ARC2 Fire Growth in Compartments

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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
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