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Fire Behavior. Photo: The Daily Galaxy. Fire Behavior. CPBM Objectives (chapter 8) Identify fire behavior terms Explain the fire triangle Discuss the major elements of the fire environment List and explain the three methods of heat transfer

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Fire behavior
Fire Behavior

Photo: The Daily Galaxy


Fire behavior1
Fire Behavior

  • CPBM Objectives (chapter 8)

    • Identify fire behavior terms

    • Explain the fire triangle

    • Discuss the major elements of the fire environment

    • List and explain the three methods of heat transfer

    • List fuel characteristics which govern combustion


Fire behavior2
Fire Behavior

  • CPBM Objectives (chapter 8)

    • Identify Fuel Models and examples in Florida

    • Explain the difference between fire intensity and severity and how both can be regulated and measured

    • Define residence time and why it is significant in Rx fire

    • Discuss indicators of erratic or potentially erratic fire behavior


Parts of a fire
Parts of a Fire

SPOT FIRE

FINGER

HEAD

Wind

POCKET

LEFT FLANK

UNBURNED

ISLAND

RIGHT FLANK

REAR


Types of fires
Types of Fires

  • Surface Fire

    • Burning in surface fuels

      • Grass, shrubs, litter

  • Ground Fire

    • Smoldering in ground fuels

      • duff, peat, roots, stumps

  • Crown Fire

    • Burning in aerial fuels

      • Crowns or canopy of the overstory

      • May or may not be independent of surface fire

Photo: Univ. of Toronto Fier Lab

Photo: News Provider


Some other fire behavior terms
Some other fire behavior terms

  • Spotting – burning or glowing embers being transported in the air.

  • Torching – Movement of fire from the surface to the crowns of individual trees.

  • Flare Up – A sudden increase in ROS and Intensity.


What is fire
What is fire?

Energy release in the form of heat and light when oxygen combines with a combustible material (fuel) at a suitably high temperature

The Fire Triangle

Fuel Oxygen

Heat


Combustion general process
Combustion: General Process

  • Photosynthesis: converts radiant energy to stored chemical energy (CO2 + H2O ---light-----> C6H12O6 + O2).

  • Combustion: reverses photosynthesis

    (C6H12O6 + O2 ---high temperature-----> H2O + CO2 + heat and light)

    (fuel) (325 C for wood)

    • Same process as decay and decomposition

    • Begins with endothermic reaction, becomes exothermic

    • Produces thermal, radiant and kinetic energy

  • Extinction: insufficient heat to sustain combustion


Flaming

Glowing

Smoldering

Pre-Ignition

4 Phases of Combustion


Phases of combustion

Pre-Ignition

Phases of Combustion

  • Pre-ignition

    • Requires heat/energy input to increase surface temperature >200˚C

    • Dehydration

    • Volatilization of waxes, oils, other extractives

    • Pyrolysis (chemical decomposition of organic matter without Oxygen– inside fuels, emits volatiles)

    • Volatiles either condense into particles (smoke) or are consumed during flaming combustion


Phases of combustion1
Phases of Combustion

  • Ignition

    • Transition to flaming combustion: gases released by pyrolysis ignite

    • Surface temperatures around 320 C (600F)

    • Heat released by combustion brings other fuels to ignition


Phases of combustion2

Flaming

Phases of Combustion

  • Flaming combustion

    • Surface temperatures 200- 500˚ C

    • Combustible volatiles ignite above surface, creating flame: the GASES are burning, not the fuel itself.

    • Combustion occurs in zone above fuel surface

    • Oxidation produces: heat, CO2, H2O and incompletely degraded organic compounds

    • Smoke includes these + other gases which condense or reform above flame zone


Phases of combustion3

Smoldering

Phases of Combustion

  • Smoldering

    • No visible flames

    • Surface temperatures < 500 C

    • Carbon buildup on surface reduces gas production that would maintain flame

    • Occurs when fuels tightly packed

    • Surface char oxidizes to CO2, H2O, ash

    • Continued oxidation of other compounds

    • Smoldering duff and ground fires raise soil temperature and can kill roots

    • Large quantities of smoke


Smoke constituents
Smoke Constituents

  • A result of incomplete combustion

  • Major constituents

    • Particulate matter

      • Solid or liquid particle suspended in atmosphere

      • Condensed hydrocarbons and tar materials

      • Entrained fragments of vegetation and ash

    • CO2 and CO

    • H2O

    • Gaseous hydrocarbons

  • Smoke/volume burned increases for:

    • Low intensity fires in moist or living fuels

    • High rates of spread (& less efficient combustion)


Glowing

  • All volatiles have already been driven off, oxygen reaches the combustion surfaces, and there is no visible smoke (products are CO2 and CO)

  • Oxidation of solid fuel accompanied by incandescence

  • This phase follows smoldering combustion, continues until temperature drops or only non-combustible ash remains


Heat transfer processes
Heat Transfer Processes

  • Radiation

    • For example, the sun, and your hand…

    • Electromagnetic waves transfer heat to fuel surface only

    • Accounts for most drying and heating of fuel surfaces ahead of flame or on opposite steep slopes– radiates in all directions

  • Radiation

    • For example, the sun, and your hand…

    • Electromagnetic waves transfer heat to fuel surface only

    • Accounts for most drying and heating of fuel surfaces ahead of flame or on opposite steep slopes– radiates in all directions


Heat transfer processes1
Heat Transfer Processes

  • Convection

    • Vertical (or other direction) movement of gas or liquid, as heat rises

    • Heats plant foliage above surface fires and fuels ahead of the flame on steep slopes, or if wind driven

    • Carries firebrands away from fire; spotting potential

    • Can create enormous columns and drive fire behavior

  • Convection

    • Vertical (or other direction) movement of gas or liquid, as heat rises

    • Heats plant foliage above surface fires and fuels ahead of the flame on steep slopes, or if wind driven

    • Carries firebrands away from fire; spotting potential

    • Can create enormous columns and drive fire behavior


Heat Transfer Processes

  • Conduction

    • Transfer by molecular activity within a solid object

    • Primary method for raising temperatures within large fuels

    • Occurs between objects/fuels that are in contact

    • Transfers heat in dense fuels, requiring additional heat to reach ignition


Measures
Measures

  • Rate of spread (ROS): rate at which fire front advances through forest fuel (ft/sec, chains/min)

  • Residency Time: Duration for flaming combustion to pass a specific location.

  • Flame Length & Depth

Residency Time = Flame Depth/ROS


Measures1
Measures

  • Intensity – rate of heat energy during combustion

    • Reaction intensity: per unit area (BTU·ft-2·min-1)

    • Fireline Intensity: per unit length of the fire front

      (BTU·ft-1·min-1)

I = h·w·r

I fireline intensity

h fuel heat content

w weight of fuel consumed per unit area

r rate of spread

  • *Flame Length is a good estimate of intensity


Measures2
Measures

  • Severity: Impact of fire on the environment

    • Plants, animals, soils, water

HIGH

Backing fire in long unburned longleaf pine

Stand replacing fire in mixed conifer forests

SEVERITY

Head fire in frequently burned longleaf pine

Chaparral Brush Fires

LOW

LOW

HIGH

INTENSITY



Fuels
Fuels

  • Surface Fuels

    • Grasses

    • Shrubs

    • Litter (leaves)

    • Woody debris


Fuels1
Fuels

  • Ground Fuels

    • Duff (partially decomposed)

    • Peat

    • Roots

    • Stumps

litter

fermentation layer

Duff

humus

mineral soil


Fuels2
Fuels

  • Aerial Fuels

    • Crown or canopy of overstory

  • Ladder Fuels (located between crown and surface fuels)

    • Smaller trees

    • Vines


Fuel characteristics
Fuel Characteristics

  • Size and Shape

    • Surface area:volume ratio

      • Grasses

      • Palmetto

      • Branches

      • Logs

1000:1

40:1

  • Particle Density


Fuel characteristics1
Fuel Characteristics

  • Heat Content (stored energy)

    • 6,000-12,000 BTU/lb

  • Fuel Chemistry

    • Volatile oils

  • Mineral Content

    • Dampening effect on combustion


Fuel characteristics2
Fuel Characteristics

  • Fuel Arrangement

    • Vertical

      • Grasses & shrubs

    • Horizontal

      • Litter

      • Downed woody debris


Fuel characteristics3
Fuel Characteristics

  • Fuel Loading

    • By size classes

  • Compactness

    • Bulk density (fuel load/fuelbed volume)

    • Packing ratio (fuelbed density/particle density)

  • Continuity

    • Vertical

    • Horizontal

ALL FUELBED PROPERTIES


Fuel characteristics4
Fuel Characteristics

  • Fuel Moisture Content (FMC)

    • Large dampening effect on combustion

    • Heat sink

      • FMC changes hourly, daily, and seasonally!

Fuel Moisture Content (%) = (Water Weight / Dry Fuel Weight) x 100


Fuel moisture content
Fuel Moisture Content

  • What influences FMC

    • In Dead Fuels

      • Precipitation (amount and duration)

      • Temperature

      • Relative humidity

      • Wind


Fuel moisture content1
Fuel Moisture Content

  • Equilibrium Moisture Content

    • For a given temperature and RH dead fuel will reach a FMC at equilibrium.

    • Environmental conditions are not constant

    • Fuel is constantly changes FMC to reach EMC

    • The lag time to reach EMC depends on particle size


Fuel moisture content2
Fuel Moisture Content

  • Timelag categories for dead woody fuels

Timelag, or “response time”, is the time it takes for 63% of the change to occur between one EMC and a second EMC when a fuel in equilibrium with a stable environmental condition is suddenly exposed to a different stable environmental condition.


Fuel moisture content3
Fuel Moisture Content

  • Small diameter fuels react quickly to hourly and daily changes.

    • Important to monitor.

  • Large diameter fuels react more to seasonal changes

    • California versus Florida?

  • Fine fuels drive fire behavior


Fuel moisture content4
Fuel Moisture Content

  • Moisture of Extinction

    • Dead: 12-40%

    • Live: >120%

  • Available Fuel


Fuel moisture content5
Fuel Moisture Content

  • Florida Fine Fuel Moisture Calculation Chart

  • http://www.fl-dof.com/wildfire/rx_training.html#cbc


Fuel moisture content6
Fuel Moisture Content

  • Live Fuels

    • FMC can be much higher than dead fuels (100-300%)

    • Influenced by:

      • Drought (KBDI)

      • RH

      • Wind

*Ignition of live fuels may largely depend the combustion characteristics of other fuels (e.g. dead surface fuels).


Fuel moisture content7
Fuel Moisture Content

  • Duff Moisture

    • Very dry to very moist

    • <30% FMC duff can burn on its own

    • Potential for tree mortality in burning long unburned forests

    • May smolder for long durations

    • May cause lots of smoke


Weather
Weather

  • FMC

  • Wind

    • Increases O2

    • Bends flames

    • Increases ROS

    • Dries fuels

wind

convection

radiation

conduction


Topography
Topography

  • Slopes

    • Similar effect as wind

    • Bends flames

    • ROS higher upslope

      Slope Position

      top, middle, bottom


Topography1
Topography

Aspect


Topography2
Topography

  • Other topographic features

    • Valleys

    • Box Canyons

    • Steep draws

    • Elevation


Topograpy
Topograpy

ELEVATION


Erratic fire behavior
Erratic Fire Behavior

  • Indicators (on a Rx burn)

    • KBDI>500

    • FMC (fine) <7%

    • RH<30%

    • Cold front approaching

    • Gusty winds

    • Dust devils/fire whirls

    • Just inland from seabreeze

    • Well-defined convection column

    • Thunderstorms

    • Spotting

    • DI approaching 70


Can we predict fire behavior
Can we predict fire behavior?

  • Fire Behavior Prediction Models (e.g. BehavePlus)

  • INPUTS OUTPUTS

    Fuel characteristics Rate of Spread

    FMC Fireline Intensity

    Slope Flame Lengths

    Wind and more…


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