOx, and O2 were also measured at the tail end of every flame. Flames were digitally photographed to support the point measurements with the visual observations. Fifteen different stability points were examined, which were the results of three bluff body variants and five coflow velocities. The results show that a blue-colored ring flame is formed, especially at high coflow velocities. The results also illustrate that, depending on the mixing at the bluff-body wake, the flames exhibit two modes of combustion regimes, namely fuel jet- and coflow-dominated flames. In the jet-dominated regime, the flames become longer compared to the flames of the coflow-dominated regime. In the latter regime, emissions were largely reduced due to the dilution by the excess air, which also surpasses their production. The final study examines the thermal characteristics of turbulent nonpremixed methane flames stabilized by four burner heads with the same exit diameter but different heights. The fuel flow rate was kept constant with an exit velocity of 15 m/s, while the co-flow air speed was increased from 0 to 7.6 m/s. The radial profiles of the temperature and flame visualizations were obtained to investigate the stability limits. The results evidenced that the air co-flow and the cone angle have essential roles in the stabilization of the flame: Increase in the cone angle and/or the co-flow speed deteriorated the stability of the flame, which eventually tended to blow-off. As the cone angle was reduced, the flame was attached to the bluff body. However, when the cone angle is very small, it has no effect on stability. The mixing and entrainment processes were described by the statistical moments of the temperature fluctuations. It appears that the rise in temperature coincides with the intensified mixing, and it becomes constant in the entrainment region.