Numerical studies and experimental observations of whirling flames

• Published in: International journal of heat and mass transfer 2004-06, Vol.47 (12), p.2523-2539
• Main Authors: Snegirev, A.Yu, Marsden, J.A., Francis, J., Makhviladze, G.M.
• Format: Article
• Language: English
• Subjects: Applied sciences; Combustion. Flame; Energy; Energy. Thermal use of fuels; Exact sciences and technology; Miscellaneous; Theoretical studies. Data and constants. Metering
• ISSN: 0017-9310; 1879-2189
• DOI: 10.1016/j.ijheatmasstransfer.2004.02.002

The experimental observations and modelling of buoyant whirling flames in a room-size enclosure are presented. The periodic formation and destruction of whirling core, and the increase of the time-averaged burning rate have been observed in the experiments. To interpret the mechanism of development of buoyant whirling flames, the concepts and results of existing theory of rotating flows have been used, and the conditions necessary for flame rotation to occur have been identified. The CFD model is then discussed which is modified to represent the response of buoyant turbulent diffusion flame on the imposed circulation through decrease of turbulent mixing. The model is first applied to simulate unconfined flames above a round fuel source approximating the pool fire studied in the experiments. Elongation of whirling flames observed in published and our own experiments has been reproduced. The predicted flame characteristics (radius of whirling core, angular velocity, swirl number) dependence on the magnitude of the imposed external circulation was found in qualitative agreement with the simplified theoretical model of whirling flow. Also, the change of flame shape due to rotation resulted in reduced predicted radiative heat flux incident to fuel surface. This indicates the need of additional physical mechanisms to explain and predict the experimentally observed increase in burning rate when the rotation occurs. A possible mechanism, namely entrainment intensification of the air into the fuel rich region near the fuel surface, has been identified. Finally, the CFD model is applied to simulate the experimentally studied whirling flames in the enclosure. The simulation results recreated periodic precession, formation and destruction of the whirling flame as observed in the experiments. The period of oscillations was found to decrease if the fuel supply rate increases.