LWF PROFESSIONAL BULLETIN FIRE GROWTH IN COMPARTMENTS Introduction For those involved in innovative or complex building designs, the legislative requirement for conforming to the requirements of British Standards BS 5588 and Approved Document B with regard to fire safety measures can often result in modifications to initial design concepts, or clients brief, resulting in less desirable design solutions. An alternative to modifying design concepts, in order to acquire Building Control approval, is that of the application of fire engineering techniques to demonstrate compliance. In order to apply a fire engineering solution to design problems, it is essential that the designer understands the factors which affect fire growth and development. Essentially, fires can be divided into two specific categories; (1) those burning in open air, and, (2) compartment fires. For fires burning in open air, there is an unlimited amount of oxygen available for fire growth, and the rate of fire development will be dependant on the local fuel load, and space separation between adjacent fuel sources. Fires burning in open air will generally cause little damage, although this is not always the case e.g. forest fires in dry climates where prevailing winds have caused rapid fire growth and substantial damage. The fires which are of most concern in this bulletin are compartment fires. Principal Stages of Fire Growth Subsequent to ignition, a fire in a compartment will behave much as it would in the open, but as the fire grows, local environmental factors will play a significant role in determining the subsequent rate of growth. It is generally recognised that there are three fundamental phases during the life of a fire, although the terminology used sometimes varies; 1) The growth stage The average temperature in the compartment is low, although local temperatures around the burning zone are high, and the fire is localised in the vicinity of the point of fire origin. Further growth from ignition will be dependant on the local fuel and oxygen supply. This stage may last up to several hours or a matter of minutes. Given suitable conditions, a ready supply of fuel-oxygen-heat, accelerated fire growth may follow. As a fuel source burns, hot gases are expelled, which form a plume of gases emanating from the fire. These gases rise and collect at ceiling level, where smoke and hot gas spread occurs across the ceiling. Initially, 70% of the heat expended from the fire will be transferred into the plume thus raising its temperature, the remaining 30% is absorbed by room linings etc. Unless there is heat/smoke ventilation from the compartment the temperature of the layer continues to rise, and as it does, the level of radiation given off by the smoke layer increases. In essence, the smoke layer acts as a large radiant panel, which heats the other combustible items in the room toward their ignition temperature. Thermodynamics and heat transfer principles can be applied to determine the heat flux at any point in the room under the smoke layer, using room geometry, configuration factors and the following expression, q = εσΤ4 (where q is the heat flux, ε = emmisivity, σ = Stefan Boltzmann constant) to determine if the heat flux at a given point in the room is adequate to spontaneously ignite an object at that point for a specific smoke layer temperature. These items may also ignite, by pilot (flame or spark) ignition at a specific heat flux. The transition from the fire growth phase to the fully developed stage is often referred to as ”flashover”. Flashover can be defined as the transition from localised burning to a general conflagration within the compartment when all fuel surfaces are burning. This transitional period is normally short in comparison to the main stages of the fire. For fire engineering assessments, if the smoke layer temperature at ceiling level reaches 600°C, then it can be assumed that flashover will occur. Using expressions and correlation’s from over 100 experiments, it was found that the temperature rise of the smoke layer gas could be evaluated using Q2 ∆TG = 6.85 Av H v hk AT 1 3 where Q is heat output, hk = heat transfer coefficient, and AT is the internal surface area of the room. This equation is used within the confines of limits of applicability, and the confidence limit and the 80% confidence interval for model uncertainty parameter (β) is expected to be in the range 0.85 < β < 1.25 where β= Predicted Value Expected Value 2) The fully developed fire During the fully developed stage of a fire, the rate at which heat is released from the fire will reach a maximum. The temperatures within the enclosure will continue to rise, but at a slower rate than during the transitional period. It is also during this stage that subsequent threat of further internal or external fire spread to other sections of the building, or to adjacent buildings, is at its greatest. In addition to the obvious threat to occupants still remaining in the building at this stage of the fire, there is also the threat of partial or complete structural failure, as load bearing capacity of structural members decreases with increasing temperature. 3) The decay stage By the onset of the decay stage of the fire, most of the combustible contents of the room will have been depleted of their volatiles (i.e. the combustible gases released from materials, which burn). The temperatures within the enclosure will also fall steadily. At this stage, the fire will either have been manually suppressed by the fire brigade, or will have become depleted of oxygen or fuel. For fires starved of oxygen, it may appear that burnout has occurred, but in some instances, smouldering combustion within the hot embers may continue at a very slow rate due only to low levels of oxygen concentration. If normal oxygen levels are reinstated to the smouldering materials, by means of opening a door or breaking a window and thus providing fresh air, rapid resurgence of combustion can occur. This phenomenon is often referred to as “backdraft”. Factors affecting Fire Growth and Development The main factors which affect fire growth, and the interaction between these factors for a “real” fire scenario, are often difficult to assess as fire is a very complex phenomenon. Much research has been conducted world wide, to assess the affect of varying factors on preflashover and post-flashover fires, in order to determine the rates of fire growth, and the subsequent fire severity. The main factors affecting compartment fire growth are outlined briefly below: a) Fuel supply Control of fuel supplies is one method used to reduce fire growth and spread potential. This has long been recognised within building regulations and other regulatory documents and codes of practice. Materials are tested in standard fire tests, and rated as either Class 0 - Class 3 in relation to their “surface spread of flame” performance. The requirements for limiting fire spread, using the above classifications, are detailed in the Building Regulations, where certain surface linings within buildings are required to meet specific standards of resistance to fire spread. Fire resistance, in terms of integrity and insulation, should not be confused with surface spread of flame classification. Materials can be of Class 0 surface spread of flame but provide no fire resistance. The physical and thermal characteristics of the fuel load are the major items of concern in terms of assessing likely fire growth rates and subsequent destructive potential. The rate at which heat is expended from the burning item, (Heat Release Rate HRR), the thermal capacity of other combustibles and non-combustibles within the fire enclosure e.g., wall linings etc., conductive, convective and radiative heat transfer coefficients, are significant in determining whether a fire can grow to flashover stage. b) The Ventilation Factor and Burning Regime As already noted, the fuel load, and its distribution throughout the compartment is a major determining factor in fire growth. Adequate levels of oxygen are also required to sustain combustion. As a fire grows, the oxygen in the air in the compartment is consumed, and unless sufficient ingress of air is available for combustion, the fire will self extinguish. In most compartments, there will be sufficient air leakage around doors and windows to sustain smouldering combustion. Experiments have been conducted on compartments with different sizes of ventilation openings, and it was found that the burn rate was strongly dependant on the size and shape of the opening, and strongly correlated with the expression; m = 55 . Av Hv (kg/s) where m is the burn rate, and A and H are the area and height of the ventilation opening. The term A H is more commonly known as the ventilation factor. In further tests conducted, it was noted that the burn rate divided by the ventilation factor is not a constant (i.e. does not equal 5.5 over a wide range of conditions). Within a certain range, the rate of ARC 2 – FIRE GROWTH IN COMPARTMENTS burning is controlled by the rate at which air can flow into the compartment, and this scenario is referred to as a “ventilation controlled” fire. In well sealed compartments, the temperature profile will be low, and the fire will eventually self extinguish due to oxygen starvation. If there is sufficient oxygen available for the all the fuel to be consumed, and the relationship between the ventilation rate, fire loading, compartment geometry and thermal characteristics of the surface linings are such that maximum temperatures are achieved. It is under these conditions that flashover and further fire spread are most likely to occur. The third scenario is that there is excess ventilation, and the excess air is entrained into the fire process (plume and flames) thus having a cooling affect. This further reduces the degree of radiated heat from the boundary enclosure. any height in a vertical plume is 2.2 times greater than an open plume, and in the side wall plume excess temperatures are 1.7 times greater than the open plume. Conclusions As can be seen from the brief commentary provided within this bulletin, fire growth and development is a very complex phenomenon. It is essential that architects, designers and engineers are aware of the factors affecting fire growth when involved in innovative or complex building designs, which require the application of fire safety engineering techniques, to demonstrate compliance with traditional prescriptive requirements. Lawrence Webster Forrest Copyright Lawrence Webster Forrest Ltd As the ventilation rate increase, it passes the point where the burn rate is independent of the ventilation rate, and becomes dependent of the surface area and characteristics of the burning fuel. This scenario is known as a “fuel controlled” fire. It is important to be able to distinguish between fuel controlled and ventilation controlled fires, as the fuel controlled fire will generally be less severe, except in comparison with fires where the ventilation rate is very poor. A guideline used for fires involving cellulose materials is: Ventilation Controlled if ρ gA H < 0.235 Afloor where ρ is air density, and g is acceleration due to gravity (9.81ms-2) If the above factor is greater than 0.290, then the fire is fuel controlled. The transition between these limits, However, is ill-defined. c) Fire Location and Room Geometry As can be seen from the previous equations, the size and shape of the room, and its openings, influence fire growth. As a fire grows flames eventually reach ceiling height, and spread out under the ceiling. It is important to note that the horizontal length of a flame under a ceiling is five times greater than its height if the ceiling confinement were not there. This increases the temperature of the smoke layer, rapidly resulting in increased levels of downward radiation, and subsequent fire spread. The location of fire origin within the compartment is also significant. This is due to the effects of cooling by entrainment of air into a fire plume. Where a fire is located within the centre of the room, the rate of entrainment will be twice that for a fire at a wall, and four times greater than a fire in a corner. Tests conducted have shown that the excess temperature at ARC 2 – FIRE GROWTH IN COMPARTMENTS