Effects of circulation and buoyancy on the transition from a fire whirl to a blue whirl

Abstract

The relative influence of circulation and buoyancy on fire whirls (FWs), blue whirls (BWs), and the transition between these regimes of a whirling flame is investigated using a combination of experimental data and scaling analyses. FWs are whirling, turbulent, cylindrical yellow (sooting) flame structures that form naturally in fires and are here created in laboratory experiments. In contrast, a BW is a laminar, blue flame (nonsooting) with an inverted conical shape. Measurements of the circulation and heat-release rate are combined with measurements of the flame geometry, defined by the flame width and the height, to provide characteristic length scales for these whirling-flame regimes. Using these, a nondimensional circulation (* f) and a heat-release rate (Q * f) were defined and shown to correspond to azimuthal and axial (buoyancy driven) momenta, respectively. The ratio R * = * f /Q * f , a quantity analogous to the swirl number used to characterize swirling jets, was evaluated for FWs and BWs. For FWs, R * < 1, so that axial momentum is greater than azimuthal momentum and the flame is dominated by buoyant momentum. For BWs, R * > 1, so that the flame is circulation dominated. This is argued to be consistent with vortex breakdown being an important part of the transition of FWs to BWs. This work presents a basis for predicting when a BW will form and remain a stable regime.

Effects of Natural and Forced Entrainment on PM Emissions from Fire Whirls

“…Comparing these quantities based on scaling analyses in the literature, , the ratio */Γ* is shown in Figure . In general, the quantity */Γ* is generally much lower for fire whirls under FE than NE.…”

“…For instance, at small length scales (5 cm), the blue whirl is a regime that burns completely without soot formation . The blue whirl regime, however, occurs at low heat-release rates, − and transition from the fire whirl to the blue whirl requires high azimuthal momentum when compared to the axial momentum. , At slightly larger length scales (15 cm), the effects of over-rotation are strongly evident when forming fire whirls in microgravity conditions, , causing an increase in particulate emissions.…”

“…The resulting intensification of combustion imposes higher heat feedback to the fuel surface, increasing the burning rate. , Fire whirls can be generated using a variety of mechanisms, including the use of physical walls, flow forcing devices such as fans, topographical features as in the case of wildfires, or even obstacles such as large buildings . Hariharan et al showed that the regime of a fire whirl can be determined by the relative magnitudes of the buoyant and azimuthal momenta. Regardless of generation mechanism, the burning behavior and emissions from a fire whirl are sensitive to the dynamics of air entrainment.…”

Comparison of particulate-matter emissions from liquid-fueled pool fires and fire whirls

“…A number of open questions regarding the emissions from fire whirls remain to be answered before practical applications are considered. While soot-free whirling flames such as the "blue whirl" are known in the literature [48,49,50], they form at very small length scales [40] and the transition to the blue whirl is highly sensitive to experimental conditions [51,52]. Fire whirls, on the other hand, can be formed more robustly across a large range of length scales.…”