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Encyclopedia > Deflagration

## Applications

In engineering terms, deflagrations are easier to control than detonations. Consequently, they are better suited when the goal is to move an object (a bullet in a gun, or a piston in an engine) with the force of the expanding gas. Typical examples of deflagrations are combustion of a gas-air mixture in a gas stove or a fuel-air mixture in an internal combustion engine, a rapid burning of a gunpowder in a firearm or pyrotechnic mixtures in fireworks. .357 Magnum cartridges, containing bullets A bullet is a solid projectile propelled by a firearm and is normally made from metal (usually lead). ... Combustion or burning is a complex sequence of chemical reactions between a fuel and an oxidant accompanied by the production of heat or both heat and light in the form of either a glow or flames. ... A stove is a heat-producing device. ... The internal combustion engine is an engine in which the burning of a fuel occurs in a confined space called a combustion chamber. ... Smokeless powder Gunpowder, whether black powder or smokeless powder, is a substance that burns very rapidly, releasing gases that act as a propellant in firearms. ... Fourth of July fireworks in San Diego, California New Years Day fireworks at Seaport Village, California Preparing fireworks at Sayn Castle A firework is classified as a low explosive pyrotechnic device used primarily for aesthetic and entertainment purposes. ...

## Flame physics

We can better understand the underlying flame physics by constructing an idealized model consisting of a uniform one-dimensional tube of unburnt and burned gaseous fuel, separated by a thin transitional region of width $delta ;$ in which the burning occurs. The burning region is commonly referred to as the flame or flame front. In equilibrium, thermal diffusion across the flame front is balanced by the heat supplied by burning. Physics (from the Greek, (phÃºsis), nature and (phusikÃ©), knowledge of nature) is the science concerned with the discovery and understanding of the fundamental laws which govern matter, energy, space, and time. ... It has been suggested that this article or section be merged with Fire. ...

There are two characteristic timescales which are important here. The first is the thermal diffusion timescale τd, which is approximately equal to $tau_d simeq delta^2 / kappa$ where $kappa ;$ is the thermal conductivity. In physics, thermal conductivity, k, is the intensive property of a material that indicates its ability to conduct heat. ...

The second is the burning timescale τb, which is approximately equal to $tau_b simeq epsilon / dot {w}$ where ε is the total energy released by burning per unit mass, and $dot {w}$ is the burn rate (i.e., the rate of increase of specific thermal energy).

In equilibrium, these two rates are equal: The heat generated by burning is equal to the heat carried away by heat transfer. This lets us find the characteristic width δ of the flame front : In thermal physics, heat transfer is the passage of thermal energy from a hot to a cold object. ...

$tau_b simeq tau_d$

$delta simeq sqrt {epsilon kappa / dot {w}}$

Now, the thermal flame front propagates at a characteristic speed Sl, which is simply equal to the flame width divided by the burn time :

$S_l simeq delta / tau_b simeq sqrt {kappa dot {w} / epsilon}$

This simplified one-dimensional model neglects the possible influence of turbulence. As a result, this derivation gives the laminar flame speed -- hence the designation Sl. In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic, stochastic property changes. ... laminar and turbulent water flow over the hull of a submarine In fluid dynamics, laminar flow is a flow regime characterized by high momentum diffusion, low momentum convection, and pressure and velocity independence from time. ...

## Damaging deflagration events

Damage to buildings, equipment and people can result from a large-scale short-duration deflagration. The nature of the damage is primarily a function of the total amount of fuel burned in the event (total energy available), the maximum flame velocity that is achieved, and the manner in which the expansion of the combustion gases is contained.

In free-air deflagrations, there is a continuous variation in deflagration effects relative to maximum flame velocity. When flame velocities are low, the effect of a deflagration is the release of heat. Some authors use the term flash fire to describe these low-speed deflagrations. At flame velocities near the speed of sound, the energy released is in the form of pressure and the results resemble a detonation. Between these extremes both heat and pressure are released. A flash fire is an unexpected, sudden intense fire caused by ignition of flammable solids, liquids or their vapors, gases, or dust. ... A weapons cache is detonated at the East River Range on Bagram Airfield, Afghanistan Detonation is a process of supersonic combustion that involves a shock wave and a reaction zone behind it. ...

When a low-speed deflagration occurs within a closed vessel or structure, pressure effects can produce damage due to expansion of gases, as a secondary effect. The heat released by the deflagration causes the combustion gases and excess air to try to expand thermally as well. The net result is that the volume of the vessel or structure needs to either expand/fail to accommodate the hot combustion gases, or build internal pressure to contain them.

Results from FactBites:

 Pulse Detonation Engine (1247 words) Deflagration is what happens when you burn a mixture of air and fuel at reasonably low pressure. Deflagration is, believe it or not, a relatively gentle process which is simply the rapid burning the fuel. One of the primary attributes of deflagration is that the flame travels at a speed significantly lower than the speed of sound.
 Deflagration - Wikipedia, the free encyclopedia (385 words) Deflagration is a process of subsonic combustion that usually propagates through thermal conductivity (hot burning material heats the next layer of cold material and ignites it). Deflagration is different from detonation which is supersonic and propagates through shock compression. Typical examples of deflagrations are combustion of a gas-air mixture in a gas stove or a fuel-air mixture in an internal combustion engine, a rapid burning of a gunpowder in a firearm or pyrotechnic mixtures in fireworks.
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