Unit 4: Fire Dynamics Flashcards
is the scientific study of fire
behavior (e.g., ignition, flames, flame
spread, fire growth, extinguishment).
Fire Dynamics
A rapid oxidation
process,
which is a
chemical reaction
resulting in the evolution of
light and heat
in varying
intensities.
Fire
Fire Triangle
Fuel
Oxygen
Heat
Fire Tetrahedron
Fuel
Heat
Oxygenizing Agent
Uninhibited Chemical chain reaction
Basic Combustion Reaction
Fuel + Oxidizer = Combustion Products
Fuels include= Carbon, Hydrogen
Oxidizer include= Air, Oxygen Oxidizers
Combustion Products = Carbon Dioxide, Water
Physical Process of Fire
Heat absorbed by Fuel
Vapors Generated
Vapors Ignite
Flames radiate to Heat Surface
Air is approximately __ percent oxygen
and ___ percent nitrogen.
21% Oxygen
79% Nitrogen
Only _______ reacts with the fuel.
Oxygen
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
Flaming fires require at least ________
percent oxygen.
14 to 16%
Smoldering combustion requires as little as _______ percent oxygen.
8%
No change in chemical structure; liquids
change back to a solid when cooled (candle
wax).
Melting
No change in chemical structure; gases
change back to a liquid when cooled.
Evaporation
– Irreversible changes to the structure of the
material due to the effects of heat (pyrolysis).
Thermal decomposition
– Solid material changes directly into a vapor
naphthalene, methenamine
Sublimation
Fuels exists in different forms:
– Solids.
– Liquids.
– Gases.
Solid oxidizers:
– Materials that contain oxygen.
– Chemicals: nitrates, chlorates,
sulfates, phosphates, etc.
Minimum temperature required to cause
combustion.
ignition temperature
Types of ignition
Piloted ignition.
Autoignition.
External ignition
source ignites
flammable vapors.
Piloted ignition
Initiation by heat without direct contact from a flame, spark or hot surface.
autoignition
Occurs once the
fuel reaches its auto
ignition temperature
(AIT).
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.
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.
Minimum concentration of fuel vapors in air that will
burn.
Lower explosive limit (LEL).
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
