The blue whirl: Boundary layer effects, temperature and OH measurements

• Published in: Combustion and flame 2019-05, Vol.203, p.352-361
• Main Authors: Hariharan, Sriram Bharath, Anderson, Paul M., Xiao, Huahua, Gollner, Michael J., Oran, Elaine S.
• Format: Article
• Language: English
• Subjects: Blue whirl; Boundary layer transition; Chemiluminescence; Combustion; Fire whirl; Flame structure; Liquid fuels; Metal plates; Obstructions; Organic chemistry; Pyrometry; Vortex breakdown
• ISSN: 0010-2180; 1556-2921
• DOI: 10.1016/j.combustflame.2019.02.018

The blue whirl is a small flame with an inverted conical shape, first observed as it developed from fire whirls formed using liquid fuels burning on a water surface. Here, it is shown that the water surface is not critical for a transition from a fire whirl to a blue whirl, but that the surface over which the whirl is formed must be flat without any obstructions to the incoming flow. This observation highlights the importance of the radial boundary layer formed at the base of the whirl. The transition therefore also occurs over a flat metal plate, over which temperature maps of blue whirls are obtained using thin-filament pyrometry. Visualization of the reaction front, by imaging spontaneous OH* chemiluminescence, shows that a significant fraction of the combustion occurs in the small, visibly bright, blue ring. The temperature maps are consistent with the burning structure seen with chemiluminescence, and they are qualitatively similar for blue whirls formed over both water and metal plates. High frame-rate images of the transition process show the presence of a recirculation zone within the flame. Together, these observations distinguish the blue whirl as a flame structure unique from the fire whirl and present a basis for understanding the physical processes which control blue whirl formation and its structure.