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

flammable A combustible material is something that can burn (i.e., ''combust'') in air. A combustible material is flammable if it ignites easily at ambient temperatures. In other words, a combustible material ignites with some effort and a flammable mat ...

gas and

air The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing f ...

(or

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...

) 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 a mixture of fuel and oxidizer. Deflagrations can only occur in pre-mixed fuels. Most fires found in daily life are diffu ...

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. Combusti ...

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 overpressures, 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 A flash fire is a sudden, intense fire caused by ignition of a mixture of air and a dispersed flammable substance such as a solid (including dust), flammable or combustible liquid (such as an aerosol or fine mist), or a flammable gas. It is charact ...

. 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 e ...

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

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

above the

autoignition The autoignition temperature or kindling point of a substance is the lowest temperature in which it spontaneously ignites in a normal atmosphere without an external source of ignition, such as a flame or spark. This temperature is required to su ...

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 med ...

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 Rudolf Diesel, is an internal combustion engine in which ignition of the fuel is caused by the elevated temperature of the air in the cylinder due to mechanical compression; thus, the diesel engine is a so-call ...

, 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 at present which can predict the transition phenomenon.

Examples

A deflagration to detonation transition has been a feature of several major

industrial accident A work accident, workplace accident, occupational accident, or accident at work is a "discrete occurrence in the course of work" leading to physical or mental occupational injury. According to the International Labour Organization (ILO), more tha ...

s: * 1970 Propane vapour cloud explosion in Port Hudson * Flixborough disaster *

Phillips disaster of 1989 On 23 October, 1989 at approximately 1:05 PM Central Daylight Time, a series of explosions occurred at Phillips Petroleum Company's Houston Chemical Complex in Pasadena, Texas, near the Houston Ship Channel. The initial blast registered 3.5 on th ...

in Pasadena, Texas * Damage observed in the

Buncefield fire The Buncefield fire was a major fire at an oil storage facility that started on 11 December 2005 at the Hertfordshire Oil Storage Terminal, located near the M1 motorway, Hemel Hempstead, in Hertfordshire, England. The terminal was the fifth la ...

* 2020 Beirut Explosions

Applications

The phenomenon is exploited in pulse detonation engines, 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 The Shchelkin spiral is a device that assists the transition from deflagration (subsonic combustion) to detonation in a pulse detonation engine. The spiral is named after Kirill Ivanovich Shchelkin, a Russian physicist who described it in his 1965 ...

in the

combustion chamber A combustion chamber is part of an internal combustion engine in which the fuel/air mix is burned. For steam engines, the term has also been used for an extension of the firebox which is used to allow a more complete combustion process. Interna ...

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, a vacuum bomb or a fuel air explosive (FAE), is a type of explosive that uses oxygen from the surrounding air to generate a high-temperature explosion. The fuel–air explosive is one of the be ...

Related phenomena

An analogous deflagration to detonation transition (DDT) has also been proposed for thermonuclear reactions responsible for supernovae initiation. This process has been called a "

carbon detonation Carbon detonation or carbon deflagration is the violent reignition of thermonuclear fusion in a white dwarf star that was previously slowly cooling. It involves a runaway thermonuclear process which spreads through the white dwarf in a matter of se ...

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