Formation of a blue whirl controlled by tangential and radial airflows

This paper explores the formation of a blue whirl by controlling the incoming tangential and radial airflows. Utilizing an innovative fire whirl apparatus with forced ventilation, which allows independent adjustment of the fuel supply rate and tangential and radial airflow rates at the inlets, the blue whirl can be formed directly upon ignition without experiencing the transient phase of a regular fire whirl, which was required previously. Moreover, the conditions under which a blue whirl can form are significantly extended, resulting in the discovery of a series of novel flame regimes, including the sooted blue whirl, lifted blue whirl, spiral blue whirl, and lifted spiral fire whirl. Further investigation into the characteristics and evolution processes of different flame regimes reveals that the blue whirl's formation and transition between regimes are significantly influenced by the relative rate between tangential and radial incoming flows. Through dimensional analysis, the underlying mechanism governing the flame regime transition can be interpreted as a competition between centrifugal and radialconverging effects, expressed in terms of two effective pressure differences, Delta P*U theta and Delta P*Ur, respectively. This leads to the definition of a pressure-based swirl number, SP = Delta P*U theta/Delta P*Ur, as a criterion determining the blue whirl's emergence, which has been validated against a wide range of flame conditions.