Detonation
Encyclopedia
Detonation involves a supersonic
Supersonic
Supersonic speed is a rate of travel of an object that exceeds the speed of sound . For objects traveling in dry air of a temperature of 20 °C this speed is approximately 343 m/s, 1,125 ft/s, 768 mph or 1,235 km/h. Speeds greater than five times the speed of sound are often...

 exothermic front accelerating through a medium that eventually drives a shock front
Shock Front
Shock Front is the first full length album by Converter, released November 22, 1999 . The album is released in two editions, the first featuring a special metal plate packaging , the second featuring a regular cardboard booklet format .-Track listing:# "Conqueror"–8:05# "Shock Front"–7:15#...

 propagating directly in front of it. Detonations are observed in both conventional solid and liquid explosives, as well as in reactive gases. The velocity of detonations in solid and liquid explosives is much higher than that in gaseous ones, which allows far clearer resolution of the wave system in the latter.

Gaseous detonations normally occur in confined systems but are occasionally observed in large vapor clouds. They are often associated with a gaseous mixture of fuel and oxidant of a composition, somewhat below conventional flammability limits. There is an extraordinary variety of fuels that may be present as gases, as droplet fogs and as dust suspensions. Other materials, such as acetylene, ozone and hydrogen peroxide are detonable in the absence of oxygen, fuller lists are given by both Stull and Bretherick. Oxidants include halogens, ozone, hydrogen peroxide and oxides of nitrogen and chlorine.

In terms of external damage, it is important to distinguish between detonations and deflagration
Deflagration
Deflagration is a term describing subsonic combustion that usually propagates through thermal conductivity; hot burning material heats the next layer of cold material and ignites it. Most "fire" found in daily life, from flames to explosions, is deflagration...

s where the exothermic wave is subsonic and maximum pressures are at most a quarter of those generated by the former. Processes involved in the transition between deflagration and detonation are covered thoroughly by Nettleton.

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
Chapman-Jouguet condition
The Chapman–Jouguet condition holds approximately in detonation waves in high explosives. It states that the detonation propagates at a velocity at which the reacting gases just reach sonic velocity as the reaction ceases....

 (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 W. Doering. This theory, now known as ZND 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. With a reference frame of a stationary shock, the following flow is subsonic, so that an acoustic reaction zone follows immediately behind the lead front, the Chapman-Jouguet condition
Chapman-Jouguet condition
The Chapman–Jouguet condition holds approximately in detonation waves in high explosives. It states that the detonation propagates at a velocity at which the reacting gases just reach sonic velocity as the reaction ceases....

.
There is also some evidence that the reaction zone is semi-metallic in some explosives.

Both theories describe one-dimensional and steady wave fronts. 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. Indeed, such waves are quenched as their structure is destroyed. The Wood-Kirkwood detonation theory can correct for some of these limitations.

Experimental studies have revealed some of the conditions needed for the propagation of such fronts. In confinement, the range of composition of mixes of fuel and oxidant and self-decomposing substances with inerts are slightly below the flammability limits and for spherically expanding fronts well below them. The influence of increasing the concentration of diluent on expanding individual detonation cells has been elegantly demonstrated. Similarly their size grows as the initial pressure falls. Since cell widths must be matched with minimum dimension of containment, any wave overdriven by the initiator will be quenched.

Mathematical modeling has steadily advanced to predicting the complex flow fields behind shocks inducing reactions. To date none has adequately described how structure is formed and sustained behind unconfined waves.

Applications

The main cause of damage from explosive devices is due to a supersonic blast front (a powerful shock wave
Shock wave
A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...

) in the surrounding area. Therefore, the detonation is primarily associated with explosives and the acceleration of various projectiles. However, detonation waves may also be utilized for less destructive purposes like deposition of coatings to a surface or cleaning of equipment (e.g. slag removal). Pulse detonation engine
Pulse detonation engine
A pulse detonation engine, or "PDE", is a type of propulsion system that uses detonation waves to combust the fuel and oxidizer mixture. The engine is pulsed because the mixture must be renewed in the combustion chamber between each detonation wave initiated by an ignition source. Theoretically, a...

s utilize the detonation wave for aerospace propulsion. The first flight of an aircraft powered by a pulse detonation engine took place at the Mojave Air & Space Port on January 31, 2008.

In engines and firearms

Unintentional detonation when deflagration
Deflagration
Deflagration is a term describing subsonic combustion that usually propagates through thermal conductivity; hot burning material heats the next layer of cold material and ignites it. Most "fire" found in daily life, from flames to explosions, is deflagration...

 is desired is a problem in some devices. In internal combustion engines it is called engine knocking
Engine knocking
Knocking in spark-ignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front.The...

 and causes loss of power and excessive heating of certain components. In fire arms, it may cause catastrophic and possibly lethal failure.

See also

  • Carbon detonation
    Carbon detonation
    Carbon detonation is the violent re-ignition of thermonuclear fusion in a dead star, which produces a Type Ia supernova. A white dwarf undergoes carbon detonation only if it has a normal binary companion which is close enough to the dwarf star to dump sufficient amounts of matter onto the dwarf,...

  • Detonator
    Detonator
    A detonator is a device used to trigger an explosive device. Detonators can be chemically, mechanically, or electrically initiated, the latter two being the most common....

  • Detonation of an explosive charge
  • Detonation diamond
  • Detonation flame arrester
    Detonation flame arrester
    A detonation flame arrester is a device fitted to the opening of an enclosure or to the connecting pipe work of a system of enclosures and whose intended function is to allow flow but prevent the transmission of flame propagating at supersonic velocity and characterized by a shock wave...

  • Sympathetic detonation
    Sympathetic detonation
    A sympathetic detonation , also called flash over, is a detonation, usually unintended, of an explosive charge by a nearby explosion. Sympathetic detonation is caused by a shock wave, or impact of primary or secondary blast fragments.The initiating explosive is called donor explosive, the initiated...

  • Nuclear testing
    Nuclear testing
    Nuclear weapons tests are experiments carried out to determine the effectiveness, yield and explosive capability of nuclear weapons. Throughout the twentieth century, most nations that have developed nuclear weapons have tested them...

  • Predetonation
  • Chapman-Jouguet condition
    Chapman-Jouguet condition
    The Chapman–Jouguet condition holds approximately in detonation waves in high explosives. It states that the detonation propagates at a velocity at which the reacting gases just reach sonic velocity as the reaction ceases....

  • Engine knocking
    Engine knocking
    Knocking in spark-ignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front.The...

  • Deflagration
    Deflagration
    Deflagration is a term describing subsonic combustion that usually propagates through thermal conductivity; hot burning material heats the next layer of cold material and ignites it. Most "fire" found in daily life, from flames to explosions, is deflagration...


External links

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