Chapter 4: Fire Dynamics

Position: A function of Fire Spread

- Horizontal ~ Flame is spread by radiation, fire gases rise due to convection, point of ignition and slower spread of fire - Vertical ~ wood surface is preheated by convection and radiation, point of ignition and faster fire spread

The fire tetrahedron

- Reducing agent (fuel) - chemical chain reaction (center) - oxidizing agent - heat

Heat transfer due to convection is dependent upon three factors:

- area being heated - temp. Difference between the hot fluids or gas and the material being heated - turbulence and velocity of moving gases

Heat transfer due to conduction is dependent upon three factors:

- area being heated - temperature difference between the heat source and the material being heated - thermal conductivity of the heated material

Types of Energy:

- chemical - thermal - mechanical - electrical - light - nuclear - sound

What are the key terms that are among rapid fire development?

- flashover = rapid transition from the growth stage to the fully developed stage - backdraft = instantaneous explosion or rapid burning of superheated gases that occurs when oxygen is introduced into an oxygen-depleted confined space. The stalled combustion resumes with explosive force; may occur because of inadequate or improper ventilation procedures.

Organic fuels can be divided into hydrocarbon- based fuels, such as:

- gasoline - plastics - fuel oil - cellulose-based materials (wood and paper)

Numerous factors that influence radiant heat transfer, include:

- nature of the exposed surfaces- dark-colored materials emit and absorb heat more effectively than light-colored materials; smooth or highly-polished surfaces reflect more radiant heat than rough surfaces - distance between the heat source and the exposed surfaces- increasing distance reduces the effect of radiant heat - temperature of heat source- unlike other methods of heat transfer that depend on the temperature of both the heat source and exposed surface, radiant heat transfer primarily depends on the temperature of the heat source. As temp and heat release rate of the heat source increases, the radiant energy also increases.

Building compartmentation is the layout of the various open spaces in a structure and include:

- number of stories above or below ground - floor plans - openings between floors - continuous voids or concealed spaces - barriers to fire spread

The fire triangle

- oxygen - heat - fuel

Electrical heating can occur in these ways:

- resistance heating- electrical current flowing through a conductor produces heat. - overcurrent or overload- when the current flowing through the conductor exceeds its design limits, the conductor may overheat and present an ignition hazard - arcing- an arc is a high temp luminous electrical discharge across a gap or through a medium such as charred insulation. - sparking- when an electrical arc occurs, luminous (glowing) particles can form and splatter away from the point of arcing.

For self-heating to progress to spontaneous ignition, the following factors are required:

- the insulation properties of the material immediately surrounding the fuel must be such that the heat cannot dissipate as fast as it is generated - the rate of heat production must be great enough to raise the temperature of the material to its auto ignition temperature the available air supply in and around the heated material must be adequate to support combustion

Why is the fire now in a state of ventilation-limited decay?

- there is not enough oxygen to maintain combustion - the heat release rate has decreased to the point that fuel gases will not ignite.

Four common elements of flashover:

- transition in fire development= flashover represents a transition from the growth stage to the fully developed stage - rapidity = although it is not an instantaneous event, flashover happens rapidly, often in a matter of seconds, to spread fire completely throughout the compartment - compartment= there must be an enclosed space such as a single room or enclosure. - pyrolysis of all exposed fuel surfaces= fire gases from all of the combustible surfaces in the enclosed space ignite, provided that there is sufficient oxygen to support flaming combustion.

Specific Gravity of Liquids

- water - 1.0- middle - Gasoline- less than 1.0 top - salt water- greater than 1.0 bottom

Heat Release Rates in Watts

- watts (candle) - Kilowatts (trash can) - Megawatts (sofa)

All combustible materials in the buildings construction compromise the fuel load, such as:

- wood framing - floors - ceilings - furnishing - combustible materials within the building

A British thermal unit is the amount of heat required to raise the temperature of 1 pound of water by?

1 degree Fahrenheit

The quantity of heat required to change the temperature of 1 gram of water by 1 degree of Celsius is?

4.2 joules

Exposure fires

A fire ignited in fuel packages or buildings that are remote from the initial fuel package or building of origin

The fire triangle and the fire tetrahedron?

Are models used to explain the elements of fire and how fires can be extinguished

Self-Sustained Chemical Reaction

As flaming combustion occurs, the molecules of a fuel gas and oxygen break apart from free radicals(electrically charged, highly reactive parts of molecules) . Free radicals combine with oxygen or with the elements released from the fuel gas to form new substances (molecules) and even more free radicals. The process also increases the speed of the oxidation reaction. Flaming combustion is one example of a chemical chain reaction. Sufficient heat will cause fuel and oxygen to form more free radicals and initiate the self-sustained chemical reaction. The fire will continue to burn until it consumes the fuel or oxygen or an extinguishing agent, applied in sufficient quantity, interferes with the ongoing reactive chemical flame inhibition occurs when an extinguishing agent, such as dry chemical or halon-replacement agent, interferes with this chemical reaction, forms a stable product, and terminates the combustion reaction.

Decay Stage

As the fire consumes the available fuel or oxygen and the heat release rate begins to decline, the fire enters the decay stage. Fuel- limited fires may self-extinguish in this phase and reduce to smoldering fires

Growth Stage

As the fire transitions from the incipient stage to the growth stage, more of the initial fuel package becomes, involved and the production of heat and smoke increases. If there are other fuels close to the initial fuel package, radiant heat from the fire may begin to proline nearby fuels which could spread the fire to new fuel packages. The fire may continue to grow to become fully developed or may enter an early stage of decay depending upon available oxygen.

Surface to mass ratio note

As the surface to mass ratio of a fuel becomes higher (increases), the energy required for ignition is lower (reduced)

Another unit of measurement?

British thermal unit (BTU)

Electrical energy

Can generate temperature high enough to ignite any combustion materials near the heated area.

Most structural fires involve multiple types of fuels:

Carbon based fuels( wood, cotton), hydrocarbon fuels (plastics, synthetic materials, synthetic fibers)

All energy can what?

Change from one form to another

Sources of Thermal Energy

Chemical, electrical and mechanical energy are common sources of heat that result in the ignition of a fuel. They can all transfer heat, cause the temperature of a substance to increased and are most frequently the ignition sources of structure fires.

Firefighters should have a scientific understanding of:

Combustion, fire, heat and temperature

Flaming Combustion

Commonly referred to as a fire. Occurs when a gaseous fuel mixes with oxygen in the correct ratio and heats to an ignition temperation. Requires liquids or solid fuels to be converted to the gas phase through the addition of heat (vaporization or pyrolysis)

Heat transfers from one body to another by three mechanisms:

Conduction, convection and radiation

Products of combustion

Described as heat and smoke

Vapor density

Describes the density of gases in relation to air. Air has a vapor density of 1. Gases with a vapor density of less than 1, such as methane, will rise while those having a vapor density of greater than 1, such as propane, will sink.

Reactions that absorb energy as they occur are?

Endothermic reaction

Reactions that emit energy as they occur are?

Exothermic reactions

Conversation of temperature:

F= (C x 1.8)+ 32 C=(F - 32)/1.8

Asphyxiation

Fatal levels of oxygen deficiency in the blood

Liquids terminology

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. Fire point- is the temperature at which a piloted ignition of sufficient vapors will begin a sustained combustion reaction. Firefighters must know how liquid fuels react with water. Solubility describes the extent to which a substance will mix with water. Materials that are miscible in water will mix in any proportion. Some liquids are lighter that water and don't nix with it. Such as hydrocarbon fuels. Flammable liquids called polar solvents, such as alcohol will mix readily with water.

Gases

For flaming combustion to occur, fuels must be in the gaseous state. Thermal energy is required to change solids and liquids into the gaseous state. Vapor is the common term used to describe the gaseous state of a fuel that would normally exist as a liquid or a solid at standard temperature and pressure. Gaseous fuels such as methane (natural gas), hydrogen and acetylene, can be the most dangerous of all fuel types because they are already in the physical state required for ignition.

Mechanical Energy

Friction and compression generate mechanical energy. The movement of two surfaces against each other creates heat of friction that generates heat and/ or sparks. Heat is generated when a gas is compressed.

All fire involve a heat-producing chemical reaction between some type of:

Fuel and an oxidizer

Compartment Fire Development

Fuel- limited - fire with adequate oxygen in which the heat release rate and growth rate are determined by the characteristics of the fuel, such as, quantity and geometry. Ventilation- limited - fire with limited ventilation in which the heat release rate or growth is limited by the amount of oxygen available to the fire

Difference Between Heat Release Rate and Temperature: Heat

Heat is the thermal energy needed to release the potential chemical energy in a fuel. As heat begins to vibrate the molecules of fuel, the fuel begins a physical change from a solid or liquid to a gas. The fuel emits flammable vapors which can ignite and release thermal energy.

Heat Transfer:

Heat transfers from warmer objects to cooler objects because heated materials will naturally return to a state of thermal equilibrium in which all areas of an object are a uniform temperature. Objects at the same temperature do not transfer heat.

Ceiling Jet

Hot gases in the plume phase rise until they encounter the ceiling and then begin to spread horizontally.

Four Stages of Fire development:

Incipient, growth, fully developed and decay

Smoke is a product of what?

Incomplete combustion. Combustion is incomplete when any of the fuel is left after combustion occurred.

Oxidation

Is a chemical reaction involving the combination of an oxidizer, such as oxygen in the air, with other materials

Combustion

Is a chemical reaction while flaming combustion is only one possible form of combustion. Combustion can occur without visible flames. Two models of combustion: non-flaming and flaming.

Self-heating

Is a form of oxidation, is a chemical reaction that increases the temperature of a material without the addition of external heat. Self-heating can lead to spontaneous ignition which is ignition without the addition of external heat.

Carbon Dioxide (CO2)

Is a product of complete combustion or organic materials. It replaces oxygen and creates a less oxygen rich environment.

Carbon Monoxide

Is a toxic and flammable product of the incomplete combustion of organic (carbon-containing) materials. CO is colorless, odorless gas present at almost every fire. CO molecules attach to hemoglobin, decreasing the blood's ability to carry oxygen.

Hydrogen cyanide (HCN)

Is a toxic and flammable substance produced in the combustion materials containing nitrogen, commonly found in smoke. It kills the cells in the body when it is inhaled.

Note: a dangerous misconception:

Is that temperature is an accurate predictor or measurement of heat transfer

Pressure

Is the force per unit of area applied perpendicular to the surface.

Autoignition temperature

Is the minimum temp. At which a fuel in the air must be heated in order to start self-sustained combustion. The auto ignition temp of a substance is always higher than its piloted ignition temperature.

Piloted Ignition

Is the most common form of ignition and occurs when a mixture of fuel and oxygen encounter an external heat source with sufficient heat or thermal energy to start a combustion reaction.

Chemical Energy

Is the most common source of heat in combustion reactions. The potential for oxidization exists when any combustible fuel is in contact with oxygen. The oxidization process almost always results in the production of thermal energy.

Conduction

Is the transfer of heat through and between solids. Conduction occurs when a material is heated as a result of direct contact with heat source. Conduction results from the increased molecular motion and collisions between a substances molecules, resulting in the transfer of energy through the substance.

Convection

Is the transfer of thermal energy by the circulation or movement of a fluid (liquid or gas). In the fire environment, convection usually involves transfer of heat through the movement of hot smoke and fire gases. The heat flows from hot fire gases to the cooler structural surfaces, building contents and air. Convection may occur in any direction.

Radiation

Is the transmission of energy as an electromagnetic waves, such as light waves, radio waves or x-rays, without an intervening medium. Radiant heat can become the dominant mode of heat transfer as the fire grows in size and can have a significant effect on the ignition of objects located some distance from the fire. Radiant heat transfer is also a significant factor in the fire development and spread in compartments.

Heat From a Fire:

It increases pressure of the surrounding gases. This increased in pressure will seek to expand and equalize with areas of lower pressure. Heated gases will rise, remain aloft (buoyant) and generally travel up and out. At the same time, cooler , fresh air will generally travel inward toward the fire. This exchange of air creates a convective flow.

In liquids,?

It is a physical change called vaporization.

Fuel

It is an oxidized or burned material or substance in the combustion process. A fuel may be found in any of three physical states of matter: gas, liquid or solid.

Energy is measured in?

Joules (j)

Liquid Notes:

Liquids that are less dense (lighter) than water are more difficult to extinguish using water as the sole extinguishing agent. Because the liquid fuel is less dense and will not mix with water, adding water to the liquid fuel may disperse the burning liquid instead of extinguishing it.

The interface between the hot gas layers and cooler layer of air is commonly referred to as?

Neutral plane

Oxidizers are:

Not combustible but will support or enhance combustion.

Nonflaming combustion

Occurs more slowly and at a lower temp. Producing a smoldering glow in the materials surface.

Fully Developed Stage

Occurs when all combustible materials in the compartment are burning at their peak heat release rate based on the available oxygen. The fire is consuming the maximum amount of oxygen that it can.

Autoignition

Occurs without any external flame or spark to ignite the fuel gases or vapors

Oxygen

Oxygen in the air is the primary oxidizing agent in most fires. Normally, air consists of about 21 percent oxygen. The energy release in the fire is directly proportional to the amount of oxygen available for combustion. When a fire ignites in an open area where air is plentiful, the fire will release energy based on the given surface area.

Heat of combustion

Potential energy available for release in the combustion process

In solids, off-gassing is a chemical change known as?

Pyrolysis

Flashover notes:

Rapid transition from the growth stage to the fully developed stage is known as flashover. When the flashover occurs, the combustible materials and fuel gases in the compartment ignite almost simultaneously; the result is full-room fire involvement. The environment of the room changes from a two-layer condition (hot on top of cold) to a single, well mixed hot gas condition from floor to ceiling. Gas temps can reach 1,100 degree F.

Potential Energy

Represents the amount of energy that an object can release at some point in the future.

A significant indicator of flashover is rollover:

Rollover is when a condition where the unturned fire gases that have accumulated at the top of a compartment ignite and flames propagate through the hot gas layer or across the ceiling.

Warning about smoke:

Smoke is fuel and is potentially flammable. Smoke can be oxygen deficient and contain chemicals, which may be acutely toxic, and/or carcinogens which may cause cancer.

Liquids

Specific gravity is the ratio of the mass of a given volume of a liquid compared to the mass of an equal volume of water at the same specific temperature. To burn, liquids must vaporize. Vaporization is the transformation of a liquid to vapor or a gaseous state. For vaporization to occur, the escaping vapors must be at a greater pressure than atmospheric pressure. The pressure that vapors escaping from a liquid exert is known as vapor pressure. They are different from gases. They will not expand to the entire room, they will all flow to the low point.

Incipient Stage

Starts with ignition when the three elements of the fire triangle come together and the combustion process begins. At this point, the fire is small and confined to a small portion of the fuel first ignited.

Difference Between Heat Release Rate and Temperature: Temperature

Temperature is the measure of heat. Temperature is the measurement of the average kinetic energy in the particles of a sample of matter. A block of wood at room temperature has stable molecules and is in no danger of ignition. When thermal energy transfers to the wood, the wood is heated, and the temperature of the wood rises because the molecules have begun to vibrate and move more freely and rapidly.

A working knowledge of fire dynamics requires an understanding of:

Temperature, energy and power or heat release rate - heat release rate= total amount of heat released per unit time. The heat release rate is typically measured in kilowatts (kW) or megawatts MW of output.

Combustion Zone

The area where sufficient air is available to feed the fire.

Note Flashover

The auto ignition temperature of CO, the most abundant fuel gas created in most fires, is approximately 1,100 degrees F

Kinetic Energy

The energy that a moving object possesses.

Note : Flashpoint vs. Fire Point in Gasoline

The flashpoint of a liquid indicates the temperature at which the liquid will ignite temporarily, while the fire point indicates the temperature at which the liquid, once ignited will continue to burn.

Note:

The increased heat release rate results in an increased heat transfer rate to an object. This energy flow to a unit area (heat flux) is measured in kilowatts per square meter.

Oxygen Notes Extended

The range of concentrations of the fuel vapor and air is called the flammable (explosive) range. The fuel's flammable range is reported using the percent by volume of gas or vapor in air for the lower explosive (flammable) limit (LEL) and for the upper explosive (flammable) limit (UEL)

Heat Transfer 2:

The rate at which heat transfers is related to the temperature differential of the bodies and the thermal conductivity of the materials involved. The greater the temperature differences between the bodies, the greater the transfer rate.

Surface-to-mass ratio

The ratio of the surface area of the fuel to the mass of the fuel

The fuel in a combustion reaction is known as?

The reducing agent

Flow Path

The space between at least one intake and one exhaust outlet. The difference in pressure determines the direction of the flow of gases through this space. Heat and smoke in a high pressure area will flow toward areas of lower pressure.

Fuels must be in a gaseous state to burn;

Therefore, solids and liquids must be converted in gaseous in order for ignition to occur.

Ventilation Controlled Fire

Timeline: ignition growth decay begins as oxygen is depleted change in ventilation allows oxygen to reach the fire fully developed decay begins as fuel is depleted decay

Fuel Controlled Fires

Timeline: ignition growth fully developed decay begins as fuel is depleted decay

Entrainment

Unconfined fires draw air from all sides and the entrainment (drawing in) of air cools the plume or hot gases reducing flame length and vertical extension.

Solids

When solids such as wax and metals will change their state and melt, while other such as wood and plastics will not. When solid fuels are heated, they begin to pyrolyses (off-gas) and release fuel gases and vapors. The solid fuels begin to decompose and emit combustible vapors. If there is enough fuel and heat, the process of pyrolysis generates sufficient flammable vapors to ignite in the presence of sufficient oxygen or another oxidizer.

Thermal Layering

is the tendency of gases to form into layers according to temperature, gas density and pressure.