Deflagration to detonation transition (DDT) refers to a phenomenon in ignitable mixtures of a

flammable A combustible material is a material that can burn (i.e., sustain a flame) in air under certain conditions. A material is flammable if it ignites easily at ambient temperatures. In other words, a combustible material ignites with some effort ...

gas and

air An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

(or

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

) when a sudden transition takes place from a

deflagration Deflagration (Lat: ''de + flagrare'', 'to burn down') is subsonic combustion in which a pre-mixed flame propagates through an explosive or a mixture of fuel and oxidizer. Deflagrations in high and low explosives or fuel–oxidizer mixtures ma ...

type of

combustion Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion ...

to a

detonation Detonation () is a type of combustion involving a supersonic exothermic front accelerating through a medium that eventually drives a shock front propagating directly in front of it. Detonations propagate supersonically through shock waves with ...

type of explosion.

Description

is characterized by a subsonic flame propagation velocity, typically far below , and relatively modest

overpressure Overpressure (or blast overpressure) is the pressure caused by a shock wave over and above normal atmospheric pressure. The shock wave may be caused by sonic boom or by explosion An explosion is a rapid expansion in volume of a given amoun ...

s, typically below . The main mechanism of combustion propagation is of a flame front that moves forward through the gas mixture - in technical terms the reaction zone (chemical combustion) progresses through the medium by processes of

diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...

of heat and mass. In its most benign form, a deflagration may simply be a flash fire. In contrast, a

is characterized by

supersonic Supersonic speed is the speed of an object that exceeds the speed of sound (Mach 1). For objects traveling in dry air of a temperature of 20 °C (68 °F) at sea level, this speed is approximately . Speeds greater than five times ...

flame propagation velocities, perhaps up to , and substantial overpressures, up to . The main mechanism of detonation propagation is of a powerful

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...

wave that compresses the unburnt gas ahead of the wave to a

temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

above the autoignition temperature. In technical terms, the reaction zone (chemical combustion) is a self-driven

shock wave In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a me ...

where the reaction zone and the shock are coincident, and the chemical reaction is initiated by the compressive heating caused by the shock wave. The process is similar to ignition in a

Diesel engine The diesel engine, named after the German engineer Rudolf Diesel, is an internal combustion engine in which Combustion, ignition of diesel fuel is caused by the elevated temperature of the air in the cylinder due to Mechanics, mechanical Compr ...

, but much more sudden and violent. Under certain conditions, mainly in terms of geometrical conditions (such as partial confinement and many obstacles in the flame path that cause turbulent flame eddy currents), a subsonic flame front may accelerate to supersonic speed, transitioning from deflagration to detonation. The exact mechanism is not fully understood, and while existing theories are able to explain and model both deflagrations and detonations, there is no theory which can predict the transition phenomenon.

Examples

A deflagration to detonation transition has been a feature of several major industrial accidents: * 1970 propane vapor cloud explosion in Port Hudson * Flixborough disaster * Phillips disaster of 1989 in Pasadena, Texas * Damage observed in the

Buncefield fire The Buncefield fire was a major fire at an oil storage facility that started at 06:01 UTC on Sunday 11 December 2005 at the Hertfordshire Oil Storage Terminal, located near the M1 motorway, Hemel Hempstead, in Hertfordshire, England. The ter ...

* 2020 Beirut explosions

Applications

The phenomenon is exploited in

pulse detonation engine A pulse detonation engine (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 wav ...

s, because a detonation produces a more efficient combustion of the reactants than a deflagration does, i.e. giving a higher yields. Such engines typically employ a Shchelkin spiral in the

combustion chamber A combustion chamber is part of an internal combustion engine in which the air–fuel ratio, fuel/air mix is burned. For steam engines, the term has also been used for an extension of the Firebox (steam engine), firebox which is used to allow a mo ...

to facilitate the deflagration to detonation transition. The mechanism has also found military use in

thermobaric weapon A thermobaric weapon, also called an aerosol bomb, or a vacuum bomb, is a type of explosive munition that works by dispersing an aerosol cloud of gas, liquid or powdered explosive. The fuel is usually a single compound, rather than a mixture o ...

Related phenomena

An analogous deflagration to detonation transition (DDT) has also been proposed for thermonuclear reactions responsible for

supernovae A supernova (: supernovae or supernovas) is a powerful and luminous explosion of a star. A supernova occurs during the last evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion. The original ob ...

initiation. This process has been called a " carbon detonation".

References

* {{cite book , last = Lea , first = CJ , author2=HS Ledin , title = A Review of the State-of-the-Art in Gas Explosion Modelling, HSL/2002/02 , publisher = UK Health and Safety Laboratories , year = 2002 , url = http://www.hse.gov.uk/research/hsl_pdf/2002/hsl02-02.pdf Combustion Industrial fires and explosions Explosives engineering

Description

Examples

Applications

Related phenomena

See also

References