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Numerical Simulation of Rapid Combustion in an Underground Enclosure.


pdf icon 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.