Deflagration to detonation transition
Encyclopedia
Deflagration to detonation transition (DDT) refers to a phenomenon in ignitable
mixtures of a flammable gas and air (or oxygen
) when a sudden transition takes place from a deflagration
type of combustion
to a detonation
type of combustion. The effects of a detonation are usually devastating.
A deflagration
is characterized by a subsonic
flame propagation velocity, typically far below 100 m/s, and relatively modest overpressure
s, say below 0.5 bar
. 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 of heat and mass. In its most benign form, a deflagration may simply be a flash fire
. In contrast, a detonation
is characterized by supersonic
flame propagation velocities, perhaps up to 2000 m/s, and substantial overpressures, up to 20 bars. The main mechanism of combustion propagation is of a powerful pressure
wave that compresses the unburnt gas ahead of the wave to a temperature
above the autoignition temperature. In technical terms, the reaction zone (chemical combustion) is a self-driven shock wave
where the reaction zone and the shock are coincident, and the chemical reaction is initiated by the compressive heating caused by the shock wave.
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 current
s, a subsonic flame 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.
A deflagration to detonation transition has been a feature of several major industrial accidents
The phenomenon is exploited in pulse detonation engine
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
to facilitate the deflagration to detonation transition.
The mechanism has found military use in the thermobaric weapon
.
A deflagration to detonation transition (DDT) has also been proposed for thermonuclear reactions responsible for supernovae initiation; see also Carbon detonation
.
Apart from the name, this phenomenon is completely unrelated to the chemical combustion and flame acceleration phenomenon.
Combustion
Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...
mixtures of a flammable gas and air (or oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
) when a sudden transition takes place from a 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...
type of combustion
Combustion
Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...
to a detonation
Detonation
Detonation involves a supersonic exothermic front accelerating through a medium that eventually drives a shock front propagating directly in front of it. Detonations are observed in both conventional solid and liquid explosives, as well as in reactive gases...
type of combustion. The effects of a detonation are usually devastating.
A 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 characterized by a subsonic
Speed of sound
The speed of sound is the distance travelled during a unit of time by a sound wave propagating through an elastic medium. In dry air at , the speed of sound is . This is , or about one kilometer in three seconds or approximately one mile in five seconds....
flame propagation velocity, typically far below 100 m/s, and relatively modest overpressure
Overpressure
The term Overpressure is applied to a pressure difference, relative to a "normal" or "ambient" pressure, in various circumstances:* In engineering: the pressure difference over the wall thickness of a pressure vessel...
s, say below 0.5 bar
Bar (unit)
The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...
. 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 of heat and mass. In its most benign form, a deflagration may simply be a flash fire
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 , flammable or combustible liquid , or a flammable gas...
. In contrast, a detonation
Detonation
Detonation involves a supersonic exothermic front accelerating through a medium that eventually drives a shock front propagating directly in front of it. Detonations are observed in both conventional solid and liquid explosives, as well as in reactive gases...
is characterized by 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...
flame propagation velocities, perhaps up to 2000 m/s, and substantial overpressures, up to 20 bars. The main mechanism of combustion propagation is of a powerful pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
wave that compresses the unburnt gas ahead of the wave to a temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
above the autoignition temperature. In technical terms, the reaction zone (chemical combustion) is a self-driven 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...
where the reaction zone and the shock are coincident, and the chemical reaction is initiated by the compressive heating caused by the shock wave.
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 current
Eddy current
Eddy currents are electric currents induced in conductors when a conductor is exposed to a changing magnetic field; due to relative motion of the field source and conductor or due to variations of the field with time. This can cause a circulating flow of electrons, or current, within the body of...
s, a subsonic flame 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.
A deflagration to detonation transition has been a feature of several major industrial accidents
- 1970 Propane vapour cloud explosion in Port Hudson1970 Propane vapour cloud explosion in Port HudsonThe 1970 Propane vapor cloud explosion in Port Hudson resulted from a propane pipeline break, which led to the formation of a large, dense vapor cloud. Upon ignition, the vapor cloud exploded with tremendous force...
- The Flixborough disasterFlixborough disasterThe Flixborough disaster was an explosion at a chemical plant close to the village of Flixborough, England, on 1 June 1974. It killed 28 people and seriously injured 36.-Background:...
- The 1989 Phillips DisasterPhillips DisasterThe Phillips Disaster refers to a devastating series of explosions and fire on October 23, 1989 near the Houston Ship Channel in Pasadena, Texas, USA. The initial blast registered 3.5 on the Richter Scale, and the conflagration took 10 hours to bring under control...
in Pasadena, Texas - The damage observed in the Buncefield fire, see the 2005 Hertfordshire Oil Storage Terminal fire2005 Hertfordshire Oil Storage Terminal fireThe Buncefield fire was a major conflagration caused by a series of explosions on 11 December 2005 at the Hertfordshire Oil Storage Terminal, an oil storage facility located near the M1 motorway by Hemel Hempstead in Hertfordshire, England. The terminal was the fifth largest oil-products...
The phenomenon is exploited in 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 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
Shchelkin spiral
The Shchelkin spiral is a device that assists the transition from deflagration to detonation in a pulse detonation engine. The spiral is named after Kirill Ivanovich Shchelkin, a Russian physicist who described it in his 1965 book Gas Dynamics of Combustion.In pulse detonation engines, direct...
in the combustion chamber
Combustion chamber
A combustion chamber is the part of an engine in which fuel is burned.-Internal combustion engine:The hot gases produced by the combustion occupy a far greater volume than the original fuel, thus creating an increase in pressure within the limited volume of the chamber...
to facilitate the deflagration to detonation transition.
The mechanism has found military use in the thermobaric weapon
Thermobaric weapon
A thermobaric weapon, which includes the type known as a "fuel-air bomb", is an explosive weapon that produces a blast wave of a significantly longer duration than those produced by condensed explosives. This is useful in military applications where its longer duration increases the numbers of...
.
A deflagration to detonation transition (DDT) has also been proposed for thermonuclear reactions responsible for supernovae initiation; 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,...
.
Apart from the name, this phenomenon is completely unrelated to the chemical combustion and flame acceleration phenomenon.