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.