Properties of turbulent non-premixed methane/air flames in a miniature-scale swirl burner under different coaxial airflow swirl numbers

This study investigates the dynamics and properties of non-premixed methane/air flames under three swirl numbers by segmenting flame images using the Otsu thresholding technique. Under three operating conditions, the lean blow out (LBO) and flame length, lift-off height, maximum width, flame angle, and flame pulsing displacements in terms of flame center of gravity, length, and width are measured and compared. A high-speed camera is used to record video of flames, and the image processing of frames collected from a high-speed video was accomplished by using the intermittency distribution method to quantitatively compare flame attributes. The findings show that increasing the swirl number from 0.5 to 0.7 generally has an unfavorable effect on the LBO at given fuel flow rates, and the LBO of flames under 35° (0.6 swirl number) and 40° (0.7 swirl number) swirlers has decreased up to about 15% and 40%, respectively when compared with a 30° swirler (0.5 swirl number). Additionally, observations indicate that the flame length (𝐿) and lift-off height (𝐿𝑂) drop as the swirl number rises, although the flame width (𝑊) and angle (𝛼) show an ascending tendency. Besides, flame lift-off reveals an increasing-decreasing trend with an increment in the airflow, and flame length decreases as the airflow rate increases. It was also observed that flame pulsating displacements in terms of center of gravity (𝛿𝐶𝐺), length (𝛿𝐿), and width (𝛿𝑊) increases with an increase in the fuel flow rate, and as the swirl number is increased, 𝛿𝐶𝐺 and 𝛿𝐿 lessens, while 𝛿𝑊 increases.