Extinction of Opposed Flame Spread and Regression Burning of Thick Solid Fuels

There are several flammability markers associated with solid combustible materials, although extinction is a critical one once a fire has ignited. The determination of the extinction of a material is often based on the oxygen volume fraction below which the material doesn’t burn. Testing is performed in the Limiting Oxygen Index apparatus (LOI- ASTM 2863), by measuring the oxygen concentration below which a flame spreading downward over a material extinguishes. The corresponding oxygen volume fraction is referred to as the limiting oxygen index, or concentration (LOI/LOC). The test however doesn’t differentiate between a spreading flame spread or a flame sustained by the fuel burning behind the flame, which has been found to occur at two different LOC’s. Although significant research has been conducted on this topic, there is still a need for further work to predict the extinction of a flame, particularly in non-earth atmospheres such as in a spacecraft or space habitats. In this work an analytical model is presented describing the problem. For both, flame spread and fuel regression extinction, the analysis predicts two oxygen molar fraction boundaries for extinction: a “heat transfer” boundary and a “chemical kinetics” boundary. The former is given by the conditions needed to heat the solid to its pyrolysis temperature, and the latter by the conditions needed for the gas phase reaction to reach the critical Damkohler number for extinction. The combination of two boundaries provides overall LOC for extinction of both flame spread (FS) and fuel regression (FR), as a function of material properties, gas flow velocity, ambient pressure, gravity level, and external heating. The model predictions for extinction of opposed flame spread over for polymethylmethacrylate (PMMA) sheets are compared with data available in the literature. The results show a qualitative agreement between the model predictions and experimental data.