Critical Mass Pyrolysis Rates For Extinction Of Fires Over Solid Materials

Abstract

New flame extinction conditions for the critical mass pyrolysis rate are developed when extinction occurs by interaction of flames with the pyrolyzjng surface of a condensed m a t e d The extinction conditions provide the critical mass pyrolysis rate and the corresponding convective heat flux to the surface. A novel formulation shows that the sum of fuel mass fraction near the surface and the ambient oxygen mass fraction corrected for stoichiometry and incompleteness of combustion is constant. The extinction conditions are derived from simple analysis 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 the literature in the regime of low straining rates for a stagnation flow on a cylinder. The present approach can be used to obtain the critical extinction conditions from measurements in a standard flammability apparatus.

Prediction of the Limiting Oxygen Index Using Simple Flame Extinction Theory and Material Properties Obtained from Bench Scale Measurements

“…With a further decrease in the mass flux, the convective heat flux decreases as the flame approaches the surface because of quenching of the chemical reaction. As an analogy to the combustion of a solid material, the mass supply of the fuel in the porous burner may be related to the rate of mass pyrolysis and the mass flux corresponding to the maximum of the convective heat flux is the critical mass flux at which extinction occurs [13]. For a solid material, when the rate of mass pyrolysis becomes lower than the critical mass flux, the energy balance at the surface of the material does not allow to sustain a flame and extinction conditions are reached.…”

“…1, which shows the calculated flame heat flux feedback to the surface versus the normalized mass flux at the wall for both infinitely fast kinetics and when this heat flux is maximum and also by asymptotic analyses for plane counter-flow flame situation carried elsewhere [13][14][15][16]. The approximate expression of the convective heat flux in Eq.…”

Extinguishment of a PMMA fire by water spray with high droplet speeds

“…As discussed in the literature [8], fuel mass flux would decrease when mass pyrolysis rate decreases, and surface blowing effect would then decrease to increase the convective heat transfer. Therefore, maximum convective heat flux from the flame to the fuel surface is found at extinction.…”

“…Method B: Empirical values are used directly by taking h to be independent of the fuel surface diameter D for turbulent conditions, and proportional to D −1/4 for laminar conditions [8].…”

“…Also, fine water spray (water mist) has been used for suppressing solid fires in recent years [5]. Experimental and numerical investigations have been conducted on plastic fire extinguishment by water spray [6][7][8][9].…”