1 a time-scale analysis of opposed-flow flame spread – the foundations subrata (sooby)...
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A Time-Scale Analysis of Opposed-Flow Flame Spread – The Foundations
Subrata (Sooby) Bhattacharjee
Professor, Mechanical Engineering Department
San Diego State University, San Diego, USA
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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)
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Overview
• What is opposed-flow flame spread?• Flame spread in different
environment. • Mechanism of flame spread.• Length scales and time scales.• Spread rate in normal gravity.• Spread rate in microgravity• The quiescent limit
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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.
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AFP: = 0.08 mm
= 1.8 mm/sfV
Downward Spread Experiment, SDSU Combustion Laboratory
PMMA: = 10 mm
= 0.06 mm/sfV
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•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
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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.
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Mechanism of Flame Spread
gVVf
Fuel vapor
O2/N2 mixture
The flame spreads forward by preheating the virgin fuel ahead.
Virgin Fuel
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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
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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
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Vr Vg V f
VfGas-phase conduction being the driving force,
The Leading Edge Length Scales
gxL
Lsy
sxL
gyL
gxsx LL ~
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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
~
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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 ~,
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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
f
gsres V
Lt ~,
gL
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Vr Vg V f
Vf
gL
Vaporization Temperature,
Ambient Temperature,
TTcWVQ vsshfsh ~
gL
gL
h
Energy Balance: 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
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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
Thick Fuel Spread Rate from Energy Equation
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
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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
Thin Fuel Spread Rate from Energy Equation
Fc
Vss
gosDelichatsithinf
4,,
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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
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Vr Vg V f
Vf gL
gL
gL
Gas to Environment Radiation (ger)
Time Scales Relevant to Gas Phase
h
r
ggres V
Lt ,
tcomb
gert
Available Time
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Vr Vg V f
Vf gL
gL
gL
Time Scales Relevant to Gas Phase: Thermal Regime
h
r
ggres V
Lt ,
Available Time in Gas Phase
combgresger ttt ,
Gas to Solid Radiation (gsr)
Solid to Env. Radiation (ser)
tcomb
gert
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VfgL
gL
gL
rV
h
Time Scales Relevant to Solid Phase
f
gsres V
Lt ,
tshvapt
tgsc
sfct
gsrtsert
Available Time
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VfgL
gL
gL
rV
h
Time Scales Relevant to Solid Phase: Thermal Regime
f
gsres V
Lt ,
tshvapt
tgsc
sfct
gsrtsert
Available Time
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VfgL
gL
gL
rV
h
The Thermal Regime Governing Equation
f
gsres V
Lt ,
gscsh tt ~
r
ggres V
Lt ,
gscsres tt ~,
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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 – Gas to Surface Conduction
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VfgL
gL
gL
Gas to Solid Conduction (gsc)
Fc
c
VV
L
gg
ss
r
h
f
g 1~
Substitute the two limits of
rV
h
Thermal Regime: Spread Rates Using Time Scales
gscsres tt ~,
h
2, ~ F
c
cVV
sss
gggrthickf
F
cV
ss
gthinf
~,
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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
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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
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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
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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
, ~
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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
,
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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,
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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:
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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
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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
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• A phenomenological model for opposed flow flame spread is built around two residence times, one in the gas phase and one in the solid.
• Theoretical solutions in the thermal regime are reproduced using the time scale analysis.
• Deviation from the thermal regime can be quantified by comparing the time scale of the added physics with the appropriate residence time.
• In the quiescent microgravity environment all fuels behave like thin fuels.
• A critical thickness is proposed beyond which a spreading flame cannot be sustained in such environment.
Conclusions