Deflagration velocity

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... 

The UN deflagration test consists of a Dewar vessel with a volume of about 400 cm3. The vessel is filled with preheated material (standard temperature is 50°C if the stability of the substance permits), and the substance is initiated at the top of the vessel with a flame. The propagation of deflagration is recorded by temperature sensors that are located in the substance at preset distances. From the time required for passing two temperature sensors and from the known distance between them, the deflagration velocity can be calculated. 

Control of a deflagration after initiation by a source such as a hot spot, a flame, or a spark, depends on the rate of deflagration, the confinement, and the accumulation of heat from the evolved energy. Very slow deflagrations can sometimes be controlled under nonconfined situations. Under confined conditions, pressure builds up with simultaneous energy accumulation, which increases the deflagration velocity, most likely to an unacceptable level in processing. 

The Hugoniot curve shows that in the deflagration region the pressure change is very small. Indeed, approaches seeking the unique deflagration velocity assume the pressure to be constant and eliminate the momentum equation. 

For those who have not studied fluid mechanics, the definition of a deflagration as a subsonic wave supported by combustion may sound over sophisticated nevertheless, it is the only precise definition. Others describe flames in a more relative context. A flame can be considered a rapid, self-sustaining chemical reaction occurring in a discrete reaction zone. Reactants may be introduced into this reaction zone, or the reaction zone may move into the reactants, depending on whether the unbumed gas velocity is greater than or less than the flame (deflagration) velocity. 

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... 

Detonation Velocity vs Explosion (or Ignition) Temperature See Table 8 under DETONATION (EXPLOSION AND DEFLAGRATION.) VELOCITY... 

See Table 8, under DETONATION (EXPLOSION AND DEFLAGRATION) VELOCITY and also under Detonation Velocity and Chemical Composition and Detonation Velocity as a Function of Oxygen Balance and Heat of Formation... 

Detonation Velocity Tests. See under DETONATION EXPLOSION AND DEFLAGRATION) VELOCITY, Experimental Determination of Detonation Velocity and also under CHRONOGRAPHS in Vol 3 of Eneycl, pp C304-R to C319-L... 

Detonation (expin and deflagration) velocity 4 D629-D676... 

Recent efforts to distinguish between the terms burning velocity and flame speed on the basis of Eulerian and Lagrangian coordinate systems appear to introduce confusion. Therefore, the terms are used interchangeably here, as synonyms for such terms as deflagration velocity, wave speed, and propagation velocity. They all refer to velocities measured with respect to the gas ahead of the wave. 

Detonation limits have been measured for various fuel-oxidizer mixtures. These values and comparison with the deflagration (flammability) limits are given in Table 7. It is interesting that the detonation limits are always narrower than the deflagration limits. But for H2 and the hydrocarbons, one should recall that, because of the product molecular weight, the detonation velocity has its maximum near the rich limit. The deflagration velocity maximum is always very near the stoichiometric value and indeed has its minimum values at the limits. Indeed, the experimental definition of the deflagration limits would require this result. 

The results for propagation in single crystals of /3-lead azide are interesting. Reaction fails if the crystal thickness falls below 17 fxm [133]. For crystals of thickness greater than this and up to 215 /mn the deflagration velocity increases. In the range 215 /rm to about 2 mm the velocity remains constant at... 

Methods for growing single crystals of a number of azides are now available, and the crystal structures of these have been determined (see Chapters 2 and 3). Experiments to study fast-reaction velocities as a function of crystallographic direction have not been performed. The slow-decomposition studies discussed in Chapter 6 and Section D.l. a suggest that deflagration velocities will depend on crystallographic direction in a number of azides, including sodium, thallous, and lead azide. 

In order to achieve an initiation of decomposition as uniformly as possible, the use of an ignition mixture is recommended by Gibson consisting of silicon and lead oxide in a ratio of two to one [42]. The deflagration velocities may vary quite significantly from less than 1 cm/min up to several 100 cm/min. 

Pitts, L.D. Electrical Probe Technique for Measurement of Detonation and Deflagration Velocities, 4th Symposium (hitemational) on Detonation, Silver Spring, 1965. 

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