opposed-flow flame spread in different environments

Opposed-Flow Flame Spread in Different Environments

Subrata (Sooby) Bhattacharjee

San Diego State University

Acknowledgement

• Profs. Kazunori Wakai and Shuhei Takahashi, Gifu University, Japan

• Dr. Sandra Olson, NASA Glenn Research Center.

• Team Members (graduate): Chris Paolini, Tuan Nguyen, Won Chul Jung, Cristian Cortes, Richard Ayala, Chuck Parme

• Team Members (undergraduate): Derrick, Cody, Isaac, Tahir and Mark.

(Support from NASA and Japan Government is gratefully acknowledged)

Overview

• What is opposed-flow flame spread?

• Flame spread in different environment.

• Recent experiments at MGLAB, Japan

• Mechanism of flame spread.

• Length scales and time scales.

• Spread rate in normal gravity.

• Spread rate in microgravity

• The quiescent limit

• Future plan.

Upward or any other flow-assisted flame spread becomes large and turbulent very quickly.

Opposed-flow flame spread is also known as laminar flame spread.

AFP: = 0.08 mm

= 1.8 mm/sfV

Downward Spread Experiment, SDSU Combustion Laboratory

PMMA: = 10 mm

= 0.06 mm/sfV

•Gravity Level: 1.e-6g

•Environment: 50-50 O2/N2 mixture at 1.0 atm.

•Flow Velocity: 50 mm/s

•Fuel: Thick PMMA (Black)

•Spread Rate: 0.45 mm/smm

Sounding Rocket Experiment Spread Over PMMA: Infrared Image at 2.7

Fuel: Thin AFP, =0.08 mm = 4.4 mm/sfV

Thick PMMA

Image sequence showing extinction

Vigorous steady propagation.

Experiments Aboard Shuttle: O2: 50% (Vol.), P=1 atm.

Front view camera

Side view camera

Fuel holder

O2 portN2 port

Vacuum pump portManometer port

Apparatus for normal-gravity experimentsApparatus for normal-gravity experiments

CCD camera

Air

Honeycomb

Fan

PMMA ：30mm x 10mm x 15,50,125m

Fuel holder

Igniter (Ni-Cr wire)

Apparatus for micro-gravity experiments conducted with the 4.5sec trop-tower Apparatus for micro-gravity experiments conducted with the 4.5sec trop-tower (100meter-drop) of MGLAB in Japan.(100meter-drop) of MGLAB in Japan.

Igniter (Ni-Cr wire)

Fuel holder

VacuumO2

CCD camera

Air

Igniter (Ni-Cr wire)

Vf

Vf

Fuel holder

Vg Vg~300mm/sec

PMMA ： 30mm x 10mm x 15,50,125m

Front view

Back view

Video camera

Sample holder(sample size: 6cm x 1cm)

Fan & Motor

Motor controller

Solenoid coil to remove theigniter at the onset of MG

Assemble Move to drop shaft Close the capsule

Attach the transceiverReady to drop

Time (msec)

G

-2000 -1000 0 1000 2000 3000 4000 5000 6000-2

0

2

4

6

8

10

12

Ignite the sample 1.6 sec before MG.

Remove the igniter 0.3sec before MG.

The onset of MG

Declaration G in the friction damper.

MG for 4.5 sec

Typical sequence of the drop experiment

PMMA: = 0.025mm

= 10 mm/s

(Downward spread)

fV = 4.1 mm/s

(MGLAB drop tower)

fV

O2: 30%, 1 atm.

PMMA: = 0.025mm

= 22.8 mm/s

(Downward spread)

fV = 18.9 mm/s

(MGLAB drop tower)

fV

O2: 50%, 1 atm.

Mechanism of Flame Spread

gVVf

Fuel vapor

O2/N2 mixtureFlame seeks out the stoichiometric locations

The flame spreads forward by preheating the virgin fuel ahead.

Virgin Fuel

Mechanism of Flame Spread

Vr Vg V f

Vf

O2/N2 mixture

The rate of spread depends on how fast the flame can heat up the solid fuel from ambient temperature to vaporization temperature .

Virgin Fuel

Vaporization Temperature, vT

T

vT

fgr VVV

Vf

Forward Heat Transfer Pathways: Domination of Gas-to-solid Conduction (GSC)

Preheat Layer

Pyrolysis LayerGas-to-Solid

Conduction

Solid-ForwardConduction

The Leading Edge

Vr Vg V f

VfGas-phase conduction being the driving force,

Zooming on the Leading Edge

gxL

Lsy

sxL

gyL

gxsx LL ~

Length Scales - Continued

Vr Vg V f

Vf

gxL

Lsy

gyL

gxL

2

2

~x

T

cuT

x p

2~

gxp

r

gx

rr

Lc

T

L

TV

r

ggx VL

~

Vr Vg V f

VfLsy

gxL

Heated Layer Thickness – Gas Phase

r

g

r

gxggresggy VV

LtL

~~~ ,

r

gggygx VLLL

gxL

gyL

r

gxgres V

Lt ~,

f

gxsres V

Lt ~,

Heated Layer Thickness – Solid Phase

Vf Lsy

gL

f

gsres V

Lt ~,

fr

sg

f

gs

sresssy

VVV

L

tL

~~

~ , gL

gL

fr

sgh VV

,min~

Vf

Lsy

gL

gLvT

Vr Vg V f

Vf

gL

Vaporization Temperature,

Ambient Temperature,

TTcWVQ vsshfsh ~

gL

gL

h

The Characteristic Heating Rate

Sensible heating (sh) rate required to heat up the unburned fuel from to T vT

vT

T

Heating rate due to gas-to-solid (gsc) conduction:

g

vfgggsc L

TTWLQ

~

Flame Temperature, fT

Vr Vg V f

Vf

gL

TT

TTFF

cV

v

vf

ss

g

hf where,

1~

gL

gL

Conduction-limited or thermal spread rate:

Flame Temperature, fT

Spread Rate Expressions

gscsh QQ ~

Vaporization Temperature, vT

2, ~ F

c

cVV

sss

gggrthickf

fr

sgsyh VVL

~~

For semi-infinite solid,

2,, F

c

cVV

sss

gggrdeRisthickf

h

Vr Vg V f

Vf

Lsy

gL

TT

TTFF

cLV

v

vf

ss

g

syf where,

1~

gL

gL

Conduction-limited spread rate: Flame Temperature, fT

gscsh QQ ~

Vaporization Temperature, vT

Fc

Vss

gthinf

~,

For thermally thin solid,

~h

Spread Rate Expressions

Fc

Vss

gosDelichatsithinf

4,,

gr VV

VfgL

Hang-distance, the distance between the flame front and the pyrolysis front, is ignored in de Ris solution.

Flame front.

Pyrolysis front

Fc

cVss

ghdESTthinf

4,,

Hang-Distance Correction for Thin Fuels [Bhattacharjee, Combustion and Flame, 94]

The Extended Simplified Theory (EST) retains the same form as the de Ris expression and recommends for evaluating properties.

vT

gr VV

Vf

TT

TTF

v

vf where,

Thick Fuel Spread Rate (EST):Replace the forced or buoyancy induced boundary layer with an equivalent slug flow.

vT

2,, ~ F

c

cVV

sss

gggeqvESTthickf

Extended Simplified Theory – Thick Fuels[Bhattacharjee et al., 26th Symp]

eqvr VV

The Extended Simplified Theory (EST) retains the same form as the de Ris expression and recommends for evaluating properties.

vT

Introduce a correction for the lifted flame through

'fT

gr VV

Vf

vT

There are Hardly Any Studies on Transition in Literature

gr VV

Vf

thickthincr ,

Vf

Most thin fuel studies were done with cellulose

Most thick fuel studies were done with PMMA

2, ~ F

c

cVV

sss

gggrthickf

F

cV

ss

gthinf

~,

TT

TT

VcF

L

f

v

rgg

sg

g

sthickthincr

1~~,

At low opposing velocity, critical thickness can be a hundred time larger, removing the difficulty of creating thin samples.

Vf

Thin-fuel formula

Thick-fuel formula

thickthincr ,The intersection produces:

It Maybe Easier to Study Transition in the Absence of Buoyancy

0.25atm 50% 0.5atm 50% 0.75atm 50% 1atm 50% 1atm 30% 1atm 21% Prediction

T

0.01 0.1 1 10 1000.1

1

10

100

Thoery, Numerical Simulation and Existing Data

0.001

0.01

0.1

1

10

0.01 0.1 1 10 100 [mm]

50%

70%

100%

Experiments[5][9][12]

Eqs (1-2)

Spr

e ad

Rat

e [c

m/s

]

V

VTf

f thick EST crit EST, , ,

,

min ,11

Twhere,

O2 : 50%O2 : 30%O2 : 21%O2 : 18%Prediction

Fuel half-thickness [mm]

Fla

me

spre

ad r

ate

[mm

/s]

21%

18%

30%

50%

11%

0.01 0.1 1 100.01

0.1

1

10

100

Fuel half-thickness [mm]

Fla

me

spre

ad r

ate

[mm

/s]

1.0atm 0.75atm 0.5atm 0.25atm Prediction

1.0atm

0.75atm

0.5atm

0.25atm

0.01 0.1 1 100.01

0.1

1

10

100

Vc L

Ffg

s s sy

~

V

cFf thin

g

s s, ~

V Vc

cFf thick r

g g g

s s s, ~

2

for thermally-thin fuel

and

for thermally-thick fuel

Downward spread rate vs. fuel half-thickness in normal-gravityDownward spread rate vs. fuel half-thickness in normal-gravity

V cg T T

TBC eqv BCg g c

,,

/( )

1 3

T T K cg c v BC, , . 620 0575 where

V

VTf

f thick EST crit EST, , ,

,

min ,11

Twhere

Non

-dim

ensi

onal

sp

read

rat

e

Non-dimensional fuel half-thickness

0.25atm 50% 0.5atm 50% 0.75atm 50% 1.0atm 50% 1.0atm 30% 1.0atm 21% 1.0atm 18% Prediction

0.01 0.1 1 100.1

1

10

100

Non-dimensional downward spread rate vs. non-dimensional fuel half-Non-dimensional downward spread rate vs. non-dimensional fuel half-thicknessthickness

Vr Vg V f

Vf gL

gL

gL

Solid Forward Conduction (sfc)

Gas to Solid Conduction (gsc)

Gas to Environment Radiation (ger)

Gas to Solid Radiation (gsr)

Solid to Environment Radiation (ser)

Parallel Heat Transfer Mechanisms

h

VfgL

gL

gL

Gas to Solid Conduction (gsc)

mechanismgiven by the rate Heating

tRequiremenHeat sticCharacterimechanismt

TTWLcQ vghsschar ~

Fc

c

VWL

L

TT

TTWLc

WLq

Qt

gg

ss

r

h

gg

vfg

vghss

ggsc

chargsc

1~~~

The characteristic heat is the heat required to raise the solid-phase control volume

from to . vT T

Gas-to-surface conduction time:

rV

h

Time Scales

VfgL

gL

gL

Solid Forward Conduction (sfc)

Gas to Solid Conduction (gsc)

s

gss

hg

v

vghss

hsfc

charsfc

Lc

WLTT

TTWLc

Wq

Qt

2

~

~

~

FLt

tN

gg

hs

sfc

gscsfc

1~

2

2

,max,

1~

1~~

FFL

LNN

g

s

gg

sysThicksfcsfc

rV

h

Relative dominance of GSC over SFC

VfgL

gL

gL

Solid Residence Time: f

gsres V

Lt ~,

Gas to Solid Conduction (gsc)

Solid to Environment Radiation (ser)

The radiation number is inversely proportional to the velocity scale. In the absence of buoyancy, radiation can become important.

WLTT

Qt

sxv

charser 44

~

vfrgg

v

ser

sres

TTVc

TT

t

t

44

, ~

rV

h

Radiative Term Becomes Important in Microgravity

ESTf

f

V

V

,

Mild Opposing Flow: Computational Results for Thin AFP

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.1 2.1 4.1 6.1 8.1

21%

50%

70%

100%

As the opposing flow velocity decreases, the radiative effects reduces the spread rate

vfrgg

v

ser

sres

TTVc

TT

t

t

44

, ~

Mild Opposing Flow: MGLAB Data for Thin PMMA

vfrgg

v

ser

sres

TTVc

TT

t

t

44

, ~

0

0.2

0.4

0.6

0.8

1

1.2

0 0.05 0.1 0.15 0.2

Eq. (5)

7.5 micro-m, 50%

25 micro-m, 50%

7.5 micro-m, 30%

25 micro-m, 30%

7.5 micro-m, 21%

25 micro-m, 21%

ESTf

f

V

V

,

VfgL

gL

gL

Gas to Solid Conduction (gsc)

Solid to Environment Radiation (ser)

Include the radiative losses in the energy balance equation: rV

WTT

WLTTTTWcV

vfg

gvvhssf

~

44

1~,, ThermalThinf

fthin V

V 21~

,, ThermalThickf

fthick V

V

Algebraic manipulation leads to:

Spread Rate in the Microgravity Regime

h

Vf

syL

gL

gL

Gas to Solid Conduction (gsc)

Solid to Environment Radiation (ser)

The minimum thickness of the heated layer can be estimated as:

All fuels, regardless of physical thickness, must be thermally thin in the quiescent limit.

fr VV

The Quiescent Microgravity Limit: Fuel Thickness

ggg

sss

Thinf

gs

sy

f

gs

rf

gssy

c

cF

VL

VVVL

,

),min( syh L

syL Therefore,

0fV

gL

gL

Gas to Solid Conduction (gsc)

Solid to Environment Radiation (ser)

The spread rate can be obtained from the energy balance that includes radiation.

where,

0fr VV

The Quiescent Microgravity Limit: Spread Rate

WTT

WLTTTTWcV

vfg

gvvssf

~

440

0,

00 41

2

1

2

1~

Thinf

f

V

V

TT

TT

c

c

F v

v

gss

gg44

20

1

0~0020

reduces to:

In a quiescent environment steady spread rate cannot occur for

The Quiescent Limit: Extinction Criterion

0,

00 41

2

1

2

1~

Thinf

f

V

V

2

1~ ,

4

1~For 00

imaginary. is , 4

1For 00

3

2

4 v

g

gg

ss

Tc

cF

Extinction criterion proposed is supported by the limited amount of data we have acquired thus far.

The Quiescent Limit: MGLAB Experiments

occur.not does spreadsteady

, 4.0For 0

0

0.2

0.4

0.6

0.8

1

1.2

0.01 0.1 1 10

21% O2

30% O2

50% O2

Eq. (8)

0

0

Oxygen/Nitrogen Mixture

AB

Flow Modifier

reduces the entrance length.

Average velocity Centerline velocity

Control Thermocouple: The conveyor belt holding the fuel is spooled from roller A to B so as to maintain a constant thermocouple temperature.

Igniter for opposed-flow spread. The fuel is spooled from A to B

Igniter for concurrent-flow spread. The fuel is spooled from B to A

Spot Radiometer

Imaging window backlit with IR radiation

Smoke Wire

IR Source with beam expander

IR Camera with a rotating filter wheel containing 4.3 m and 2.8 m filters of varying trasmittance.

Top View

Side View

C D

B A

C

Thin PMMA sheet (thickness 200 m or less) attached on a conveyor belt.

E

Future Work

• The MGLAB data suffers from limited low-g duration (4.5 s) to distinguish steady spread from a spreading extinction. Only space experiment can establish the microgravity and quiescent formulas proposed.

• While this work predicts extinction for fuel with thickness greater than a certain critical thickness, the pathway to extinction is not clear. Detailed infrared emission and absorption photography will be used to establish the role played by radiation.

• Numerical modeling and a comprehensive set of data with flow velocity, oxygen level, ambient pressure and fuel thickness as parameters from an ambitious flight experiment will be used to quantify the transition between thin and thick fuels, thermal, microgravity and quiescent regimes, and wind opposed and wind aided spread.

• A novel experimental set up is being built at SDSU, where the fuel is moved relative to the flame so as to keep the flame stationary with respect to the laboratory. The absorption pyrometry is being developed at Gifu.