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Fuel Controlled Fires

Introduction

￼Firefighters are now attending fewer live fire incidents particularly in domestic homes [1]. This is mostly due to the effective fire safety strategy and public education initiatives implemented by the Fire and Rescue services in both UK and Ireland. The combination of greater public awareness along with widespread uptake on domestic smoke alarms acting as an early first warning has resulted in a significant reduction in the number of house fires attended by fire fighters.

One negative consequence to this welcome downturn in domestic fires is that fire fighters are now receiving less live fire experience throughout their career than many of their predecessors. As a result of this, fire fighters have fewer opportunities to develop their live fire skills and gain a thorough understanding of the behaviour of fire development and the factors that influence its behaviour in a compartment such as quantity of fuel, compartment layout, oxygen supply and effects of ventilation both positive and negative.

In addition to the reduction in compartment ‘live fire fighting” incidents attended by fire fighters, there has inversely been an increase in the number of fires attended by fire and rescue services that are at the ventilation control stage. This is due to changes in the structural design of buildings and the modern materials they are constructed from. Modern building design making use of components such as double glazing, wall and roof space insulation, PVC doors and intumescent strips, to name but a few, has resulted in modern houses with far greater heat retention abilities coupled with excellent draught exclusion which limits airflow and air ingress. As a result of this “ anti ventilation “, a fire confined to a room of a modern dwelling can use up all available oxygen, and without an available supply to sustain it, the precursors for backdraught are set in motion. A fuel-controlled fire is the initial start of this process.

‘As synthetic materials have become more prevalent in building construction, the energy contained in fuel loads has increased substantially. At the same time, buildings have become more energy efficient, retaining heat and limiting air and smoke movement under fire conditions. As a result of changing fuel and building design characteristics, fires develop and become ventilation-controlled more quickly, with increased unburned gas phase fuel present in smoke. These conditions all result in shorter time to flashover and increased risk due to extreme fire behaviour as a result of unplanned changes in the ventilation profile. Operating safely in this dynamic environment requires that fire fighters and officers take the changing nature of the building environment into account in their strategic and tactical decision making.’ Ed Hartin October 31, 2008

Definition: The fire is said to be fuel controlled, when there is sufficient air for combustion, and the fires development is controlled entirely by the fuels properties and arrangement [2].

Outcomes of a fuel controlled fire [3]: • Fire remains localised and/or goes out • Fire fully develops. 1. Factors affecting the fire growth and development: [4] • Compartment geometry • Fuel Quantity • Fuel Arrangement • Heat Release Rate controlled by: – Oxygen availability (size of opening) Room Geometry • Materials Thermal Inertia (KpC) • Surface Direction • Surrounding Environment • Passives

2. Compartment geometry The geometry of the compartment can contribute to or slow down the development of the fire. Variables include: • Ceiling height- the higher the ceiling the more fire gasses need to be produced to fill the room and have an effect on the level of the neutral zone, heat layer and the premixed fuel-air mixture levels within the compartment • Size of compartment; The size of the compartment in relation to the size of the fire can either speed up the fires development, in the case of a large fuel loading in a small compartment e.g., a cluttered garage. Or it can help prevent the spread of fire, in the case of small fuel loading in a large compartment. • Proximity of boundary walls; If the fire starts in the corner of the room, the boundary walls will assist in the fires development by funnelling all the hot fire gasses and fuel upwards in one direction. When the compartment boundaries heat up sufficiently they will begin to radiate heat back into the compartment aiding fire development. Whereas if the fire begins in the middle of the room the fire gasses and heat will dissipate further, causing the fire to develop at a much slower rate. • Compartment layout/shape The compartment shape, much like the boundary walls can have a direct affect on the rate of fire development. If the compartment is long and narrow, e.g. A corridor, it will channel the heat and fire gasses in one direction whereas if the compartment is wide the gasses and heat will spread out more and the fire will need to reach a much higher temperature to continue development and cause pyrolysis of products further away from the seat of the fire.

3. Fuel Quantity and arrangement The rate of fire development will be determined by the arrangement of its fuel and the amount of it. • If the fuel is not near the seat of the fire it is less likely to become involved therefore the fire will develop at a slower rate and Vice Versa. • Also if the fuel is below the seat of fire then due to the buoyant nature of fire gasses the fuel is less likely to ignite. • The more fuel there is in the compartment the greater the chance it will become involved aiding the development of the fire. If there is only a small amount of fuel it is likely that the fire will burn out before it gets hot enough to cause pyrolysis of any further materials. This may also be dependent on the type of fuel involved.

4. Heat release rate [5]. As we know fire needs three elements to survive, Fuel, Heat and Oxygen. The heat release rate can be affected by a variety of factors, including room geometry, amount of available fuel and also the oxygen availability, If the fire is getting a large supply of oxygen then combustion will continue at a fast rate, this will increase the temperature within the compartment causing pyrolysis of other flammable materials providing more fuel and the reaction continues developing the fire. Whereas if the fire is only given a small amount of oxygen then the air-fuel mixture will become imbalanced, the fire will become too rich to burn and the temperature will drop causing the fire to develop at a much slower rate, it may even go out due to a lack of heat.

5. Materials Thermal inertia The term Thermal inertia refers to a materials resistance to burning. The greater the thermal inertia, the greater the material’s resistance to pyrolysis. Generally the higher the density of a fuel the higher its thermal inertia therefore it has a higher resistance to burning, e.g. paper will have a low thermal inertia in comparison to a wooden table which will have a high thermal inertia.

6. Surface direction The surface direction of the fuel involved can affect the development of the fire in a positive or a negative way. Fire can spread both vertically or horizontally. Flames spread better vertically as they are being pushed upwards due to the buoyant nature of the hot fire gasses. If the fuel is below the seat of the fire, e.g. carpet in a room where the fire is on top of a table, it is less likely to become involved as the heat and fire gasses rise upwards until they reach a horizontal surface it then spreads out dissipating the heat and fire gasses. If the fire is channelled upwards e.g. in the corner of the room the walls will assist in fire development vertically.

7. Surrounding environment The environment in the vicinity of the compartment fire can have a differing effect on the fires development. Some of the environmental factors will include: • Wind. • Rain. • Temperature. If there is a wind blowing it can aid in the supply of fresh air into the seat of the fire through any openings. Depending on which direction the wind is blowing it may also assist in ventilation via the venture principle, where it will create an area of negative pressure at an opening drawing the fire gasses out of the compartment raising the level of the neutral zone and lowering the compartment temperature. The rain can aid us in the extinction of the fire by cooling the compartment boundaries externally, e.g. a steel container will be cooled easier from outside than a compartment made from bricks/concrete. The ambient temperature can have a positive or negative effect on the development of the fire, e.g. in winter when the outside temperature will be significantly lower than summer, the fire will take longer to overcome the temperature difference and reach a higher heat release rate and vice versa on days where the outside temperature is higher this will aid in fire development by allowing the boundaries to heat up quicker allowing them to begin radiating the heat back into the compartment earlier causing pyrolysis of flammable materials and a faster rate of fire development

8. Passives During the initial stages into the growth phase, the fire must overcome some obstacles, known as passives. Examples of such are: • The environment (Humidity) • Weather conditions • Ambient temperature The fuel itself is a passive until it changes state into its gaseous form, i.e. Solids & liquids absorb heat energy from the fire before they start to pyrolysis [7]. Once they are in this gaseous state they will mix readily with available oxygen and begin to combust. As a fuel decomposes it gives off passives in the form of: • C02 • Water vapour These too must be overcome for the fire to develop.

9. Stages of fire development There are 6 phases of fire development within a fuel-controlled fire: • Initial fire- small fire starts • Growth /Development phase- fire develops getting hotter and producing more fire gasses • Flashover period (culminating in a flashover)- When flames reach ceiling height and begin to spread horizontally the flashover period has begun. When every combustible surface /material is burning flashover has occurred. • Fully developed fire- when a compartment is fully engulfed in flame • Decay period- when the fuel has been used and there is no longer anything to burn, the fire will begin to decrease in size and the temperature will begin to drop. • Extinction- the fire has gone out, as there is no more fuel or heat.

10. Flashover • ‘Stage in a compartment fire when the combined thermal radiation from the fire plume, hot gases and hot compartment boundaries causes the generation of flammable products of pyrolysis from all exposed combustible surfaces within the compartment. These flammable products, when given a source of ignition will result in the sudden & sustained transition of a growing fire to a fully developed fire’. • Flashover has occurred Flashover indicators • Fuel Controlled Fire • Lowering Of Neutral Zone • Painful Radiant Heat • Increased Rate of Pyrolysis from all combustible surfaces. • Gravity Current Speed (slow start, getting faster) • Flames visible in Gas Layer

References ￼[1] 2014, Department for Communities and Local Government. (Fire Statistics: Great Britain April 2012 to March 2013) [Pdf] London: Department for Communities and Local Government. Available at http://Https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/31 3590/Fire_statistics_Great_Britain_2012-13__final_version_. Pdf [Accessed 8th January 2015]. [2] Hartin, E. (2008) 'Applying "3D" Firefighting to the Fireground' FireRescue [Online], (November). Available at http://Http://www.firefighternation.com/article/firefighting-operations/applying-3d- firefighting-fireground [Accessed 8th January 2015] [3] Bengtsson, L G (2001) Enclosure Fires. Page 85. Sweden: NRS Tryckeri. [4] Bengtsson, L G (2001) Enclosure Fires. Page 26. Sweden: NRS Tryckeri [5] Babrauskas, V., & Peacock, R D. (1992) 'Heat Release Rate: The single most important variable in fire hazard'. Fire Safety Journal 18, (3) 255-272. [6] http://scienceworld.wolfram.com/physics/ThermalInertia.html [7] Burning of wood, InnoFireWood's website. Accessed on 2010-02-06.