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3. Fire Dynamics

NFPA 1033, 2014 edition

1.3.7* The investigator shall have and maintain at a minimum an up-to-date basic knowledge of the following topics beyond the high school level : (1) Fire science (3) Thermodynamics (5) Fire dynamics (7) Computer fire modeling (11) Fire investigation technology (13) Failure analysis and analytical tools

Topics you should know, and be prepared to answer

questions about

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1. Ignition 2. Flames 3. Compartment fires 4. Fire pattern interpretation 5. Ventilation controlled v

fuel controlled fires 6. Computer fire modeling

Ignition

For ignition to occur, the substance must first be capable of propagating self-sustained combustion. Ignition is defined as the process by which this propagation begins.

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Heat generation rate > Heat dissipation rate

Reaction rate increases as temperature increases.

Heat breaks chemical bonds Produces gases and vapors

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Piloted ignition temperatures for solids are generally in the range of 250-400 °C. (480-750 °F)

Auto ignition temperatures generally exceed 500 °C. (930 °F)

The rate of heat generation is controlled by the evolution of volatiles, and a more or less stable flame is created.

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-Thickness -Density -Specific heat capacity

For thin (<2mm) materials, when the target fuel will ignite

depends on :

-Conductivity (k) -Density (ρ) -Specific heat capacity (c)

For thick materials, when the target fuel will ignite depends

on :

k ρ c (kay-row-see)

Thermal inertia

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Lower Ignition Temperature -Poor conductors -Low density solids -Solids with a high heat capacity

1.Polyurethane foam cushion 2.Polyethylene trash can 3.Particle board desk

Critical Heat Flux: A threshold value, below which ignition will not occur. (kW/m2)

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Ignition occurs more easily in mixtures of vapors or dusts in air than on solids.

-Fuel air mixtures just slightly above stoichiometric ignite most easily. ~0.2 mJ for hydrocarbons ~0.01 mJ for hydrogen ~10-500 mJ for dust clouds, depends mostly on particle size

Special cases of ignition

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Spontaneous Ignition

Oily Rags

-Possible in residences undergoing construction or renovation -Common in restaurants, heath spas, and establishments that launder rags, towels and linens from restaurants and health spas.

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Oily Rags

-Usually requires ambient temperature > 80 °F -Often occurs in clothes dryers after they shut off -Usually requires 5 T-shirts

Oily Rags

-Vegetable oils -NOT petroleum oils

Spontaneous Ignition

•  Not instantaneous •  Generally preceded by much generation of smoke •  Minutes, hours or days

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88 hours!

Minck, C., How to Safely (and not so Safely) Dispose of Oil Soaked Rags, Fine Woodworking, May 2005

Chemical Ignition

• Pyrophoric metals • Lithium, sodium, potassium • Swimming pool sanitizers

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Cal Hypo Reactivity •  .

Flames

A flame is a luminous zone of burning gases or vapors and fine suspended matter where combustion is taking place.

Flames

Premixed or diffusion

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Flames

The flames of interest to fire investigators are almost always diffusion flames.

Diffusion Flames

In a diffusion flame, fuel gases or vapors combine with oxygen in a reaction zone because of differences in concentration.

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Diffusion Flames

• Laminar or turbulent. • Laminar flames are easier to study. • Any flame taller than a foot is likely to be turbulent.

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Flame Temperature

•  Depends on the oxygen supply •  Premixed > diffusion •  Diffusion flames are cooler because of reduced oxygen supply

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Flame Temperature

No flame ever reaches its published “adiabatic flame temperature.”

Flame Temperature

• ~1000-2200 °F (600 to 1200 °C). • ~500-600 °C in the upper layer for flashover

Flame Temperature

In ordinary combustion, flame

temperature depends on ventilation, NOT on

what is burning!

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Flame Temperature

The temperature of a well-ventilated wood fire is no different (and maybe higher) than the temperature of a well-ventilated gasoline fire.

NFPA 921 92-98 Edition Fire Patterns

•  4-8.1 Wood and gasoline burn at essentially the same flame temperature. The flame temperatures achieved by all hydrocarbon fuels (plastics and ignitable liquids) and cellulosic fuels are approximately the same, although the fuels release heat at different rates.

NFPA 921 2001 Edition Fire Patterns

•  4.8.1 Wood and gasoline burn at essentially the same flame temperature. The turbulent diffusion flame temperatures achieved by all hydrocarbon fuels (plastics and ignitable liquids) and cellulosic fuels are approximately the same, although the fuels release heat at different rates.

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NFPA 921 2004-08 Edition Fire Patterns

•  6.2.2.2 Wood and gasoline burn at essentially the same flame temperature. The turbulent diffusion flame temperatures achieved by all hydrocarbon fuels (plastics and ignitable liquids) and cellulosic fuels are approximately the same, although the fuels release heat at different rates.

June 2009 Fire Report "...This was a very hot fire as all of the

aluminum melted. I can not think of anything inside the cab that would burn that hot. In my opinion this is a suspicious fire. ...The fire department did not investigate. I asked if they took samples from the floor area and was advised that they had not. All they did was fight the fire and leave.

June 2009 Fire Report

...The fire fighter agreed with me that it seemed like a very hot fire without some flammable being used and also agrees that there is not likely anything inside the cab that would fuel a fire of this nature."

Don’t be this guy!

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NFPA 921 2011 Edition Fire Patterns

•  6.2.2.2 Wood and gasoline burn at essentially the same flame temperature. The turbulent diffusion flame temperatures achieved by all hydrocarbon fuels (plastics and ignitable liquids) and cellulosic fuels are approximately the same, although the fuels release heat at different rates. Burning metals…

NFPA 921 2011 Edition Fire Patterns

Burning metals and highly exothermic chemical reactions can produce temperatures significantly higher than

those created by hydrocarbon- or cellulosic-fueled fires.

Flammability • “Flammability” encompasses many different properties. • Many tests have been designed to measure these properties. • You might be interested in:

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Flammability

• Energy required for ignition, mJ • Critical heat flux, kW/m2 or W/cm2

• Flash point, °F or °C • Burning rate or mass loss rate, g/s • Energy content, J/g, Btu/lb, Btu/ft3

• Heat release rate, kW or MW

Flammability of Gases,

Vapors and Dusts • Energy required for ignition, mJ (Difficult to determine but many literature values are available.)

Flammability of Liquids

•  Flash point, °F °C • “Flammability” per ASTM D3065 • Limits of flammability • Auto ignition temperature (Difficult to determine but many literature values are available.)

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Flammability of Solids • Flame spread rating (0-200+) • Mass loss rate (g/s) • Heat of combustion (J/g) • Heat release rate (kW) • Auto ignition temperature (Difficult to determine but many literature values are available.) (°F or °C)

Flame Spread

• Takes place by one of two mechanisms: • Radiation • Surface burning

Flame Spread by Radiation Influenced by: •  Flame to target distance •  Angle of incidence (view) •  Presence or absence of a “pilot” •  Inherent properties of the fuel •  Orientation of the fuel •  Location of the flame on the fuel

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Flame Spread by Radiation

For cellulosic materials, the critical radiant heat flux is: ~12 kW/m2 piloted ~29 kW/m2 auto ignition

Flame Spread by Surface Burning

• A “succession of ignitions” • The result of heating the material ahead of the flame front to its piloted ignition point

Flame Spread Test ASTM E84 (NFPA 255)

a/k/a Steiner Tunnel Test

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Flame Spread Test ASTM E84 (NFPA 255)

Flame Spread Rating

•  Asbestos board is 0 •  Red oak is 100 •  It takes 5 to 6 minutes for the flame to

travel 25 feet on red oak •  Class A (BOCA/UBC Class I) 25 or less •  Class B (BOCA/UBC Class II) 26 to 75 •  Class C (BOCA/UBC Class III) 75 to 200

Flame Spread Test Radiant Panel Test, ASTM E648 (NFPA 253)

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Flame Spread Test Radiant Panel Test

Flame Spread Test Radiant Panel Test

Flame Spread Rating for Flooring

• Class I 45 kW/m2

• Class II 22 kW/m2

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Ease of Ignition

•  First fuel ignited is a very important part of most origin and cause hypotheses.

• White spruce might have twice the flame spread rating of red oak, but unless it’s in the right form, it might still be very hard to ignite.

Heat Release Rate

• How much energy does the fuel have?

• What are the units of energy content?

•  Joules per gram •  In the old days is was Btus per

pound. 8,000 for wood, 16,000 for gasoline and plastics.

Heat Release Rate

• How much energy does the fuel have? (J/g)

• What is the mass loss rate? • What are the units of mass loss

rate? • Grams per second

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Heat Release Rate

joules per gram X grams per second

Heat Release Rate

joules per gram X grams per second

Heat Release Rate

joules per gram X grams per second

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Heat Release Rate

joules per gram X grams per second

Heat Release Rate

Equals joules per second (J/s)

a/k/a

Heat Release Rate

and Energy Release Rate

mean the same thing

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Energy Content

Energy Release Rate

Energy Release Rate

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Energy Release Rate

Energy Release Rate

Energy Release Rate of Common Fuels

Cigarette, not puffed 5 W

Match, wooden 80 W

Newspaper, folded, bottom ignition 4 kW

Newspaper, crumpled double sheet, top ignition 7.4 kW

Newspaper, crumpled double sheet, bottom ignition

17.4 kW

Plastic wastebasket, with 12 milk cartons 50 kW

Plastic trash bag, filled with paper trash 120-350 kW

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Energy Release Rate of Common Fuels

1 ft2 pool of gasoline or kerosene 300 kW*

Cotton upholstered chair 300-400 kW

Polyurethane upholstered chair 1350-1990 kW

Polyurethane mattress 800-2600 kW

Polyurethane upholstered sofa 3000 kW (3 MW)

Furnished living room 4-8 MW

Compartment Fires

Fuel packages behave differently inside structures

(compartments) than they do when they are

unconfined.

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The simplistic explanation that “fire burns up and out” is not only simplistic, it

is false and misleading!

Free Burning Stage

Hot Gas Layer Forms

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Radiation Sends Heat DOWN

Flashover Occurs, Igniting Everything

Full Room Involvement

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New in 2017

Flashover Demonstration Cell

Unaccelerated Fire

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Ignition97

2minutes 98

3minutes 99

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3min15secs100

First Task Damage Assessment

Uniformity is not a criterion! Uniformity is fleeting!

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First Task Damage Assessment

Did the fire ventilate through doors or windows? (auto-ventilation)

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First Task Damage Assessment

Are the baseboards charred?

Pattern Generation Plume patterns: Truncated cones Confinement patterns: Horizons Movement and Intensity patterns Ventilation generated patterns Irregular patterns

Pattern Generation

Plume patterns: Truncated cones

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Pattern Generation

Confinement patterns: Horizons

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Pattern Generation

Movement and Intensity patterns

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Pattern Generation

Penetrations through floors.

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Pattern Generation

500 ml gasoline & kerosene on vinyl

500 ml gasoline & kerosene on carpet

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5 liter gasoline & kerosene on carpet

1 liter gasoline on carpet

500 ml gasoline on plywood

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500 ml gasoline on plywood

500 ml kerosene on plywood

500 ml kerosene on plywood

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Herndon fire patterns

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Willingham “Pour Pattern”

FIRE DYNAMICS AND FORENSIC ANALYSIS OF LIQUID FUEL FIRES

Final Report Grant No. 2008-DN-BX-K168

Prepared by: Christopher L. Mealy, Matthew E.

Benfer, and Daniel T. Gottuk

Pattern Generation

You do not see what you are

not looking for.

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You see what you expect to see

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Pattern Generation

Ventilation generated patterns

HRR 12 seconds post flashover

150 kW/m2

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O2 content 12 seconds post flashover

A floor jet?

8” above floor

O2 content 8 seconds pre flashover

Pattern Generation

Clean Burn

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Pattern Generation

Clean Burn

Pattern Generation

…when the soot and smoke condensate that would normally be found adhering to the surface is burned off. (NFPA 921)

Pattern Generation

Or maybe not.

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Pattern Generation

It may be that the “clean burn” area never accumulated any soot because it was too hot for the soot to condense.

Pattern Generation

Pattern Generation

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Pattern Generation

Pattern Generation

Pattern Generation

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Pay Attention to New Research

Carman Cox and Pijaca

Origin Matrix Analysis

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CONSIDERATIONS FOR ANALYSIS 1. Regardless of where a fire starts, post-flashover compartment fire conditions will produce ventilation induced exposures and corresponding damages in areas associated with fresh air supply vents such as doors and windows.

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2. Damages generated during post-flashover compartment fire conditions offer little insight as to where a fire originated. Such damages are only associated with the ventilation openings that provided fresh air.

3. Investigators must identify damages likely to have been generated due to post-flashover ventilation induced exposures, and then differentiate those damages from ones that may have been generated during pre-flashover fire conditions.

4. Damages that cannot be attributed to post-flashover ventilation induced exposures must be systematically evaluated in any credible origin analysis.

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CFITRAINER.NET

1. Postflashover Fires 2. A Ventilation-Focused Approach to the Impact of Building Structures and Systems on Fire Development

Pattern Generation

Irregular patterns

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Pattern Generation

Electrical patterns

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Fire Modeling

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Fire Modeling Three types:

Hand modeling Simplified equations

found in CFI Calculator

Fire Modeling Hand modeling

Babrauskas Thomas MQH

(McCaffrey, Quintiere, Harkleroad)

Fire Modeling Hand modeling Zone Modeling

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Fire Modeling Hand modeling Zone Modeling

Divides the room into an upper layer and a

lower layer CFAST

Fire Modeling

Hand modeling Zone Modeling Field Modeling

Fire Modeling Field Modeling

Divides the room into many small cubes

FDS (Fire Dynamics Simulator)

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Fire Modeling Requirements

Hand models-calculator Zone models-computer

Field models-computer and lots of time

Fire Modeling Requirements

Hand models-calculator Zone models-computer

Field models-computer and lots of time

All models-knowledge

Fire Modeling Requirements

Hand models-calculator Zone models-computer

Field models-computer and lots of time

All models-knowledge www.fire.nist.gov

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Fire Modeling Disclaimers NIST: The software package is a computer model that may or may not have predictive capability when applied to a specific set of factual circumstances. Lack of accurate predictions by the model could lead to erroneous conclusions with regard to fire safety all results should be evaluated by an informed user.

Fire Modeling Disclaimers CFI Calculator: While these equations can be very useful in hypothesis development and testing, it is important to recognize that these formulas do have limitations and are appropriately utilized to bound a problem, defining a “fence” around the realm of possibilities. …

Fire Modeling Disclaimers This tool is designed to be utilized by those individuals that have a sound knowledge of fire dynamics and fire dynamics equations and the use of this tool may require referencing additional scientific literature or consulting outside experts

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Fire Modeling

Station Nightclub World Trade Center

Experiments Demonstrations

Fire Modeling

The answers were in the back of the book!

Comparison of predictions

vs. measured values

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Pred

ictio

n

Measurement

Salley, et al. NRC 2007: Hand calculations: Hot gas layer temp + 25%

Salley, et al. NRC 2007: Hand calculations: Hot gas layer temp + 25% Heat flux + 60%

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Salley, et al. NRC 2007: CFAST, FDS: Hot gas layer temp + 13% Heat flux + 30%

Pred

ictio

n

Measurement

Predicted Measured

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This may be a sound basis for designing buildings and fire control systems

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This is NOT a sound basis for recreating historical events

Or for sending folks to prison!

2006 Dalmarnock Study

•  10 teams of modelers •  8 used FDS4, 2 used CFAST 2000

edition •  Provided with far more data than is

available to after-the-fact modelers •  Predicted time to flashover, upper layer

gas temperature and other parameters

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2006 Dalmarnock Study

“The accuracy to predict fire growth (i.e. evolution of the heat release rate) is, in general, poor.”

2006 Dalmarnock Study

“I always avoid prophesying beforehand because it is much easier to prophesy after the event has already taken place.” -Sir Winston Churchill

Conclusions

Ignition is the process by which self-sustained combustion begins. Vapors or dust clouds are more easily ignited than solids. A flame is a luminous volume where combustion occurs. Flames can be either premixed or diffusion, laminar or turbulent.

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Conclusions Flammability can describe the ease of ignition, the rate of fire spread, the heat release rate, the mass loss rate, or other properties related to the fire behavior of a fuel.

The energy released by a burning fuel depends on the chemical makeup, form, orientation with respect to the fire, and location within an enclosure.

Conclusions Compartment fires behave differently than the free burning fires with which most people are familiar. This behavior is (more or less) reproducible and can be described mathematically to predict the effects of the fire.

Conclusions

Just because a phenomenon can be (approximately) described mathematically, that is not a guarantee that the numbers used will accurately describe a particular event.

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Conclusions

If you don’t figure out the ventilation, you won’t figure out the fire!

Review

What is the name of the process by which self-sustaining combustion begins?

Review What kind of flame is typically found on a candle? Premixed or diffusion? Turbulent or laminar?

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Review What units are used to describe a fire’s heat release rate?

Review What units are used to describe radiant heat flux?

Review In the absence of compelling evidence, what is the most that can be said about a fire in a room that burns for 15 minutes beyond flashover?

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