Microgravity combustion Lecture 3

1
Microgravity combustion (Lecture 3)

• Motivation
• Time scales (Lecture 1)
• Examples
• Premixed-gas flames
• Flammability limits (Lecture 1)
• Stretched flames (Lecture 1)
• Flame balls
• Nonpremixed gas flames
• Condensed-phase combustion
• Particle-laden flames
• Droplets
• Flame spread over solid fuel beds
• Reference Ronney, P. D., Understanding
  Combustion Processes Through Microgravity
  Research, Twenty-Seventh International Symposium
  on Combustion, Combustion Institute, Pittsburgh,
  1998, pp. 2485-2506

2
Flame spread over solid fuels - motivation

• Flame spread over flat solid fuel beds is a
  useful means of understanding more complex
  two-phase non premixed flames
• Role of radiation is not fully understood, but is
  substantial, especially at reduced gravity
• Importance
• Improved understanding of fire spread at 1g
  2500 fatalities, gt 10 billion damage annually
• Radiation is main mechanism of fire spread
  between buildings
• Spacecraft fire safety - ISS will use CO2 fire
  extinguishers, but flames may spread faster at µg
  with CO2 diluent due to radiative preheating of
  fuel!

3
Schematic model of flame spread
4
Basic theory (adiabatic, fast chemstry)

• References Williams (1976), Wichman (1992)
• Flame spread rate (Sf) is determined by equating
  total heat flux to the fuel bed (q?xW, q heat
  flux to fuel bed per unit area) to rate of
  increase of fuel bed enthalpy
  (?sSfW?s)Cp,s(Tv-T8)
• Boundary layer estimates of q not applicable -
  free-stream gas is at ambient T - heat
  transferred to fuel bed comes from heat generated
  by flame
• Need to equate forward heat transfer (from flame
  to gas ahead of flame) to lateral heat transfer
  (from gas ahead of flame to fuel bed) ? Creeping
  flow (dx dy) q lg(Tf-Tv)/dy
• Thermally-thin fuels (deRis, 1968 Delichatsios,
  1986) entire fuel bed is heated uniformly ?s
  fuel bed thickness

5
Basic theory

• Thin fuels opposed-flow velocity (USf) does not
  affect ideal (adiabatic, infinitely fast
  chemistry (mixing limited)) Sf
• 1g Almost always U gtgt Sf, thus U Sf U
• µg If no forced flow, then U Sf (self-induced
  convection)
• Thus, g does affect
• Convection-diffusion zone thickness ?x ?y
  ?/(USf)
• Larger at µg
• Diffusive transport time scale (tdiff) ?/(USf)
  ?/(USf)2
• Much larger at µg
• Heat loss parameter H tdiff/trad
  ?/(USf)2trad
• Much larger at µg
• Large U tdiff lt tchem - blow-off limit (like
  blowing out a match or candle)

6
Experiments - 1g - Fernandez-Pello et al., 1980

• Non-dimensional spread rate ( Sf,expt/Sf,deRis)
  as a function of Damköhler number Da (
  tdiff/tchem)
• Experiments consistent with model at large Da
• Buoyancy leads to existence of minimum U thus
  maximum Da
• Residence-time limited extinction at large U or
  low O2 (small Da)
• Thin fuels Thick fuels

7
Experiments - µg - Olson et al., 1988, 1991

• Characteristic relative velocity - combination of
  forced and buoyant flow
• Dual-limit behavior
• Residence-time limited (large U) tdiff tchem
• Heat loss (small U) tdiff trad
• Most robust U 10 cm/s - less than 1g buoyant
  flow!
• Infinite-rate kinetics limit not achieved at 21
  O2 !

8
Experiments - µg - Honda Ronney, 1998

• Radiation not all lost if ambient atmosphere
  absorbs Honda Ronney, 1998
• O2-N2, O2-He, O2-Ar Sf(1g) gt Sf(µg)
• O2-CO2, O2-SF6 Sf(1g) lt Sf(µg)
• International Space Station uses CO2 fire
  extinguishers!
• Behavior for non-radiating diluents attributed to
  radiative loss - µg flames thicker, more volume
• Behavior for radiating diluents attributed to
• Reabsorption of emitted radiation (reduced heat
  loss)
• Re-radiation to surface (increased Sf)

9
Honda Ronney (1998)
10
Flame spread - continued

• All fronts thicker at µg (? ?/U)
• With reabsorption, difference in thickness
  between 1g and µg is larger

19 O2 in N2 (optically thin)
42 O2 in SF6 (reabsorbing)
11
Schematic of radiation reabsorption
Absorption Re-radiation (CO2 or SF6)
Radiation from flame
Oxygen
U
Flame
Fuel
Fuel Bed
12
Schematic model of flame spread with radiation
13
Combined radiation convection

• Combining radiative flux ??g and conductive flux
  ?g(Tf - Tv)/?g with ?g ?g/(USf) leads to

Whenever radiation is important, convection
decreases Sf since the radiation-free Sf is
independent of U
14
Transition from radiation to convection

• Transition from radiatively-driven to
  conduction-driven flame spread occurs when
  radiative flux ??g comparable to conductive flux
  ?g(Tf - Tv)/?g
• ?g ?g/(USf), thus transition requires
• Same for thin or thick (but of course thin or
  thick affects Sf)

15
Theory of thick fuel flame spread

• Thick fuels ?s thermal penetration depth into
  solid determine by equating gas-phase solid
  phase heat flux
• Substitute into thin-fuel equation (deRis, 1969)
• Conventional wisdom steady Sf not possible at µg
  without forced flow since Sf U - indeterminate
• Unsteady analysis Sf t-1/2, Sf decreases until
  extinction due to heat losses (Altenkirch et al.,
  1996, 1998)
• At 1g buoyant flow provides U - steady spread
  possible

16
Thick fuels at µg - Alternkirch et al., 1996, 1998
17
Effect of flame-generated radiation on thick fuels

• de Ris (1968) Radiative transfer from external
  source to fuel bed leads to steady spread over
  thick fuel bed even if U 0
• q radiative flux per unit area, ? length of
  radiating zone
• but the hot gases also radiate, especially in
  O2-CO2 O2-SF6 atmospheres
• Estimation of radiative flux from flame to fuel
  bed
• q ?(?/(USf)) (? radiative emission per unit
  volume ? qr ) with U 0 leads to combined
  effects of radiation conduction

18
Convection effects

• U ? 0 response of radiatively-affected spread
  process to convection (U ? 0) can be
  non-monotonic
• low U means large ?, thus large volume of
  radiating gas
• .but large ? also means small conductive flux
• ? thick-fuel flame spread parameter (larger ?
  ? smaller Sf) (note deRis without radiation Sf
  U/?)
• Urad characteristic gas-phase radiation
  velocity

19
Convection effects

• Small U Sf not strongly dependent on ?
• Minimum Sf at intermediate U (U/Urad 1 - 2)
• Large U Sf U/? a la deRis

20
Thick fuel experiments at µg - approach

• Problem with conventional thick fuels
• Low Sf (e.g. PMMA)
• Time scale ?/Sf2 too large for drop towers
• Length scale ?/Sf possibly too large even in
  space
• Need very low ?s?sCp,s - use foams
• Also use high pressure - ?g higher, ? higher
• Fuels
• Polyphenolic floral foam, density 0.0290 g/cm3
• No melting, no distortion, low sooting
• 1 sided and 2 sided spread
• More recently - polyurethane foam, density 0.03
  g/cm3
• Measurements
• Imaging via direct video shearing
  interferometry
• Radiometers

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DROP APPARATUS
22
Sample Holder

• Ignition via Kanthal wire imbedded in
  nitrocellulose membrane
• Interferometer to image changes in gas density
  (side view)
• Direct video (front view)
• Spot radiometers aimed at the fuel surface or
  holes in fuel surface to measure radiant flux

Kanthal wire igniter
Nitrocellulose Membrane
Hole
Camera
Radiometers
Interferometer Field of view
Fuel
Front View
Side View
23
Images at 1g and µg
Front View
Side View
µg test
1g test
40 O2 in CO2 _at_ 4 atm, polyphenolic foam, density
0.027 g/cm3

• Thicker flame at µg (d a/U, U small at µg - no
  buoyant flow)
• Really thick flames even for fast flames in
  drop tower

24
Movies - µg flame spread

• 40 O2-CO2 _at_ 4atm 40 O2-N2 _at_ 4atm
• Polyphenolic fuel - sooty!

40 O2-CO2 _at_ 1 atm Polyurethane fuel - not
sooty!
25
Flame spread rate determination

• Steady Sf possible at µg
• With foam fuel, spread seems to reach steady Sf
  even in 2.2 sec drop tower test

26
Flame spread vs. O2 concentration

• For CO2, Sf at µg is higher than at 1g,
  especially diluent low O2 concentrations,
  whereas for He and N2, µg and 1g are similar
• At µg, Sf can be higher in CO2 than N2 at the
  same O2
• For CO2 but not N2, the minimum O2 concentration
  supporting combustion is lower at µg

27
Flame Spread vs. Pressure

• For N2, Sf (µg) ltlt Sf (1g) at low P, but for CO2,
  Sf (µg) Sf (1g)
• Radiation effects more important at high P -
  shorter absorption lengths - allows Sf (µg) gt Sf
  (1g)
• Low P less reabsorption, more loss, Sf (µg) lt
  Sf (1g)

28
Flame Spread vs. Pressure

• Model with no adjustable parameters reasonably
  consistent with experiments except at
• Low pressures - radiative heat loss
• High pressures - optically thick
• (factors not considered in simple model)

29
Flame spread rate vs. thickness

• Sf is independent of thickness (t) when t gt 2 mm
  (thermally-thick behavior)
• Thermally-thin behavior at t lt 2 mm (Sf is
  dependent on t)
• For thinnest samples, Sf (1-sided) 1/2 of Sf
  (2-sided) - consistent with the simple thermal
  model for thin fuels
• but trend NOT monotonic!

30
CO2 vs. He diluent

• CO2 much better than helium at 1g, but no better
  at µg
• He may be better extinguishant at µg
• Same efficacy per mole (? storage bottle mass
  volume)
• Much better per unit mass
• No physiological impact

31
Radiometer configurations (each set)
Flame
Radiation from flame
Back-side radiometer Views through hole -
measures incident gas radiation only
Front-side radiometers (2) (A) Views hole -
outward gas radiation only (B) Offset
horizontally from hole - outward gas solid
radiation
Hole
Fuel bed
32
Radiation (CO2 diluent, µg)
Blue gas-phase radiative loss only Red
gassurface radiative loss Green gas-phase
radiation to surface

• Radiation from front rear radiometers show
  similar intensity and timing - substantial
  re-absorption and re-radiation
• Surface radiation gt gas-phase peak is later
  (after flame passage)
• Substantial radiative flux to fuel bed -
  accelerates spread

33
Radiation (N2 diluent, µg)

• Radiation to rear-side radiometer small compared
  with CO2 diluent - less importance of gas-phase
  radiation to fuel surface
• Gas-phase loss significant - higher than CO2 -
  less reabsorption
• Peak surface radiative loss similar to CO2

Blue gas-phase radiative loss only Red
gassurface radiative loss Green gas-phase
radiation to surface
34
Radiation (CO2 diluent, 1g)
Red gas-phase radiative loss only Green
gassurface radiative loss Blue gas-phase
radiation to surface

• Gas-phase loss lt µg case due to thinner front
  (less volume)
• Negligible re-radiation to surface
• Surface radiative loss similar to µg

35
Radiation (N2 diluent, 1g)
Red gas-phase radiative loss only Green
gassurface radiative loss Blue gas-phase
radiation to surface

• Gas-phase loss lt µg case due to thinner front
  (less volume)
• Negligible re-radiation to surface
• Surface radiative loss similar to µg

36
Thermocouple data

• Penetration depth tp lt 2 mm (estimate  0.07 mm)
• Vaporization temperature Tv 600K

37
Fingering flame spread at µg

• Olson et al. 1998 - space experiments
• Strong forced flow - smooth fronts, similar to 1g
• Weak or no forced flow - fingering fronts
• Radiative or conductive loss gas-phase heat
  transfer lost heat transport through solid
  phase O2 transport can only occur through gas
  phase
• Two Lewis numbers?
• High U heat transport in gas phase Leeff
  ?gas/DO2 1
• U ? 0 heat transport through solid Leeff
  ?solid/DO2 ltlt 1

38
Olson et al. 1998
39
Fingering flame spread at 1g

• Similar behavior seen at 1g (Zik Moses, 1998)
  in narrow channel (suppresses buoyancy), high O2
  (prevent extinction), low flow velocity (solid
  phase dominates heat transport)

40
Fingering flame spread at 1g

• Similar behavior seen by Zhang et al. (1992) in
  downward spreading flames at 1g in O2-SF6 and
  O2-CO2 atmospheres

41
Summary - what have we learned from µg combustion
experiments?

• Time scales
• when buoyancy, radiation, etc. is important
• Radiative loss gas-phase soot
• causes many of the observed effects on burning
  rates extinction conditions
• double-edged sword - optically thin vs.
  reabsorbing
• Dual limits (high-speed blow-off low-speed
  radiative)
• seen for practically all types of flames studied
  to date
• Spherical flames (flame balls, droplets, candle
  flames)
• long time scales, large domains of influence,
  radiative loss
• Oscillations near extinction
• common, not yet fully understood
• Chemistry
• different reactions rate-limiting for very weak
  flames

42
Challenges for future work

• Radiative reabsorption effects
• Apparently seen in many µg flames
• Relevant to IC engines, large furnaces, EGR,
  flue-gas recirculation (d aP-1)
• Need faster computational models of radiative
  transport!
• High-pressure combustion
• Buoyancy effects (tchem/tvis) increase with P for
  weak mixtures
• Reabsorption effects increase with P
• Turbulence more problematic
• Few µg studies - mostly droplets
• 3-d effects
• Flame spread - effects of fuel bed width
• Flame balls - breakup of balls
• Spherical diffusion flames - porous sphere
  experiment - advantage over droplets - can
  examine steady state conditions

43
Perspective on space flight training

• 2 types of training
• Orbiter-related
• Launch entry
• Living in space
• Photography, videography
• Payload related
• Science background
• Procedures and schedules
• Performing experiments
• On-orbit repair
• Not like The Right Stuff now - STRAIGHTFORWARD
• Toughest part - TRAVEL

44
Perspective on space flight training
45
References

• Altenkirch, R.A., Tang, L., Sacksteder, K.,
  Bhattacharjee, S., Delichatsios, M.A. (1998).
  Proc. Combust. Inst. 272515.
• Delichatsios, M. A. (1986). Combust. Sci. Tech.,
  Vol. 44, pp. 257-267.
• deRis, J. N. (1969). Twelfth Symposium
  (International) on Combustion, The Combustion
  Institute, Pittsburgh, 1969, p. 241.
• Fernandez-Pello, A. C., Ray, S.R., Glassman, I.
  (1981). Eighteenth Symposium (International) on
  Combustion, The Combustion Institute, Pittsburgh,
  pp. 579.
• Honda, L. and Ronney, P. D. (1998). "Effects of
  Ambient Atmosphere on Flame Spread at
  Microgravity, Combust. Sci. Technol 133, 267-291
  (1998).
• Olson, S. L., Ferkul, P. V., Tien, J. S. (1988).
  Twenty-Second Symposium (International) on
  Combustion, Combustion Institute, p. 1213.
• Olson, S. L. (1991). Combust. Sci. Tech. 76,
  160.
• S. L. Olson, H. R. Baum and T. Kashiwagi (1998)
  Finger-Like Smoldering over Thin Cellulosic
  Sheets in Microgravity, Proc. Combust. Inst.
  272525.
• Son, Y., Ronney, P. D. (2002). "Radiation-Driven
  Flame Spread Over Thermally-Thick Fuels in
  Quiescent Microgravity Environments," Proc.
  Combust. Inst., Vol. 29 (to appear).
• West, J., Tang, L., Altenkirch, R.A.,
  Bhattacharjee, S., Sacksteder, K., Delichatsios,
  M.A. (1996). Proc. Combust. Inst. 261335-1343.
• Wichman, I. S. (1992). Prog. Energy Combust.
  Sci. 18, 553.
• Williams, F.A. (1976). Proc. Combust. Inst.
  161281.
• Zhang, Y., Ronney, P. D., Roegner, E., Greenberg,
  J. B. (1992). "Lewis Number Effects on Flame
  Spreading Over Thin Solid Fuels," Combustion and
  Flame, Vol. 90, pp. 71-83.
• O. Zik and E. Moses (1998). Fingering
  Instability in Solid Fuel Combustion The
  Characteristic Scales of the Developed State.
  Proc. Combust. Inst. 272815.