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National Fire academy > Unit 4: Fire Dynamics > Flashcards

Unit 4: Fire Dynamics Flashcards

1
Q

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

A

Fire Dynamics

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2
Q

A rapid oxidation
process,

which is a
chemical reaction

resulting in the evolution of
light and heat

in varying
intensities.

A

Fire

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3
Q

Fire Triangle

A

Fuel

Oxygen

Heat

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4
Q

Fire Tetrahedron

A

Fuel

Heat

Oxygenizing Agent

Uninhibited Chemical chain reaction

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5
Q

Basic Combustion Reaction

A

Fuel + Oxidizer = Combustion Products

Fuels include= Carbon, Hydrogen

Oxidizer include= Air, Oxygen Oxidizers

Combustion Products = Carbon Dioxide, Water

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6
Q

Physical Process of Fire

A

Heat absorbed by Fuel

Vapors Generated

Vapors Ignite

Flames radiate to Heat Surface

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7
Q

Air is approximately __ percent oxygen

and ___ percent nitrogen.

A

21% Oxygen

79% Nitrogen

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8
Q

Only _______ reacts with the fuel.

A

Oxygen

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9
Q

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).

A

STOICHIOMETRIC

COMBUSTION

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10
Q

Flaming fires require at least ________

percent oxygen.

A

14 to 16%

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11
Q

Smoldering combustion requires as little as _______ percent oxygen.

A

8%

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12
Q

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

A

Melting

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13
Q

No change in chemical structure; gases

change back to a liquid when cooled.

A

Evaporation

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14
Q

– Irreversible changes to the structure of the

material due to the effects of heat (pyrolysis).

A

Thermal decomposition

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15
Q

– Solid material changes directly into a vapor

naphthalene, methenamine

A

Sublimation

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16
Q

Fuels exists in different forms:

A

– Solids.
– Liquids.
– Gases.

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17
Q

Solid oxidizers:

A

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

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18
Q

Minimum temperature required to cause

combustion.

A

ignition temperature

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19
Q

Types of ignition

A

Piloted ignition.

Autoignition.

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20
Q

External ignition
source ignites
flammable vapors.

A

Piloted ignition

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21
Q 
Initiation by heat 
without direct 
contact from a 
flame, spark or hot 
surface.
A

autoignition

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

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22
Q

In origin and cause, need to be able to

articulate each of these bullets.

A

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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23
Q

A competent ignition source will have:

A

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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24
Q

Minimum concentration of fuel vapors in air that will

burn.

A

Lower explosive limit (LEL).

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25
Q

Maximum concentration of fuel vapors in air that will

burn.

A

Upper explosive limit (UEL).

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26
Q

Gases require less than ______J of energy

to ignite.

A

1 mj

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27
Q

A strong static shock is on the order of

A

1 j or 1,000mj

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28
Q

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

A

Flash Point

29
Q

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

A

Flammable Liquid = < 100 Degrees Flash Point

Combustible Liquid = > 100 Degrees Flash Point

30
Q

Temperature at which burning of the

vaporizing liquid can be sustained.

A

Fire Point

31
Q

Ignition of solids

A 
•   Heat must be 
supplied to 
decompose solid 
fuel into fuel 
vapors.

• Fuel vapor/air
mixture must be
within flammability
limits.

32
Q

FACTORS THAT AFFECT

IGNITION OF SOLIDS

A
  • Shape (surface to mass ratio).
  • Density (how tightly the fuel is packed).
  • Thickness (thick versus thin materials).
33
Q

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

A

Thermal inertia (kρc)

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

34
Q

Materials with high thermal inertia (kρc) .

A

require more energy to raise their surface temperature.

35
Q 
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.

A

Temperature

36
Q

scales are based on the
temperature at which
molecular activity ceases

A

Absolute temperature scale

37
Q

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

A

Heat

38
Q

Heat is not the same as

A

Temperature

39
Q 
Energy that is 
transferred between 
objects due to a 
temperature 
difference.
A

Heat transfer

40
Q

1 Watt =

A

1 joule

Kilowatt (kW) = 1,000 W.

41
Q

– Heat transfer through a solid material.

A

Conduction

42
Q

Heat transfer between a fluid or gas and a

solid surface

A

Convection

43
Q

Heat transfer by electromagnetic waves

through open space.

A

Radiation

44
Q

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.

A

ignition

45
Q

FACTORS THAT AFFECT

RADIATION HEAT TRANSFER

A
  • Distance between radiating object and target.
  • Size of both the radiating object and target.
  • Orientation of both the radiating object and target.
46
Q

Smoke consists of

A

millions of tiny, dark, solid particles, each radiating heat.

The dirtier or darker the
smoke, the more
radiation it will emit
(higher emissivity).

47
Q

______is not a type of heat transfer.

It is a combination of convection and radiation heat
transfer.

A

Direct flame contact

48
Q

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

A

Heat flux

49
Q

Heat flux = energy per unit time per unit

area.

A

.

50
Q

is the minimum radiant

heat flux required for piloted ignition.

A

Critical heat flux

51
Q

For most materials, the critical radiant
heat flux is
.

A

10 to 20 kW/m2

52
Q

• 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.

A

Dust Ignition

53
Q

Fuel and oxidizer
are mixed prior to
combustion.

A

Premixed flames

54
Q

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

A

Diffusion Flames

55
Q

Flame velocity < speed of sound.

A

Deflagration:

56
Q

Flame velocity > speed of sound.

A

Detonation

57
Q 
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.

A

Diffusion Flames

58
Q

TYPES OF DIFFUSION FLAMES

A

• Laminar:
– Orderly, unfluctuating fluid motion.
– Only small flames (candles).

• Turbulent:
– Randomly fluctuating fluid motion.
– Flames in most real fires are turbulent.

59
Q

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

A

around 1800 degrees

60
Q

• 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.

A

SPONTANEOUS COMBUSTION

61
Q

• 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.

A

Concurrent Flame Spread

62
Q

• 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.

A

Counter Flow Flame

63
Q

• _______________ 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.

A

Heat Release Rate (HRR)

64
Q

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

A

Buoyancy

65
Q

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

A

Buoyancy

66
Q

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

A

Fuel Limited Fires

67
Q

• 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.

A

Ventilation Limited Fires

68
Q

• 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.

A

Ventilation Limited Fires

69
Q

• 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.”

A

Flashover

National Fire academy (15 decks)

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