Editing Deflagration

{{Short description|Combustion that leads on to an explosion}}
[[File:Explosions.jpg|thumb|upright=1.35|Pyrotechnic deflagrations]]
'''Deflagration'''  (Lat: ''de + flagrare'', 'to burn down') is [[Speed of sound|subsonic]] [[combustion]] in which a [[Premixed flame|pre-mixed flame]] propagates through an explosive or a mixture of fuel and oxidizer.<ref>{{Cite web |last=O'Conner |first=Brian |date=March 27, 2023 |title=Explosions, Deflagrations and Detonations |url=https://www.nfpa.org/News-and-Research/Publications-and-media/Blogs-Landing-Page/NFPA-Today/Blog-Posts/2023/03/27/Explosions-vs-Deflagrations-vs-Detonations |url-status=live |access-date=May 31, 2023 |website=National Fire Protection Association|archive-url=https://web.archive.org/web/20230328200609/https://www.nfpa.org/News-and-Research/Publications-and-media/Blogs-Landing-Page/NFPA-Today/Blog-Posts/2023/03/27/Explosions-vs-Deflagrations-vs-Detonations|archive-date=March 28, 2023}}</ref><ref>{{cite book |title=Handbook of Fire Protection Engineering |date=2016 |publisher=Society of Fire Protection Engineers |page=373 |edition=5 |url=https://www.sfpe.org/standards-guides/sfpehandbook}}</ref> Deflagrations in high and [[low explosives]] or fuel–oxidizer mixtures may [[Deflagration to detonation transition|transition to a detonation]] depending upon confinement and other factors.<ref>{{Cite web |last=McDonough |first=Gordon |date=April 1, 2017 |title=What is a high explosive |url=https://www.lanl.gov/museum/news/newsletter/2017/2017-04/high-explosives.php |url-status=live |access-date=May 31, 2023 |website=Bradbury Science Museum, Los Alamos National Laboratory|archive-url=https://web.archive.org/web/20170502001919/https://www.lanl.gov/museum/news/newsletter/2017/2017-04/high-explosives.php|archive-date=2017-05-02}}</ref><ref>{{Cite journal |last1=Rosas |first1=Camilo |last2=Davis |first2=Scott |last3=Engel |first3=Derek |last4=Middha |first4=Prankul |last5=van Wingerden |first5=Kees |last6=Mannan |first6=M.S. |doi=10.1016/j.jlp.2014.03.003|date=July 2014 |title=Deflagration to detonation transitions (DDTs): Predicting DDTs in hydrocarbon explosions |url=https://www.sciencedirect.com/science/article/abs/pii/S0950423014000412 |access-date=May 31, 2023 |journal=Journal of Loss Prevention in the Process Industries|volume=30 |pages=263–274 |bibcode=2014JLPPI..30..263R |url-access=subscription }}</ref> Most [[fire]]s found in daily life are [[diffusion flame]]s. Deflagrations with flame speeds in the range of 1&nbsp;m/s differ from [[detonation]]s which propagate [[supersonic]]ally with [[Detonation velocity|detonation velocities]] in the range of km/s.<ref>{{cite book |title=Handbook of Fire Protection Engineering |date=2016 |publisher=Society of Fire Protection Engineers |page=390 |edition=5 |url=https://www.sfpe.org/standards-guides/sfpehandbook}}</ref>

==Applications==
Deflagrations are often used in engineering applications when the force of the expanding gas is used to move an object such as a [[projectile]] down a [[Gun barrel|barrel]], or a piston in an [[internal combustion engine]]. Deflagration systems and products can also be used in mining, demolition and stone quarrying via gas pressure blasting as a beneficial alternative to high explosives.

== Terminology of explosive safety ==
When studying or discussing explosive safety, or the safety of systems containing explosives, the terms deflagration, [[detonation]] and [[Deflagration to detonation transition|deflagration-to-detonation transition]] (commonly referred to as DDT) must be understood and used appropriately to convey relevant information. As explained above, a deflagration is a subsonic reaction, whereas a [[detonation]] is a supersonic (greater than the [https://www.nde-ed.org/Physics/Sound/vibration.xhtml sound speed of the material]) reaction. Distinguishing between a deflagration or a detonation can be difficult to impossible to the casual observer. Rather, confidently differentiating between the two requires instrumentation and diagnostics to ascertain reaction speed in the affected material. Therefore, when an unexpected event or an accident occurs with an explosive material or an explosive-containing system it is usually impossible to know whether the explosive deflagrated or detonated as both can appear as very violent, energetic reactions. Therefore, the [[energetic materials]] community coined the term "high explosive violent reaction" or "HEVR" to describe a violent reaction that, because it lacked diagnostics to measure sound-speed, could have been either a deflagration or a detonation. <ref>{{Cite web |last=Squires |first=Jess |date=2023-01-22 |title=High Explosive Violent Reaction (HEVR) — DOE Directives, Guidance, and Delegations |url=https://www.directives.doe.gov/terms_definitions/high-explosive-violent-reaction-hevr#:~:text=For%20the%20purposes%20of%20the,may%20be%20subsonic%20or%20supersonic. |url-status=live |archive-url=https://web.archive.org/web/20220929215616/https://www.directives.doe.gov/terms_definitions/critical-nuclear-weapon-design-information-cnwdi |archive-date=2022-09-29 |access-date=2023-06-08 |website=www.directives.doe.gov |language=en}}</ref><ref>{{Cite web |title=What's the difference between an explosion and a detonation? |url=https://www.lanl.gov/museum/news/newsletter/2018/08/detonation.php |access-date=2023-06-08 |website=www.lanl.gov}}</ref>

== Flame physics ==
The underlying flame [[physics]] can be understood with the help of an idealized model consisting of a uniform one-dimensional tube of unburnt and burned gaseous fuel, separated by a thin transitional region of width <math>\delta\; </math> 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.<ref>Williams, F. A. (2018). ''Combustion theory''. CRC Press.</ref><ref>Landau, L. D., & Lifshitz, E. M. (1959). ''Fluid Mechanics''. Course of Theoretical Physics, 6.</ref><ref>Linan, A., & Williams, F. A. (1993). ''Fundamental aspects of combustion''.</ref><ref>Zeldovich, I. A., Barenblatt, G. I., Librovich, V. B., & Makhviladze, G. M. (1985). ''Mathematical theory of combustion and explosions''.</ref>

Two characteristic timescales are important here. The first is the [[thermal conduction|thermal diffusion]] timescale <math>\tau_d\;</math>, which is approximately equal to

<math display="block">\tau_d \simeq \delta^2 / \kappa,</math>

where <math>\kappa \;</math> is the [[thermal diffusivity]]. The second is the [[activation energy|burning timescale]] <math>\tau_b</math> that strongly decreases with temperature, typically as

<math display="block">\tau_b\propto \exp[\Delta U/(k_B T_f)],</math>

where <math>\Delta U\;</math> is the activation barrier for the burning reaction and <math>T_f\;</math> is the temperature developed as the result of burning; the value of this so-called "flame temperature" can be determined from the laws of thermodynamics.

For a stationary moving deflagration front, these two timescales must be equal: the heat generated by burning is equal to the heat carried away by [[heat transfer]]. This makes it possible to calculate the characteristic width <math>\delta\;</math> of the flame front:

<math display="block">\tau_b = \tau_d\;,</math>

thus

<math display="block"> \delta \simeq \sqrt {\kappa \tau_b} .</math>

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

<math display="block">S_l \simeq \delta / \tau_b \simeq \sqrt {\kappa  / \tau_b} .</math>

This simplified model neglects the change of temperature and thus the burning rate across the deflagration front. This model also neglects the possible influence of [[turbulence]]. As a result, this derivation gives only the [[laminar flame speed]]—hence the designation <math>S_l\;</math>.

==Damaging events==

Damage to buildings, equipment and people can result from a large-scale, short-duration deflagration.  The potential damage is primarily a function of the total amount of fuel burned in the event (total energy available), the maximum reaction velocity that is achieved, and the manner in which the expansion of the combustion gases is contained. Vented deflagrations tend to be less violent or damaging than contained deflagrations.<ref>{{Cite journal |last1=Tarver |first1=C. M. |last2=Chidester |first2=S. K. |date=2004-02-09 |title=On the Violence of High Explosive Reactions |journal=Journal of Pressure Vessel Technology |volume=127 |pages=39–48 |doi=10.1115/1.1845474 |osti=15013892 |url=https://www.osti.gov/biblio/15013892 |language=English}}</ref>

In free-air deflagrations, there is a continuous variation in deflagration effects relative to the maximum flame velocity.  When flame velocities are low, the effect of a deflagration is to release heat, such as in a [[flash fire]].  At flame velocities near the [[speed of sound]], the energy released is in the form of pressure, and the resulting high pressure can damage equipment and buildings.<ref>{{cite book |title=NFPA 68 Standard on Explosion Protection by Deflagration Venting |date=2018 |publisher=National Fire Protection Association|page=5 |url=https://www.nfpa.org/codes-and-standards/all-codes-and-standards/list-of-codes-and-standards/detail?code=68}}</ref>

==See also==
{{Wiktionary}}
* [[Conflagration]]
* [[Deflagration to detonation transition]]
* [[Pressure piling]]

==References==
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[[Category:Combustion]]
[[Category:Explosives]]
[[Category:Physical chemistry]]
[[Category:Process safety]]