On the critical conditions for pool-fire puffing

Abstract

Pool fires are known to undergo a bifurcation to a globally unstable puffing state driven by baroclinic and buoyant vorticity production. Although the supercritical puffing regime away from the bifurcation has been studied extensively in the literature, no detailed account has been given of the critical conditions for its onset, that being the purpose of the present paper. For the relevant canonical case of round liquid pools without swirl, aside from the inherent thermochemical and transport parameters associated with the fuel, pool-fire puffing is governed by a single dimensionless number, the Rayleigh number, which scales with the cube of the pool diameter. Consequently, for a fixed fuel and under fixed ambient conditions, there is a critical fuel pool diameter, associated with a critical value of the Rayleigh number, above which the flame starts puffing. A global linear stability analysis that accounts for the axisymmetry of the prevailing instability mode is developed here to describe the bifurcation. The mathematical formulation employs the limit of infinitely fast reaction, with account taken of the nonunity Lewis number and vaporization characteristics of typical liquid fuels. Predictions of critical puffing conditions, including critical diameters and puffing frequencies, are provided for methanol and for heptane pool fires, and the results are compared with results of new small-scale experiments under controlled laboratory conditions, reported here, yielding reasonably good agreement.

Vortex-dynamical interpretation of anti-phase and in-phase flickering of dual buoyant diffusion flames

“…In the present study, we have the perspective that the flickering flame is physically caused by the dynamics of vortices, as confirmed by numerous experimental studies, and modeled the flickering frequency by calculating the formation and detachment of the main toroidal vortex that is synchronized with the dynamics of the flame [24]. In another perspective, Moreno-Boza et al [15] and others [11,46] interpreted the flickering as a hydrodynamic global instability. As such, the dynamics of the vortices are not modelled physically but treated as a perturbation, which could grow and propagate in the form of instability waves.…”

“…The flickering flame has a sub-harmonic frequency 1 , which is twice of the primary frequency. Physically, the harmonics is closely related to the breakup of the main toroidal vortices in the downstream, which was found to be sensitive to environmental disturbances in previous experiments of laminar diffusion flames [15,17,19]. Based on the result of Table 1, the setting of Case Recently, we adopted vortex-dynamical theory to analytically derive the flickering frequency of a buoyancy-dominated diffusion flame as = 0.48√ / [24], which recovers the prominent scaling relation obtained from previous experimental studies [40,41].…”

“…Probably due to its intrinsic unsteadiness, this problem has not been sufficiently understood, even for the simplest scenario-the interaction of two flames [5][6][7][8][9]. In fact, the unsteadiness could originate from a single flame, for example the "flickering" or "puffing" of a buoyant diffusion flame [10][11][12][13][14][15][16][17][18][19][20][21]. Flickering is a periodic flame oscillation phenomenon.…”

“…For single diffusion flames without or with small initial velocity, buoyancy is the main source that induces the flame to evolve into a self-sustained oscillation mode [12,13,22,23]. The occurrence of flame oscillation phenomena, such as flame pinch-off and flame flickering, can be determined by a Rayleigh number criterion and a Froude number criterion, respectively proposed by Carpio et al [14] and Moreno-Boza et al [15]. Recently, a scaling relation for the frequency of flickering diffusion flames has been analytically obtained by Xia and Zhang [24] based on vortex-dynamical analysis.…”

Comparison of combustion models based on fast chemistry assumption in large eddy simulation of pool fire

“…Oscillatory behavior of buoyancy-driven flows has been mentioned in many prior studies [12,36,37]. This phenomenon in fire simulation is known as the puffing cycle.…”