Effect of fuel nozzle geometry and airflow swirl on the coherent structures of partially premixed methane flame under flashback conditions

The effect of fuel nozzle geometry and swirling airflow on the flashback and its relationship with the coherent structures of partially premixed methane flame is investigated experimentally. Particle image velocimetry (PIV) and laser Doppler velocimetry (LDV) are used to document flow characteristics, and Schlieren imaging technique is used to study flame appearance and vortex shedding frequency downstream of the burner exit. Proper orthogonal decomposition (POD) technique is applied to capture the coherent structures, along with phase averaging of the linear superposition of the first four POD modes. Seven different fuel nozzle geometries and two swirl number (S = 0.79 and 1.15) are tested. The nozzles are categorized into three groups, with each has similar equivalent diameter; namely, a symmetric nozzle (used as a reference), nozzles with polygonal orifices (group A), and angled multi-orifice nozzles (group B). The results of the flow field inside the mixing tube show that the strength of coherent structures and flashback propensity increase with the swirling airflow Reynolds number, swirl number, nozzle bluff body area, and the number of the peripheral angled orifices of the fuel (central) nozzle. On the other hand, the results of flame appearance outside of the mixing tube indicate that methane flame experiences symmetric vortex shedding at high swirl number and low Reynolds number, while it experiences PVC near blowout conditions at low swirl number and high Reynolds number. Furthermore, the frequency of coherent structures is found to depend on the swirling airflow Reynolds number, swirl number, and fuel nozzle geometry. Additionally, the flashback's mean region inside the mixing tube is found directly proportional to the strength and frequency of the coherent structures.