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Pressure waves from deflagrations: dependence on flame velocity and acceleration

Figure 1. Dependence of shock overpressure generated
by hydrogen-air deflagration on dimensionless distance
for different flame speeds (DorofeevSB:2002).

Deflagration processes produce pressure waves ranging from extremely weak acoustic waves, which man can only hear, to extremely strong shock waves in case of detonation, which can produce consid-erable damages.

The intensity of pressure wave resulted from deflagration is defined by the intensity of the combustion process. In case of slow subsonic flames the pressure wave typically has gradually growing front with the typical peak overpressure less than 1 bar and duration of several seconds. In case of fast supersonic flames (which are usually fast turbulent flames) the pressure wave transforms into shock (which is characterized by abrupt step-wise changes in media characteristics) with typical overpressures of order of magnitude of 10 bar and its duration of 200 - 500 ms. In the limiting case of detonation a shock wave is also formed but with higher peak values and shorter duration. For example, in case of detona-tion of stoichiometric hydrogen-air mixture at normal conditions an amplitude of detonation wave reaches 17 bar with duration of less than 100 ms.

Pressure waves generated by combustion process decay while they propagate from their source. In far zone the amplitude of such pressure wave is defined only by the chemical energy released during com-bustion. This means that for the same amount of hydrogen independently on the intensity and charac-teristics of the combustion process, the overpressure value will be the same. On the other hand (see e.g., Figure 1), in the near zone the observed peak overpressure strongly depends on the speed of combustion.

Dorofeev S.B. (2002) Flame acceleration and DDT in gas explosions. Journal de Physique de France IV, 12(7):3-10.(BibTeX)

Content: Deflagration

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