3 backdraft state of art

Chapter 3

Under-Ventilated Fires: Backdraft Phenomenon

Table of Contents

1. Introduction................................................................................35 2. State of the Art ..........................................................................35 3. Definition of Backdraft Phenomenon ..........................................38 4. Typical Backdraft Evolution.......................................................39 4.1 Gravity Current .....................................................................41 4.2 Ignition of the Flammable Premixed Area .............................43 4.3. Visual Sequence of Backdraft Deflagration............................43 5. Backdraft Scenarios ....................................................................45 6. Temperature Evolution in Backdraft ..........................................45 7. Grey Area Between Various Phenomena ....................................46 7.1 Differentiating between Flashover and Backdraft...................46 7.2 Differentiating between Smoke Gases Auto-Igniting in the Opening and Backdraft................................................................47 7.3 Differentiating between Backdraft and Smoke Gas Explosion 47 7.4 Differentiating between Flashover and Smoke Gas Explosion 47

7.5 Warning Signs of Backdraft ...................................................47 8. Backdraft Fatalities and Case Histories ......................................48 8.1 Backdraft Fatalities ...............................................................48 8.2 Backdraft: Case Histories .......................................................49 8.2.1 Backdraft in Cinema of London (1994) ............................49 8.2.2 Backdraft in a Two-Storey House in Wales (1996)...........50 8.2.3 Backdraft in a Supermarket in Bristol..............................50 8.2.4 Backdraft in an Apartment, New York City (1994) .........51 9. Backdraft Pictures ......................................................................52 10. Conclusions ...............................................................................53

3. Under-Ventilated Fires: Backdraft Phenomenon

1. Introduction Backdraft is a special fire phenomenon that takes place in a limited-ventilation building, often as a result of fire-fighting operations. The consequence of this phenomenon is a deflagration growth in the intensity of the fire in the form of a fireball. When this happens, it normally causes fire fighter fatalities and may provoke structure collapse.

This phenomenon is still confused with other rapid spread flame processes such as smoke explosions or flashover. Due to this confusion, this chapter tries to clarify what a backdraft is as well as to establish the difference between flashover and gas explosions. Furthermore, the features included in a backdraft phenomenon and the evolution of a fire compartment for reaching backdraft is also described.

2. State of the Art Prior to 1991, the word backdraft was known only to fire fighters and a few researchers. However, in 1991 Universal Studios released a major motion picture called BACKDRAFT and almost overnight, backdraft became a household word. Prior to the movie there was a great deal of confusion about the concept of backdraft; since the release of the movie, the situation has become even more confused.

A recent study carried out in 2002 by a Website poll (P. Grimwood, 2003) at www.firetactics.com shows an example of this existing confusion among fire experts. More than 300 fire experts were asked to offer their opinion of what event was occurring in the picture below, Figure 3-1. The results obtained concluded that the phenomenon is not really well understood. Table 3-1 shows the results.

NFPA statistics (Rita, F., 2001) show that a few cases of backdraft are reported every year. However, due to the existing confusion shown in Figure 3.1 and the fact that the statistics are based on “incident report forms”, which do not include a checkbox for backdraft (D. Gojkovic, 2001), one could expect that this phenomenon do in fact occur more often than reported.

A review of fire service literature reveals many narrative articles on backdrafts and the terrible consequences for fire fighters caught in this rapid flame spread phenomenon (P. Grimwood, 2003). Some of these backdraft events are reported in Section 9.2: “Backdraft: Case Histories”. Having a look at these events, one can easily realise how dangerous this phenomenon is, not only for the safety of such as individuals or people called to deal with emergencies, especially fire fighters, but also for the integrity of structures.

35

3. Under-Ventilated Fires: Backdraft Phenomenon 120 Votes

100

Votes

Votes

80 60 40 Votes [%]

20

Votes [%]

Votes [%]

0 Flashover

Backdraft

Phenomenon

Percentage

Flashover Backdraft Smoke explo.

29 35 34

Fire gas ignition

Votes 91 108 105

Figure 3-1: Result of the Website poll. Flashover, backdraft or smoke explosion? Photograph of a rapid flame spread process submitted to expert opinion.

Articles on backdraft, sometimes mistakenly called “smoke explosions”, first appeared in the literature in 1914. This first article described backdraft as a dust explosion caused by the carbon particles in the smoke. Other more recently proposed theories explaining backdraft that require impossible conditions such as instantaneous transport of oxidizer to reactant when a compartment is opened, auto-ignition of gases at impossible compartment temperatures, and ignition of soot particles at temperatures less than 500K. Since 1993, the backdraft phenomenon has started to be studied in further detail. The most elaborate work was carried out by Fleischmann: “Backdraft Phenomena” in 1993. He conducted backdraft experiments in a half-scale compartment (1.2 m x 2.4 m x 1.2 m) with both middle-slot and window openings at one end. Two different types of gases were used in his experiments: methane and propane. All of the experiments carried out with propane resulted in backdraft. Fleischmann also conducted small-scale saltwater experiments using flow visualization in order to visualise the gravity current prior to backdraft. The scale compartment (0.3 m x 0.15 m x 0.15 m) was fitted with a variety of end-opening geometries: full, middle slot, door, and window. The results showed that the values of non-dimensional parameter, Eq. (3-1), for a variety of end opening geometries were independent of the density difference ratio, Eq. (3-2). v* =

v β gh1

(3-1)

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3. Under-Ventilated Fires: Backdraft Phenomenon

β=

( ρo − ρ1 ) ρ1

(3-2)

Here, 1 indicates initial conditions in the compartment (fresh) and 0 indicates conditions in the ambient (saltwater). h1 is the height of the compartment; g is the magnitude of gravitational acceleration and ρ is the density.

Weng and Fan, 2003 describe recent experimental work carried out at the University of Science and Technology of China. They used a compartment representing a 1/4-scale residential room (1.2 m × 0.6 m × 0.6 m) and found that the key parameter determining the occurrence of backdraft is the mass fraction of unburned fuel. They estimated that the critical value of the mass fraction of unburned fuel (methane) for backdraft to take place was 9.8%, compared to a value of 15% reported by Fleischmann. He also carried out further studies on the gravity current prior to backdraft with saltwater modelling using flow visualization and Digital Particle Image Velocimetry (DPIV). The results obtained, regarding the non-dimensional parameter and the density difference ratio, also agree with the Fleischmann’s results. Ceiling opening geometries were also analysed experimentally with the objective of elucidating the effects of fire fighter ventilation tactics (Weng, 2002).

J.A. Foster, 2003 conducted backdraft experiments in a half-scale compartment (1.2 m x 2.4 m x 1.2 m) with a middle-slot opening at one end. The effects of delaying the ignition in a compartment as well as the influence in position of the igniter were analysed.

Not only small-scale experiments were conducted. Bollinger, 1995 performed a full-scale experimental study of backdrafts, in order to determine the effect of compartment scaling. The results from the study were compared with the work carried out by Fleischmann. Gojkovic et al., 2001, also performed full-scale experiments of backdraft. He used natural gas as fuel in a 5.2 m x 2.2 m x 2.2 m compartment with a middle-slot end opening. The purpose of these backdraft experiments was to create a scenario that could be used as a demonstration device for fire fighters, as well as to study in depth this phenomenon.

Gottuk et al., 1995 describe the results of a real-scale experimental series of tests designed to study the development and mitigation of backdraft.

Experiments were carried out on an ex-US Navy ship. The study showed that the key parameter for backdraft development is the fuel mass fraction in the vitiated compartment prior to venting. Using diesel fuel for the fire, it was found that a fuel mass fraction of at least 16% was required for backdraft to occur.

This test was carried out in two compartments. One compartment was used to produce safe and reliable backdraft scenarios that could be used to produce safe and reliable backdraft experiments onboard a naval ship. The other compartment with different geometries and ventilation conditions was adjacent to the backdraft compartment in order to study the intensity of a backdraft on the adjacent boundaries and ventilation conditions.

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3. Under-Ventilated Fires: Backdraft Phenomenon Much research has been carried out by FIRENET project. This project began in June 2002 and will finish in May the 2006. A consortium of eight partner organisations plus one subcontractor undertook the project. This thesis is the result of the research carried out by the University of Liege.

3. Definition of Backdraft Phenomenon Backdraft, as a phenomenon, has still not yet been defined by International Standard Organisations. That is the reason why each fire organization has its own definition and tries to include it in the fire safety world. For example, the Fire Research Station, FRS, has proposed the following definition for backdraft (Chitty, 1994):

Limited ventilation can lead to a fire producing smoke gases containing significant proportions of unburnt gases. If these unburnt gases accumulate, then the admission of air when an opening is made to the compartment can lead to a sudden deflagration, moving through the compartment and out of the opening.

This deflagration is known as a backdraft.

The National Fire Protection Association, NFPA, has also proposed a definition for this phenomenon:

A kind of ventilation-controlled fire characterized by a rapid burning of heated gases that occurs when oxygen is introduced into a building that has not been properly ventilated and has a depleted supply of oxygen due to fire.

The Institute of Fire Engineers, IFE, uses the following:

An explosion of greater or lesser degree, caused by the inrush of fresh air from any source or cause, into a burning building, in which combustion has been taking place in a shortage of air.

In some of the above the term “explosion” is included. This indicates that an explosion could take place but it is not necessarily the case because backdraft often occurs in a fairly “quiet” way (Goran, 2001). In others, the term “ignition source” is not included. This avoids an important phase of the process of the backdraft phenomenon. Without ignition, backdraft is not possible.

The author of this work proposes a new definition for this limited ventilation phenomenon in which the weaknesses of the previous definitions are corrected:

A ventilation-controlled fire characterized by a burning process of an unburntburnt gaseous mixture accumulated in the compartment due to an excess of pyrolyzates. This mixture becomes flammable by the inrush of fresh air created by a sudden opening in the compartment. If an ignition source comes into contact with the flammable mixture, a deflagration occurs pushing the yet unburnt gases outside the compartment and combusting them in a fireball.

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3. Under-Ventilated Fires: Backdraft Phenomenon Nevertheless, more important than a definition is to understand the process and/or parameters that lead to backdraft, as well as to identify all the features included in this phenomenon. A list of these features is shown below (C. PérezJiménez, 2005):

• • • • • • •

Fire developed in a ventilation-controlled situation.

Large quantity of unburnt gases accumulated in the compartment.

Supply of fresh air to the compartment (gravity current).

Mixing process between the hot, fuel-rich gases inside the compartment and the entering fresh air.

Ignition of the flammable gas due to an ignition source.

Deflagration inside the compartment.

Fireball ejected out the room.

4. Typical Backdraft Evolution Backdraft can develop from fires of either ordinary combustibles or ignitable liquids that become oxygen starved yet continue to generate a fuel-rich environment inside the compartment owing to the high temperature reached by the burning material.

An oxygen-starved or a ventilation-controlled situation is characterized by a burning with a lack of oxygen, which is due to the presence of small openings or leakages in the compartment. In this situation a large quantity of unburnt gas is released as a result of the incomplete combustion that takes place within the fire itself and therefore, a considerable accumulated inside the compartment. A fuelcontrolled situation is not conducive to backdraft since there are not so many released unburnt gases and its accumulation is negligible.

As the fire develops in a ventilation-controlled situation, the level of oxygen decreases as the hot layer descends towards the flame region. The flames then start to die out progressively until totally dying out when the oxygen concentration reaches a certain percentage. During this period, the energy release and the gas temperature start to decrease. However, the pyrolysis of the burning material may still continue at a relatively high rate, causing. This type of combustion without flames is known as smouldering combustion and it finishes when all the burning material has been consumed or has lost its thermal inertia.

The type of burning material plays a crucial role in the amount and type of unburnt gas species released into the compartment. For instances, some materials emit pyrolysis products more easily than others. A list of some pyrolysis products and their possible origin is given in Chapter 1: “Fundamentals of Fire Compartments”.

If an opening is created, a flow of fresh air enters the compartment. This inflow of fresh air is known as gravity current and is defined as the flow of one fluid into another one caused by a difference in density.

39

3. Under-Ventilated Fires: Backdraft Phenomenon

The fresh air and hot fuel-rich gases will mix along the interface of the two flow streams. This mixing process may produce a localized premixed flammable mixture that can be ignited if it comes into contact with an ignition source such as small flames, glowing particles or an electric spark.

The flammable region depends on the temperature and pressure of the mixture as well as the concentration of inert species (burnt gas). A rise in temperature or in pressure causes the flammable range to broaden, meaning an enlargement of the concentration range where an explosion can occur (see Chapter 8: “Flammability Limits of Flammable Mixtures”). The inert species has the opposite impact.

The ignition of the premixed flammable portion does not occur immediately after opening the compartment; on the contrary, there is a time delay, caused by the gravity current propagation. This creates a hazard, especially for the fire fighter facing a fire in intervention manoeuvres.

If ignition takes places, a deflagration inside the compartment will occur (see Chapter 9: “Ignition of the Flammable Region: Backdraft Deflagration”). The deflagration causes the inner gases to heat and creates a volumetric expansion within the fire compartment, thus forcing the unburnt gases out of the vents ahead of the flame front. When the propagated flame reaches the opening, the expelled unburnt gases are combusted in a dramatic fireball, Figure 3-2.

Figure 3-2: Backdraft deflagration in a shipping container: Fireball.

Typically, a backdraft deflagration lasts for a very short time, in a matter of seconds. The pressure and temperature reached in a backdraft depend on the size of the openings and of the compartment and obviously on the type of unburnt gas. As an order of magnitude and considering a compartment of 27 m3 in which methane is used as the unburnt gas, the pressure can range from 100 to 200 Pa (overpressure) and the temperature can range from 700 to 1200ºC.

40

3. Under-Ventilated Fires: Backdraft Phenomenon Backdraft can be followed by flashover, since the thermal insult will ignite all combustible material that remains non-consumed in the enclosure. However Backdraft after flashover is rare.

4.1 Gravity Current When an opening is created, a gravity current of fresh air enters the compartment. As fresh air is denser than the hot, fuel-rich gases, it will flow into the compartment through the bottom of the opening while the hot fuel-rich gases will flow out from the top. A hot fuel rich gas-air mixture is formed along the interface of the two streams.

Figure 3-3 is only for qualitative purposes, taken from the simulations of Chapter 7: “Gravity Current Prior to Backdraft�. This figure shows the inflow and outflow of the fresh air and hot fuel-rich gas, respectively, as well as the turbulent mixing process that takes place along the interface. Figure 3-3 (a) represents the gravity current when it has not reached the rear wall of the compartment, while Figure 3-3 (b) represents the gravity current when it has been reflected by the rear wall. Red indicates the hot fuel-rich gas and blue represents fresh air. Green and yellow represent the premixed hot fuel-rich gasair mixture in any other concentration than 100%.

(a)

(b)

Figure 3-3: Gravity current simulation in a fire compartment at different times.

The hot fuel-rich air mixture is created as a result of the instabilities along the shear interfaces between the outflow and the inflow, and it is carried across the compartment by the movement of the gravity current. Figures 3-4 and 3-5 represent the lobes, clefts and billows formed in a gravity current.

The rate at which the air flows in and the rate of mixing depend on the following factors:

41

3. Under-Ventilated Fires: Backdraft Phenomenon

• • •

Kind of opening; different types of opening produce different mixing processes.

Density difference between the hot fuel rich gas inside the compartment and the fresh air.

Obstacles found in the open doorway, i.e. a fire fighter standing in the doorway, and obstacles found inside the compartment.

See Chapter 7: “Gravity Current Prior to Backdraft” for further details on the effect of the type and position of the openings and the aspect ratio of the compartment on the gravity current prior to backdraft as well as the effect of obstacles on the gravity current prior to backdraft.

PLAN VIEW

Figure 3-4: Lobes and clefts formed in the gravity current structure.

PROFILE VIEW

Figure 3-5: Billows formed in the gravity current interface.

For the mixture to be ignited and for the flame to be able to propagate through the compartment, three conditions must be reached (C. Pérez Jiménez, 2004):

•

The inner gaseous mixture just before creating the opening must already or must be able to become flammable with a supply of air.

42

3. Under-Ventilated Fires: Backdraft Phenomenon

• •

An ignition source with enough energy to start the chemical reaction must come into contact with the premixed flammable mixture.

The heat and the turbulence created by the ignition of the premixed flammable mixture must be enough to ignite the mixture around it.

4.2 Ignition of the Flammable Premixed Area Two types of flame are found in a backdraft deflagration, namely premixed and diffusion flames. See Chapter 2 “Fundamentals of Fire Compartments” for further details.

Premixed flames occur when a fuel is well mixed with an oxidant (within the flammable region), normally air. For ignition to occur in this kind of flame, energy must normally be supplied to the system as a spark or small flame. A self-sustaining flame will be established around the ignition source and propagate outwards in all directions. Auto-ignition is also possible at high temperatures.

Strictly speaking, the term “deflagration” applied to backdraft refers especially to the propagation of flames through a pre-mixture of the fuel and air. Green and yellow in the Figure 3-3 represent the premixed area, subject to possible ignition.

Diffusion flames occur at the interface, where the fuel vapour and air meet.

Unlike premixed flames, the fuel vapour and the air are separate prior to burning. The dominant process in diffusion flames exists only along the fuel-air interface.

Goran, 2001 and J.A. Foster, 2003 argue that the influence of the position and time of the ignition relies on the quantity and distribution of the premixed flammable mixture formed inside the compartment at this time.

4.3. Visual Sequence of Backdraft Deflagration A backdraft deflagration is represented in Figure 3-6 by means of pictures taken from backdraft experiments (D. Gojkovic, 2001) carried out in a shipping container of 5.5 m x 2.2 m x 2.2 m (length x width x height).

The pictures represent the event from the time of ignition to the time of the ejected fireball. It can be observed:

• • • •

The ignition of the flammable mixture.

The yet unburned gases expelled outside the container as a result of the volumetric expansion.

The propagation of the flame through the container And the expelled unburned gas outside the container combusting in a dramatic fireball.

43

3. Under-Ventilated Fires: Backdraft Phenomenon

Gravity current

Propagation of flame

Flames reaching the opening

Ignition

Pushing out of unburnt and burnt gases

Propagation of flames in pushed gases

Fireball

Figure 3-6: Sequential deflagration of the backdraft experiment. Gojkovic, 2001.

The gaseous combustible used in this experiment was natural gas (98% methane). A small spark igniter placed along the rear wall is used to obtain the ignition of the flammable gas. To simulate this backdraft deflagration the following procedure was followed:

• • • •

The compartment was pre-heated.

All openings into the compartment were closed.

A 600kW flame was ignited using a pilot light and safety control system.

This diffusion flame was allowed to burn. It died out as it became oxygen-starved.

44

3. Under-Ventilated Fires: Backdraft Phenomenon

•

• • •

After the flame was extinguished, the gas was allowed to flow into the compartment. This procedure is used to simulate a pyrolysis process where a high concentration of combustion gas is released into the compartment.

When the desired concentration is reached the gas flow is turned off.

The hatch that covers the opening is opened and fresh air is allowed to enter the containers.

The spark igniter was turned on.

5. Backdraft Scenarios According to the explanation given in Section 4: “Typical Backdraft Evolution”, there are only two types of fire scenarios that can lead to backdraft. Obviously, both must be developed in ventilation-controlled fire with a great accumulation of unburnt products. These scenarios are:

•

If flames exists in the compartment: Only localised fires can lead to backdraft. In the case of fully-developed fires, all the oxygen entering the compartment will be consumed by the flames, preventing the formation of a gravity current and, consequently, the premixed mixture along the interface.

•

If flames does not exist in the compartment Every kind of fire scenario can lead to backdraft.

6. Temperature Evolution in Backdraft All the features concerning backdraft explained in the previous sections can be identified in Figure 3-7, which represents a typical temperature evolution of a backdraft.

At the beginning, the fire develops in a fuel-controlled situation. However, since the openings are small, the hot layer will soon descend towards the flame region and eventually cover the flame. When the fire reaches point A, it tries to start the transition period towards flashover, but due to the lack of oxygen the energy release and the gas temperature decrease, seen at point B. It is possible that in this period A-B, the fire may die out from oxygen starvation. However, the pyrolysis may continue at a relatively high rate causing an accumulation of unburnt gases. If an opening is provided, at point C, the temperature will decrease even more due to the entering fresh air (“Air enters” region). If the ignition of the flammable gases occurs, point D, a great rise in the temperature develops in the compartment. At this stage, backdraft will occur.

45

3. Under-Ventilated Fires: Backdraft Phenomenon Temp. [ºC] Smouldering 1000

500

Growth

Oxygen

phase

deplexion

A

B

Backdraft deflagration

C

D Grey area

0 Flashover period

Air enters

Time [s]

Figure 3-7: Backdraft evolution as a function of the temperature of the compartment.

As one way readily observe, a grey zone is shown in Figure 3-7, representing the region in which it is difficult to distinguish different phenomena such as flashover, backdraft and smoke explosions. In the following section, this is explained in further detail.

7. Grey Area Between Various Phenomena We pointed out in Chapter 2: “Fundamentals of Fire Compartments” that a fire can develop in different ways. In real life, we sometimes encounter situations that involve aspects of different phenomena. A description of the fire scenarios that can be difficult to differentiate is presented below.

7.1 Differentiating between Flashover and Backdraft As it was introduced in Chapter 1: “Fundamentals of Fire Compartments”, in a flashover the fire heats up the room slowly, then all the objects in the room suffer from the intense heat radiating from the fire plume, hot gases, hot compartment boundaries and flames, causing them to pyrolysis, to evaporate and to heat up beyond their ignition point.

Backdraft originates in a completely different way, occurring only when there is a limited airflow in the compartment, and the combustion continues with a limited oxygen supply. This leads to the accumulation of a quantity of unburnt gases, which can ignite at a later stage when air is once again supplied.

46

3. Under-Ventilated Fires: Backdraft Phenomenon

7.2 Differentiating between Smoke Gases Auto-Igniting in the Opening and Backdraft A smoke gases auto-igniting phenomenon occurs when the gases have a temperature higher than the thermal ignition point.

In a backdraft phenomenon, the ignition of the flammable gases is due to an ignition source such as an electric spark. Furthermore, ignition does not happen just after opening: there is an ignition delay, whereas in smoke explosions, ignition and opening time happen almost simultaneously.

7.3 Differentiating between Backdraft and Smoke Gas Explosion Backdraft and smoke explosions originate in entirely different ways. Backdraft occurs in a room where the ventilation condition has changed. This may be, for instance, a room where the panes are cracking, allowing air in. A smoke gas explosion, on the other hand, occurs most often in an area adjacent to the fire room.

7.4 Differentiating between Flashover and Smoke Gas Explosion Both scenarios are the simplest to differentiate between. Flashover involves diffusion flames, whereas only premixed flames are involved in a smoke gas explosion. A flashover always happens in the fire compartment itself, while gas explosion takes place in adjacent rooms.

7.5 Warning Signs of Backdraft The indicators below, taken together, should be regarded as warning signs for an imminent backdraft.

Before a fire fighter creates the opening in a fire compartment, the following points need to be taken into consideration:

• • • • • • •

Blackened windows with oil deposits, which are a sign that pyrolysis products have condensed on cold surfaces. Also a sign of an underventilated fire.

Pulsating smoke gases form small openings in the room, which are also a sign of under-ventilated conditions.

Whistling sounds around doors and windows. It may be related to the fire’s pulsating.

High room temperature.

Little or no visible flames in the room.

Large amounts of unburnt gases accumulated.

Hot doors and windows indicating that the fire has been burning a while, perhaps with limited ventilation.

47

3. Under-Ventilated Fires: Backdraft Phenomenon After a fire fighter creates the opening in a fire compartment, the following points need to be taken into consideration:

• •

•

An orange glow or a small fire can indicate that the fire has been burning with a lack of oxygen for a long time.

Smoke gases drawn back through the opening, indicating that a gravity current has entered the room. Hot smoke gases will leave the room, perhaps through another opening and the replacement of air will be drawn in through the opening.

The neutral plane is close to the floor.

These signs together can provide warning signs for imminent backdraft.

8. Backdraft Fatalities and Case Histories 8.1 Backdraft Fatalities Rapid-fire events are a significant killer of fire fighters. NFPA statistics (Rita F., 2001) show that a total of 47 US fire fighters lost their lives to rapid-fire process between 1985 and 1994.

Of the 87 fire fighters killed in the US during the period between 1990-1999 who reportedly died of smoke inhalation while operating inside structures, the major causes of injury were, Figure 3-8:

• • •

Became lost inside the structure and ran out of air: 29 deaths.

Caught by the progress of the fire, backdraft or flashover: 23 deaths.

Caught in structural collapses: 18 deaths.

Furthermore, of the 31 US fire fighters who reportedly died of burns inside a structure during the same period, the major causes of injury were, Figure 3.9:

• •

Rapid-fire progress (backdraft or flashover): 14 deaths.

Structural collapse: 12 deaths.

As we can see, rapid-fire processes are very dangerous, occurring in 30% of the fires provoking injuries and deaths.

Observing this statistical study, backdraft is a fire situation very seldomly reported. The reason of that can be found in some of the following points:

• • •

Experienced fire fighters recognise the signs of impeding backdraft and deal with it.

Observers may have difficulty describing the phenomena and may easily mistake it for other phenomena.

Weak backdraft can be mistaken for other phenomena.

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3. Under-Ventilated Fires: Backdraft Phenomenon

Figure 3-8: Fire fighters killed by smoke inhalation from 1990-1999. Major causes of injury.

Figure 3-9: Fire fighters dying from burns inside structure, 1990-1999. Major causes of injury.

8.2 Backdraft: Case Histories Some accidents which occurred between 1990-1999 are commented on below.

8.2.1 Backdraft in Cinema of London (1994) At 17:39 on February 26, 1994 London fire fighters responded to a fire in a private cinema club in the central city area. On arrival four people were seen trapped at a third-storey window and one man had already jumped from this window prior to their arrival. As a ladder was positioned for the rescue a further

49

3. Under-Ventilated Fires: Backdraft Phenomenon three men jumped from the window and another three were eventually assisted out and down the ladder. With reports of additional people trapped inside the structure fire fighters in SCBA advanced a hose line towards the interior stairs.

As they reached the stairs a very loud roaring and intense fire escalated in the stair-shaft and the crew were beaten back. A total of three people had jumped from the third storey and portable ladders and an aerial tower ladder were used to rescue a further 17. An additional six men died in the third storey cinema area. The classic roaring sounds experienced by fire fighters attempting to reach the upper floors by the interior stair shaft demonstrated a backdraft situation where fire gases were burning off in the shaft as air rushed in from the doorway access.

8.2.2 Backdraft in a Two-Storey House in Wales (1996) On February 1, 1996 in Blaina, Wales, a fire involved the ground floor kitchen at the rear of a two-storey house during the early hours. The initial crew of six fire fighters were faced with the predicament of children reported missing and trapped upstairs. The building was heavily charged with smoke, which was seen issuing from the eaves on arrival. They chose to attempt the rescues first and in doing so, no interior fire attack or fire isolation strategy was undertaken. Two hoselines (19mm hose-reels) were brought to the structure but neither was put into use prior to the backdraft occurring five minutes after arrival. Flames were seen issuing from the rear kitchen window and the compartment fire had progressed to a post flashover stage. However, a distinct gravity current was in progress with a heavy volume of thick black smoke exiting at the front entrance doorway. A resulting backdraft took the lives of two fire fighters as the fire developed unchecked for several minutes.

8.2.3 Backdraft in a Supermarket in Bristol Just three days later, another fire fighter (female) was killed by an ensuing backdraft that occurred in a large supermarket in Bristol. As four fire fighters (including the victim) entered through the main entrance to tackle the fire the heavy black smoke layer was in motion, continually rising and falling. Just five minutes after entry an intense howling wind was seen entering the main doorway causing flames to bend inwards. The resulting ignition of the fire gases moved across the wide expanse of the store both under and within the suspended fibre-board ceiling at an estimated five metres per second (high velocity gas combustion). The accompanying pressure wave knocked one fire fighter off his feet.

Should fire fighters have entered these conditions in the first place? The continuous rise and fall of the smoke layer is most likely a result of the pulsation cycle caused by brief ignitions (oscillatory combustion) in the fuel-rich gas layers. This may also be linked to the puffing phenomena noted by Sutherland. As these ignitions occur intermittently the repeated thermal 50

3. Under-Ventilated Fires: Backdraft Phenomenon expansions of fire gases may cause the smoke interface to rise and lower and such a process must be viewed as a classic warning sign for backdraft.

8.2.4 Backdraft in an Apartment, New York City (1994) On March 28, 1994, the New York City Fire Department (FDNY) responded to a report of smoke and sparks issuing from a chimney at a three-store apartment building in Manhattan. The officer in charge ordered three-person hose teams to create an entry into the first- and second-floor apartments while the truck company ventilated the stairway from the roof. When the door to the first-floor apartment was forced open, a large flame issued from the apartment and up the stairway, engulfing the three fire fighters at the second-floor landing. The flame persisted for at least 6½ minutes, resulting in their deaths. The FDNY requested the assistance of the National Institute of Standards and Technology (NIST) to model the incident in the hope of understanding the factors that produced a backdraft condition of such duration. The CFAST model was able to reproduce the observed conditions and supported a theory of the accumulation of significant quantities of unburnt fuel from a vitiated fire in an apartment, which had been insulated and sealed for energy efficiency. This demonstrated that backdraft is not always, as commonly believed, a transient event involving short, possibly violent, releases of energy from the fire, which are not normally sustained.

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3. Under-Ventilated Fires: Backdraft Phenomenon

9. Backdraft Pictures

Photograph from: http://haileyburyfiredepartment.com

Photograph from: http://www.firetactics.com/

Photograph from: http://www.firetactics.com/

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3. Under-Ventilated Fires: Backdraft Phenomenon

10. Conclusions In this chapter, the typical backdraft evolution in fire compartments has been explained. A new definition of this phenomenon in which the weaknesses of the existing ones are corrected is also provided. The difference between this and other rapid-fire processes that could be difficult to differentiate such as flashover and gas explosions has been commented.

An extensive review of the studies carried out on backdraft phenomenon has been presented in Section 2: “State of the Art�. One may well remark that 90% of the studies have been done in the experimental field. The principal conclusion obtained from these experimental studies show that backdraft is closely linked to the mass concentration of hydrocarbon accumulated just before the opening.

However, certain aspects remain still unanswered such as what is the influence of the partitions on the backdraft phenomenon, with which fuels the risk of backdraft is higher, what about the effect of the size of the openings on the backdraft deflagration, which parameters influences the size of the fireball or for example how the available energy for a backdraft is distributed inside and outside the compartment when deflagration occurs. This thesis intends to clarify these and other questions using a simple numerical model developed by the author.

Thus, the pyrolysis rate and the smouldering combustion of the burning material are explained in Chapter 4. This chapter introduces a graphical method for obtaining the accumulation of unburnt products in a compartment that can be very useful for evaluating the risk of backdraft. The Chapter 5 deals with the combustion products released in a fire. Chapter 6 deals with a model for predicting the breaking times of glass windows in a fire. The mixing process of fresh air and hot smoke is analysed by CFD simulations in Chapter 7. Chapter 8 introduces a new model for computing the flammable region of flammable mixture (fuel(s)-inert(s)-air) in backdraft conditions. Chapters 9 and 10 deal with the ignition and the deflagration of the flammable mixture. Chapter 11 introduces the basics of OZone and explains the implementation involving the models developed in the previous chapter. Chapter 12 deals with the application of the model to a hypothetical fire in a building. The last chapter deals with the conclusions obtained from this research.

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3. Under-Ventilated Fires: Backdraft Phenomenon

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