Detonation

Detonation

[det-n-ey-shuhn]

Detonation is a process of supersonic combustion in which a shock wave is propagated forward due to energy release in a reaction zone behind it. It is the more powerful of the two general classes of combustion, the other one being deflagration. In a detonation, the shock compresses the material thus increasing the temperature to the point of ignition. The ignited material burns behind the shock and releases energy that supports the shock propagation. This self-sustained detonation wave is different from a deflagration, which propagates at a subsonic speed (i.e., slower than the sound speed of the explosive material itself), and without a shock or any significant pressure change. Because detonations generate high pressures, they are usually much more destructive than deflagrations.

Detonations can be produced by high explosives, reactive gaseous mixtures, certain dusts and aerosols.

Etymology

French détoner, to explode; from Latin detonare, to expend thunder; from de-, ~off + tonare, to thunder

Theories

The simplest theory to predict the behavior of detonations in gases is known as Chapman-Jouguet (CJ) theory, developed around the turn of the 20th century. This theory, described by a relatively simple set of algebraic equations, models the detonation as a propagating shock wave accompanied by exothermic heat release. Such a theory confines the chemistry and diffusive transport processes to an infinitely thin zone.

A more complex theory was advanced during World War II independently by Zel'dovich, von Neumann, and Doering. This theory, now known as ZND (explosion) theory, admits finite-rate chemical reactions and thus describes a detonation as an infinitely thin shock wave followed by a zone of exothermic chemical reaction. In the reference frame in which the shock is stationary, the flow following the shock is subsonic. Because of this, energy release behind the shock is able to be transported acoustically to the shock for its support. For a self-propagating detonation, the shock relaxes to a speed given by the Chapman-Jouguet condition, which induces the material at the end of the reaction zone to have a locally sonic speed in the reference frame in which the shock is stationary. In effect, all of the chemical energy is harnessed to propagate the shock wave forward.

Both CJ and ZND theories are one-dimensional and steady. However, in the 1960s experiments revealed that gas-phase detonations were most often characterized by unsteady, three-dimensional structures, which can only in an averaged sense be predicted by one-dimensional steady theories. Modern computations are presently making progress in predicting these complex flow fields. Many features can be qualitatively predicted, but the multi-scale nature of the problem makes detailed quantitative predictions very difficult.

Applications

Detonations are hard to control and are used primarily for demolition and in warfare. A great deal of research is conducted on achieving or preventing detonation in various materials to improve the performance of explosives and engines. An experimental form of jet propulsion, the pulse detonation engine, uses a series of well-timed detonations to generate thrust.

Detonation in reciprocating engines (sometimes called engine knock) is the uncontrolled supersonic explosion of the fuel-air charge, and may be caused by excessively high combustion chamber temperatures. It may occur prior to the firing of the spark plug in Otto-cycle engines. High compression ratios or high temperatures can cause the flame speed of combustion to exceed the sound barrier, resulting in an explosion or detonation and a pressure shockwave. Detonations are extremely destructive to common piston engines, and may result in mechanical deformation of the cylinder walls, piston rings, holes blown through the top of pistons, or cracks in cylinder heads. Detonation can also burn out a spark plug, reducing or eliminating its ability to produce a spark. Detonation is most destructive if it occurs during times in which the explosion forces the piston downwards while it is traveling upwards, which relates to engine timing.

References

For a good layman's introduction to the theory of detonation, see:

Davis, William C., "The detonation of explosives," Scientific American, vol. 256, pages 106-112, 128 (May 1987).

See also

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