Fire is described as the visible result of an oxidation process in the chemical reaction known as combustion. Combustion occurs between an oxidant (source of fuel) and oxygen which results in the release of heat and the source of fuel being converted into another form (Grimwood et al., 2010). When heat is released, light is produced either as a flame or glowing. The fuels which bring about combustion are mainly organic compounds like hydrocarbons which may be in the solid, liquid, or gaseous phase.
The part of the fire which is visible is known as the flame. It is made of gases which are glowing and hot. In case the temperatures are high enough, these gases may ionize into plasma (Azhakesan, 2003). The color and intensity of a fire are dependent on the substances burning and their purity. The color of a flame depends on the temperature: white represents the hottest temperature; yellow is also quite hot but slightly lower temperature than the white; orange is cooler; red is cooler still. When combustion is not taking place, the carbon particles create black smoke is visible (Azhakesan, 2003).
The Growth of Fire in a Compartment
For fire to occur, an ignition process must take place. Ignition is where the oxygen and fuel vapor are heated into the temperature where the fuel will be ignited into a flame (Grimwood et al., 2010). The ignition temperature varies according to the fuel or oxidant. At normal temperature, the oxidation process is basically very slow to an extent that it cannot be noticed through human observation. For example consider the rusting process which can extend for a long period of time for changes in the reaction to be realized. However, with the increase in the level of temperature above the normal range, the oxidation speed usually increases and the result is heat generation. After reaching the temperature of ignition, the result is an appearance of a flame. This is the stage at which ignition has occurred. After the ignition, there is the combustion process which follows afterwards.
Modes of transmission
From the initial process of ignition, until the time when there is full compartmental involvement, there are different modes of heat transfer. Heat needs to be transferred to the fuel so that the ignition process can begin. According to Azhakesan, (2003), heat is transferred from materials with higher temperature to those with lower temperature. The modes of heat transfer include radiation; convection; and conduction (Naruse et al., 2004)). In the conduction process, heat is transferred through direct contact of a hot object with another with lower temperature. In the initial stages where fire is developing, conduction is the major mode through which heat is transferred. As the fire develops, conduction occurs when hot gases come into contact with cooler fuel (Neruse et al., 2004). Radiation on the other side is where heat is transferred in form of electromagnetic radiation. In this process, heat moves from a heated object in many or all directions. In the development of fire in a compartment; radiant heat is one of the main mechanisms through which the spreading of the heat occurs. Through convection, heat is transferred in process whereby a fluid medium like air is heated. When the medium for example air is heated, it reduces in density, expands and then rises.
Early stages of growth and spread of fire and limiting factors
In the early stage of fire expansion, the incipient process usually occurs. This is basically what begins at the ignition and during this time, the flame is mostly limited. This is also the stage at which is also characterized with regulation of the fuel which ends up limiting the growth of fire. This implies that the spread of fire is not controlled by the accessible oxygen. The spread is actually determined by the mass, pattern, together with the fuel geometry. At the early stage, the oxygen level is basically within a standard range and the normal temperatures still prevails.
As the fire continues to develop, a spiral of hot fire smokes begin ascending towards the upper room portions. As the plume continues to rise because of convection, more oxygen is drawn into the flames bottom. Carbon monoxide and Sulphur dioxide are some of the fire gases that start accumulating in the room. If above this flame there is a solid fuel, the result will be an outward spread which will result in production of a ‘V’ shaped burning on the erect surfaces. Therefore, the result in this stage is a spread that is not limited (a free burning stage). This occurs mainly because there is consumption of more fuel coupled with the intensification of the fire. The flames have already extended in all the sides through the process of conduction, convection together with uninterrupted impingement. In addition, there is a continuous accumulation of a layer of fire gases and smoke which starts radiating downwards. In the upper layer of the smoke, various gases are usually accumulated. These gases are inclusive of hydrogen chloride, hydrogen cyanide, carbon monoxide and arcolein among others (Azhakesan, 2003). Unless there is a tight sealing at the origin compartment, there will be spread of the fire gases and smoke in the entire vessel and even outside.
The process of fire growth is continuous and intensive. As this process continues, there is continuous accumulation of a layer of fire gases and soot which continues dropping towards the lower level. The accumulation of the inflammable gases and soot continues until the ignition temperature is reached by one of the fuels. What follows is a rollover after the upper layer ignition results in the extension of the fire towards the upper levels. As a result of this rollover, the overhead temperature increases at a rate which is even higher. In addition, the radiated heat is amplified and descends into the compartment. All this time, the fire is still regulated by fuel.
Energy origin and impact on the growth and spread of fire
Chemical heat energy: According to HM Fire Service Inspectorate (2000), this energy results from oxidation. To the engineers involved with the protection of fire, the chemical heat energy is usually there main concern. The production of heat through the chemical procedure involves various techniques. One of the techniques is combustion. The combustion heat is the amount of heat that is usually released when a fuel undergoes oxidation process. This heat is in most cases used domestically and by industries. In addition, the heat is basically limited by the level of air supply (Karlsson & Quintiere, 2000).
Another technique is that of spontaneous heat. It is practical that to all the substances that have the capability of combining with the oxygen, there will be oxidation which will result in the generation of heat. At the standard temperature, the oxidation speed is usually slow with the released heat being moved to the surroundings. Through this process, the temperature is basically maintained at low level coupled with prevention of ignition.
Electrical heat energy: This heat is produced when there is a continuous flow of an electric current through a conductor (DASH). One of the ways through which the electric heat energy is produced is through resistance. This process takes place when a resistance is offered by an electric conductor therefore resulting in generation of heat. The result from this heat is ignition and oxidation of the combustibles located in the surrounding. As a result, a fire is created. This type of fire is very common in the areas whereby electricity is used as a source of energy.
Another way of generating electric heat energy is through induction process. Induction heating occurs when a current is passed through a wire in alternation process. Therefore, there is induction of a current in another parallel wire. Through resistance to the energy flow, the result is production of inducted heat. A good example of this form of heat is the one produced in a microwave oven (Grimwood et al., 2010).
Nuclear heat energy: This form of heat comes from the nucleus of an atom. Through the strong force released as a result of nucleus bombardment with the energy particles, the nucleus contracts (Azhakesan, 2003). As a result of the bombardment process, the energy is released in form of tremendous pressure and heat. The nucleus fission also results in the energy being released. On the other side, the fusion results in release of energy when two nucleuses combine.
White smoke significance
Basically, it is clearly known that a white smoke is in most cases a sign to warn of an awaiting smoke flare-up. There is a high possibility that such a phenomenon will come from the nearby compartment or even from the ignition compartment itself. In addition, the pyrolysis components have the possibility of appearing as white smoke in the early stages of the live burn. This is clearly experienced as the fibre board is heated by fire resulting in the formation of an overhead gas layer that is very highly combustible. This layer is finally ignited as the gas mixes depending on the air that is unfilled within the compartment.
With extremely low temperatures, it is hard to sustain the flaming ignition. Alternatively with the level of oxygen dropping below 17%, what follows is the breakdown of the fuel package. As a result, a white colored smoke is eventually produced. It should importantly be noted that as the fire continues to develop, there is transfer of heat to the nearby compartments. The result is breakdown of the fuel package and eventual accumulation of a white smoke in which most of the fuel is not burnt. Basically, the white color of the smoke is an indication of pyrolysis accumulation as a result of increase in the compartment temperatures (Naruse et al., 2004). This is basically what is also witnessed in the rooms that are nearby the fire compartment.
Once the average temperature of 595oc is reached by the upper layer, the heat generated is enough to result in concurrent ignition of the other fuel present within the compartment. This is what is usually referred to as flashover. After the occurrence of flashover, it is usually impossible to survive in such an area for a long time. Within this space, the temperatures increases to a level of more than 1000oc. in addition, this flashover point is also characterized with regulation of the fuel. Nevertheless, as the fire continues being confined in the origin compartment, it results to being regulated by oxygen. Basically, after the occurrence of the flashover, the next process involves the full vessel.
As a result of the flashover, there is powerful burning which consumes the whole compartment and the contents in it. Therefore, the result of thisflashover will be a substantial deck level combustion. In other cases, the flashover will also have a high likelihood of resulting in burning all the contents in the external part of the compartment.
The process of fire development is usually very complicated. In addition, there are basically various factors that affect the spread, growth and development. Some of the components which have been noted to play a very important role in the spread of fire are inclusive of: the fuel physical state and shape, the oxygen availability together with the heat transmission. Basically, each fire is usually different although the patterns that are followed by all the fires are predictable. With an investigator having a clear understanding of the fire, it is possible to provide a logical basis through which the cause and origin of fire can be established.
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HM Fire Service Inspectorate, (2000). Fire service manual volume 4, fire service training, guidance and compliance framework for compartment fire behavior training. Norwich, UK: The Stationary Office.
Karlsson, B., & Quintiere, J. (2000).Enclosure Fire Dynamics. CRC Press, Boca Raton, Florida,
Naruse, T. et al. (2004). Compartment fire behavior under limited ventilation in Fire and Explosion Hazards.Fire SERT, University of Ulster. Northern Ireland. pp. 109–120.