Fire Dynamics Flashcards
Why is it important for firefighters to understand fire dynamics?
To anticipate how the fire will behave in the future and how that affects fire operations.
True of False: All fires involve a heat-producing chemical reaction between some type of fuel, an oxidizer, and combustion.
False. All fires involve a heat-producing chemical reaction between a fuel and an oxidizer. The combustion IS the chemical reaction.
What is an oxidizer?
An oxidizer is a material that readily yields oxygen or other oxidizing gas, or that readily reacts to promote or initiate combustion of combustible materials. The common oxidizer is oxygen in the air. However, there are a few other common oxidizers:
What is a physical versus a chemical change?
A physical change is when a substance changes forms such as in size, shape, appearance but remains the same chemically. For example, water melting (from ice to liquid) and water freezing (from liquid to gas).
A chemical change is when a substance changes from one type of matter to another. For example, this occurs when two or more substances form a compound.
What is oxidation?
A chemical reaction involving an oxidizer, such as oxygen in the air, and other materials. Rust is an example of slow oxidation while explosions are rapid oxidation.
What is potential energy?
The amount of energy that an object can release in the future. The potential energy available for release in the combustion process is known as the heat of combustion.
What factors limit the release of potential energy?
- Chemical composition
- Arrangement
- Density of the fuel
- Availability of the oxidizer
What is kinetic energy?
The energy that moving objects possess.
What unit of measurement is used for energy?
Energy is most commonly measured in joules. The amount of energy required to heat 1 gram of water by 1 degree celcius is 4.2 joules.
However, the british thermal unit (Btu) is still commonly used in the fire service as well. A Btu is the amount of energy it takes to heat 1 pound of water by 1 degree fahrenheit.
When comparing the two: 1,055 J = 1 Btu
What is an exothermic reaction versus an endothermic reaction?
An exothermic reaction emits heat as it occurs. Fire is an exothermic chemical reaction. In contrast, endothermic reactions absorb heat. Converting water from a liquid to a gas (steam) is an example of an endothermic reaction.
What is the fire triangle?
A model used to explain the elements of fire and how fires can be extinguished. The fire triangle is the oldest and simplest model. It shows the three elements necessary for combustion to occur: fuel, oxygen, and heat.
What is the fire tetrahedron?
This demonstrates the four components needed for a self-sustaining fire: fuel, oxygen, heat, a chemical chain reaction.
What is a synonym for fuel in a combustion reaction?
Reducing agent
Can liquids and solids burn?
No. Fuels must be in a gaseous state to burn. Therefore solids and liquids must become gaseous in order for ignition to occur. This process (known as off-gassing) happens when heat is transferred to the liquid or solid. In solids, off-gassing is known as pyrolysis. In liquids, it is called vaporization.
What is the difference between piloted ignition and autoignition?
Piloted ignition is the most common type of ignition. It occurs when a mixture of fuel and oxygen encounter an external heat source with enough thermal energy to start combustion.
In contrast, autoignition occurs without any external flame or spark to ignite the fuel gases or vapors. Instead, the surface of the substance is heated to a point at which combustion occurs.
What is the autoignition temperature?
The minimum temperature at which a fuel in the air must be heated in order to start self-sustained combustion. The autoignition temperature of a substance is always higher than its piloted ignition temperature.
Can combustion occur without visible flames?
Yes. Flaming combustion is only one form of combustion. It can also be non-flaming.
What are the main differences between flaming and nonflaming combustion?
Nonflaming combustion occures at a lower temperature and burns slower. For example, smouldering coals are a type of nonflaming combustion.
Flaming combustion is what people know as fire. It requires the perfect combination of all elements of the fire tetrahedron. Solid and liquid fuels are tranformed into gas by heating. Oxygen comes into the reaction through the inlet flow and the exhaust leaves through the outlet flow.
Why does smoke rise from the fire?
The density of smoke is less than the surrounding air, so it “floats” on top.
What are the two by-products of fire?
Heat and smoke.
What is smoke?
Smoke is the product of incomplete combustion. Therefore, smoke is leftover fuel and has the potential to burn.
True or false: Less oxygen creates more smoke in a structure fire.
True. Less oxygen results in higher levels of incomplete combustion, which produces more smoke.
What are the two major types of fuels in structure fires?
- Carbon-based (wood)
- Hydrocarbon (plastics)
True or false: All smoke is toxic to a certain degree.
True. While some are far worse than others, it is never advised to consume smoke of any kind.
What is asphyxiation?
Death due to low oxygen levels in the blood.
What are common products of combustion and their toxic effects?
What is the unit of measurement for pressure?
Pressure is the force per unit of area applied perpendicular to a surface. It’s measured in kilopascals. The atmosphere of the earth is 101 kPa.
How does the gas pressure within a structure affect fire?
At standard atmospheric pressure, gases remain calm. However increases or decreases in pressure create movement in gases. Note that gases always move from areas of high pressure to low pressure. As the pressure difference between high and low pressure areas increases, the speed at which gases will move from high to low also increases. This results in heated gases traveling up and away from the fire while cool, fresh air travels towards it.
What is inlet and outlet flow?
In a fire, flaming combustion can be compared to a pump. Fresh oxygen is “pumped in” and mixes with fuel gases. As it burns the fire “pumps out” combustion products that have larger amounts of mass and a higher level of energy than the inlet air. In the case of open burning, the “pump” does not have a well-defined inlet or outlet as the air is being entrained (drawn in) from all around.
What is the difference between heat release rate and temperature?
Temperature is the measurement of heat. Specifically, it’s the average kinetic energy in the particles of a sample of matter. This is measured in degrees Celsius or Fahrenheit. In contrast, the heat release rate is the amount of heat released per unit time. This is usually measured in kilowatts (kW) or megawatts (MW). In the case of an interior fire, the temperature may be within tolerances for PPE but the heat flux to the PPE from the fire indicates how long it will protect you. That is, the temperature indicates if it’s safe to enter and the heat release rate indicates how long you can stay inside.
True or false: temperature is a good predictor of heat transfer.
False. One candle burns at the same temperature as ten candles, but the heat released by ten candles in ten times greater than one so the heat transfer is not the same.
What is the measurement of heat transfer to a unit area (heat flux)?
Kilowatts per square meter.
What are the three main sources of thermal energy?
- Chemical - the most common source of heat in combustion reactions.
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Electrical - can also generate temperatures high enough to ignite combustible materials nearby. Electric heating can occur via:
- Resistance heating
- Overcurrent or overload
- Arcing
- Sparking
- Mechanical - friction and compression generate mechanical energy.
What is self-heating?
Oxidation of a substance that increases the temperature of a material without the addition of external heat. This can lead to spontaneous ignition, which is ignition without the addition of external heat.
Rags soaked in linseed oil can self-heat to the pont that they ignite spontaneously.
What is thermal equilibrium?
Heat transfers from warmer objects to cooler objects because heated materials will naturally return to a state of equilibrium in which all areas of an object are a uniform temperature. Thermal equilibrium is the point at which two regions that are in thermal contact no longer transfer heat between them because they have reached the same temperature.
Note that the rate at which heat transfers depends on the temperature differential of the bodies and the thermal conductivity of the materials involved. Therefore the greater the temperature differential, the greater the rate of transfer.
What three ways is heat transferred?
- Conduction - the transfer of heat between solids. This occurs when a material is heated as a result of direct contact with a heat source. For example, in a fire, if you touch a hot door with your hand, then the heat is transferred from the door to your hand through the contact.
- Convention - the transfer of thermal energy through the circulation or movement of a fluid (liquid or gas). In a fire, this usually happens through the movement of smoke and fire gases. Vertical movement is due to the bouyancy of the smoke and fire gases while horizontal movement is the result of pressure differences.
- Radiation - the transfer of heat by electromagnetic waves without another medium to transfer the heat energy. Radiant heat can become the dominant mode of heat transfer as a fire grows in size and is a significant factor in fire development and spread in compartments.
True of false: white materials absorb more radiation heat that dark materials.
False. Dark-colored materials absorb emit and absorb heat more effectively than light-colored materials.
At what speed does radiated heat energy travel?
At the speed of light. The sun is the best example of this.
What is a common cause of exposure fires?
Radiation. As a fire grows, it radiates more energy which other objects absorb as heat. In large fires it’s possible for the radiated heat to ignite buildings and other fuel packages a considerable distance away.
How do methods of heat transfer interact in a fire?
Conduction, convection, and radiation rarely occur individually in a fire. Instead, they occur in unison. The fire radiates heat, causes convection of heat through hot fuel gases, and conducts heat through burning materials or metals involved in the fire.
What is a synonym for fuel in a combustion reaction?
Reducing agent
What is power and how is it measured?
Power is the rate at which energy transfers. It’s measured in watts. One watt is 1 joule per second. In terms of fire behavior, power is the heat release rate during combustion.
What is vapor density?
The density of gases in relation to air. This is important to understand to anticipate how vapor (a.k.a. the gaseous state of fuel that usually exists in liquid or solid form) will travel.
What is specific gravity?
The density of liquids in relation to water. If they are denser than water, they’ll sink to the bottom. If they’re less dense, then they’ll rise to the top. Most flammable liquids, such as gasoline, are lighter than water and will float on its surface.
What is a flash point and fire point?
A flash point is the minimum temperature at which a liquid gives off sufficient vapors to ignite, but not sustain combustion, in the presence of a piloted ignition source.
A fire point is the temperature at which a piloted ignition of sufficient vapors will begin a sustained combustion reaction.
How does the progression of a fuel-limited fire differ from that of a ventilation-limited fire?
A fuel-limited fire will usually progression through each stage of fire development in order. In contrast, ventilation-limited fires tend to enter an early stage of decay at the end of the growth stage since there’s no longer enough oxygen for the fire to become fully developed. However, a change in ventilation can quickly change the fire’s progression from an early stage of decay to fully developed.
During the growth stage of fire development, what influences the entrainment of air into the fire?
Unconfined fires draw air from all sides and the entrainment (drawing in) of air cools the plume of hot gases, reducing flame length and vertical extentsion. In a compartment fire, the location of the fuel package in relation to the compartment walls affects the amount of air that is entrained and thus the amount of cooling that takes place. Fires located:
- In the middle of the room can entrain air from all sides
- Near a wall can only entrain air from 3 sides
- In corners can only entrain air from 2 sides
