Chapter 5 Flashcards
Firefighting personnel should have an understanding of combustion and fire dynamic principles and be able to use them for fire scene size up and assessment of fire conditions both upon initial arrival and continuously over the course of the incident.
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The combustion reaction can be characterized by four components: the fuel, the oxidizing agent, the heat, and the uninhibited chemical chain reaction.
The difference between the fire tetrahedron model and the fire triangle model of combustion is the inclusion of the chemical chain reaction.
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The chemical chain reaction provides the ability to sustain flames.
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The fire triangle will only support a flash or flame or combustion in the condensed phase, such as glowing embers or hot charcoal.
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A fuel is any substance that sustains combustion under specified environmental conditions.
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The majority of fuels encountered are organic, which means that they are carbon-based and may contain other elements such as hydrogen, oxygen, and nitrogen in varying ratios.
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Examples of inorganic fuels would include combustible metals, such as magnesium or sodium.
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The term fuel load is used to describe the amount of fuel present within a defined space, usually within a compartment.
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Increased synthetic fuel loads and new construction materials with higher heat of combustion lead to higher heat release rate.
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The state of a given material depends on the temperature and pressure and can change as conditions vary.
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Fuels also exist in various states of matter under standard atmospheric temperature and pressure conditions.
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For flames to exist, the fuel must be in a gaseous form to mix with the oxygen in gaseous form to allow the combustion to occur.
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Since solids cannot burn in their current state, the solid must be pyrolyzed.
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Pyrolysis is a process in which the solid fuel is decomposed, or broken down, into simpler molecular compounds by the effects of heat alone.
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Pyrolysis precedes combustion and continues to support the combustion after ignition occurs.
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The application of heat causes vapors or pyrolysis products to be released where they can burn when in proper mixture with air and a sufficient ignition source is present, or if the fuel’s autoignition temperature is reached.
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If the thermal exposure to the fuel is increased, the rate of pyrolysis may increase.
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Fuels that exist as a gas under atmospheric temperature and pressure do not require vaporization or pyrolysis before combustion can occur.
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Heats of combustion typically range from 10 Mj/kg to 45Mj/kg with hydrocarbon-based products having two to three times higher values than natural products.
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Air in the earth’s atmosphere is made up of approximately 21 percent oxygen and 78 percent nitrogen.
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In order for a fire to burn, fuel and sufficient oxygen must be combined.
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Fire can occur in the absence of atmospheric oxygen, when fuels are mixed with chemical oxidizers.
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Every fuel air mixture has an optimum ratio at which point the combustion will be most efficient. This ratio occurs at or near the mixture known by chemists as the stoichiometric ratio.
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When the amount of air is in balance with the amount of fuel, the burning is referred to as stoichiometric.
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Visible smoke is an indication of inefficient combustion.
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The heat release rate of a fire is variable over time and is dependent on the fuel load characteristics, oxygen available, and enclosure characteristics.
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Heat flux is the measure of the rate of heat transfer to a surface, expressed in kilowatts per meter squared.
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The higher the heat flux from a fire to surface, the faster the temperature of the surface will increase.
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Slow oxidation such as rust or the yellowing of newspaper, produces heat so slowly that combustion does not occur.
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Self-sustained combustion occurs when sufficient excess heat from the exothermic reaction radiates back to the fuel to produce vapors and cause ignition in the absence of the original ignition source.
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Phase changes most relevant in fire are melting and vaporization.
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Thermal decomposition involves irreversible changes in the chemical structure of a material due to the effects of heat.
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Thermal decomposition of a solid or liquid most often results in the production of gases.
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At more moderate heating conditions, flexible polyurethane decomposes to a char and flammable gases or vapors.
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Structure fires would be an example of diffusion burning.
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Premixed burning occurs when fuel vapors mix with air in the absence of an ignition source and the fuel-air mixture is subsequently ignited.
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Examples of premixed fuel and air include a natural gas release into the environment and evaporation of gasoline.
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Deflagration velocities normally range from cm/sec to m/sec, though velocities into the hundreds of m/sec are possible.
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Detonation velocities are normally in the thousands of m/sec.
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Premixed flame propagation in confined volume is normally considered a smoke explosion.
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In order for flammable gases and vapors of ignitable liquids to ignite, they must be mixed with a sufficient amount of oxidizer to allow the combustion reaction to occur.
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Diffusion flame burning is the ordinary sustained burning mode in most fires.
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The lowest oxygen concentration in nitrogen is termed the limiting oxygen index.
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For most fuel vapors, the LOI is in the range of 10 percent to 14 percent by volume at ordinary temperatures.
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Transitions from premixed burning to diffusion flame burning are common during the ignition of liquid and solid fuels.
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On average, 13.1Mj of heat is produced for every kg of oxygen consumed.
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Complete combustion of hydrocarbon fuels containing only hydrogen and carbon will produce carbon dioxide and water.
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When less air is available for combustion, as in ventilation-limited fires, the production of carbon monoxide increases as does the production of soot and unburned fuels and pyrolyzates.
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The application of water can produce large volumes of condensing vapor that will appear white or gray when mixed with black smoke from the fire.
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White smoke from a fire compartment may be unburned pyrolyzate.
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The higher velocity of the fire plume causes a local reduction in pressure.
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Heat transfer is measured in terms of energy flow per unit of time.
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The energy that causes a change in the temperature of an object is referred to as sensible heat, while the transfer of energy that results in phase change is called latent heat.
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When heat energy is transferred to an object, without a phase change the temperature increases.
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Conduction is heat transfer within solids or between contacting solids.
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Firefighters often experience conductive heat transfer when wearing ppe during firefighting operations.
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As the ppe absorbs heat energy from the fire environment, it is transferred conductively through the various layers of material and to the firefighter’s body.
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Convection is the transfer of heat energy by the movement of heated liquids or gases from the source of heat to a cooler part of the environment.
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During a fire, heat is transferred by convection to a solid when hot gases pass over cooler surfaces or when hot smoke mixes with atmospheric air.
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The higher the velocity and turbulence of the gas, the greater the rate of convective heat transfer.
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Radiation is a line-of-sight transfer of heat energy from a hot surface or gas to a cooler material by electromagnetic waves.
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Although flame is often the greatest source of radiant heat transfer during a compartment fire, the smoke and hot gases that collect at ceiling level is also a source of radiant heat and often contributes to the ignition of materials.
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Even though considered a light hazard, a residential room could easily have 5 MW to 15 MW of potential peak, HRR, provided sufficient oxygen/ventilation is available.
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The potential HRR is determined by multiplying the mass of fuel by the heat of combustion of the fuels.
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Fuel load can be used in conjunction with the size of vent openings to estimate the duration of fully developed burning in a compartment.
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The term fuel load density is the potential combustion energy output per unit floor area or the mass of fuel per unit floor area.
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