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Numerical Simulation of the Dynamics of Large Fire Plumes and the Phenomenon of Puffing.


pdf icon Numerical Simulation of the Dynamics of Large Fire Plumes and the Phenomenon of Puffing. (3086 K)
Ghoniem, A. F.; Lakkis, I.; Soteriou, M.

Combustion Institute, Symposium (International) on Combustion, 26th. Proceedings. Volume 1. July 28-August 2, 1996, Napoli, Italy, Combustion Institute, Pittsburgh, PA, 1531-1539 pp, 1996.

Sponsor:

National Institute of Standards and Technology, Gaithersburg, MD

Keywords:

predictive models; combustion; fire plumes

Abstract:

A vortex-based computational simulation of an axisymmetric fire plume is presented. The physical model accounts for unsteady buoyancy dynamics, conductive and convective heat transfer, and fast combustion. The effect of radiation is modeled by reducing the effective heat release from the combustion zone and raising the temperature of the evaporate from the pool. Numerical solutions are obtained using a modified vortex method in which the element vorticity is updated every time step according to source terms modeling the impact of gravity and pressure gradients in variable density flows. Attention is focused on the unsteady dynamics of the fire plume, or puffing, its dependence on the pool diameter, and the observed similarity between the unsteady behavior of isothermal and fire plumes. The numerical results show that, except for very low Reynolds numbers, fire and isothermal plumes oscillate at a frequency that depends most strongly on the pool diameter. The oscillations are accompanied by the shedding of large burning structures at a point located approximately one pool diameter. We find that the origin of the instability is a Kelvin-Helmholtz type mechanism of the vortex sleeve that forms at the interface between the fuel and air sides of the plume. The numerical results agree with experimental data on the shedding frequency, the average size of the structures and average flame height. We also find that while the buoyancy flux variation with height is different in fire and isothermal plumes, over a range of conditions, both cases have similar density fields near the pool and thus similar shedding characteristics.