2 fundamental of fires

Chapter 2

Fundamentals of Fire Compartments

Table of Contents

1. Introduction..............................................................................11 2. Definition of Fire: Fire Triangle................................................11 3. Fire Evolution in a Compartment.............................................12 4. Factors Influencing Fire Development in a Compartment ........14 5. Physics and Chemistry of Fire Source ......................................17 5.1

Thermal Mechanism in a Fire.............................................17

5.1.1 Conduction.......................................................................18 5.1.2 Convection .......................................................................18 5.1.3 Radiation .........................................................................18 5.2

Rate of Heat Release ..........................................................18

5.3

Combustion Heat of a Fuel.................................................19

5.4

Fire Load............................................................................20

5.5

Fire Load Density...............................................................21

5.6

Ignition of a Fuel................................................................21

5.6.1 Ignition Temperature .......................................................22

5.6.2 Auto-Ignition Temperature ..............................................22 5.7

Surface Flame Spread .........................................................23

6. Pyrolysis Process and Unburnt Products in Fires .....................24 6.1

Pyrolysis Rate ....................................................................24

6.2

Content of Unburnt Products.............................................24

7. Types of Flames: Premixed and Diffusion.................................27 8. Flammability Limits of Gaseous Mixture..................................28 9. Rapid Flame Spread Process ....................................................30 9.1

Flashover ............................................................................30

9.2

Smoke Gas Explosions ........................................................30

9.3

Backdraft Phenomenon.......................................................31

2. Fundamentals of Fire Compartments

1. Introduction Before examining the major objective of the present thesis, we find it necessary to be sure that all the basics are understood. Therefore, the aim of this chapter is to introduce to the reader the terms and concepts that will be used throughout the thesis (i.e. ventilation and fuel-controlled fires, rapid flame spread process, the factors influencing fire evolution, the physics and chemistry of fire and the basics of combustion).

This section will simply serve as a reminder to some and a quick introduction to others. Nevertheless it is important for all readers to have some background knowledge of fire science.

2. Definition of Fire: Fire Triangle Fire is defined as an exothermic reaction that may occur when three elements are brought together: fuel, oxygen and a heat source. These three elements are frequently referred to as the “fire triangle”. To exist and to be sustained, a fire needs: • • •

Enough oxygen for combustion.

Enough heat to raise the material to its ignition temperature.

Some sort of fuel or combustible material.

If one of these elements is missing, a fire will not take place or it will go out.

Other authors have extended the necessary elements to sustain a fire to five i.e. the “fire pentacle” (see Figure 2-1).

Fuel

Fuel

Oxygen

Oxygen

Energy

Mixing Ratio

Energy

Inhibidor Inhibitor

Figure 2-1: Fire triangle and Fire pentacle.

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2. Fundamentals of Fire Compartments

3. Fire Evolution in a Compartment A fire can develop in many different ways. However, it is mainly affected by the quantity and arrangement of combustible material in the fire compartment, and by the oxygen supply.

If the fire takes place in a situation where the ventilation is great enough to have no influence on the fire source development, the fire is said to be fuelcontrolled. In this situation there is sufficient oxygen available for combustion and the fire depends mainly on the amount of available combustible material.

However, if the ventilation is small, relative to the size of the fire, and there is not enough oxygen to combust all the pyrolysis fuel, the fire is said to be ventilation-controlled. In this situation the rate of heat released depends mainly on the amount of available oxygen and therefore on the ventilation conditions.

Fires are often discussed in terms of temperature evolution in the compartment.

Figure 2-2 shows the evolution of the average temperature in a fire compartment. Here, the horizontal axis represents the time and the vertical axis the temperature. Four different fire phases can be distinguished in this figure: • • • •

Ignition phase.

Growth phase.

Steady-state phase.

Decay phase.

Temp. [ºC]

Pre-flashover

Post-flashover

1000 Steady-state phase 500

(full-developed fire) Growth phase

Decay phase

Ignition 0 Time [s] Figure 2-2: Temperature evolution in a fire compartment. Fire phases.

A brief explanation of these phases is given below. For further details, readers are invited to refer to the book Enclosures Fire Dynamic (Karlsson, 2000).

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2. Fundamentals of Fire Compartments Ignition phase Every fire starts with the ignition of the burning material, usually in a single location of the compartment. Well-known exceptions are arson fires that are often ignited in several locations. To ignite a combustible material, a certain amount of energy must be provided. This energy comes from any kind of ignition source such as an electric spark or an open flame. See Section 5.6: “Ignition of fuels”.

Growth phase In this phase the fire starts to propagate within the compartment. It is characterized by an exponentially increasing heat release rate that depends on the type of combustion, fuel, interaction with the surroundings, and access to oxygen; see Section 4: “Factors Influencing the Fire Development in a Compartment”.

As the fire propagates, a fire plume is formed above the fire source as well as a hot smoke layer in the upper part of the compartment. The temperature rises in the compartment and, from here, the situation can evolve towards one of the following situations: •

Fuel-controlled situation: a) The gas temperature becomes so elevated that, after a certain period of time, it causes the sudden ignition of every object and unburnt gas in the compartment. This phenomenon is called flashover.

The period leading up to flashover is often called pre-flashover phase during which the fire is usually localised. After flashover, the fire phase is called post-flashover, during which the fire is fully developed, Figure 2-2. b) There is no spread of the fire to the whole compartment, because the propagation is so slow that the temperature rise is not sufficient to cause flashover, or because the fire can find no combustible material in its close vicinity. The fire remains localised and, with time, dies out.

•

Ventilation-controlled situation: a) If air is not allowed to enter the compartment, the fire will die out. This fire development is shown in lines 2 and 3 of Figure 2-3. b) If a new opening is created, an air current will enter the compartment.

This can result in a rapid flame process such as a flashover, a backdraft or a smoke explosion. The terms flashover, backdraft and smoke explosion are defined in Section 9: “Rapid Flame Spread Process”. This fire development is represented in line 1 of Figure 2-3.

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2. Fundamentals of Fire Compartments The fully developed phase This phase is characterized by a heat release rate, which is relatively unchanging, leading to small variations in temperature.

Decay phase This phase is characterised by a continuous deceleration in the heat release rate and thus, the average temperature in the compartment. In this period, the fire may go from a ventilation-controlled to a fuel-controlled situation.

Figure 2-3 shows the evolution of the average temperature in a fire compartment as a function of time. Line 1, 2 and 3 represents the different ventilation-controlled situations explained in previous paragraphs. The horizontal axis represents the time and the vertical axis the temperature.

Temp. [ÂşC] Fuel-controlled

Ventilation-controlled

1

1000

500

3

2

0 Time [s] Figure 2-3: Temperature evolution according to the fire situation (Fuel or ventilation-controlled).

4. Factors Influencing Fire Development in a Compartment The factors influencing the fire development in a compartment can be divided into two main groups: those that have to do with the compartment itself and those that have to do with the fuel (Karlsson, 2000).

These factors are: •

Regarding to the fuel: o Size and location of the ignition source.

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2. Fundamentals of Fire Compartments o Fuel packages (e.g. type, amount, position). •

Regarding to the compartment: o Geometry. o Openings. o Material properties of partitions.

A brief commentary on these factors is given below.

Ignition source Ignition is the first visible sign of combustion. The ignition source can consist of a spark or a heated surface. The source of energy can be chemical, electrical or mechanical. The greater the energy of the source is, the quicker the subsequent fire growth on the fuel source will be.

Its position of the ignition source also has an important role in the growth of the fire. For example, an ignition at the lower part of a curtain causes a faster fire growth than if it is placed at the top or an ignition of unburnt gases next to the opening will result in lower pressure than if it is ignited in another place in the compartment.

Fuel The type and amount of fuel is one of the principal factors determining the fire development in a compartment. The fuel can be found as a solid (wood pallets), liquid (gasoline) or gas (methane). A solid fuel such as wood-based furniture usually results in a slow fire growth but can burn for a long time. Liquids and above all gaseous fuels result in more rapid fire growth but burn for a shorter time (smoke explosions are considered as this kind of fire).

As well as the ignition source, the position of the fuel package influences the fire development. Figure 2-4 shows temperature measurements above the fires in 1.22 m high stacks of wood pallets. Other factors having a marked effect on the fire development are the orientation and the surface area. For example, large surface of fuel packages will burn more rapidly that an otherwise equivalent fuel package with a small surface area.

Compartment geometry The hot smoke layer and the upper bounding surfaces of the enclosure will radiate towards the burning fuel, increasing the pyrolysis rate. Other combustible objects in the compartment will also be heated up due to this radiation.

In addition to the temperature and thickness of the hot layer, the temperature of the upper bounding surfaces has a considerable impact on the fire growth (Karlsson, 2000). If the compartment is small, the burning material will cause high temperatures and rapid fire growth due to high radiation. However, if the 15

2. Fundamentals of Fire Compartments same burning material is placed in a large compartment, then lower gas temperatures, longer smoke-filling times, less feedback to the fuel as well as slower fire growth will occur.

Figure 2-4: Temperature of the plume as a function of the height above a burning stack of wood pallets

Compartment openings A fire compartment can develop in a ventilation-controlled situation or in a fuel-controlled fire depending on the size of the openings. However, it is when the fire becomes ventilation-controlled that the size and position of the opening first become all-important.

Kawaoge, 1958 found that the rate of burning depended very strongly on the ventilation factor, defined in Eq (2-1), where Ao is the area of the opening and Ho is the height.

Vf = A o H o

(2-1)

An increase in the ventilation factor will lead to an equal increase in the burning rate. This is valid up to a certain limit when the burning rate becomes independent of the ventilation factor and the burning becomes fuel-controlled.

Properties of partitions The properties of the partitions of a compartment affect the gas temperature as well as the amount of fire gases released into the compartment.

The property controlling the heat flow through a construction is called thermal

16

2. Fundamentals of Fire Compartments

inertia and is given as the product of the conductivity, density, and heat capacity: λρc. Table 2-1 shows these properties for normal wood and lightweight concrete.

Wood Concrete

λ [W/mK]

ρ [kg/m3]

Cp [J/kgK]

0.1 0.8

450 1600

1113 840

Table 2-1: Thermal inertia properties of wood and lightweight concrete.

Figure 2-5 is an OZone simulation that shows the gas temperature for a cubic compartment, 8.0 m3, when the partitions are made of wood or lightweight concrete. The higher thermal inertia, the lower gas temperature.

1200

Wood

1000

[C]

800 600

Ligth concrete

400 200 0 0

250

500

750

1000

1250

1500

1750

2000

2250

2500

Time [s]

Figure 2-5: Comparison of the average gas temperature according to the partition material.

5. Physics and Chemistry of Fire Source 5.1 Thermal Mechanism in a Fire There are three thermal mechanisms present in a fire compartment: • • •

Conduction.

Convection.

Radiation.

In the following sections brief commentaries about each thermal mechanism are given. Major review articles may be found in the SFPE Handbook (Atreya, 1995; Rockett and Milke, 1995; Tien et al., 1995).

17

2. Fundamentals of Fire Compartments

5.1.1 Conduction Conduction is the mode of heat transfer associated with solids. It is direct thermal energy transfer due to contact. This energy is passed on from one molecule to the next. The empirical relationship (Fourier, 1955) is expressed as:

qɺ "x = −λ

∆T ∆x

(2-2)

where λ is the thermal conductivity, expressed in W/mK and ∆T is the temperature difference, K, over a distance ∆x, in m.

5.1.2 Convection Convection is heat transfer transmitted through a liquid or gaseous medium.

This transfer is caused by the density difference of hot molecules compared to cold ones. The empirical relationship first discussed by Newton is: qɺ " = h∆T

(2-3)

where h is known as the convective heat transfer coefficient, expressed in W/m2K. The evaluation of this parameter has been one of the major problems in heat transfer and fluid dynamics. Typical values lie in the range of 5.0 – 25.0 W/m2K for free convection and 10.0-500.0 W/m2K for forced convection in air, (Drysdale, 1999).

5.1.3 Radiation Radiation is the electromagnetic wave transfer of heat to an object. Waves travel in all directions from the fire and may be reflected or absorbed by a surface. Absorbed heat raises the temperature of the material. It may cause pyrolysis, augmenting the material’s temperature, and even ignition.

It is expressed, according to Stephan Bolzmann as: qɺ " = σεT4

(2-4)

where ε is a measure of the efficiency of the surface as a radiator and σ is the Stefan-Boltzmann constant, 5.67·10-8 W/m2K4.

5.2 Rate of Heat Release The rate of heat release is the quantity of energy that is released by the fire per second. The RHR depends on the type and quantity of fuel present in the compartment, on the ventilation conditions, and on the phase of the fire (growth, steady, decay). Its value is time dependent.

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2. Fundamentals of Fire Compartments It controls a considerable extent all phenomena that occur during the first stages of a fire such as the plume flow or the hot layer temperature. In subsequent fire stages, other parameters become very important: ventilation conditions, excess of pyrolysis, thermal properties of partitions, etc., but the rate of heat release remains of primary importance. ɺ fi , is the quantity of the mass of solid fuel that is The pyrolysis rate, define as m transformed into combustible gases per second. In other words, it is the mass loss rate of fuel. The rate of heat release is related to the pyrolysis rate by Eq (2-5), where the effective combustion heat is defined in Section 5.4: “Fire Load”.

ɺ fi (t) RHR(t) = Hc,eff (t) m

(2-5)

Eq (2-5) is only valid for free burning fires, i.e. when the oxygen does not limit the amount of heat released by the fire. It is very common to use the expression “burning rate” as a synonym of pyrolysis rate. However, this usage is inappropriate because mass loss and burning might be not proportional in under-ventilated conditions. Thus in this text, the terms pyrolysis rate or mass loss rate are preferred to burning rate.

Models for the estimation of the rate of heat release exist for different types of burning items (Babrauskas, 1995 & 2000): wood cribs; pool fires; various types of furniture (sofas, mattresses); Christmas trees; television sets; etc.

5.3 Combustion Heat of a Fuel The energy released by the combustion of one unit of mass of fuel in an oxygen bomb calorimeter under high pressure and in pure oxygen is the complete (or net) combustion heat of the fuel, Hc,net. Under these conditions, all the fuel is burnt, leaving no residue and releasing all its potential energy.

In real fires, the energy that the same unity of mass is able to release is lower than Hc,net. Usually about 80% of the complete combustion heat is released.

Table 2-2 shows the heat of combustion of some fuels at 25ºC (Drysdale, 1999).

A part of the combustible material is not pyrolysed leaving some soot and, in addition, not all of the volatiles produced by pyrolysis are completely oxidised, resulting in unburnt products. The effective combustion heat of fuel is defined as the ratio between the heat release rate during a real fire and the rate of mass of fuel loss during this real fire, Eq (2-6).

H c,eff (t) =

RHR(t) ɺ fi (t) m

(2-6)

The efficiency of the combustion is represented by the combustion efficiency factor m, which is the ratio between the effective and the complete combustion heat of the fuel, Eq (2-7).

19

2. Fundamentals of Fire Compartments m(t) =

H c,eff (t)

(2-7)

H c,net

The values of the effective combustion heat and therefore of the combustion efficiency factor depend on many parameters, therefore the temperature in the compartment or the means of fuel storage, and actually vary with time t.

Fuel species Carbon monoxide Methane Ethane Ethene Ethyne Propane n-Butane n-Pentane n-Octane c-Hexane Benzene Methanol Ethanol Acetone D-Glucose Cellulose Polyethylene Polypropylene Polystyrene Polymethlmethacrylate Polyacrylonitrile

CO CH4 C2H6 C2H4 C2H2 C3H8 n-C4H10 n-C5H12 n-C8H18 c-C6H12 C6H6 CH3OH C2H5OH (CH3)2CO C6H12O6

∆Hc ∆Hc [KJ/mol] [KJ/g] 283 10.10 800 50.00 1423 47.45 1411 50.35 1253 48.20 2044 46.45 2650 45.69 3259 45.27 5104 44.77 3680 43.81 3120 40.00 635 19.83 1232 26.78 1786 30.79 2772 15.4 --16.09 --43.28 --43.31 --39.85 --24.89 -30.80

∆Hc,air [KJ/g(air)] 4.10 2.91 2.96 3.42 3.65 2.97 2.97 2.97 2.97 2.97 3.03 3.07 2.99 3.25 3.08 3.15 2.93 2.94 3.01 3.01 3.16

∆Hc,ox [KJ/g(O2)] 17.69 12.54 11.21 14.74 15.73 12.80 12.80 12.80 12.80 12.80 13.06 13.22 12.88 14.00 13.27 13.59 12.65 12.66 12.97 12.98 13.61

Table 2-2: Heat of combustion of selected fuels at 25ºC. Drysdale (1999).

In Chapter 10: “Energy Released in Backdraft Phenomenon”, a method for evaluating the combustion heat at temperatures other than 25ºC is introduced.

5.4 Fire Load The fire load in a compartment is defined as the total energy that could theoretically be released into the compartment in case of fire. It consists of the different elements present in the compartment (furniture, etc.), construction elements, partition linings and, in general, all the combustible content. The fire load is usually expressed in Joules and is the sum of the product of the mass Mi of each item present in a compartment and its heat of combustion, Hc,i, Eq (28). It is also very common to use the equivalent wood mass, i.e. the mass of

20

2. Fundamentals of Fire Compartments wood that would release the same amount of energy as the fire load (specifically, the fire load in J divided by the combustion heat of wood in J/kg). m fi =

∑H

c,i

Mi

(2-8)

i

5.5 Fire Load Density The fire load density qf,k is the fire load per unit area related to the floor area. It is obtained by a survey of real compartments. Data are available for different types of occupancies of compartments (Robertson, 1970; Culver, 1976; Thomas, 1995; Kumar, 1995 & 1997; Korpela, 2000; EN1991-1-2, 2002). In order to obtain these data, the mass of all types of combustible materials present in compartments is measured or estimated. This mass is then multiplied by the combustion heat of the materials that constitute the fire load. Next, this quantity is divided by the floor area of the compartment, according to Eq (2-9). q f,net =

1 Af

∑H

c,i

Mi

(2-9)

i

The combustion heat used in Eq (2-9) is either the net or the effective combustion heat, depending on the author. The fire load density may also be defined as the fire load per unit area related to the surface area of the total enclosure, including openings. In this case, the fire load density is noted as qt.

5.6 Ignition of a Fuel Ignition is the first visible sign of combustion. As it is the first step in any compartment fire, ignition is probably the most important phenomenon: without ignition, there is no fire, and, therefore, there is no rapid flame spread process.

There are different ways that a combustible material can be ignited. Ignition can occur either in the solid or in the gas phases.

If the reaction is initiated by an ignition source (open flame, electrical spark, etc.), the ignition is normally categorised as piloted. If ignition occurs without a pilot, the process is normally referred to as spontaneous or auto ignition. For this last type of ignition to occur, the fuel must be at an elevated temperature.

Several reviews of the ignition of solid combustible have been published describing the different experimental observations and theoretical models of the process (Kanury, 1971, 1988; Williams, 1985; Drysdale, 1999, Vilyunov and Zarko, 1989). Readers are referred to these works for additional information about this topic.

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2. Fundamentals of Fire Compartments

Exothermic process

⇒

+

O2 ⇒ Energy + H2O + CO2 + CO…

Ignition Figure 2-6: Ignition process.

5.6.1 Ignition Temperature The ignition temperature of a substance is the minimum temperature to which the substance exposed to air must be heated in order to cause combustion.

Flashpoint refers to the lowest temperature of a liquid at which it gives off sufficient vapour to cause a flammable mixture with the air near the surface of the liquid or within the vessel used that can in turn be ignited by a spark or energy source. A specific ignition temperature for solids is difficult to determine because this depends upon multiple aspects such as humidity (wet wood versus dry wood), composition (treated or non-treated wood) and physical form (dust or shavings or a log of wood).

Quintiere, 2000 reports that the ignition temperature of wood exposed to the minimum heat flux possible for ignition is around 250°C, depending on the kind of wood and the direction of the grain. A surface temperature of 500-600ºC is needed before auto-ignition (Goran, 2001).

5.6.2 Auto-Ignition Temperature The auto-ignition temperature is the lowest temperature at which a solid, liquid or gas will self-ignite without an ignition source. Such conditions can occur due to external heating, as in the case of a frying pan that overheats, causing the oil to auto-ignite. The auto-ignition temperature of substances exceeds its flashpoint. Table 2-3 and Table 2-4 represent the auto ignition temperature and the flash point of some common products (Drysdale 1999).

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2. Fundamentals of Fire Compartments

Type of gas Diesel fuel Ether Frying fat Gasoline

Auto-ignition Temp [ºC] 250-400 190 350 260

Flashpoint [ºC] 40-100 -41 250-380 -45 to -18

Explosion limits (vol. %) 0.5-7 1.7-48 -1-7

Vapour density (Relation to air) 6-8 2.6 -3.5

Table 2-3: Flashpoint and auto-ignition temperature of selected gaseous fuels.

Type of Solid fuel Polyvinylchloride (PVC) Nylon Polyethylene (PE) Polystyrene (PS) Polyurethene (PUR) Wool

Auto-ignition Temp [ºC] 470 450 350 490 420 570

Type of Solid fuel Teflon Wood Paper Charcoal Coal Cotton

Auto-ignition Temp [ºC] 600 250-350 200-350 140-300 +/- 350 300-400

Table 2-4: Auto-ignition temperature of selected solid fuels.

5.7 Surface Flame Spread The spread of a flame over a surface of a solid combustible is subject of interest in fire safety because it influences the initial fire development, the rate of heat release and the pyrolysis rate. For a flame to spread, enough heat must be transferred from the flame to the unburnt material ahead of the flame to pyrolyse it. The vaporised fuel is then diffused and convected away from the surface, mixing with the oxidizer and generating a flammable mixture ahead of the flame’s leading edge, which is then ignited by the flame. The rate of flame spread is therefore determined by the ability of the flame to transfer the necessary heat to pyrolyse the solid and to ignite the combustible mixture ahead of it (Fernandez-Pello, 1995).

The normal process of fire spread in a compartment is known as the cube model (Goran, 2001). This model explains that if all the compartment partitions are equal, the ceiling will be more affected due to the exposure to the rising heat. A less likely fire spread will be the vertical one, i.e. along the walls. And the least probable fire spread will be a downward spread through the floor. Figure 2-7 represents the cubic model.

Several reviews have been published describing the differential experiment observations and theoretical models of the flame spread process (Sirignano, 1972; Williams, 1985; Fernandez-Pello, 1985; Wichman, 1992).

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2. Fundamentals of Fire Compartments

Ceiling (upwards)

Walls

Fire

(sidewards)

Walls (sidewards)

Floor (downwards)

Figure 2-7: Cubic model propagation.

6. Pyrolysis Process and Unburnt Products in Fires 6.1 Pyrolysis Rate A fire is a highly complex combustion system that can be divided into a number of distinct but interdependent processes. Heat transfer from the flame and from the environment back to the fuel induces decomposition and/or evaporation of the fuel, producing a stream of gaseous fuel; this phenomenon is the pyrolysis process.

Drysdale, 1999 gives a simple description of how the thermal processes occurring in a compartment fire will enhance the pyrolysis of the combustible material.

In Chapter 4: “Pyrolysis Rate and Smouldering Combustion�, a simplified model for estimating the pyrolysis rate and the smouldering combustion is developed and implemented in OZone.

Several reviews have been published describing the process of pyrolysis in under ventilated conditions (Delicharius, 2004; Kawaoge, 1958; Fernandez-Pello, 1995; Quintiere, 1988).

6.2 Content of Unburnt Products Unburnt gases are always formed if combustion takes place where the oxygen supply is insufficient. However, even when the oxygen supply is sufficient for all the fuel to combust, there will always be some unburnt gases formed.

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2. Fundamentals of Fire Compartments The combustion products found in smoke gases originate from: • •

Pyrolysis from materials that are not in contact with the actual source of a fire.

Incomplete combustion from the actual source of a fire.

The more incomplete combustion is, the more unburnt products are found in the smoke gases. In addition, the poorer the access to air, the more incomplete the combustion process. Thus, this increases the likelihood of the smoke gas layer igniting (e.g. as flashover, backdraft or smoke explosions).

If combustion takes place with a good oxygen supply, a large amount of carbon dioxide and water will be released. Numerous other products can also be formed, depending on the material’s constituents. Some of these are described below. Although the present work is not focused on the toxicity of the unburnt gases, in the following paragraphs, some general values will be given.

Carbon monoxide (CO) is one of the most common gases that can be found in a fire, together with carbon dioxide and water. This gas is highly combustible and has a broad flammability range. CO is a colourless and odourless gas, which makes it difficult to detect. The CO content can vary from 0% to 15% with certain fuel arrangements. Table 2-5 represents the harmful effects of this gas as a function of the CO content.

In addition, hydrogen cyanide (HCN) is produced when products such as wool, silk, nylon and polyurethane do not combust completely. This gas is highly combustible and toxic and quickly causes death by asphyxiation. This gas is flammable when its concentration falls within 4.0 – 40.0 % in the air.

Normally, nitrogen dioxide (NO2) is formed at the same time as hydrogen cyanide. This gas and other oxides of nitrogen are produced in small quantities from fabrics and in large quantities from materials such as viscose. Nitrogen dioxide causes severe irritation to the lungs and can result in immediate death.

CO content [%] Harmful effect 0.1 – 0.12 Unpleasant after 1 hour (dizziness, headaches) 0.15 – 0.2 Dangerous when inhaled for more than 1 hour (paralysis, loss of consciousness) 0.3 Dangerous when inhaled for ½ hour 1.0 Lethal when inhaled for 1 minute Table 2-5: Harmful effects as a function of the CO content.

Unburnt hydrocarbons are formed when hydrocarbons compounds are combusted incompletely. They contain C and H in different combinations and are colourless. The unburnt hydrocarbons are easily combusted in certain temperature and air concentration condition. They are in great measure responsible for the occurrence and severity of phenomena such as flashover, backdraft and smoke explosion.

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2. Fundamentals of Fire Compartments A list of other unburnt products and their origins is shown in Table 2-6.

Product

Origin

Toxicological effect

Carbon dioxide

Common combustion products

Non-toxic, can available oxygen

Carbon monoxide

Common combustion products

Asphyxiant

Hydrogen sulphide

Rubber, crude oil, sulfur containing compounds

Toxic gas, repugnant smell

Hydrogen bromide

Some fire retardant materials

Respiratory irritant

Hydrogen fluoride

Fluoropolymers

Toxic, irritant

Sulfur dioxide

Materials containing sulfur

Strong irritant

Isocyanates

Polyurethane polymers

Respiratory irritant

Acrolein & adehydes

Polyolefins,‌common products in combustion

Respiratory irritant

Ammonia

Wool, silk, nylon, melamine, normally only in small concentrations at building fires

Irritant

Phosgene

Chlorinated salts, some chlorinated hydrocarbons

Toxic, irritant, skin burns

Polyaromatic hydrocarbons

Common combustion products e.g. in soot

Long term effects

Dioxins

Combustion of PCB containing recipients,‌

Long term effects

deplete

Table 2-6: Common unburnt products in fires and their origins.

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2. Fundamentals of Fire Compartments

7. Types of Flames: Premixed and Diffusion Flame is usually described as a region where a reaction takes place between fuel and the air. There are two types of flames: premixed and diffusion flames. Both have different properties (Beyler, 1995).

•

Premixed flames

This type of flame occurs when the fuel and air are already mixed together and the mixture falls within the flammability range before ignition occurs.

The term “premixed” is used to mean that the fuel is evenly distributed and mixed with air.

To provide a better understanding, an example of this type of flame is given.

If gas leaks from a pipe at high pressure, mixing will occur very quickly, which can result in part of the mixture falling within the flammability range.

If it is ignited, a premixed flame occurs.

•

Diffusion flames

The principal characteristic of this type of flame is that the fuel and oxidizer are initially separate and combustion occurs in the zone where both gases mix. The classical diffusion flame can be demonstrated using a simple Bunsen burner with the air inlet port closed. The stream of fuel issuing from the burner chimney mixes with the air by entrainment and diffusion and, if ignited, will burn wherever the concentration of fuel and oxygen are within the appropriate flammability limits (see Section 8:“Flammability Limits of Gaseous Mixture”).

The appearance of the flame will depend on the nature of the fuel and the velocity of the fuel jet with respect to the surrounding air. Thus, hydrogen burns with a flame that is almost invisible, while all hydrocarbon gases yield flames that have the characteristic yellow luminosity arising from incandescent carbonaceous particles formed within the flame (Drysdale, 1999).

Diffusion flames can be presented as laminar or turbulent diffusion flames.

Laminar diffusion flames are obtained at low flow rates where the fuel and oxygen mix together laminarly and combustion occurs evenly in the reaction layer. Turbulent diffusion flames are obtained at high flow rates. During the mixing process occur whirls (see Figure 2-8).

27

2. Fundamentals of Fire Compartments

(a)

(b)

Figure 2-8 (a) Laminar diffusion flame; (b) Turbulent diffusion flame.

Much more information about these types of flames, premixed and diffusion, can be found in the literature, i.e. McCaffreay, 1979; Cox and Chitty, 1980; Zukoski, 1981 (Drysdale, 1999).

8. Flammability Limits of Gaseous Mixture It is a common practice to refer to gases and vapour such as methane, propane, etc as flammable. However, their mixtures with the air will only burn if the fuel concentration lies within well-defined limits. The range at which a vapour or gas can ignite and explode is known as the flammable range.

The limits of the flammable range are known as the lower flammability limits, LFL, (lower explosion limit, LEL) and the upper flammability limits, UFL, (upper explosion limit, UEL). Figure 2-9 represents the flammability limits of some gases at 1atm and 20ยบC. Data of flammability limits of flammable gases and vapours is found in the literature. The most extensive review is that of Zabetakis, 1950.

A fuel-air mixture below the LFL will not ignite when brought into contact with an ignition source; it is too lean to ignite. A fuel-air mixture above the UFL will also not ignite; it is too rich in mixture. There are a few materials like ethylene oxide that are able to decompose and burn when no oxygen is present.

However, a fuel-air mixture within the flammable range will ignite if the ignition source presented has enough energy. The minimal energy for igniting a common fuel varies between 0.01 and 0.30 milli Joule. These values can be found in the literature, (Goran, 2001). Common sparks (electric, mechanical) produce energy above 0.1 milli joules. This amount of energy can provoke the ignition of gaseous fuels such as carbon monoxide, carbon sulphide, acetylene, ethylene oxide, and hydrogen.

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2. Fundamentals of Fire Compartments

Fuel concentration with UFL and LFL Methane Propane Ethylene Hydrogen

Vol. % fuel in fuel-air

Figure 2-9: Upper and lower flammability limits of several fuel-air mixtures at 20ÂşC. The ambient conditions have an effect on the flammability limits of fuel-air mixtures. For example, a rise in temperature causes the flammable range to broaden, enlarging the concentration range where an ignition can take place.

The same effect will occur with a rise in pressure (Beyler. C, 1999). Figure 2-10a-b represent the flammability limits of methane as a function of temperature and pressure, respectively.

70 Fuel volum en [% ] in air

Fuel volum e % in air

22.5 20.0 17.5 15.0 12.5

Upper flammability limit

10.0 7.5

Lower flammability limit

5.0 2.5 0.0

60 50 40

Upper flammability limit

30 20 10

Lower flammability limit

0

0

50

100

150 200 250 Temperature °C

300

350

400

0

5

10

15

20 [atm]

25

30

35

40

Figure 2-8: (a) Flammability limits of methane as a function of temperature; (b) Flammability limits of methane as a function of pressure

Several reviews have been published describing the flammability limits of fuelinert-air mixture. The problem entailed in obtaining the flammability limits of a fuel-inert-air mixture, is that laboratory experiments are needed for each specific mixture (defined as a function of temperature, pressure and concentration) and these results cannot be extrapolated for other mixtures.

There are few studies about flammable mixtures with more than one inert species and even fewer studies if these inert species are other than typical ones 29

2. Fundamentals of Fire Compartments (N2, CO2, H2O y He). In the chapter 8: “Flammability Limits of Flammable Mixtures”, a new model for calculating the flammability limits of any fuel(s)inert(s)-air mixture is developed.

9. Rapid Flame Spread Process Three terms, flashover, backdraft and smoke gas explosion, would suffice to describe the most common hazardous phenomena that occur in building fires when the fire exhibits a very sudden and dramatic change in development (Rapid Flame Spread Process, RFSP).

These phenomena themselves are closely linked and can be difficult to distinguish in some circumstances. A brief explanation of these phenomena is given below.

9.1 Flashover Flashover phenomenon evolves as follows: the fire heats the room up slowly, then all the objects in the room suffer from the intense heat radiating from the fire plume, hot gases, hot compartment boundaries and the flames, causing them to initiate pyrolisation, to evaporate and to heat up beyond their ignition point. At a certain moment, this effect causes flashover to engulf the whole room in flames, thereby spreading the fire rapidly until it reaches a ventilationcontrolled state.

Flashover can be defined as the culmination of the fire growth phase and occurs when the ceiling temperature reaches around 600°C or the heat flux is around 20kW. However these values are an approximation as is shown in an overview of experimental flashover studies (Peacock et al., 1999), where the upper layer temperature and the heat flux received by the fuel have been collected. The temperature values range from 450 to 800°C with most values between 600 and 700°C. The heat flux values range from 15 to 33kW/m². As one may well observe, these results show the high level of uncertainty in defining the flashover phenomenon.

9.2 Smoke Gas Explosions When smoke gas leaks into an enclosure adjacent to the fire compartment, they can mix well with the air. This mixture can expand into the whole or parts of the volume of the compartment and fall within the flammability range. If the mixture ignites, the rise in pressure may be significant. This process is called smoke gas explosion. Note that for a smoke explosion to occur, the ventilation condition does not need to be changed.

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9.3 Backdraft Phenomenon Backdraft can be defined as 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.

In this phenomenon, a fire can smoulder as a result of the lack of oxygen, producing large amounts of unburnt gases. As the fire develops in a ventilationcontrolled situation (small opening), a great quantity of these unburnt gases accumulates inside the compartment. When a sudden opening is created by breaking a window or by the fire service, a new supply of fresh air is created.

The unburnt gases will mix with the fresh air and fall within the flammability region. If the flammable mixture comes into contact with a spark or glowing ember, ignition will occur.

Then, a flame will propagate throughout the compartment, creating a volumetric expansion that pushes out the yet unburnt gases outside the compartment known as deflagration. When a propagating flame reaches the opening, the expelled unburnt gases are combusted in a dramatic fireball.

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