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Fire Growth and Spread

Fire is defined as “a rapid, self-sustaining, exothermic oxidation reaction that produces heat and light in varying degrees” (University of Maryland University College, n.d., para.8). This definition differentiates fire from other slower oxidative reactions such as browning and rusting. Self-sustaining allude to the capability of the process to prolong even after the elimination of the initiating source of energy. The quantity of heat and light produced during the reaction relies on the nature of the object (the fuel) as well as the energy sent out. The flame is the noticeable section of the blaze and is made up of burning hot gases. At increased temperature, the gases that result are capable of ionizing and producing gaseous plasma. Electric failures, careless smoking, inappropriate use of candles, and arson attack generally cause most fires. These actions have immeasurable consequences as they cause widespread costly outcomes whenever and wherever they take place. This paper analyzes the growth and spread of fire from the point of origin to the point when the fire stops burning.

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Heat transfer refers to the physical process by which thermal energy moves from a hotter region to a cooler region. When a material is at a different temperature than its surrounding or another material, transfer of heat takes place. The heat exchange occurs according to the ability of the object to transfer heat, which is referred to as its thermal conductivity. An object with a higher thermal conductivity generally transfers heat much faster. The heat transfer takes place until the object and its surroundings reach thermal equilibrium. The major modes that heat transfer are conduction, convection, and radiation.

Conduction takes place when heat moves due to the direct contact of molecules of matter. The exchange of heat occurs mainly through elastic impact as in fluids. It can also occur through free electron diffusion or phonon vibration as predominant in metals or insulators respectively. The transition of thermal energy by conduction occurs when neighboring atoms vibrate against each other, or as the movement of electrons takes place from one atom to the other. This is the main mode of heat transfer from a fire’s point of origin to the surrounding areas or objects. As the fire continues to spread due to fluid circulation, heat is mainly transferred by convection. The flow of the medium is due to density differences within the fluid, which makes them less dense hot gases rise by buoyant flow. As the fire starts to develop, the plume of gaseous substance coming from the fire is hot, and rises, whereas the cooler air sinks and comes within the range of the fire, which makes it to be hotter and less dense. This process generates a convective current within the scene of the fire whereby the hot air rises while the cold air sinks. Exchange of heat takes place when hot fluids meet other objects.

The impact of a fully developed fire is felt due to heat transfer by radiation. Radiation occurs when electromagnetic waves move from a heat source to an absorbing substance. Radiation exerts heat energy all around a hot material. The transmissions of electromagnetic waves do not need any medium. While the layer of hot air extends across the upper borders of a compartment, the energy in that layer also radiates outward. This process affects fuels below the gas layer and away from the spot where the fire started.

In the early stages of the development of a fire, the availability of heat, fuel, and oxygen limits its growth and spread. These three elements, commonly referred to as the fire triangle, are vital for creating and sustaining fire. A heat source at a considerable temperature is important for the fire to start. It is also required to maintain the fire and allow it to spread. It achieves this by getting rid of the moisture in the vicinity of the fuel, increasing the temperature of the neighboring air, as well as preheating the fuel in its path. This makes the fire spread with greater ease. Fuel refers to any type of combustible substance. It is typified by the amount of moisture it has, size and shape, and quantity. As the fire starts, the moisture content of the fuel determines the growth and spread of the fire. In the absence of an adequate amount of oxygen (at least 16%), a fire cannot start and spread. Oxygen aids the chemical processes that take place during combustion. As the fuel is being consumed, the process of oxidation takes place whereby oxygen reacts with the incoming air to give out heat and other products. In the later stages as the fire grows and spreads, the deprivation of these elements makes the fire to be suppressed. This can occur when a sufficient amount of heat is not created to maintain the process, when the fuel is drained out, or when the oxidation process receives inadequate oxygen supply.

As a fire grows and spreads, it gives off heat energy (DeHaan, 2007). A chain reaction occurs where the fire can maintain its own heat through the further discharge of heat energy in the process of combustion. The discharge of heat energy may propagate when a sufficient supply of oxygen and fuel is available. The released energy plays a pivotal role in the growth and the spread of the fire since it supplies sufficient energy to the fuel to initiate the oxidation process. The minimum quantity of energy required to accomplish this reaction is called ignition energy. It varies according to the properties of the fuel, for example, thermal inertia, which determines the rate at which the surface temperature of an object will rise on heating. The process of increasing the temperature of the fuel to its desired ignition temperature can occur either through piloted ignition or through auto-ignition. In the former method, a pilot flame or spark is introduced to the fuel. This increases its energy to its minimum ignition level, hence kicking off the combustion process. In the latter method, an energy source other than a flame or a spark increases the level of energy in the fuel. The increase takes place up to the point when self-sustaining oxidation is initiated. Auto-ignition can take place by means of the application of radiant heat from the falling hot gas layer of a remote fire. It can also be due to the placement of combustible substances at close proximity to an electric space heater.

The white smoke coming from the right side of the couch and foreground objects at about two minutes into the video is a classic warning sign for an impending smoke explosion that is about to take place. Pyrolysis products usually present themselves as white smoke. This takes place when the temperature has reduced to a level that cannot support the burning process or when oxygen levels fall to below 16%. Therefore, the fuel package disintegrates (pyrolysis) without active combustion and nearly all the carbon does not leave the object. This results in the production of white-colored smoke. It is necessary to note that as the process of combustion progresses, transition of heat occurs to the surrounding compartments. Pyrolysis of the contents results in the build-up of white smoke that has a high concentration of poorly burnt fuel. In other words, the presence of white smoke suggests that there is a build-up of products of pyrolysis because of rising compartment temperatures.

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The occurrence of flashover is feared as the most unsafe period of a fire incident since the whole enclosed area bursts into flame. Scientifically, flashover is described to be due to radiation feedback of heat energy. The heat emanating from the building fire is absorbed into the upper walls and contents of the enclosed area. The combustible gases and furnishings are then heated to their auto-ignition temperature. This increase of heat in the enclosed area initiates the occurrence of a flashover. Flashover indicates a number of significant changes in a fire and it marks the culmination of any fruitful attempt to rescue persons trapped inside the enclosed area. It signals the demise of any individual still trapped in the blaze. It also signals the culmination of the growth phase and that the blaze has reached the next phase of ignition, which is called the fully developed phase. Lastly, flashover indicates the transition from contents to a structure fire. It is the onset of the collapse danger.

In the video clip, a flashover occurs in the compartment. This is because the blaze involving the initial materials generates a layer of hot smoke that extends across the ceiling in the enclosed space. The hot buoyant smoke increases in depth since the walls of the enclosed space confine it. The radiated heat coming from this layer heats all the combustible objects within the enclosed area. This makes them produce flammable gases by pyrolysis. These flammable gases burst into flames at the intensification of the surface temperatures.

Fire has formed an important ingredient of societal cultures and religions, from historic times to our present world, and has played a key role in the advancement of civilization. Fire has taken a variety of trends all through the history of our world. The common type of flame is capable of causing conflagration that may cause harm to the body through burning. However, most fatalities due to fire do not result from burns, but due to inhalation of smoke. Most of the time, smoke disables individuals to the point that they are unable to overcome and escape to safety as quickly as possible. Besides generating smoke, fire can disable or cause death by lowering the levels of oxygen, either by utilizing the oxygen or by changing it with other less desirable gases. The generation of heat during the burning process is also harmful to the respiratory system, as highly heated gases burn it. In situations when the air is exceedingly hot, one breath is capable of causing death.

Reference List

DeHaan, J. D. (2007). Kirk’s fire investigation (6th ed.). Upper Saddle River, NJ: Pearson Education.

University of Maryland University College. (n.d.). Fire analysis. 2010, Web.

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