Deflagration

A log in a fireplace.

Deflagration (Lat: de + flagrare, "to burn down") is a technical term describing subsonic combustion that usually propagates through thermal conductivity (hot burning material heats the next layer of cold material and ignites it). Combustion or burning is a complex sequence of Exothermic chemical reactions between a Fuel and an Oxidant accompanied by the production of In Physics, thermal conductivity, k is the property of a material that indicates its ability to conduct Heat. Most "fire" found in daily life, from flames to explosions, is technically deflagration. Fire is the heat and light energy released during a Chemical reaction, in particular a combustion reaction. A flame is often defined as the visible (light-emitting part of a Fire. An explosion is a sudden increase in Volume and release of Energy in an extreme manner usually with the generation of high Temperatures and the release Deflagration is different from detonation which is supersonic and propagates through shock compression. Detonation is a process of Supersonic Combustion in which a Shock wave is propagated forward due to energy release in a reaction zone behind it For other uses see Supersonic. The term supersonic is used to define a speed that is over the Speed of sound ( Mach 1 For the music album by Converter see Shock Front For the 1977 horror film see Shock Waves A shock wave (also called

1 Applications
2 Flame physics
3 Damaging deflagration events
4 See also
5 References

Applications

In engineering applications, 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 internal combustion engine) with the force of the expanding gas. A bullet is a solid Projectile propelled by a Firearm or Air gun and is normally made from metal (usually Lead) The internal combustion engine is an engine in which the Combustion of Fuel and an Oxidizer (typically air occurs in a confined space called a 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. Combustion or burning is a complex sequence of Exothermic chemical reactions between a Fuel and an Oxidant accompanied by the production of In Cooking, a gas stove is a Cooker which uses Natural gas as a Fuel source The internal combustion engine is an engine in which the Combustion of Fuel and an Oxidizer (typically air occurs in a confined space called a Gunpowder is a an explosive mixture of Sulfur, Charcoal and Potassium nitrate (also known as saltpetre/saltpeter that burns rapidly producing volumes 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. Physics (Greek Physis - φύσις in everyday terms is the Science of Matter and its motion. The burning region is commonly referred to as the flame or flame front. A flame is often defined as the visible (light-emitting part of a Fire. In equilibrium, thermal diffusion across the flame front is balanced by the heat supplied by burning.

There are two characteristic timescales which are important here. The first is the thermal diffusion timescale $\tau_d\;$ , which is approximately equal to

$\tau_d \simeq \delta^2 / \kappa$ ,

where $\kappa \;$ is the thermal diffusivity. In Heat transfer analysis thermal diffusivity (symbol \alpha\ but note that the symbols \kappa D and k are all commonly The second is the burning timescale $τ b$ that strongly decreases with temperature, typically as

$\tau_b\propto \exp[\Delta U/(k_B T_f)]$ ,

where $\Delta U\;$ is the activation barrier for the burning reaction and $T_f\;$ is the temperature developed as the result of burning that can be found from thermodynamics (the so-called "flame temperature").

For a stationary moving deflagration front, these two timescales are equal: The heat generated by burning is equal to the heat carried away by heat transfer. In thermal physics, heat transfer is the passage of Thermal energy from a hot to a colder body This lets us find the characteristic width $\delta\;$ of the flame front:

$\tau_b = \tau_d\;$ ,

thus

$\delta \simeq \sqrt {\kappa \tau_b}$ .

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

$S_l \simeq \delta / \tau_b \simeq \sqrt {\kappa / \tau_b}$ .

This simplified model neglects the change of temperature and thus the burning rate across the deflagration front. Also this model neglects the possible influence of turbulence. In Fluid dynamics, turbulence or turbulent flow is a fluid regime characterized by chaotic Stochastic property changes As a result, this derivation gives the laminar flame speed -- hence the designation $S_l\;$ . Laminar flow, sometimes known as streamline flow occurs when a fluid flows in parallel layers with no disruption between the layers

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. A flash fire is an unexpected sudden intense Fire caused by ignition of flammable solids (including Dust) liquids or gases At flame velocities near the speed of sound, the energy released is in the form of pressure and the results resemble a detonation. Detonation is a process of Supersonic Combustion in which a Shock wave is propagated forward due to energy release in a reaction zone behind it Between these extremes both heat and pressure are released.

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. The risks of deflagration inside waste storage drums is a growing concern among storage facilities ^[1].

see drum deflagration videos

References

^ Drum Deflagration Risk Reduction: Current Research and Testing

Dictionary

-noun

The act of deflagrating; an intense fire; a conflagration or explosion. Specifically, combustion that spreads subsonically via thermal conduction.

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