Numerical Simulation of the Dynamics of Large Fire Plumes and the Phenomenon of Puffing.
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.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899