Opposed-Flow Flame Spread in Different Environments

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Opposed-Flow Flame Spread in Different
Environments

• Subrata (Sooby) Bhattacharjee
• San Diego State University

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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.
• 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.

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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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Downward Spread Experiment, SDSU Combustion
Laboratory
PMMA 10 mm 0.06 mm/s
AFP 0.08 mm 1.8 mm/s
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Sounding Rocket Experiment Spread Over PMMA
Infrared Image at 2.7m

• 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/s

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Experiments Aboard Shuttle O2 50 (Vol.), P1
atm.
Image sequence showing extinction
Fuel Thin AFP, 0.08 mm 4.4 mm/s
Vigorous steady propagation.
Thick PMMA
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Apparatus for normal-gravity experiments
Apparatus for micro-gravity experiments conducted
with the 4.5sec trop-tower (100meter-drop) of
MGLAB in Japan.
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Front view
Solenoid coil to remove the igniter at the onset
of MG
Video camera
Sample holder (sample size 6cm x 1cm)
Fan Motor
Motor controller
Back view
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Assemble
Move to drop shaft
Close the capsule
Attach the transceiver
Ready to drop
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MG for 4.5 sec
The onset of MG
Ignite the sample 1.6 sec before MG.
Declaration G in the friction damper.
Remove the igniter 0.3sec before MG.
Typical sequence of the drop experiment
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O2 30, 1 atm.
PMMA 0.025mm
10 mm/s (Downward spread)
4.1 mm/s (MGLAB drop tower)
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O2 50, 1 atm.
PMMA 0.025mm
22.8 mm/s (Downward spread)
18.9 mm/s (MGLAB drop tower)
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Mechanism of Flame Spread
Flame seeks out the stoichiometric locations
O2/N2 mixture
Fuel vapor
Virgin Fuel
The flame spreads forward by preheating the
virgin fuel ahead.
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Mechanism of Flame Spread
O2/N2 mixture
Vaporization Temperature,
Virgin Fuel
The rate of spread depends on how fast the flame
can heat up the solid fuel from ambient
temperature to vaporization temperature
.
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Forward Heat Transfer Pathways Domination of
Gas-to-solid Conduction (GSC)
The Leading Edge
Gas-to-Solid Conduction
Pyrolysis Layer
Preheat Layer
Solid-Forward Conduction
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Zooming on the Leading Edge
Gas-phase conduction being the driving force,
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Length Scales - Continued
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Heated Layer Thickness Gas Phase
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Heated Layer Thickness Solid Phase
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The Characteristic Heating Rate
Sensible heating (sh) rate required to heat up
the unburned fuel from to
Flame Temperature,
Vaporization Temperature,
Heating rate due to gas-to-solid (gsc)
conduction
Ambient Temperature,
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Spread Rate Expressions
Conduction-limited or thermal spread rate
Vaporization Temperature,
For semi-infinite solid,
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Spread Rate Expressions
Conduction-limited spread rate
Vaporization Temperature,
For thermally thin solid,
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Hang-Distance Correction for Thin Fuels
Bhattacharjee, Combustion and Flame, 94
Hang-distance, the distance between the flame
front and the pyrolysis front, is ignored in de
Ris solution.
Flame front.
Pyrolysis front
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Extended Simplified Theory Thick
FuelsBhattacharjee et al., 26th Symp
Replace the forced or buoyancy induced boundary
layer with an equivalent slug flow.
Thick Fuel Spread Rate (EST)
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There are Hardly Any Studies on Transition in
Literature
Most thin fuel studies were done with cellulose
Most thick fuel studies were done with PMMA
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It Maybe Easier to Study Transition in the
Absence of Buoyancy
Thin-fuel formula
Thick-fuel formula
At low opposing velocity, critical thickness can
be a hundred time larger, removing the difficulty
of creating thin samples.
The intersection produces
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Thoery, Numerical Simulation and Existing Data
where,
Spread Rate cm/s
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where
and
for thermally-thin fuel
for thermally-thick fuel
Downward spread rate vs. fuel half-thickness in
normal-gravity
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Non-dimensional downward spread rate vs.
non-dimensional fuel half-thickness
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Parallel Heat Transfer Mechanisms
Gas to Environment Radiation (ger)
Gas to Solid Radiation (gsr)
Solid to Environment Radiation (ser)
Gas to Solid Conduction (gsc)
Solid Forward Conduction (sfc)
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Time Scales
The characteristic heat is the heat required to
raise the solid-phase control volume from to
.
Gas to Solid Conduction (gsc)
Gas-to-surface conduction time
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Relative dominance of GSC over SFC
Gas to Solid Conduction (gsc)
Solid Forward Conduction (sfc)
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Radiative Term Becomes Important in Microgravity
Solid to Environment Radiation (ser)
The radiation number is inversely proportional to
the velocity scale. In the absence of buoyancy,
radiation can become important.
Gas to Solid Conduction (gsc)
Solid Residence Time
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Mild Opposing Flow Computational Results for
Thin AFP
As the opposing flow velocity decreases, the
radiative effects reduces the spread rate
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Mild Opposing Flow MGLAB Data for Thin PMMA
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Spread Rate in the Microgravity Regime
Solid to Environment Radiation (ser)
Include the radiative losses in the energy
balance equation
Gas to Solid Conduction (gsc)
Algebraic manipulation leads to
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The Quiescent Microgravity Limit Fuel Thickness
Solid to Environment Radiation (ser)
The minimum thickness of the heated layer can be
estimated as
Gas to Solid Conduction (gsc)
All fuels, regardless of physical thickness, must
be thermally thin in the quiescent limit.
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The Quiescent Microgravity Limit Spread Rate
Solid to Environment Radiation (ser)
The spread rate can be obtained from the energy
balance that includes radiation.
Gas to Solid Conduction (gsc)
reduces to
where,
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The Quiescent Limit Extinction Criterion
In a quiescent environment steady spread rate
cannot occur for
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The Quiescent Limit MGLAB Experiments
Extinction criterion proposed is supported by the
limited amount of data we have acquired thus far.

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Average velocity Centerline velocity
Igniter for concurrent-flow spread. The fuel is
spooled from B to A
Igniter for opposed-flow spread. The fuel is
spooled from A to B
Oxygen/Nitrogen Mixture
Smoke Wire
Spot Radiometer
Flow Modifier reduces the entrance length.
C
D
A
B
E
Imaging window backlit with IR radiation
Control Thermocouple The conveyor belt holding
the fuel is spooled from roller A to B so as to
maintain a constant thermocouple temperature.
Side View
Top View
A
B
IR Source with beam expander
C
IR Camera with a rotating filter wheel containing
4.3 mm and 2.8 mm filters of varying trasmittance.
Thin PMMA sheet (thickness 200 mm or less)
attached on a conveyor belt.
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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.