physical description of forest fires - ecmwf · september 15, 2009 physical description of forest...

September 15, 2009 Physical description of forest fires

ESF Exploratory WorkshopImproved Quantitative Fire Description With Multi-

Species Inversions Of Observed PlumesFarnham Castle, 14-16 September 2009

Albert Simeoni

UMR 6134

CNRS UMR 6134 – Univerità di CorsicaBP 52 - 20250 Corte, France

+33 495 450 638 – [email protected]://spe.univ-corse.fr



Physical description ofForest Fires




1

Introduction

• Physical description of forest fires aim to understand the behaviour of forest fires and predict their spreading

• This research started 50 years ago in Australia, USA and USRR

• The involved mechanisms are very complex

• The operational objectives are to predict the fire spread and to estimate the impact of the fire

The predictions are done thanks to fire spread models inserted in simulators

The impacts are estimated thanks to modelling and experiment




Fire scale (km)Plot scale in field experiments (100 m)

Vegetation species (20 cm to 20 m)Fuel particule (mm to cm)

Reaction (m)

Wind (O2)

Combustion

Difficulty to model the time

scales

Difficulty in modelling the huge variable variability (fuel gases…)

Fire Spread Modelling

Where does the difficulty in the modelling of forest fire lay? The Physical laws are knownGIS and weather models provide the Environmental data

Difficulty to model the

space scales

Wind, Topography

Heat transferts

2




Three kinds of modelling:

• Empirical (Mc Arthur, 1966; simulator = Fire grass meter)

• Semi-empirical (Rothermel, 1972; simulator = Farsite or Behave)

• Physical

• Simplified (Albini, 1981; simulator in development by our team)

• Detailed (Grishin, 1996; simulator = Firetec or Firestar)


The problems in the modelling of forest fires are due to the numerical solving of mixed complex problems at different scales

3




Drawbacks: less accurate and less general than detailed models Advantages: possibility to implement them at the field scale

- Simple models that only take into account the main mechanisms involved in fire spreading

Drawbacks: small times and lengthsAdvantages: detailed description of the phenomena

- Detailed models that describe as finely as possible the mechanisms

Two ways to describe fire spreading with physical modelling:


Knolwedge and prevention

Simulation

4




Gas Phase

• Convection • Turbulence• Gas mixture• Combustion• Radiation• Soot

Solid Phase

• Heat transfer • Drying• Pyrolysis• Combustion• Radiation

Transfers

MassHeat

MomentumEnergy


Detailed models – A multiphase model

Fuel layer

Gas phaseControl volume

Solidphase

5




For each phase: Mass, species, momentum and energy balances

Interface relationships

Radiative Transfer Equation

Sub-models

Example – Mass balance:

kgkggggg MV

t

.

Example – Arrhenius type laws

prksurfkkk MM

t

0.. LaLadSenLgLe gggggk p S

gggg

pk

prksurfkgk MMM

Detailed models – A multiphase model


Example – Interface equation for mass:

6




Radiation

Heat release

Losses

Advection

Radiation Radiation

Assumptions:

•One single equivalent phase

• Assumed flame geometry


Medium equivalent to the fuel

Burned area Unburned areaCombustion area

Simplified models – A semi-Physical model

7




Rt

QTKTTkTVktT k

agv

)(.

Energy balance

t

Vx

VxV k

g

zgg

g

xgxg

1,,,

Mass conservation

vS

flvfl

flflvdvfl dSdSr

TBafl

24 coscos

Radiative transfer

sla

zg gTTV cos12,

Buoyancy

aag TT Perfect gas


8




Meeting “Combustion Theory and modelling”Institute of Physics – Combustion Physics Group

London, 24 June 2009

9





410June 24, 2009 Theoretical and Experimental Tools for Combustion in Forest Fires





burnedunburnedempty

Modelling Heterogeneity – Non flammable areas


burnedunburnedempty

11




12

Experiments under controlled conditions (heat flux, flow)

BRE Centre for FireSafety Engineering

Fire Impact

0

2

4

6

8

10

12

0 50 100 150Time (s)

HR

R (k

W)

0% - Ph26% - Ph63% - Ph0% - Pp26% - Pp63% - Pp

Fire intensity:

0

500

1000

1500

2000

2500

0 50 100 150 200 250 300Time(s)

CO

2 (p

pm)

-5

0

5

10

15

20

25

30

CO

(ppm

)

Exhaust CO2 Molar FractionExhaust CO Molar Fraction

Fire emissions:




Smouldering

Fire Impact

13




Fire Spread and Severity

0 200 400 6000

20

40

60

80

100 COCO2

Yiel

d(%

)

Moisture content (%)20 30 40 50 60 70 80

0

20

40

60

80

100 COCO2

Yiel

d(%

)

Heat flux (KW/m²)

Emissions

Fire Impact

BRE Centre for FireSafety Engineering

Smouldering

14




Fire Impact

Thermal impact (but also influence on vegetation and wind on fire spread)

15




0

1000

2000

3000

4000

5000

6000

7000

8000

0 50 100 150 200 250 300 350 400time (s)

heat

flux

(W/m

²)

1st spread - 5 m1st spread - 10 m2d spread - 5 m2d spread - 10 m

Limits of exposure

Firefighter: 7000 W/m² (90 s)

Unprotected person: 6400 W/m² (8 s)

Fire Impact

16

Thermal impact (but also influence on vegetation and wind on fire spread)




17

Conclusions and perspectives

• Physical modelling of forest fires is a young science

• Two kind of tools arte developed:

• Fire spread simulation

• Fire impact estimation

• Many improvements are necessary but simple answers can be provided

• A strong perspective is the coupling between fire and atmosphere:

• Integration of satellite data to evaluate the fire

• Using fire simulations to detect fires

• Characterize the fire as a source of gases




Albert Simeoni

UMR 6134

CNRS UMR 6134 – Univerità di CorsicaBP 52 - 20250 Corte, France

+33 495 450 638 – [email protected]://spe.univ-corse.fr



Physical description ofForest Fires

physical description of forest fires - ecmwf · september 15, 2009 physical description of forest...

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