Basic Fire Science Flashcards
What are the four components of the fire tetrahedron?
Fuel
Heat
Uninhibited chemical chain reactions
oxidizing agent
Fires can be prevented or suppressed by controlling or removing
one or more of the sides of the tetrahedron.
A fuel is
any substance that can undergo combustion.
The majority of fuels encountered are
organic. Which simply means that they are carbon based
Combustion of liquid fuels and most solid fuels takes place
above the fuel surface in a region of vaports created by heating the fuel surface.
Gaseous fuels do not require ______ or _____ before combustion can occur.
vaporization or pyrolysis
In most fire situations, the oxidizing agent is
the oxygen in the earth’s atmosphere.
Many chemical oxidizers contain readily released oxygen. List three examples:
Ammonium nitrate fertilizer
potassium nitrate
hydrogen peroxide
Every fuel-air mixture has an optimum ratio at which point the combustion will be most efficient. This ratio occurs at or near the4 mixture known by chemists as the
Stoichiometric ratio
Self sustained combustion occurs when
sufficient excess heat from the exothermic reaction radiates back to the fuel to produce vapors and cause ignition in the absence of the original ignition source.
Lower explosive (flammable) Limit
LEL
The minimum percentage of fuel in air in which combustion can occur is the lower explosive limit of the material.
Upper Explosive Limit
UEL
There is also a maximum percentage of fuel in air in which combustion can occur.
Complete combustion of hydrocarbon fuels containing only hydrogen and carbon will produce
carbon dioxide and water
When less air is available for combustion, as in ventilation controlled fires, the production of ______ increases as does the production of ______
Carbon monoxide increases as does the production of soot and unburned fuels.
Combustion products exist in all three states of matter:
solid, liquid and gas.
Solid material makes up the ash and soot products that represent the
visible “smoke”
Many of the other products of incomplete combustion exist as vapors or as extremely small tarry droplets or aerosols. These vapors and droplets often condense on surfaces that are cooler than the smoke, resulting in
smoke patterns that can be used to help determine the origin and spread of fire.
Soot and tarry products often accumulate more heavily on
ceramic-tiled surfaces than on other surrounding surfaces due to the heat conduction properties of ceramic tile. Those surfaces that remain the coolest the longest tend to collect the most condensate.
Smoke is generally considered to be
the collection of the solid, liquid, and gaseous products of incomplete combustion.
_______ flows occur because hot gases are less dense than cold gases. This causes the hot gases to rise, just as a hot air balloon rises.
Buoyant Flows
What causes a fire plume to increase in diameter as it rises?
As the hot gases rise, they mix with or entrain the surrounding air so that the flow of gases in the plume increases with height above the fire and at the same time the temperature of the plume is reduced by the entrainment of air. It is the entrainment of air into the plume that causes the plume to increase in diameter as it rises.
When a fire plume reaches the ceiling of a room, the gases turn to move laterally along the ceiling jet as a
ceiling jet.
The buoyancy of gases in a compartment fire causes flow into and out of a compartment through vents. In a compartment fire with a single vent opening, hot gases flow out through the upper portion of the opening, and fresh air enters in the lower portions of the opening.
Vent flows
The heat transfer rate per unit area
Heat flux
_____ is a measure that expresses the degree of molecular activity of a material compared to a reference point, such as the freezing point of water.
Temperature
____ is the energy that is needed to change the temperature of an object.
Heat
Heat transfer is measured in terms of energy flow per unit of time
kilowatts
The greater the temperature difference between the objects, the more energy transferred per unit of time and the higher the heat transfer rate.
_______ is classically defined as the transport of heat energy from one point to another caused by a temperature difference between those points.
Heat transfer
Energy is transferred from the heated area to the unheated area at a rate dependent on
(conduction)
the difference in temperature and the thermal conductivity of the material.
High-density materials conduct heat faster than
low density
Phase changes most relevant in fire are
melting and vaporization.
In _____, the material changes from a solid to a liquid with no change in the chemical structure of the material.
Melting
In _____ , the material changes from a liquid to a vapor with no change in chemical structure of the material.
Vaporization
______ involves irreversible changes in the chemical structure of a material due to the effects of heat (pyrolysis)
Thermal decomposition
_________occurs when fuel vapors mix with air in the absence of an ignition source and the fuel-air mixture is subsequently ignited.
Premixed burning
Premixed flame spread can proceed as a _____ or as a _____.
deflagration (subsonic combustion) or as a detonation (supersonic combustion)
Deflagration velocities normally range from _____to ____/sec
cm/sec to m/sec
Detonation velocities are normally in the ____ of m/sec.
thousands of c/sec
In order from flammable gases and vapors of ignitable liquids to ignite, they must be mixed with a sufficient amount of _______ to allow the combustion reaction to occur.
Oxidizer (typically atmospheric oxygen)
The percentage of the mixture of gaseous fuel to air by volume must be within a specific range for combustion to occur. This is known as the ______ range of the fuel.
flammable or explosive range of the fuel.
The flammable or explosive range of a fuel is expressed as
a percentage of ignitable gas or vapor in air by volume.
The minimum percentage of fuel in air (by volume) in which combustion can occur is the
lower explosive limit (LEL)
In a mixture that is below its LEL no combustion will occur. This is because below the LEL there are insufficient fuel molecules in the mixture. The mixture can be said to be
“to lean”
There is also a maximum percentage of fuel in air (by volume) in which combustion can occur. This is called the
upper explosion limit (UEL)
In a mixture that is above the UEL, no combustion will occur because there are insufficient oxygen molecules in the mixture. These mixtures can be said to be
“too rich”
The difference between the lower and upper limits is called the
flammable or explosive range.
Fuel vapors and oxidizers are separate and combustion occurs in the region where they come together.
diffusion flame burning
The _____ of a material is a measure of the amount of heat that will flow across a unit area with a temperature gradient of 1 degree per unit of length.
Thermal conductivity
____ have high thermal conductivities.
metals
The ____ of a material is a measure of the amount of heat necessary to raise the temperature of a unit mass 1 degree, under specified conditions.
Heat capacity
____ and ____ have low thermal conductivities.
Plastics and glass
The _____ of a material is a measure of how easily the surface temperature of the material will increase when heat flows into the material.
Thermal inertia
In the ____ scale, water freezes at 32 and boils at 100 degrees.
Fahrenheit
What are the most familiar empirical temperature scales?
Fahrenheit and Celsius (Centigrade)
The ______ scales are based on the lowest possible temperature of absolute zero, and therefore are called the absolute temperature scales.
Thermodynamic (absolute) Temperature scales
The simplest definition of ______ is a theoretical lowest feasible temperature characterized by the absence of heat and molecular motion.
Absolute zero
Thermodynamic temperature scales differ from empirical scales in that they are based on
the fundamental laws of thermodynamics or statistical mechanics instead of the scaling of properties of water.
Thermodynamic temperature is measured in
Kelvins
Many engineering fields in the US hoever, measure thermodynamic temperature using the Rankine scale (symbol: R)
The term _____ is used to describe the amount of fuel present, usually within a compartment.
Fuel load
As the oxygen contained within a structure is reduced, the ____ of the fire decreases and as a result, the gas temperatures within compartments in the structure decrease.
HRR (heat release rate)
The _____ of flame is not necessarily an accurate indicator of what is burning, or of the temperature of the flame.
color
The following flame height definitions define the three regions of a fire:
Continuously flaming region
(lower portion of visible flame)
Intermittently flaming region
(upper portion of the visible flame)
Plume region
(above the visible flame)
If a fuel package is placed against a wall or in a corner, air entrainment into the plume can be restricted, creating an imbalance in the airflow. As a result of the imbalance in airflow, the flame and thermal plumes will?
bend toward the restricting surface
_____ is known as solid phase combustion, also known as smoldering.
Solid phase combustion
flameless form of combustion whose principal heat source is char oxidation.
_____ produces more toxic compounds than flaming combustion per unit mass burned, and it provides a chance for flaming combustion from a heat source too weak to produce flame directly.
Smoldering
Some liquids are capable of being oxidized in the liquid phase. Most often tis only leads to ignition when the liquid is supported on a _____ substrate.
Porous substrate
linseed oil on rags.
_____ is a solid phase burning process, which normally includes a thermal decomposition step to create a char, followed by solid phase burning of the char produced.
Smoldering
Generally, self-heating and spontaneous combustion (self-ignition) are commonly encountered in
organic materials, such as animal and vegetable fats and oils.
Because these materials contain polyunsaturated fatty acids. Such fatty acids react with oxygen to generate heat. Unsaturated molecules contain carbon to carbon double bonds which are reactive.
Self heating and spontaneous combustion of oils containing mostly saturated hydrocarbons, such as motor oil or lubricating oil, occur only
under elevated temperature conditions (e.g., an oil soaked rag wrapped around a steam pipe)
Self heating to ignition requires a
a porous, permeable, and oxidizable material _ the material must have all three properties.
The most commonly encountered forms of elf-heating and self-ignition by fire investigators are the following:
Polymerization of fatty acids (animal fats, cooking oils, and drying oils) in cellulose materials (wood, cloth, and paper)
Oxidation of carbonaceous materials (coal and charcoal)
Biologically induced oxidation (hay bales and compost)
Heat-induced oxidation of lingocellulose materials (usually wood fiber and cloth)
Polymerization reactions (plastics, rubbers, adhesives, and paint overspray particles)
An _____agent is a chemical substance that , while not necessarily combustible by itself, can rapidly increase the rate of burning of other substances, or result in spontaneous combustion when combined with other substances.
Oxidizing agent
Materials that undergo spontaneous combustion upon exposure to air are known as
pyrophoric
Certain elements, particularly white phosphorus, sodium, potassium, and some finely divided metals, such as zirconium, spontaneously ignite when exposed to
air
The range of ignition temperatures for solids ranges from about
270 degrees C to 450 degrees C
Broadly speaking flame spread can be classified as
concurrent flame spread or counterflow flame spread
These terms relate the direction of flame spread compared to the direction of gas flow.
Flames may spread differently on different portions of the same fuel package, for example, a flame may spread slowly across the horizontal surface of a chair cushion with spread increasing rapidly as the back of the chair becomes involved. If a material melts, it is likely to
drip and pool on horizontal surfaces. The dripping of liquefied polyurethane foam from furniture can form a pool fire under that furniture.
The dominant method of spreading fire from one remote location to another remote location is through
radiation.
Fire spread can also occur as a result of ______. This occurs when a flaming material drops down and ignites combustibles that it falls on or near.
“drop down”
While glass cracks when the glass reaches 60-100 C, glass does not generally fall out until
flashover,
except when flames directly imping on the glass.
The post flashover condition is called
full room involvement
The onset of flashover occurs when
the hot gas laver imposes radiant energy levels (flux) on unignited fuels sufficient to ignite them.
Poorly ventilated compartments or very large compartments may be fully consumed without
ever involving all the fuel packages at the same time.
Flashover times of ___to ___ minutes are not unusual in residential room fires tests and even shorter times to flashover have been observed in nonaccelerated room fires.
3 to 5 minutes
Development of a ceiling layer of sufficient temperature to cause radiant ignition of exposed combustible fuels is necessary for
flashover.
When a burning fuel package is away from a wall, air is free to flow into the plume from all directions and mix with the fuel gases. This brings air for combustion into the flame zone and ____ the upper part of the plume by entrainment.
cools
If the fuel package or the fire plume is against a wall, a given fire size will lead to a ___ percent greater hot layer absolute temperature than the same fire size away from the wall.
30%
When the same fuel package is placed in a corner, a given fire size will lead to a ___% greater hot later absolute temperature than the same fire size away from walls or corners.
70%
_____ is the visible or measurable physical changes or identifiable shapes formed by a fire effect or group of fire effects.
Fire pattern
______are the observable or measurable changes in or on a material as a result of exposure to the fire.
Fire effects
______ and gasoline burn at essentially the same flame temperature.
Wood
Identifiable temperatures achieved in structural fires rarely remain above _____C for long periods of time.
1040
The binder in paint will ______ and _____ the color of the painted surface.
Char and darken
The presence of large shiny blisters (alligator char) is not evidence that a liquid accelerant was present during the fire, or
that a fire spread rapidly or burned with greater intensity.
The rate of charring of wood varies widely depending upon variable, including the following:
Rate and duration of heating Ventilation effects Surface area to mass ratio Direction, orientation, and size of wood rain Species of wood Wood density Moisture content nature of surface coating Oxygen concentration of the hot gases Velocity of the impinging gases
The rate of charring and burning of wood in general has no relation to
its age once the wood has been dried.
Thus, old, dry wood is no more combustible than new kiln-dried wood if they have both been exposed to the same atmospheric conditions.
Analysis of the depth of charring is more reliable for evaluating ______, rather than for the establishment of specific burn times or intensity of heat from adjacent burning materials.
fire spread
The ___________ from point to point is the key to appropriate use of charring, locating the places where the damage was most severe due to exposure, ventilation, or fuel placement.
relative depth of char
(The investigator may then deduce the direction of fire spread, with decreasing char depths being farther away from the heat source.)
_____ is characterized by the loss of surface material resulting in cracking, breaking, and chipping or in the formation of craters on concrete, masonry, rock, or brick.
Spalling
___________ is the expansion or contraction of the surface while the rest of the mass expands or contracts at a different rate; one example is the rapid cooling of a heated material by water.
A mechanism of spalling
________ is the basic chemical process associated with combustion.
Oxidation
________ of some non combustible materials can produce lines of demarcation and fire patterns of use to fire investigators.
Oxidation
The higher the temperature and the longer the time of exposure, the ____ pronounced the effects of oxidation will be.
more
When uncoated iron or steel is oxidized in a fire, the surface first acquires a _______dullness.
blue-gray
At elevated temperatures, iron may also combine with oxygen to form
Black oxides
Copper forms a dark ____ or black oxide when exposed to heat.
red
The color is not significant. What is significant is that oxidation can form a line of demarcation. The thickness of the oxide depends upon the duration and intensity of the heat exposure. The more it is heated, the greater the oxidation.
Rocks and soil, when heated to very high temperatures, will often change colors that may range from
yellowish to red
Nearly all materials ____ when heated.
expand
Bending and buckling of steel beams and columns occurs when the steel temperature exceeds approximately ____ degrees
500 C
Carbon based fuels produce particles that are predominantly
carbon (soot)
Enhanced soot deposition (acoustic soot agglomeration) is a phenomenon whereby the
(in many cases, the nature of soot deposition on certain surfaces of typical single or multiple station smoke alarms can show that the smoke alarm sounded or did not sound during a fire.
soot particulate in smoke forms identifiable patterns on such surfaces of the smoke alarm as the internal and external surfaces of the smoke alarm cover near the edges of the “horn” outlet, the edges of and “horn” sound outlets of the interior “horn” enclosures if present, and surfaces of the “horn” disks themselves
Smoke alarms should be taken into evidence when smoke alarm performance may be an issue.
The alarm should be collected as evidence after being photographed in place.
Should not be altered by applying power, removing or inserting batteries or pushing the test button.
Alarms still on the wall or ceiling should be secured intact with mounting hardware, electrical boxes, and wired connections. Removing ____ with the alarm may be needed to preserve the condition of the alarm and all electrical power connections.
a section of wall material
A _____ is a distinct and visible fire effect generally apparent on non-combustible surfaces after combustible layers (such as soot, paint, and paper) have been burned away.
Clean Burn.
Clean burn patterns produced by burning away of soot can be produced by
direct flame contact or intense radiated heat.
_____ is used by fire investigators to describe numerous chemical and physical changes that occur in gypsum wall board surfaces during a fire.
Calcination
Calcination involves a chemical change of the gypsum to another mineral,
anhydrite
The deeper the calcination into the wallboard the
greater the total amount of heat exposure (heat flux and duration)
Fire rated gypsum wall board contains ______ or ______ particles embedded in the gypsum to preserve the strength of the wall board during fire exposure.
Mineral fibers or vermiculite particles
The fibers add strength to the wall board even after it has been thoroughly calcined
Glass that has received an impact will exhibit a characteristic ______ pattern.
Cobweb
The cracks will be in straight lines and numerous. The glass may have been broken before during or after the fire.
_______ is a term used to describe a complicated pattern of short cracks in glass. These cracks may be straight or crescent-shaped. and may or may not extend through the thickness of the glass.
Crazing
A result of rapid cooling.
______ glass fragments are more uniformly shaped than the complicated pattern of short cracks of crazing.
Tempered
Incandescent lightbulbs can sometimes show the direction of heat impingement. As the side of the bulb facing the source of heating is heated and softened, the gases inside the bulb can begin to expand and bubble out the softened glass.
1
The bulged or pulled portion of the bulb will be n the direction of the source of the heating.
In evaluating a distorted light bulb, the investigator should
be careful to ascertain that the bulb has not been turned in its socket or that the socket itself has not turned as a result of coming loose during or after the fire.
Oily substances, which do not mix with water, float and create interference patterns on the surface of water. This results in a _____ appearance.
“rainbow” or “sheen”
Although ignitable liquids will create a rainbow effect, the observation of a rainbow effect should not be interpreted as an indication of the presence of ignitable liquids unless confirmed by a lab analysis.
Body fat can melt and burn as a
liquid fuel.
Muscle can change shape, char and burn. Heat causes dehydration and shortening of tendons and muscles. Bulkier flexor muscles, such as the biceps in the arms and quadriceps in the legs, shorten and contract, causing a body position known as the
pugilistic posture.
Shorter muscles of the torso cause
arching of the neck and back.
The _____ posture is a common post mortem response of muscle to heat and is not indicative of a behavioral response to events prior to or during the fire.
Pugilistic
Other responses by the body to heat could be criminal attempts to restrain the body.
The skull can exhibit a _______ appearance regardless of the presence or absence of pre existing traumatic injury.
Fragmented or fractured
A ______ is the visible or measurable physical changes or identifiable shapes formed by a fire effect or group of tire effects.
Fire pattern
______ are the underlying data that are used by the investigator to identify fire patterns.
Fire effects
_______ are the borders defining the differences in certain heat and smoke effects of the fire on various materials. They appear between the affected area and adjacent, less affected areas.
Lines or areas of demarcation
