Modeling of Turbulent Wall Fires - Stanford University · 2012. 7. 19. · ¾Impact of smoke layer:...

Modeling of Turbulent Wall Fires

Arnaud Trouvé

Department of Fire Protection EngineeringUniversity of Maryland, College Park, MD 20742 (USA)

• OutlineOverview of Compartment Fire Dynamics

Multi-physics problemRole of wall flames

Laminar Wall FiresAnalytical solution of a canonical wall flame problem – the Emmons problem (non-spreading; constant wall temperature; no thermal radiation)

Turbulent Wall FiresLarge eddy simulation of a canonical wall fire problem (non-spreading; vertical wall)


Compartment Fires

• Example: Station Nightclub Fire, 02/20/03, Rhode IslandSequence of events: music band on stage using pyrotechnics; ignition of the insulation foam lining walls and ceiling around stage; rapid fire spread over dance floor (no sprinkler system); egress blocked bycrowding at main entrance; 100 dead peopleTechnical investigation conducted by the National Institute of Standards and Technology (Final Report published in June 2005)

• Example: Station Nightclub Fire, 02/20/03, Rhode Island

Compartment Fires

• Main features: fire is an unusual combustion process in which the fuel supply corresponds to a large list of flammable objectsand materials, usually in solid or liquid form

solids (wood, plastics, foams, fabric, linings, etc)liquids (engine fuels, LNG, melted solids, etc)

Compartment Fires

Liquid fuel Gaseous fuelSolid fuel FIREFIRE

ENGINE

• Main features: fire is an unusual combustion process in which the fuel supply is unknown

Typical production of flammable vapors in a fire:• Consider a flammable solid object/material that is a potential fuel

source• At ambient temperature, the fuel is in solid form, the oxygen (from

air) in gaseous form, and there is no combustion• At moderately elevated temperatures (typically 200-400 degrees

Celsius), a complex thermal degradation process is initiated in the solid object/material, that corresponds to a phase change and produces fuel in gaseous form. This gasification process is called pyrolysis.

Compartment Fires

HeatGaseousfuel mass

Solid Fuel

Ambient air


Typical production of flammable vapors in a fire:• The fuel gasification rate is determined by a heat feedback

mechanismFuel gasification is an endothermic process and heat comes from the gas-to-solid heat transferThe fuel gasification rate is controlled by the rate of gas-to-solid heat transfer

Compartment Fires

netpyroF qHm ′′≈Δ′′ &&Fm&

Fuel gasificationrate

Heat releaserate fireQ&

Temperaturedistribution

Heat flux tofuel surface

Heat

Gaseousfuel mass

PyrolysisEvaporation


Typical production of flammable vapors in a fire:

Compartment Fires

The gas-to-solid thermal feedback controls the fuel mass loss rate and thereby the overall fire sizeThe fraction of energy fed back to the fuel source is typically a small fraction of the energy released by combustion:

The thermal feedback has 2 components corresponding to convective and radiativeheat transfer

10.001.0 −≈Δ

Δ≈

comb

pyrofeedback H

Hχ

D

radq ′′& convq ′′&

FuelFm& ′′

• Main features: fire is a buoyancy-driven, relatively-slow, non-premixed combustion process

Example: pool fire configurationFuel source velocity is small (a few cm/s)Buoyancy effects accelerate the flow up to several m/s; flow regime corresponds to moderate turbulence intensitiesFlame corresponds to diffusion combustion and to a thin reaction sheet where fuel and air meet in stoichiometric proportionsSlow velocities and long residence times promote soot formation and radiant losses

Compartment Fires

Fuel

DiffusionFlame

air air

Plumeoverfireregion

underfireregion

D

%35~)/( combradrad QQ &&=χ


Example: pool fire configurationFlame height scales with Froude number

Compartment Fires

FFFp

comb

FF

f

stFF

FrdgdTc

QQ

gdTT

ZuFr

~

])()[(

)(

2,

*

2/12/1

2/3

∞∞∞

∞∞

=

Δ=

ρ

ρρ

&&

FFrQ ~*&


Example: pool fire configuration

Compartment Fires

)/( Ff dL

B M

M

1)/(,5,105* >>>> Fff dLFrQ&

B

)1()/(,5,105* OdLFrQ Fff =<<&

Buoyancy-driven flame regime:low velocities, large diameters

Momentum-driven flame regime:high velocities, small diameters

FIRE

ENGINE

• Main features: flow confinement and buoyancy forces lead to the formation of a ceiling-level smoke layer

Compartment Fires

time

Ignition andearly growth

Smokefilling

Quasi-steadyregime


Smoke layer composition: hot combustion products mixed with ambient air; depending on fuel type and combustion conditions, may contain significant amounts of soot

Compartment Fires

Soot particlesProduct of incomplete combustionPhase: solidChemical composition: primarily made of carbon atomsParticle size distribution: from a few nanometers (10-9 m) to several microns (10-6 m)Particles geometry: complex shapes (agglomerates of elementary spherical particles)


Smoke layer depth: depends on fire size and vent flow rates

Compartment Fires

Accumulation of smokenear ceiling

Fast fire growth andsmoke descent to floor

Smoke filling andloss of visibility


Impact of smoke layer: increases (radiation-driven) heat feedback to fuel sources

Compartment Fires

Heat Fuel

Solid Fuel

Upper layer (UL)),,(, ,,, 22 ULvULCOULOHUL fxxT

Flame

{ {

{ { {

{

))()((

))exp(1(

componentsoot

,

component gas

,,

coef absorptionmeanPlanck

lengthbeammean

coef absorptionmeanPlanck emissivity

4

emissivitypower emissive

2222

43421

444444 3444444 21

ULULvsoot

COULCOOHULOHUL

ULULUL

ULULUL

TfC

TaxTaxp

d

TG

+

+=

×−−=

=

κ

κε

σε

ULG

often dominant


Impact of smoke layer: increases heat feedback to fuel sources, therefore increases fuel gasification rate and heat release rate

Compartment Fires

time

Fire spread

Fire growth


Impact of smoke layer: increases heat feedback to fuel sources, therefore increases fuel gasification rate and heat release rate (fire growth and fire spread)Possible transition to flashover (rapid series of ignition events involving all flammable objects/materials present in the fire room)

Compartment Fires

Small fire

Flammableobjects

Flashover !

time


Possible transition to flashover: may trigger in turn a transition to under-ventilated combustion

Compartment Fires

Flames extending out of the compartment of fire origin


Possible transition to under-ventilated combustionFlame location: (1) near the fuel source; (2) near the vents

Compartment Fires

(1) Over-ventilated combustion, Vent

AirFuel

Flame Smoke

(2) Under-ventilated combustionVent

AirFuel

Flame

Smoke


Possible transition to under-ventilated combustionClassical Burke-Schumann problem (1928): 2 possible regimes

Compartment Fires

FuelAir

Walls

Air

Laminar flame-flow configuration

Flameover-ventilated

flame

under-ventilatedflame


Possible transition to under-ventilated combustionSmoke layer composition: (1) products of complete combustion mixed with air; (2) products of incomplete combustion

Compartment Fires

(1) Over-ventilated combustion, Vent

AirFuel

Flame Smoke

(2) Under-ventilated combustionVent

AirFuel

Flame

Smoke

CO2, H2OExcess air

CO2, H2OCO, uHCsoot

Vent

AirFuel

Flame


Impact of smoke layer: air vitiation as the compartment fire system evolves from well-ventilated to under-ventilated combustionAir vitiation reduces the flame intensity and promotes flame extinction

Compartment Fires

Fuel

Flame

Oxidizer∞∞22

, OO TY∞∞FF TY ,

airO

airOO

TT

YY

≥

≤∞

∞

2

22 ,

Air vitiation


Reduced-scale compartment fire experiments (Utiskul & Quintiere)Vent size: variable width and heightFuel pan: variable diameter

Compartment Fires

Heat Flux Gauge

Heat Flux Gauge

Upper Gas Tube

Lower Gas TubeHeat Flux Gauge

Stand to Load Cell with Water Seal

Fuel Pan

Heat Flux Gauge

Center Thermocouple

Top Vent

Bottom Vent

Front Wall Thermocouples

Inner Size: 40x40x40 cm3

Adjustable Back Wall


Reduced-scale compartment fire experiments (Utiskul & Quintiere)Steady under-ventilated fire (flame stabilized at the vents)

Compartment Fires

Vent

Compartment Fires

• Role of wall flamesControl the ignition and fire growth/spread processes

Ignition and earlyvertical spread ofwall flames

Vertical wall flamesimpinging on ceiling

Horizontal spread

Compartment Fires

• SummaryCompartment fires feature rich multi-physics dynamics

Buoyancy-driven, low-to-moderate Reynolds number turbulent flowNon-premixed combustionPyrolysis processesSoot formation/oxidationThermal radiation transport

Wall flames are an essential ingredient of the fire dynamics


Multi-physics problemRole of wall fires




• A variety of configurations:Solid wall material

Chemically inert (e.g., concrete)Flammable (e.g., plastic, wood, fabric)

Wall orientationFloor fire (pool fire)

Wall fire

Ceiling fire

Wall thicknessThermally thin

Thermally thick

Laminar Wall Fires

• A variety of configurations:Propagation

Non-spreading wall flameBuoyancy-driven (u∞ small) or momentum-driven (u∞large) flow

Laminar Wall Fires

px

Gravity g

yx

flx

Free streamvelocity u∞

BoundaryLayer

Flame

Fuel

pxx ≤≤0

flxx ≤≤0

xx fl ≤

• Pyrolysis region

• Inert wall region

• Wall flame region

• Wall plume region

xxp ≤


Spreading wall flame with upward spread (flow-aided, concurrent spread)

Laminar Wall Fires

px

Gravity g

yx

flx

ROS

BoundaryLayer

Flame

Fuelbox


• Inert wall regions



)()( txxtx pbo ≤≤

)()( txxtx flbo ≤≤

xtx fl ≤)(

x(t)x(t)xx pbo ≤≤ and


Spreading wall flame with downward spread (flow-opposed spread)

Laminar Wall Fires

px

Gravity g

yx

flx

ROS

BoundaryLayer

Flame

Fuel

box


• Inert wall regions



)()( txxtx bop ≤≤

)()( txxtx flp ≤≤

xtx fl ≤)(

x(t)x(t)xx bop ≤≤ and

• Emmons problemA canonical boundary layer combustion problem corresponding to a non-spreading laminar wall flame with forced convection (external flow)Assumptions:

Constant and uniform free stream conditionsNo gravityBoundary layer approximationInfinitely fast, single-step chemistry (mixture-fraction based combustion model)Constant wall temperatureNo thermal radiation

H.W. Emmons (1956) “The Film Combustion of Liquid Fuel”, Z. Angew. Math. Mech., 36:1 pp. 60-71.

Laminar Wall Fires

PrOrF ss )1(2 +→+

• Flat (liquid or solid) fuel surface burning in a laminar gaseouscross-flow

Modified Blasius solution for the velocity field:Variable mass densityFinite normal velocity at the fuel surface

Infinitely-fast chemistry

Laminar Wall Fires

Flame

Liquid/solid Fuel

HeatFuel

HeatOxygen

Fuel side

Oxidizer sideFree streamvelocity u∞

0)0,( ≠xv),( yxρ

• Emmons solutionGoverning equations for the flow field

Boundary conditions

)(

0)()(

yu

yyuv

xuu

vy

ux

∂∂

∂∂

=∂∂

+∂∂

=∂∂

+∂∂

μρρ

ρρ

∞=∞=

uxuxu

),(0)0,(

Laminar Wall Fires

• Emmons solutionGoverning equations for the mixture composition

Laminar Wall Fires

combFpp

FsO

OOO

FF

FFF

HyT

yyTvc

xTuc

ry

YD

yyY

vx

Yu

yYD

yyYv

xYu

Δ+∂∂

∂∂

=∂∂

+∂∂

−∂

∂

∂∂

=∂

∂+

∂

∂

−∂∂

∂∂

=∂∂

+∂∂

ωλρρ

ωρρρ

ωρρρ

&

&

&

)(

)(

)(

2

2

22

DcDD pOF ===+ )/( 2

ρλ

PrOrF ss )1(2 +→+

• Emmons solutionBoundary conditions at fuel surface (y = 0)

Total mass:

Fuel mass:

Oxygen mass:

Energy:

Laminar Wall Fires

wwF

wwwFww my

YDYv ′′=∂∂

− &

4444 34444 21gas

, )(ρρ

0,2=wOY

43421

&

gas

)( wwpyrow yTHm∂∂

=Δ′′ λ

{ www mv ′′= &gas

ρ

• Emmons solutionMixture fraction formulation

where

Laminar Wall Fires

)(yZD

yyZv

xZu

∂∂

∂∂

=∂∂

+∂∂ ρρρ

∞

∞

∞∞

∞∞

∞

∞

−=

−Δ−

Δ−+−=

+

+−=

uuu

rYHTTcHTTcrYY

rYYrYrYY

Z

sOcombwp

FpsOO

sOwF

sOsOF

)/(/)(/)(/)(

)/()/()/(

,

,

,,

,

2

22

2

22

• Emmons solutionBoundary conditions for mixture fraction:

andwith

Laminar Wall Fires

0),(1)0,(=∞=

xZxZ

wwww myZDB ′′=∂∂

− &])([ ρ

pyro

combsOwp

wF

sOwF

HHrYTTc

YrYY

BΔ

Δ+−=

−

+= ∞∞∞ )/()(

1)/( ,

,

,, 22

(Spalding B number) Ratio of heat released by unit mass of oxidizer divided by heat required to gasify unit mass of fuel

• Emmons solutionIntroduce a modified stream function

Look for a similarity solution

Re-formulate the problem as an ODE problem

Laminar Wall Fires

),( yxψ

xv

yu

∂∂

−=∂∂

=ψρψρ ;

)(1),( ; )(),(

)(),(

)(Re

)(),(00

ηη

ηη

ηνρψ

νρρ

ρρη

ddGyxZ

ddGuyxu

Gxuyx

xudy

xdyyx

yxy

−==

=

==

∞

∞∞∞

∞

∞

∞∞∫∫

02 =′′+′′′ GGG

1)(0)0(

0)0()0(2

=∞′=′

=+′′

GG

GGB

• Emmons solutionFuel mass loss rate

Scaling

Laminar Wall Fires

BG

Gumx

w

offunction ))0((

))0((Re2=−

−=′′ ∞∞ρ&

∞′′

′′

um

xm

s

s

~

/1~

&

&

• Emmons solution

Laminar Wall Fires

)5 m/s; 5.0 flow;(air ==∞ Bu

Streamwise velocity field Temperature field (Ts = 630 K)

• Emmons solution

Laminar Wall Fires

)5 m/s; 5.0 flow;(air ==∞ Bu

Flame structure in y-direction (x = 0.5 m)


Scaling: effect of material properties

Laminar Wall Fires

))0((~ Gmw −′′&

pyro

combsO

pyro

combsOwp

HHrY

HHrYTTc

BΔ

Δ≈

Δ

Δ+−= ∞∞∞ )/()/()( ,, 22

BG

Gumx

w

offunction ))0((

))0((Re2=−

−=′′ ∞∞ρ&

15.08.1)1(

BBLn +

Emmons solution

• Emmons solution: provides a metric to evaluate fire risk associated with different materials

n-heptane (liquid fuel)

Polymethylmethacrylate (PMMA)

Wood (douglas fir)

Laminar Wall Fires

2.6;52.3 ;MJ/kg 48.0 ;MJ/kg 45 ≈=≈Δ≈Δ BrHH spyrocomb

45.1;92.1 ;MJ/kg 2 ;MJ/kg 24 ≈=≈Δ≈Δ BrHH spyrocomb

45.0;95.0 ;MJ/kg 8.6 ;MJ/kg 4.12 ≈=≈Δ≈Δ BrHH spyropyro

pyro

combsO

HHrY

BΔ

Δ≈ ∞ )/( ,2

• Emmons solution: provides a metric to evaluate fire risk associated with different materials

n-heptane (liquid fuel)

Polymethylmethacrylate (PMMA)

Wood (douglas fir)

Laminar Wall Fires

13~HRRPUA

9.3~HRRPUA

1=HRRPUA

combcombw HB

BLnHmHRRPUA Δ+

Δ′′= 15.0

)1(~&

• Emmons solution: extension to buoyancy-driven flowConsider a vertical solid flammable surfaceF. J. Kosdon, F. A. Williams & C. Buman (1969) “Combustion of vertical cellulosic cylinders in air”, Proc. Combust. Inst., 12:253-264.J. S. Kim, J. De Ris & F. William Kroesser (1971) “Laminar free-convective burning of fuel surfaces”, Proc. Combust. Inst., 13:949-961.

Laminar Wall Fires


Scaling

Laminar Wall Fires

4/1/1~ xmw′′&

BG

Gx

Grm xwww

offunction ))0((

))0(()(2

3 4/1

=−

−=′′νρ

&( )

∞

∞−=

TxTTgGr

w

wx 2

3

)(ν

• Emmons solution

Laminar Wall Fires

)5( =B

Streamwise velocity field Temperature field (Ts = 630 K)

Gravity gGravity g

• Limitations of Emmons solutionAssumes laminar flowNeglects role of thermal radiation

Laminar Wall Fires

wwpyrow yTHm )(∂∂

=Δ′′ λ& radwwwpyrow qyTHm ,)( ′′+∂∂

=Δ′′→ && λ

combHΔ combrad HΔ−→ )1( χ

• Limitations of Emmons solutionNeglects role of pyrolysis processes occurring inside condensed phase

Laminar Wall Fires

wwpyrow yTHm )(∂∂

=Δ′′ λ& radwwwws

wspyrow qyT

yTHm ,, )()( ′′+

∂∂

=∂∂

+Δ′′→ && λλ

p

pyrocombsOw c

HBHrYTT

Δ−Δ+= ∞

∞

)/( ,2 constant≠→ wT

∫ Δ−′′′=′′→

0),( dxtxm gw ω&&

in-depth processes (rather than surface processes)

• Pyrolysis modeling: description of fuel mass loss rateFinite-rate chemistry model: explicit treatment of thermal decomposition chemistryThermal degradation across flammable solid described by a local one-dimensional problem in the direction normal to the exposed surface

Laminar Wall Fires

43421&

43421&

flow gasby transportconvectiveongasificatiby

consumedenergy

)(xTcmH

xTk

xtTc s

ggpyrogs

ss

ss ∂∂′′−Δ′′′−

∂∂

∂∂

=∂∂ ωρ

G

44 344 214444 34444 214434421 convectionradiation

44

n)(conductiointerior wallflux toheat

)),0(()),0((),0( ∞∞ −+−+−=∂∂

− TtThTtTGtxTk ss

ss εσε

• Pyrolysis modeling: description of fuel mass loss rateFinite-rate chemistry model: explicit treatment of thermal decomposition chemistryThermal degradation across flammable solid described by a local one-dimensional problem in the direction normal to the exposed surface

Laminar Wall Fires

ggg

gs

xm

t

t

ωρ

ωρ

′′′=∂

′′∂+

∂

∂

′′′−=∂∂

&&

& mass conservation(solid phase)

mass conservation(gas phase)

{ccg ηηη

charvolatilessolidvirgin 1kg 1

+→−=4342143421

)/exp(0svsg RTEAx

vs−=′′′ ρω&

G

∫ Δ−′′′=′′

0),()( dxtxtm gf ω&&

J/kg 0J/mol 101.256E

1/s 105.250A

0.9

K-J/kg 2520

kg/m 133

kg/m 663

K- W/m126.0

,

5

7

00

30

30

00

=Δ×=

×=

=

==

=

=

==

gvH

cc

kk

cvs

c

vs

cvs

ε

ρ

ρ

Back surface (Δ = 2.5 cm): adiabaticComputational grid: Δx = 200 μm

MLR (kg/s/m2)

time (s)

2kW/m 25=G

MLR

secondMLR peak

initialMLR peak

• Pyrolysis modeling: description of fuel mass loss rateExample: particle board (Novozhilov, Moghtaderi, Fletcher & Kent, Fire Safety J. 27 (1996) 69-84)

Laminar Wall Fires

• SummaryWall fires are characterized by a strong coupling between gas and solid phase processes

the gas-to-solid heat transfer drives pyrolysisthe solid-to-gas fuel mass transfer drives combustion

The Emmons solution identifies the B number as the main parameter to compare and rank different materials

Laminar Wall Fires


Multi-physics problemRole of wall fires




• Experimental database: simplifiedvertical wall flame configuration (J.L. de Ris, FM Global 1999)

Porous burners with gaseous fuel (prescribed fuel mass loss rate); use different fuels (propylene, ethylene, ethane, methane) and different fuel injection ratesQuasi two-dimensional configurationMain diagnostics: total wall heat flux (water cooling system); radiative wall heat flux (radiometers); soot layer thickness (soot deposition on glass rod); temperature profiles (thermocouples)

Turbulent Wall Fires

• Ongoing large eddy simulation study (N. Ren, S. Vilfayeau, Y. Wang, A. Trouvé)

• Experimental database (J.L. de Ris, FM Global 1999)

Temperature profiles for different values of MLR

Convective/radiative/total wallheat fluxes at different elevations

totalwq ,′′&

radwq ,′′&

cwq ,′′&

fm ′′& increasing


FireFOAM

• Open-source fire model supported by FM Global, USA• Based on OpenFOAM: OpenFOAM is a general-purpose

advanced CFD solver developed by OpenCFD, UK (http://www.opencfd.co.uk)

• Main featuresLibrary of solvers: LES approach for turbulence; compressible flow formulationAdvanced physical models for LES, turbulent combustion, heat transferNumerical methods: finite volume scheme (2nd order in space); implicittime integration (2nd order in time)Advanced meshing capabilities: structured or unstructured (polyhedral) computational grid (built-in mesh generation capability)Software engineering: public domain (http://code.google.com/p/firefoam-dev/); open source (object-oriented C++ environment); Linux OS; massively parallel capability (MPI-based)Post-processing: third-party visualization with ParaView (open source)

• LES computational grid design (wall-resolved LES)Two characteristic length scalesBoundary layer thicknessViscous sub-layer thickness

Momentum-driven flow

Buoyancy-driven flow

BLδ

VSLδ

(Hölling & Herwig, J. Fluid Mech., 2005)

BLδ

yVSLδ

Temperature orvertical flow velocity


mm 2.0)/( 2/1 ≈≈

ww

wVSL ρτ

νδ

mm 5.0

)()/(Pr)/(

4/14/1,,

4/3

≈

′′≈

wwpwcw

wVSL gcq βρ

νδ&

• Numerical configurationBaseline case(propylene fuel C3H6; )(isothermal wall; Tw = 348 K)Block-structured grid (0.9 million cells)Near-wall resolutionΔy1 = 2 mm


2g/s/m 17=′′fm&

mm 55.2 1 =Δ×=Δ=Δ yxz

• Modeling choices (FireFOAM v.1.7.x)

Turbulence: dynamic k-equation eddy viscosity model coupled with WALE modelCombustion: Eddy Dissipation Concept modelSoot: mixture-fraction-based flameletcorrelationRadiation: RTE solver; finite volume model (angular space discretized using 64 solid angles)



Turbulence: dynamic k-equation eddy viscosity model coupled with WALE model (Nicoud & Ducros 1999)


( )

( ) ( )( ) ( ) 4/52/5

2/32

21 ,

21

31

dij

dijijij

dij

dij

wsgs

i

j

j

iij

i

j

j

iij

mnmnmnmnijkjikkjikdij

SSSS

SSC

xu

xu

xu

xuS

SSSSS

+Δ=

⎟⎟⎠

⎞⎜⎜⎝

⎛

∂

∂−

∂∂

=Ω⎟⎟⎠

⎞⎜⎜⎝

⎛

∂

∂+

∂∂

=

ΩΩ−−ΩΩ+=

ρμ

δ


Combustion: Eddy Dissipation Concept model (Magnussen & Hjertager1976)


( )ddEDCt

sOFF CC

rYY/ ,/min)/~;~min(

2

ττρω ×=′′′&

)/( and )/( where 22/1thdsgst Dk Δ=Δ= ττ

PrOrF ss )1(2 +→+


Radiation: RTE solver; finite volume model (angular space discretizedusing 64 solid angles)


{AbsorptionEmission

4 )/( ITdsdI κπσκ −=

43421

),,( zyx

sr

ΩdPlanck mean absorption coefficient [m-1]

where:ap,i is the Planck mean absorption coefficient

for species i [m-1 atm-1] and is obtained fromtabulated dataκsoot is the soot mean absorption coefficient [m-1]

TfC vsootsoot ×=κ

sootCOCOOHOH axaxp κκ ++= )(2222


Laminar-to-turbulent flow transition:Flame at the base of the wall is laminar Laminar region is over-estimated in the LES simulationsForce transition by using a forcing scheme acting on the fuel flow rate at the base of the wall


Constantflow rate

Perturbedflow rate

• ResultsWall heat flux versus elevation z


totalwq ,′′&

radwq ,′′&

cwq ,′′&

Experimental data

FMG Model

FMG Model

Solid lines: LES laminar turbulent

• ResultsWall-normal profile of mean temperature


Comparison between raw (uncorrected) thermocouple measurements and LES “thermocouple data”LES thermocouple (TC) data are obtained from a virtual instrumentmodel:

( ) ( )TCgasTCTC

TC

TC

TCTCTC

TThTG

dtdT

AVC

−+−

=

4 σε

ρ

Experimental data

LES

cm 77=z

• ResultsEvaluation of self-similarity in wall-normal profiles of mean temperature


Experimental data LES

• ResultsEvaluation of self-similarity in wall-normal profiles of mean z-velocity


gzw 2/~ ><

δ/z

LES

• Results: grid convergence studyWall heat flux versus elevation z

FireFOAM Results

cwq ,′′&

mm 41 =Δymm 31 =Δymm 21 =Δy

radwq ,′′&

FMG ModelFMG Model

• Results: grid convergence studyWall-normal profile of mean temperature and z-velocity

FireFOAM Results

><T~ >< w~

mm 41 =Δymm 31 =Δymm 21 =Δy

Expt data

• SummaryFirst step in a model development/validation strategy: wall-resolved LES of a simplified configuration (non-spreading vertical wall flame; prescribed fuel flow rate)

Features: low (transition) Reynolds number conditions; laminar base region and a weakly turbulent downstream regionResults are encouraging. But: the size of the laminar base region is overestimated in the simulations; and the LES combustion and radiation models lack accuracy under laminar-like conditions

The experimental database is limited to first-order statistical moments and there is a need to extend the scope and quality of experimental data (velocity measurements, second-order moments, etc)

Conclusion

Modeling of Turbulent Wall Fires - Stanford University · 2012. 7. 19. · ¾Impact of smoke layer:...

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