An experimental study of the blue whirl onset

This paper focuses on the blue whirl's formation conditions and onset mechanism. The blue whirl is experimentally generated using a forced-ventilation fire whirl apparatus, where the swirl intensity can be actively adjusted. It also allows the blue whirl to form in the laminar regime, without requiring a turbulent fire whirl as the inducer. Transverse particle image velocimetry (PIV) measurement is performed to understand the swirling flow near the vortex core and to quantify the swirl intensity in terms of circulation. The result demonstrates that a stable blue whirl can be formed only when the circulation is above a threshold value of about 0.07 m²/s, for the current experimental setup. Then, by examining the formation process during the blue whirl onset, a strong correlation can be confirmed between the circulation jumping and the flame regime transformation from a weak fire whirl to a blue whirl. To understand the critical condition for vortex breakdown, we further propose a new swirl number, S_P, based on balancing the pressure differences associated with the centrifugal and the radial-converging effects. As a result, the turning point in the S_P curve agrees well with the emergence of the recirculation zone, verifying the relevance of the proposed vortex breakdown mechanism in determining the blue whirl onset.