OPPOSEDFLOW FLAME SPREAD THE QUIESCENT MICROGRAVITY LIMIT Subrata

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 1

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) 2

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

Downward Spread Experiment, SDSU Combustion Laboratory AFP: = 0. 08 mm = 1. 8 mm/s PMMA: = 10 mm = 0. 06 mm/s 4

Experiments Aboard Shuttle: O 2: 50% (Vol. ), P=1 atm. Fuel: Thin AFP, = 4. 4 mm/s Image sequence showing extinction =0. 08 mm Vigorous steady propagation. Thick PMMA 5

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

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

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

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

Length Scales - Continued 10

Heated Layer Thickness – Gas Phase 11

Heated Layer Thickness – Solid Phase 12

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, 13

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

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

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) 16

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: 17

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

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

Mild Opposing Flow: MGLAB Data for Thin PMMA 20

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. 21

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, 22

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

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

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

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

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: 27