Deflagration front

A.C. McIntosh. Deflagration fronts and compressibility. Philosphical Transactions of the Royal Society of London A, 357 3523-3538, 1999. 

In the 1950s, the more descriptive schlieren records of the interactions between pressure waves and deflagration fronts were obtained [16-18], and Oppenheim [9] introduced the hypothesis of the "explosion in the explosion" (of the detonating mixture) occurring in the regime of accelerating flame to explain the sudden change in the velocity of the combustion wave observed in the experiments. 

Fig. 5.24. Maximum temperatures and densities calculated for an outward propagating deflagration front in a model (model W7) of an SN la explosion from accretion on to a CO white dwarf with initial mass 1 M at a rate of 4 x 10-8 M yr 1. Zones of different burning conditions are indicated. After Thielemann, Nomoto and Yokoi (1986). Courtesy Ken-ichi Nomoto.

In the trough test, the sample (only solids) is introduced in a horizontal wire mesh cage with an inner volume of 11 liters. The substance is initiated at one end of this trough by a gas burner or electrical heating source and the propagation of the deflagration front is established and noted. 

L.D. Pitts, "Electrical Probe Technique for Measurement of Detonation and Deflagration Velocities , 4thONRSympDeton(1965), 616-26. In these experiments the probe consisted of a length of resistance wire sandwiched between two strips of insulating material. After placing the probe adjacent to the wall of the metallic test cylinder, a constant current was forced thru the probe. Detonation, or deflagration front pressure... 

Nevertheless, the most typical general feature of a reaction is the existence of fronts of chemical transformation which are able to propagate, without being extinguished, in a hot mixture with a constant velocity at subsonic speed for a laminar flame (or deflagration front), at supersonic speed for a detonation wave (see below for a more detailed discussion of this paper). 

Deflagration. In a deflagration, the flame front travels through the flammable mixture relatively slowly, i.e., at subsonic velocity. 

Deflagration A propagating chemical reaction of a substance in which the reaction front advances into the unreacted substance at less than the sonic velocity in the unreacted material. Where a blast wave is produced that has the potential to cause damage, the term explosive deflagration may be used. 

Deflagration to Detonation Transition A reaction front that starts out with velocities below the speed of sound and subsequently accelerates to velocities higher than the speed of sound in the unreacted material is said to have undergone a Deflagration to Detonation Transition. The possibility of transition is enhanced by confinement/turbulence generators in the path of the reaction front. 

Deflagration The ehemieal reaetion of a substanee in whieh the reaetion front advanees into the unreaeted substanee at less than sonie veloeity. Where a blast wave is produeed that has the potential to eause damage, the term explosive deflagration is used. 

Chemical explosives detonate, or deflagrate. Detonating explosives (e.g., TNT or dynamite) rapidly decompose to produce high pressure and a shock front (travels faster than the velocity of sound). Deflagrating explosives (e.g., black and smokeless powders) bum fast, prodr er... 

Chemical explosions are uniform or propagating explosions. An explosion in a vessel tends to be a uniform explosion, while an explosion in a long pipe is a propagating explosion. Explosions are deflagrations or detonations. In a deflagration, the burn is relatively slow, for hydrocarbon air mixtures the deflagration velocity is of the order of 1 m/s. In contrast, a detonation flame shock front is followed closely by a combustion wave that releases energy to sustain the shock wave. A... 

It is common practice in the chemical process industries to provide isolation devices for stopping flame fronts, deflagration pressures, pressure piling, and flame-jet ignition between process equipment interconnected by pipes or ducts. There are several devices for providing this isolation as follows ... 

FIGURE 4-3. Differences between deflagration and detonation flame fronts. 

A radial force on the pipe wall ahead of the deflagration wave. There is a varying pressnre between the aconstic wave and the flame front where the pressnre bnilds from near atmospheric pressnre, Pi (step change at the wave front) to eight times Pi (or higher) at the flame front. The pressnre ratios depend on the flame acceleration. There is no snch effect with a detonation. 

A radial force on the pipe watt behind the flame front. For a closed pipe, this remains constant (at abont STi) everywhere for a deflagration, bnt decays from abont 20Ti to STi behind the detonation over most of the pipe length. 

Deflagration Isolation A method employing equipment and procedures that interrupts the propagation of a deflagration flame front past a point (usually in a pipe). 

Flame Front Diverter A device that opens in response to the pressure wave preceding the flame front of the deflagration, venting the flame front and pressure wave. 

The preceding section described the state of transition expected in a deflagration process when the mixture in front of the flame is sufficiently preconditioned by a combination of compression effects and local quenching by turbulent mixing. However, additional factors determine whether the onset of detonation can actually occur and whether the onset of detonation will be followed by a self-sustaining detonation wave. 

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