Radiative ignition process

The initial conditions vary with time because the physicochemical process of ignition varies according to the ignition energy supplied to the ignition surface of the energetic material. A typical example of a radiative ignition process is shown below ... 

The classical analysis corresponding to the ignition process assumes a linear approximation for the surface re-radiation. The radiative term is then defined as... 

When high-intensity radiative energy strikes the surface of a propellant, the surface absorbs the heat and the temperature increases. If the propellant is optically translucent, part of the radiation energy penetrates into the propellant by in-depth absorption. When the temperature of the propellant surface reaches the decomposition temperature, the subsequent ignition process is the same as convective ignition. 

The ignition of Magnesium/Teflon/Viton (MTV) by radiative or conductive heat transfer has been investigated in order to understand processes in both bulkhead and laser igniters (Table 7.1). Lombard appears to be the first to have tested radiative ignition of MTV. He used a solar furnace that allowed a maximum irradiance, H = 100 W cm [2]. Mg/PTFE (polytetrafiuoroethylene) samples of unknown stoichiometry were tested at = 20 W cm. The ignition temperature was determined from radiometric measurements and was found to be quite low, 217 °C. This corresponds to 29 s irradiation at the reported irradiance level and translates into a heating rate of 6 K s. ... 

At present there is no small-scale test for predicting whether or how fast a fire will spread on a wall made of flammable or semiflammable (fire-retardant) material. The principal elements of the problem include pyrolysis of solids char-layer buildup buoyant, convective, tmbulent-boundary-layer heat transfer soot formation in the flame radiative emission from the sooty flame and the transient natme of the process (char buildup, fuel burnout, preheating of areas not yet ignited). Efforts are needed to develop computer models for these effects and to develop appropriate small-scale tests. 

Fig. 13.1 Ignition transient process by conductive, convective, and radiative heat.

Once an ignition has occurred, a pool fire results and the dominant mechanism for damage is via thermal effects, primarily via radiative heat transfer from the resulting flame. If the release of flammable material from the process equipment continues, then a jet fire is also likely (see Section 3.7). If the ignition occurs at the very beginning of the release, then inadequate time is available for the liquid to form a pool and only a jet fire will result. 

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