1. 1 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

2. 2 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 )

3. 3 Overview Opposed-flow flame spread. The thermal limit. The quiescent limit. The extinction criterion. Flammability maps. Future work.

4. 4 AFP: = 0.08 mm = 1.8 mm/s Downward Spread Experiment, SDSU Combustion Laboratory PMMA: = 10 mm = 0.06 mm/s

5. 5 Fuel: Thin AFP, =0.08 mm = 4.4 mm/s Thick PMMA Image sequence showing extinction Vigorous steady propagation. Experiments Aboard Shuttle: O2: 50% (Vol.), P=1 atm.

6. 6 Mechanism of Flame Spread in Lab. Coordinates Fuel vapor O 2 /N 2 mixture The flame spreads forward by preheating the virgin fuel ahead. Virgin Fuel

7. 7 Mechanism of Flame Spread in Flame-Fixed Coord O 2 /N 2 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,

8. 8 Forward Heat Transfer Pathways: Domination of Gas-to-solid Conduction (GSC) Preheat Layer Pyrolysis Layer Gas-to- Solid Conduction Solid-Forward Conduction The Leading Edge

9. 9 Gas-phase conduction being the driving force, The Leading Edge Length Scales

10. 10 Length Scales - Continued

11. 11 Heated Layer Thickness – Gas Phase

12. 12 Heated Layer Thickness – Solid Phase

13. 13 Vaporization Temperature, Ambient Temperature, Energy Balance: Characteristic Heating Rate Sensible heating (sh) rate required to heat up the unburned fuel from to Heating rate due to gas-to-solid (gsc) conduction: Flame Temperature,

14. 14 Conduction-limited or thermal spread rate: Flame Temperature, Thick Fuel Spread Rate from Energy Equation Vaporization Temperature, For semi-infinite solid,

15. 15 Conduction-limited spread rate: Flame Temperature, Vaporization Temperature, For thermally thin solid, Thin Fuel Spread Rate from Energy Equation

16. 16 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

17. 17 Solid Residence Time: 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. Radiative Term Becomes Important in Microgravity

18. 18 Gas to Solid Conduction (gsc) Solid to Environment Radiation (ser) Include the radiative losses in the energy balance equation: Algebraic manipulation leads to: Spread Rate in the Microgravity Regime

19. 19 Mild Opposing Flow: Computational Results for Thin AFP As the opposing flow velocity decreases, the radiative effects reduces the spread rate

20. 20 Mild Opposing Flow: MGLAB Data for Thin PMMA

21. 21 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. The Quiescent Microgravity Limit: Fuel Thickness

22. 22 Gas to Solid Conduction (gsc) Solid to Environment Radiation (ser) The spread rate can be obtained from the energy balance that includes radiation. where, The Quiescent Microgravity Limit: Spread Rate reduces to:

23. 23 In a quiescent environment steady spread rate cannot occur for The Quiescent Limit: Extinction Criterion

24. 24 Extinction criterion proposed is supported by the limited amount of data we have acquired thus far. The Quiescent Limit: MGLAB Experiments

25. 25 Empty symbols stand for extinction and filled symbols for steady spread. The Quiescent Limit: Flammability Map for PMMA No steady flame over PMMA beyond this half- thickness even in a pure oxygen environment

26. 26 Empty symbols stand for extinction and filled symbols for steady spread. The Quiescent Limit: Flammability Map for AFP No steady flame over Ashless Filter Paper beyond this half- thickness even in a pure oxygen environment

27. 27 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: Conclusions