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Scientific
JARGON - a firefighters guide!
Last update on 06 September 2006 |
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HOME FLASHOVER STRATEGY
& TACTICS - BLAINA CFBT
OPEN FORUM
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Auto-ignition - The auto-ignition point is the temperature at which a flammable mixture ignites spontaneously in air. Auto-ignition temperatures refer to near stoichiometric mixtures for which the AIT is a minimum. Backdraft (Backdraught) - The closest definition to date is perhaps 'the explosive or rapid burning of heated gases (unburnt pyrolysis products) that occurs when oxygen has been introduced into a compartment or building that has a depleted supply of oxygen due to an existing fire'. However, there is also a further range of conditions that have been associated under this definition such as 'smoke explosion' and 'blow-torch' effect as examples that may not necessarily require the addition of oxygen for such phenomena to occur. Blue Flames - Noted by Grimwood as a warning sign preceding backdraft. Conversion Data - Click HERE Dancing flames - See Ghosting flames. Diffusion flame - Most flames in a fire are diffusion flames - the principal characteristic of a diffusion flame is that the fuel and oxidiser (air) are initially separate and combustion occurs in the zone where the gases mix. Flashover - A generic term that may have several scientific references or definitions. The term is used in general by firefighters to describe an element of rapid fire progress although scientists are somewhat at conflict as to any specific meaning. The originator (P.H. Thomas) admitted the term is imprecise and may be used to mean different things in different contexts. Flammability of Fire Gases - Fire gases are capable of burning in both diffusion and pre-mixed states. The smoke given off in a fire is flammable. Particulate smoke is a product of incomplete combustion and may lead to the formation of a flammable atmosphere which, if ignited, may lead to an explosion. Flammable limits applies generally to vapors and gases and are defined as the concentration range in which a flammable substance can produce a fire or explosion when an ignition source (such as a spark or open flame) is present. The concentration is generally expressed as percent fuel by volume.Above the upper flammable limit (UFL) the mixture of substance and air is too rich in fuel (deficient in oxygen) to burn. This is sometimes called the upper explosive limit (UEL). Below the lower flammable limit (LFL) the mixture of substance and air lacks sufficient fuel (substance) to burn. This is sometimes called the lower explosive limit (LEL). Forward Induced Explosion - Floyd Nelson (USA) introduced a definition for a term he referred to as Forward-induced Explosions. In effect, this definition described the ignition of pockets of fire gases as they transported throughout a structure/compartment. The phenomena differed from that of backdraft in that fresh air (oxygen) is the moving force in a backdraft whilst the gases themselves are the moving force in a 'forward-induced' explosion as they move towards a supply of air. This can occur in many ways inside a fire involved structure, for example, where a collapsing ceiling forces fire gases to transport outwards from the area of collapse. On mixing with pockets of air they may come into the flammable range and can ignite with varying explosive effects. Fuel Controlled Fire - Free burning of a fire that is characterised by an air supply in excess of that which is required for complete combustion of the fuel source or available pyrolates. Ghosting flames - A description of flames which are not attached to the fuel source and move around an enclosure to burn where the fuel/air mixture is favourable. Such an occurrence in an under-ventilated situation is a sure sign that precedes backdraft. Also termed Dancing flames. Gravity Current - also termed gravity wave - An opposing flow of two fluids caused by a density difference (termed by firefighter John Taylor as an air-track). In firefighting terms this is basically referring to the under-pressure area where air enters a building or compartment and the over-pressure area where smoke, flame or hot gases leave - the mixing process between fresh air and combustible fire gases. Heat Release Rate - The amount of energy (fire intensity) released by burning materials is recorded in Kw or Mw/sq.m. In a compartment fire a minimum level of HRR is normally required before 'flashover' can occur - this can be increased by - (1) an increase in the area of the ventilation opening; (2) an increase in the compartment size; (3) an increase in hk which depends on the thermal conductivity of the compartment boundary. High Velocity Gases - Where the ignition and movement of super-heated fire gases are accelerated through narrow openings, corridors etc, or are deflected, the effects can be dramatic. The deep levels of burning (referred to in the UK as a local deepening) will cause unusual patterns of burn as if an accelerant has been used to increase fire intensity. On occasions, where high-velocity gases escape to the outside without being deflected, their flow is such that they may cross an entire street creating a flame-thrower effect from a window or doorway. Hot Layer Interface - Often referred to as the NPP (neutral pressure plane) - it is assumed that the hot smoky upper layer that forms below the ceiling and the lower cool layer that shrinks as the hot layer descends are joined at a distinct horizontal interface (computer model). This is obviously a simplification because the turbulence within a fire compartment would prevent any true formation of such an interface. Also, highly turbulent plumes and hot layers, as well as strong vent flows, may cause the destruction of a clear interface. However, a noticeable change in conditions from the upper layer to the lower has been observed in many compartment fire experiments. The hot layer interface plane and neutral plane are not the same. The interface is the vertical elevation within the compartment, away from the vent point, at which the discontinuity between the hot and cold layer is located. The neutral plane (or point) is the vertical location at the vent at which the pressure difference across the vent is zero. Limits of Flammability - Ignition of fuel vapour and air is only possible within certain limits (ie; the ratio of the mixture). The resulting flame will be pre-mixed and the concepts of 'limits of flammability' applies only to pre-mixed flames. However, emperically clear parallels exist between diffusion and pre-mixed limits. (See also - Flammability of Fire Gases above). Local Deepening - See High Velocity Gases.
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Pre-mixed flame - In
pre-mixed burning gaseous fuel and oxidiser (air) are intimately mixed
prior to ignition - the flame propagation through the mixture is a
deflagration (eg; Smoke explosion).
Pulsation Cycle - An indication of the presence of unburned fuel vapours within a compartment with the potential for pre-mixing and a potential explosion - A warning sign for backdraft as smoke 'pulses' intermittently in and out at a ventilation/entry point Pyrolysis - The second stage of ignition during which energy causes gas molecules given off by a heated solid fuel to vibrate and break into pieces. Regardless of whether a fuel was originally a liquid or solid, the overall burning process will gasify the fuel. With liquids, the supply of gaseous fuel is a result of evaporation at the surface from the heat generated by the flames. Solids entail a significantly more complex process involving chemical decomposition (pyrolysis) of large polymeric molecules. Certain combustible solids such as sodium, potassium, phosphorus, and magnesium can even be oxidized directly by oxygen in the air without the need of pyrolysis. Rapid Fire Progress - An NFPA definition of all types of rapid fire escalation that may occur and be linked to the above phenomena and their associates. Regimes of Burning - (1) Fuel controlled; (2) Ventilation controlled; (3) Stoichiometric. Rollover - The extension of the fire plume or tongues of flame that have become detached ahead of the plume at ceiling level signalling the effect of 'rollover' - a recognised warning sign that the compartment fire is rapidly progressing towards 'flashover'. Smoke Explosion - The ignition of a pre-mixed pocket of fire gases and oxygen that may occur when an ignition source is introduced. This may occur, for example, when a hot brand or spark is directed on a convection current into an area, possibly near the ceiling, where the pre-mixed gases exist, or where an ignition source is uncovered in an area that is harboring such a gas/air mix. Steady State Fire - Most fires can be characterized by one or a combination of three unique phases related to the fire’s rate of heat release. These are the Growth Phase, Steady State Phase and Decay Phase. The early stage of a fire during which fuel and oxygen are virtually unlimited is the Growth Phase. This phase is characterized by an exponentially increasing heat release rate. The middle stage of a fire is the Steady State Phase. This phase is characterized by a heat release rate which is relatively unchanging. Transition from the Growth Phase to the Steady State Phase can occur when fuel or oxygen begins to be limited or when suppression activity begins to impact on the fire. The final stage of a fire is the Decay Phase, which is characterized by a continuous deceleration in the heat release rate leading to fire extinguishment. Step Events - The Heat Release Rate (HRR) is either controlled by the supply of fuel or the supply of air. Therefore, in principle, four transitions (steps) are possible - (1) Fuel control to new fuel control; (2) Fuel control to air control; (3) Air contro to new air control; (4) Air control to fuel control. In each of these cases the new fire is SUSTAINED. The event defined as FLASHOVER is usually related to Step 2 although it may also occur through an increase in ventilation (Step 3). Stoichiometric - In terms of flammability limits of gas/air mixtures the stoichiometric mixture is the 'ideal' mixture that will produce a most complete combustion - ie; it is somewhere between the UEL (upper) and LEL (lower) explosive limits and an ignition at the stoichiometric point may result in the most severe deflagration, in relation to those near the upper and lower limits of flammability. Tactical
Firefighting Tactical Ventilation - A concept of safe practice originally introduced during the 1980s and defined by Paul Grimwood as 'venting actions by on-scene firefighters, used to gain control of a fire building's internal environment to the advantage of firefighting and rescue teams working within. Such actions may include attempts to release or direct smoke, super-heated and burning gases from the building by either natural or forced means via vertical or horizontal openings made or existing in the structure. These actions may also include the 'closing down' of a structure in an attempt to reduce the flow of air towards the fire. This tactic is termed 'Anti-Ventilation' by the Swedish Fire service'. It is essential that firefighters remember the most dangerous opening they may create in the structure exists at the point of entry to the building. Thermal Balance - The degree of thermal balance existing in a closed room during a fire's development is dependant upon fuel supply and air availability as well as other factors. The hot area over the fire (often termed the fire plume or thermal column) causes the circulation that feeds air to the fire. However, when the ceiling and upper parts of the wall linings become super-heated, circulation slows down until the entire room develops a kind of thermal balance with temperatures distributed uniformly horizontally throughout the compartment. In vertical terms the temperatures continuously increase from bottom to top with the greatest concentration of heat at the highest level. Transient Events - These are short, possibly violent, releases of enegy from the fire which are NOT sustained - (1) adding fuel; (2) adding air/oxygen (backdraft); (3) adding heat (smoke explosion). Under-Ventilated Fire - Unlike the ventilation controlled fire an under-ventilated fire is not recognised as a burning regime but rather a situation where fuel-rich conditions have accumulated within a compartment. The situation may not involve a fully developed fire and may only be in a state of smouldering. The conditions may or may not present warning signs related to backdraught. Ventilation Controlled Fire - Sometimes referred to as an 'under-ventilated fire' although this may be incorrect (see 'under-ventilated' fire) - most fully developed fires that occur under confinement or within a compartment are ventilation controlled and burn under fuel-rich conditions. In these situations the highest temperatures are normally noted at the ventilation openings. The rate of air supply is insufficient to burn all the fuel vapours within the compartment, possibly leading to much external flaming.
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