Unit 4: Fire Dynamics Flashcards by ronnie barbee

is the scientific study of fire
behavior (e.g., ignition, flames, flame
spread, fire growth, extinguishment).

Fire Dynamics

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A rapid oxidation
process,

which is a
chemical reaction

resulting in the evolution of
light and heat

in varying
intensities.

Fire

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Fire Triangle

Fuel

Oxygen

Heat

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Fire Tetrahedron

Fuel

Heat

Oxygenizing Agent

Uninhibited Chemical chain reaction

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Basic Combustion Reaction

Fuel + Oxidizer = Combustion Products

Fuels include= Carbon, Hydrogen

Oxidizer include= Air, Oxygen Oxidizers

Combustion Products = Carbon Dioxide, Water

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Physical Process of Fire

Heat absorbed by Fuel

Vapors Generated

Vapors Ignite

Flames radiate to Heat Surface

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Air is approximately __ percent oxygen

and ___ percent nitrogen.

21% Oxygen

79% Nitrogen

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Only _______ reacts with the fuel.

Oxygen

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Optimum ratio of fuel and air mixture:

No excess fuel or air remains after
combustion.

Combustion of hydrocarbon fuels results
only in the formation of carbon dioxide
(CO2) and water vapor (H2O).

STOICHIOMETRIC

COMBUSTION

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Flaming fires require at least ________

percent oxygen.

14 to 16%

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Smoldering combustion requires as little as _______ percent oxygen.

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No change in chemical structure; liquids
change back to a solid when cooled (candle
wax).

Melting

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No change in chemical structure; gases

change back to a liquid when cooled.

Evaporation

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– Irreversible changes to the structure of the

material due to the effects of heat (pyrolysis).

Thermal decomposition

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– Solid material changes directly into a vapor

naphthalene, methenamine

Sublimation

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Fuels exists in different forms:

– Solids.
– Liquids.
– Gases.

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Solid oxidizers:

– Materials that contain oxygen.
– Chemicals: nitrates, chlorates,
sulfates, phosphates, etc.

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Minimum temperature required to cause

combustion.

ignition temperature

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Types of ignition

Piloted ignition.

Autoignition.

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External ignition
source ignites
flammable vapors.

Piloted ignition

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Initiation by heat 
without direct 
contact from a 
flame, spark or hot 
surface.

autoignition

Occurs once the
fuel reaches its auto
ignition temperature
(AIT).

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In origin and cause, need to be able to

articulate each of these bullets.

Sufficient temperature.

Sufficient energy.

Will be in contact with the fuel long
enough to raise it to the fuels ignition
temperature.

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A competent ignition source will have:

Sufficient temperature.

Sufficient energy.

Will be in contact with the fuel long
enough to raise it to the fuels ignition
temperature.

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Minimum concentration of fuel vapors in air that will

burn.

Lower explosive limit (LEL).

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Maximum concentration of fuel vapors in air that will | burn.

Upper explosive limit (UEL).

Gases require less than ______J of energy | to ignite.

1 mj

A strong static shock is on the order of

1 j or 1,000mj

Lowest temperature of a liquid at which the liquid gives off vapors at a sufficient rate to support a momentary flame across its surface.

Flash Point

Flash point < 100 °F: – Flash point > or = 100 °F: Combustible liquid.

Flammable Liquid = < 100 Degrees Flash Point Combustible Liquid = > 100 Degrees Flash Point

Temperature at which burning of the | vaporizing liquid can be sustained.

Fire Point

Ignition of solids

``` • Heat must be supplied to decompose solid fuel into fuel vapors. ``` • Fuel vapor/air mixture must be within flammability limits.

FACTORS THAT AFFECT | IGNITION OF SOLIDS

* Shape (surface to mass ratio). * Density (how tightly the fuel is packed). * Thickness (thick versus thin materials).

is a measure of how fast the surface temperature of a material will rise.

Thermal inertia (kρc) It is a product of the material’s thermal conductivity (k), density (ρ), and specific heat (c).

Materials with high thermal inertia (kρc) .

require more energy to raise their surface temperature.

``` Expresses the degree of molecular activity relative to some reference point (e.g., freezing point of water). ``` • “Hotter” objects have molecules that move faster than “colder” objects.

Temperature

scales are based on the temperature at which molecular activity ceases

Absolute temperature scale

_________is a form of energy that results from the random motion of molecules.

Heat

Heat is not the same as

Temperature

``` Energy that is transferred between objects due to a temperature difference. ```

Heat transfer

1 Watt =

1 joule Kilowatt (kW) = 1,000 W.

– Heat transfer through a solid material.

Conduction

Heat transfer between a fluid or gas and a | solid surface

Convection

Heat transfer by electromagnetic waves | through open space.

Radiation

Conduction of heat into a material is an important aspect of ignition. • The surface temperature of the fuel must be raised sufficiently to release enough fuel vapors for combustion to occur. • When a fuel is heated, heat is conducted away from the fuel’s surface to its interior. • A fuel’s ability to dissipate the heat away from its surface will affect how easily the fuel is ignited.

ignition

FACTORS THAT AFFECT | RADIATION HEAT TRANSFER

* Distance between radiating object and target. * Size of both the radiating object and target. * Orientation of both the radiating object and target.

Smoke consists of

millions of tiny, dark, solid particles, each radiating heat. The dirtier or darker the smoke, the more radiation it will emit (higher emissivity).

______is not a type of heat transfer. It is a combination of convection and radiation heat transfer.

Direct flame contact

________ is the rate of heat transfer to a defined area.

Heat flux

Heat flux = energy per unit time per unit | area.

is the minimum radiant | heat flux required for piloted ignition.

Critical heat flux

For most materials, the critical radiant heat flux is .

10 to 20 kW/m2

• Very small solid particles. • Typically occurs in unoccupied spaces due to the high concentration of particles required. • Fuel (dust)/air mixture must be within flammability limits. • Mechanical sparks, static, hot bearings, arcing are all competent ignition sources.

Dust Ignition

Fuel and oxidizer are mixed prior to combustion.

Premixed flames

Fuel and air mix or diffuse together at the combustion region.

Diffusion Flames

Flame velocity < speed of sound.

Deflagration:

Flame velocity > speed of sound.

Detonation

``` Combustion occurs in a thin reaction zone where the unmixed fuel vapors and air come together. ``` Diffusion flame burning is the ordinary, sustained burning mode in most fires.

Diffusion Flames

TYPES OF DIFFUSION FLAMES

• Laminar: – Orderly, unfluctuating fluid motion. – Only small flames (candles). • Turbulent: – Randomly fluctuating fluid motion. – Flames in most real fires are turbulent.

For most common fuels, flame temperature is essentially the same, regardless of fuel type.

around 1800 degrees

• Self-heating occurs when the fuel is unable to dissipate heat generated by a chemical reaction within the bulk of the fuel. • As the temperature of the fuel increases, it causes a faster chemical reaction, eventually leading to thermal runaway. • Ignition occurs when the temperature reaches the ignition temperature of the material.

SPONTANEOUS COMBUSTION

• Also known as wind-aided flame spread. • Flame spread direction is the same as the gas flow or wind direction. • Generally quite rapid as a result of the direct contact of the flame with the fuel ahead of the flame front.

Concurrent Flame Spread

• Also called opposed flow flame spread. • Flame spread direction is counter to or opposed to the gas flow. • Generally slow as a result of the limited ability of the flame to heat the fuel ahead of the flame front.

Counter Flow Flame

• _______________ is the amount of heat energy released by the fire per unit time. • The “size” of a fire is often specified by the HRR. • It is the single most important factor in characterizing fire behavior.

Heat Release Rate (HRR)

_________ causes the vertical motion of hot gases and combustion products.

Buoyancy

__________exists when two adjacent fluids (gas or liquid) exist at different densities.

Buoyancy

• Also known as over-ventilated fires. • More oxygen supplied to the fire than required to react with all of the fuel.

Fuel Limited Fires

• Also known as under-ventilated fires. • Insufficient oxygen to react with all of the fuel (more fuel than air), which results in the incomplete combustion of the fuel. • Compartment fires can transition from fuel limited to ventilation limited.

Ventilation Limited Fires

• The majority of enclosure fires that you will investigate are ventilation limited. • The size and location of fire is determined by ventilation. • Rule of thumb: If the smoke layer is from floor to ceiling, generally the fire is ventilation limited.

Ventilation Limited Fires

• Transition from “a fire in a room” to “a room on fire.” • Several flashover definitions: – 500 °C to 600°C (930°F to 1,100°F) upper layer gas temperature. – 20 kW/m2 incident heat flux to the compartment floor. – “Everything that can burn is burning.” – “Flames out the door.”

Flashover

Unit 4: Fire Dynamics Flashcards

(69 cards)