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 (${\mathrm{\Gamma }}_f^{*}$) and a heat-release rate (${\dot{Q}}_f^{*}$) were defined and shown to correspond to azimuthal and axial (buoyancy driven) momenta, respectively. The ratio ${\mathcal{R}}^{*}={\mathrm{\Gamma }}_f^{*}/{\dot{Q}}_f^{*}$, a quantity analogous to the swirl number used to characterize swirling jets, was evaluated for FWs and BWs. For FWs, ${\mathcal{R}}^{*}<1$, so that axial momentum is greater than azimuthal momentum and the flame is dominated by buoyant momentum. For BWs, ${\mathcal{R}}^{*}>1$, 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.

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• Combustion
• Reacting flows
• Vortex flows