Explosions: Deflagration Versus Detonation

An Explosion
Deflagration and detonation are two forms of combustion. Ken Hermann/Getty Images

Combustion takes different forms. When a decomposition reaction or combination reaction releases a lot of energy in a very short span of time, an explosion may occur. Deflagration and detonation are two ways energy may be released. If the combustion process propagates outward at subsonic speeds, it's a deflagration. If the explosion moves outward at supersonic speeds, it's a detonation.

Deflagration

Everyday fire and most explosions are examples of deflagration.

The flame propagation velocity is less than 100 m/s (usually much lower) and the overpressure is less than 0.5 bar. Because it is controllable, deflagration can be harnessed. Examples of deflagrations include:

  • internal combustion engine
  • gas stove
  • fireworks and other pyrotechnics
  • gunpowder in a firearm
  • grease fire or chip pan fire

Detonation

A detonation is a dramatic, often destructive form of an explosion. It is characterized by a supersonic exothermic front (in excess of 100 m/s up to 2000 m/s) and significant overpressure (up to 20 bars). The front drives a shockwave ahead of it.

Although technically a form of oxidation reaction, a detonation doesn't require combination with oxygen. Unstable molecules release considerable energy when they split and recombine into new forms. Examples of chemicals that produce detonations include any high explosives, such as:

  • TNT (trinitrotoluene)
  • nitroglycerine
  • dynamite
  • picric acid
  • C4

Deflagration to Detonation Transition

In some situations, a subsonic flame may accelerate into a supersonic flame. This deflagration to detonation is difficult to predict but occurs most often when eddy currents or other turbulence are present in the flames. This can happen if the fire is partially confined or obstructed.

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Helmenstine, Anne Marie, Ph.D. "Explosions: Deflagration Versus Detonation." ThoughtCo, Mar. 23, 2017, thoughtco.com/explosions-deflagration-versus-detonation-607316. Helmenstine, Anne Marie, Ph.D. (2017, March 23). Explosions: Deflagration Versus Detonation. Retrieved from https://www.thoughtco.com/explosions-deflagration-versus-detonation-607316 Helmenstine, Anne Marie, Ph.D. "Explosions: Deflagration Versus Detonation." ThoughtCo. https://www.thoughtco.com/explosions-deflagration-versus-detonation-607316 (accessed November 20, 2017).