OPPOSED-FLOW FLAME SPREAD - THE QUIESCENT MICROGRAVITY LIMIT

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OPPOSED-FLOW FLAME SPREAD - THE QUIESCENT
MICROGRAVITY LIMIT

• Subrata (Sooby) Bhattacharjee
• Professor, Mechanical Engineering Department
• San Diego State University, San Diego, USA
• JSME Microgravity Symposium, Oct. 28-30, 2001,
  Sendai, Japan

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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,
  Dave, Monty and Mark.

(Support from NASA and Japan Government is
gratefully acknowledged)
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Overview

• Opposed-flow flame spread.
• The thermal limit.
• The quiescent limit.
• The extinction criterion.
• Flammability maps.
• Future work.

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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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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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Mechanism of Flame Spread in Lab. Coordinates
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 in Flame-Fixed Coord
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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The Leading Edge Length Scales
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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Energy Balance 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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Thick Fuel Spread Rate from Energy Equation
Conduction-limited or thermal spread rate
Vaporization Temperature,
For semi-infinite solid,
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Thin Fuel Spread Rate from Energy Equation
Conduction-limited spread rate
Vaporization Temperature,
For thermally thin solid,
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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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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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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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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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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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The Quiescent Limit Flammability Map for PMMA
No steady flame over PMMA beyond this
half-thickness even in a pure oxygen environment
Empty symbols stand for extinction and filled
symbols for steady spread.
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The Quiescent Limit Flammability Map for AFP
No steady flame over Ashless Filter Paper beyond
this half-thickness even in a pure oxygen
environment
Empty symbols stand for extinction and filled
symbols for steady spread.
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Conclusions

• In a completely quiescent environment all fuels
  behave like thermally thin fuels.
• The spread rate in a quiescent environment
• The critical thickness above which there cannot
  be any steady flame spread is