Abstract
This paper presents an experimental study on the flame and flow structures of the blue whirl, in pursuit of an understanding of the blue ring's formation. The flame front was identified based on the OH* chemiluminescence signal and the flow field was obtained using high-frequency particle image velocimetry (PIV). The flame and flow field reveals the presence of a radial-inward boundary layer and the corner flow lifting the blue whirl. In the case of turbulent blue whirl, a discernible recirculation zone manifests within the flame, which offers direct evidence supporting the dominance of the vortex breakdown in blue whirl's formation. Furthermore, we find that the blue whirl's cone angle approximately equals the angle of the zero-vorticity line of the flow field, whereas the height of the blue ring aligns with the height of the upward deflection point on the radial-stagnation line. These findings demonstrate a strong coupling between the flame structures and the flow field. To understand the formation mechanism of the blue ring, the flow flux across the radial-stagnation line is calculated to estimate the equivalence ratio of the mixture forming the blue ring. It reveals that the blue ring is formed where the stoichiometric reaction occurs. This happens as the corner flow induces a strong shear to facilitate the mixing within the air-fuel mixture, causing the inner layer of the mixture to be diluted to the equivalence ratio of approximately 1 at the blue ring's location.
Published Version
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