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Critical Mass Pyrolysis Rates for Extinction in Fires Over Solid Materials.


pdf icon Critical Mass Pyrolysis Rates for Extinction in Fires Over Solid Materials. (2408 K)
Delichatsios, M. A.

NIST GCR 98-746; FMRC J.I. 0Y1J0.RU/0Y1N3.RU; 44 p. April 1998.

Sponsor:

National Institute of Standards and Technology, Gaithersburg, MD

Available from:

National Technical Information Service
Order number: PB98-146137

Keywords:

solids; pyrolysis rate; extinction; charring; solid fuels; heat flux; inert gases; nitrogen

Abstract:

New flame extinction conditions for the critical mass pyrolysis rate are developed when extinction occurs by interaction of flames with the pyrolyzing surface of a condensed material. The extinction conditions provide the critical mass pyrolysis rate and the corresponding convective heat flux to the surface. A novel chemistry based formulation for extinction is provided which shows that the sum of fuel mass fraction near the surface and the ambient oxygen mass fraction (correctedfor stoichiometry and combustion efficiency) is constant. The extinction conditions are derived from simple anslysis of combustion and heat transfer, and they are shown to be applicable for various experimental conditions such as fuel dilution by inert gas, oxygen dilution by inert gas, effects of external heat flux, material preheating, transient (charring) pyrolysis, including geometric effects which influence the critical mass pyrolysis rate through an effective heat transfer coefficient. Additional validation of the proposed extinction conditions is provided by numerical simulation reported in literature in the regime of low straining rates for a stagnation flow on a cylinder. The present approach can be used to measure critical extinction conditions in a flammability apparatus and allow them to be applied in other conditions such as in microgravity. The critical extinction conditions are needed to calculate transient decay pyrolysis of solids after an extinguishment agent is applied.