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Numerical Simulation of Rapid Combustion in an Underground Enclosure.

Numerical Simulation of Rapid Combustion in an
Underground Enclosure.
(776 K)

McGrattan, K. B.; Baum, H. R.; Deal, S.

NISTIR 5809; 16 p. April 1996.

### Available from:

National Technical Information Service

Order number: PB96-183132

### Keywords:

combustion; enclosures; fluid dynamics; high
temperature; tests; accelerants; zone models; predictive
models; fire models; field models

### Abstract:

*
The scenario of interest is a two second firing of a
rocket engine in an underground enclosure intended to
mimic the effect of burning a high temperature
accelerant (HTA). Because of the unusual nature of the
problem, at least in the context of typical fire
scenarios, two types of numerical models have been
applied to the problem. The first, a zone model,
divides each room in the enclosure into one or two
control volumes, and the transport of mass and energy
from the burn room is estimated from the basic
conservation laws. The second model, a field model
designed for relatively low Mach number flows, solves
the conservation equations of mass, momentum and energy
discretized over hundreds of thousands of cells. The
first approach has the advantage of providing a fast,
robust description of the overall thermodynamic
quantities of interest. The second approach provides a
much more detailed description of the temporal and
spatial evolution of these quantities. The energy
release for the two second firing of the rocket is
enormous. In all, 245 kg (540 lb) of solid fuel is
consumed in two seconds. The total energy released is
given as 1093 cal/g (4575 kJ/kg). Of this, it is
estimated that about half is lost to the walls or
converted to kinetic energy. The remaining energy
creates a tremendous pressure and temperature rise
throughout the facility. Both the zone model (CFAST2.0)
and the field model (NIST Large Eddy Simulation) predict
that the pressure in the enclosure after the 2 s firing
will rise about 1 atmosphere, and the temperature about
1500 C. Both models simulate one minute following
ignition, by which time the pressure in the entire
enclosure has returned to atmospheric and the
temperature to several hundred degrees over ambient,
depending on location. There is little convective
motion by this time, and the temperature decrease is
largely dependent on the absorption of heat by the
walls.
*