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Backdraft
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A backdraft (North American English), backdraught (British English)[1] or smoke explosion is the abrupt burning of superheated gases in a fire caused when oxygen rapidly enters a hot, oxygen-depleted environment; for example, when a window or door to an enclosed space is opened or broken. Backdrafts are typically seen as a blast of smoke and/or flame out of an opening of a building. Backdrafts present a serious threat to firefighters. There is some debate concerning whether backdrafts should be considered a type of flashover.
Burning
[edit]When material is heated enough, it begins to break down into smaller compounds, including flammable or even explosive gas, typically hydrocarbons. This is called pyrolysis, and does not require oxygen. If oxygen is also provided, then the hydrocarbons can combust, starting a fire.
If material undergoing pyrolysis is later given sufficient oxygen, the hydrocarbons will ignite, and therefore, combustion takes place.
Cause
[edit]
A backdraft can occur when a compartment fire has little or no ventilation. Due to this, little or no oxygen can flow into the compartment. Then, because fires reduce oxygen, the oxygen concentration decreases. When the oxygen concentration becomes too low to support combustion, some or all of the combustion switches to pyrolysis. Nonetheless, the hydrocarbons and smoke (primarily particulate matter) remain at a temperature hot enough to auto-ignite. If oxygen is then re-introduced to the compartment, e.g. by opening a door or window to a closed room, while the gasses are still hot enough to auto-ignite, combustion will restart, often abruptly or even explosively, as the gasses are heated by the combustion and expand rapidly because of the rapidly increasing temperature, combined with the energy released from combustion.
The colour and movement of smoke is used by firefighters to infer fire conditions, including the risk of backdraft.[2] Characteristic warning signs of a backdraft include yellow or brown smoke, smoke that exits small holes in puffs (a sort of breathing effect) and is often found around the edges of doors and windows, and windows that appear brown or black when viewed from the exterior due to soot from incomplete combustion. This is an indication that the room lacks enough oxygen to permit oxidation of the soot particles. Firefighters often look to see if there is soot on the inside of windows and in any cracks in the window (caused e.g. by the heat). The windows may also have a slight vibration due to varying pressure within the compartment due to intermittent combustion.
If firefighters discover a room sucking air into itself, for example through a crack, they generally evacuate immediately, because this is a strong indication that a backdraft is imminent. Due to pressure differences, puffs of smoke are sometimes drawn back into the enclosed space from which they emanated, which is how the term backdraft originated.
Backdrafts are very dangerous,[3] often surprising even experienced firefighters. The most common tactic used by firefighters to defuse a potential backdraft is to ventilate a room from its highest point, allowing the heat and smoke to escape without igniting.
Common signs of imminent backdraft include a sudden inrush of air upon creating an opening into a closed compartment, no visible signs of flame in a hot compartment (fire above its upper flammability limit), "pulsing" smoke plumes from openings, and auto-ignition of hot gases at openings as they mix with oxygen in the surrounding air.
Backdrafts and flashovers
[edit]ISO 13943[4] broadly defines flashover as a "transition to a state of total surface involvement in a fire of combustible materials within an enclosure." This definition embraces several different scenarios and includes backdrafts, but there is considerable disagreement about categorizing backdrafts as flashovers.[5] In common usage, the term flashover describes the near-simultaneous ignition of material caused by heat attaining the autoignition temperature of the combustible material and gases in an enclosure. Flashovers according to this narrower definition, i.e. those caused by rising temperatures, would not be considered backdrafts since backdrafts are caused by the introduction of oxygen into an enclosed space with conditions already suitable for ignition, and are thus caused by chemical change.[6]
In popular culture
[edit]Backdrafts were publicized by the 1991 movie Backdraft, in which a serial arsonist in Chicago uses them as a means of assassinating conspirators in a scam.
In the film adaptation of Stephen King's 1408, the protagonist Mike Enslin induces one as a last-ditch effort to kill the room.
The term is also used and is the title of a scene in the 2012 video game Root Double: Before Crime * After Days.
In the second book of the light novel series Imperial Reincarnation: I Came, I Saw, I Survived, the character Carmine uses a magically produced backdraft to defend himself against an assassination attempt.
References
[edit]- ^ "Control measure - Understand signs and symptoms of backdraught". www.ukfrs.com. Fire Central Programme Office. 1 July 2021. Archived from the original on 9 July 2021. Retrieved 1 July 2021.
- ^ a b Brouwer, Ed (6 June 2008). "Trainer's Corner: The science of reading smoke". www.firefightingincanada.com. Firefighting in Canada. Archived from the original on 30 July 2019. Retrieved 30 July 2019.
- ^ Hall, Richard & Adams, Barbara (1998). Essentials of firefighting (4th ed.). International Fire Service Training Association. ISBN 0-87939-149-9.
- ^ "ISO FDIS 13943 Vocabulary". International Organization for Standardization. Archived from the original on 2012-11-08. Retrieved 2011-06-11.
- ^ "Rapid Fire Progress" (PDF). firetactics.com. Archived from the original (PDF) on 2011-07-23. Retrieved 2011-06-11.
- ^ Dunn, Vincent, Deputy Chief, F. D. N. Y. (Ret.). "Backdraft and flashover; what's the difference?" (PDF). vincentdunn.com. Retrieved 2011-06-11.[permanent dead link]
External links
[edit]- A backdraft (still image and video) (in Swedish)
- Slow Motion Backdraft video
- White Smoke Warning
- Daniel's Block Fire-BACKDRAFT
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Backdraft
View on Grokipediafrom Grokipedia
Backdraft is a firefighting term describing the rapid or explosive combustion of superheated gases that occurs when oxygen is suddenly introduced into an oxygen-depleted environment in an enclosed space where a fire has built up unburned combustible products through pyrolysis.[1]
This phenomenon typically arises in structures with limited ventilation, where the fire consumes available oxygen, leading to high temperatures and smoke-filled conditions; it is triggered by actions such as opening a door or window, allowing air influx that ignites the accumulated fuel vapors.[2] Backdrafts pose severe risks to firefighters due to their sudden intensity and potential for structural collapse or ejection of burning materials, distinguishing them from related events like flashover (total room involvement by heat) or smoke explosions (ignition of premixed fuel-air in voids).[3]
Standards such as ISO 13943 define flashover as a rapid transition to total surface involvement in an enclosure fire.[21] However, firefighting doctrine and NFPA guidelines maintain a clear operational distinction between the two for safety and response purposes, emphasizing their differing preconditions and triggers to inform tactical decisions.[4][23]
Overview
Definition
A backdraft is defined as a deflagration resulting from the sudden introduction of air into a confined space containing oxygen-deficient but fuel-rich products of combustion.[4] This phenomenon involves the abrupt combustion of superheated, oxygen-depleted pyrolysis gases when oxygen is rapidly introduced, producing an explosive blast of flame, smoke, and hot gases.[5] The term "backdraft" emerged from firefighting jargon and first appeared in fire science literature in 1914, described by P.D.C. Steward as a smoke explosion akin to a dust explosion caused by carbon particles in oxygen-starved environments. At its core, the physics of a backdraft centers on the rapid oxidation of accumulated flammable vapors—primarily unburned pyrolysis products—in a hot, confined space depleted of oxygen by prior combustion.[6] Reintroduction of oxygen facilitates immediate mixing and ignition of these vapors at elevated temperatures, generating a sudden pressure surge and deflagration.[7]Characteristics
A backdraft event manifests through striking visual indicators, beginning with a sudden influx of dense, black smoke that rapidly fills the compartment and escapes through any available openings. Upon ignition, this is followed by a violent ejection of flames, often appearing as a rolling fireball that bursts outward from doors, windows, or vents. Accompanying these visuals is an intense heat wave and a powerful pressure surge that can propel flames and debris with significant force.[4][8][9] Auditory effects are equally dramatic, featuring a loud whooshing sound or explosive roar as fresh oxygen fuels the rapid combustion of accumulated gases. Firefighters often report a palpable sensory impact, including a swift temperature spike that delivers radiant heat intense enough to cause burns or disorientation even at a distance. The pressure wave from the deflagration can feel like a physical blow, exacerbating the immediate hazards.[4][8] These characteristics typically unfold over a brief duration of seconds to a few minutes, though the "blowtorch" phase of sustained flame ejection may persist longer in some cases. Despite its short-lived nature, a backdraft can inflict severe structural damage, such as lifting roofs or shattering windows, and poses a high risk of injury or fatality to occupants and responders in confined building environments. Overpressures during the event have been measured up to 234 Pa in full-scale tests, underscoring its potential for widespread impact within the affected space.[8]Formation Mechanisms
Pyrolysis and Gas Buildup
Pyrolysis refers to the thermal decomposition of solid fuels in the absence of oxygen, resulting in the production of combustible gases such as carbon monoxide (CO), hydrogen (H₂), and various hydrocarbons. This process is fundamental in underventilated compartment fires, where heat from ongoing combustion causes materials like wood, plastics, and foams to break down chemically without sufficient oxygen for oxidation. The decomposition involves the cleavage of molecular bonds in these organic materials, releasing volatile vapors that can sustain further fire development if ignited.[10][11][8] During pyrolysis, the generated gases accumulate in low-oxygen environments, forming a stratified layer of hot, flammable vapors near the ceiling of the compartment. This buildup is exacerbated by incomplete combustion, where limited oxygen availability halts flaming but allows continued thermal decomposition, leading to a fuel-rich atmosphere with unburned pyrolyzates. Such accumulation creates a homogeneous mixture of pyrolysis products that remains stable until external factors introduce oxygen, heightening the risk of rapid combustion.[8][12] The process typically accelerates significantly between temperatures of 300–500°C, at which point the rate of thermal decomposition intensifies, producing a higher volume of combustible gases from the solid fuels. For instance, in full-scale experiments with solid furnishings, initial gas layer temperatures often below 600°C supported ongoing pyrolysis, with temperatures exceeding this threshold during the event marking significant combustion after ignition. This temperature range underscores the transition from smoldering or limited burning to a highly volatile state conducive to backdraft conditions.[8][12][13]Ignition Trigger
The ignition trigger in a backdraft occurs when oxygen-rich fresh air is suddenly introduced into a confined space filled with superheated, fuel-rich gases produced by pyrolysis, rapidly forming a flammable premixed mixture that ignites near-instantaneously. This mixing typically happens at the interface between incoming cooler air and the hot upper layer of gases, facilitated by turbulence from the gravity-driven inflow, which enhances the homogeneity of the oxygen-fuel blend. The superheated conditions (often exceeding 400°C) and the presence of ignition sources such as embers, hot surfaces, or residual flames lower the effective activation energy barrier for combustion, as described by the Arrhenius equation where reaction rate increases exponentially with temperature: , with around 50 kJ/mol for typical pyrolysis products. This leads to a deflagration that consumes the unburned combustibles almost simultaneously across the volume.[14][15][16] The combustion releases a large amount of heat, quantified by the standard enthalpy of combustion , which for common fuels like hydrocarbons is on the order of -40 to -50 MJ/kg, driving rapid gas expansion and a significant pressure rise within the compartment. This overpressure, typically ranging from 100 Pa to over 280 Pa depending on the fuel load and confinement, generates a blast wave that propels flames and hot gases outward through the opening, often producing a visible fireball extending several meters. The pressure dynamics follow from the ideal gas law under rapid heating, where shifts dramatically as temperature surges, expelling unburned gases and intensifying the event's destructive potential. Experimental studies confirm that smaller openings amplify the overpressure by restricting outflow, while the blast wave can cause structural damage or injure firefighters nearby.[16][17][15] The propagation of the backdraft flame occurs at high velocities, often reaching up to 100 m/s in turbulent, confined conditions, far exceeding laminar burning speeds of 0.4-2 m/s for typical fuels like propane or methane. This acceleration is due to flame wrinkling and stretching induced by the incoming air turbulence, allowing the deflagration front to traverse the compartment in seconds—ignition delays as short as 0.3-6.3 s after oxygen introduction have been observed in controlled tests. Recent experiments (as of 2024) show that lower-level openings and higher initial temperatures reduce these delays, influencing the timing of backdraft onset. Numerical simulations under normal gravity show the gravity current of oxygen propagating at speeds scaling with (approximately 1-2 m/s initially), but the subsequent flame front accelerates dramatically upon ignition, consuming the fuel layer overhead before ejecting through the vent. Such rapid progression distinguishes backdraft from slower fire growth, emphasizing its explosive nature.[17][18][15][19]Preconditions and Causes
Environmental Requirements
A backdraft requires significant oxygen depletion within the fire compartment, typically to levels below 15-16% by volume, which occurs in sealed or poorly ventilated spaces where initial combustion consumes available oxygen without replenishment.[8][20] This low-oxygen environment halts flaming combustion but allows pyrolysis to continue, producing unburned fuel gases that accumulate.[8] Sustained high temperatures, generally exceeding 300°C in the upper layer of the compartment, are essential to drive the pyrolysis process and maintain the viability of accumulated fuels.[8] These conditions are supported by an abundant fuel load of ordinary combustibles, such as furniture, wood furnishings, or structural building materials, which provide the necessary pyrolyzates without being fully consumed due to oxygen starvation.[8] The geometry of the compartment plays a critical role, favoring fully or partially enclosed areas with limited outlets, like rooms, attics, or void spaces, which trap heat, smoke, and gases while promoting stratified layering of hot, fuel-rich upper layers over cooler lower ones.[8] Such configurations, often found in residential or industrial buildings, exacerbate oxygen depletion and gas buildup, setting the stage for rapid combustion upon air introduction.[17]Common Triggers
Common triggers for backdraft typically involve sudden introductions of oxygen into an oxygen-starved, fuel-rich fire environment, often occurring in enclosed structure fires where ventilation has been limited. These events can arise from both human interventions and unintended environmental changes, rapidly shifting the fire from a smoldering state to explosive combustion.[21] Human actions during firefighting or rescue operations are among the most frequent triggers, particularly when personnel open doors, windows, or break glass to gain access or ventilate without recognizing the risk of oxygen influx. For instance, forcing entry into a sealed commercial storefront or residential space can abruptly supply fresh air to superheated gases, igniting a backdraft. Such incidents are common in structure fires, where firefighters may inadvertently create a pathway for air without coordinated ventilation strategies.[22][2] Natural or structural failures also precipitate backdrafts by allowing uncontrolled air inflow, such as wind gusts forcing oxygen into an under-ventilated compartment or the activation of HVAC systems that circulate air through fire-affected areas. Roof collapses or other structural breaches can similarly open sealed spaces, enabling a rush of external air that meets accumulated pyrolyzates. These failures exacerbate risks in tightly constructed buildings, where poor initial ventilation—such as in residential basements—allows fuel buildup prior to the triggering event.[21][23]Comparisons to Similar Phenomena
Backdraft vs. Flashover
Flashover is defined as the near-simultaneous ignition of all combustible surfaces within an enclosure when the radiant heat flux causes these materials to reach their autoignition temperatures, typically in the range of 500–600°C.[4][21] This event marks a transition to full room involvement, driven by thermal radiation from the accumulating hot gas layer near the ceiling, leading to a sustained post-flashover fire.[23] In contrast, backdraft occurs when oxygen is suddenly introduced to a oxygen-depleted environment filled with pre-formed, superheated pyrolysis gases and unburned combustibles, resulting in a rapid deflagration or explosion.[23][4] Unlike flashover, which is primarily heat-driven and involves surface ignition across the entire compartment, backdraft is oxygen-driven, often localized to the point of air entry, and typically arises during the decay phase of a fire where ventilation has been restricted.[21] This distinction is critical in firefighting tactics: flashover demands cooling and upper-level ventilation to prevent total involvement, while backdraft requires cautious, controlled ventilation to avoid triggering the influx of air at lower levels.[23]| Aspect | Flashover | Backdraft |
|---|---|---|
| Primary Driver | Radiant heat buildup | Sudden oxygen introduction |
| Ignition Mechanism | Surface autoignition of combustibles | Deflagration of accumulated pyrolysis gases |
| Fire Stage | Growth to fully developed | Decay (oxygen-starved) |
| Extent | Total room involvement, sustained burning | Explosive, often localized with fireball |
| Tactical Response | Ceiling-level cooling/ventilation | Avoid low-level air entry; positive pressure |