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Microgravity combustion (Lecture 3)

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

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Microgravity combustion (Lecture 3)

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  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 AME 514 - October 21, 2004

  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, > $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! AME 514 - October 21, 2004

  3. Schematic model of flame spread AME 514 - October 21, 2004

  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-T∞) • 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 AME 514 - October 21, 2004

  5. Basic theory • Thin fuels: opposed-flow velocity (U+Sf) does not affect ideal (adiabatic, infinitely fast chemistry (mixing limited)) Sf • 1g: Almost always U >> 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 ~/(U+Sf) • Larger at µg • Diffusive transport time scale (tdiff) ≈ /(U+Sf) ≈ /(U+Sf)2 • Much larger at µg • Heat loss parameter H ~ tdiff/trad = /(U+Sf)2trad • Much larger at µg • Large U: tdiff < tchem - “blow-off” limit (like blowing out a match or candle) AME 514 - October 21, 2004

  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 AME 514 - October 21, 2004

  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 ! AME 514 - October 21, 2004

  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) > Sf(µg) • O2-CO2, O2-SF6: Sf(1g) < 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) AME 514 - October 21, 2004

  9. Honda & Ronney (1998) AME 514 - October 21, 2004

  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) AME 514 - October 21, 2004

  11. Schematic of radiation & reabsorption Absorption & Re-radiation (CO2 or SF6) Radiation from flame Oxygen U Flame Fuel Fuel Bed AME 514 - October 21, 2004

  12. Schematic model of flame spread with radiation AME 514 - October 21, 2004

  13. Combined radiation & convection • Combining radiative flux g and conductive flux g(Tf - Tv)/g with g = g/(U+Sf) leads to Whenever radiation is important, convection decreases Sf since the radiation-free Sf is independent of U AME 514 - October 21, 2004

  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/(U+Sf), thus transition requires • Same for thin or thick (but of course thin or thick affects Sf) AME 514 - October 21, 2004

  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 AME 514 - October 21, 2004

  16. Thick fuels at µg - Alternkirch et al., 1996, 1998 AME 514 - October 21, 2004

  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 ~ (/(U+Sf)) ( = radiative emission per unit volume = • qr )with U = 0 leads to combined effects of radiation & conduction: AME 514 - October 21, 2004

  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 AME 514 - October 21, 2004

  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 AME 514 - October 21, 2004

  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 AME 514 - October 21, 2004

  21. Drop Frame Shearing Window Digital Image Processing System Window Plate Mirror Test Chamber » Camera Beam Laser Mirror Diffuser Expander Mirror Fiber-optic Link VCR Side View DROP APPARATUS AME 514 - October 21, 2004

  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 AME 514 - October 21, 2004

  23. Images at 1g and µg Front View Side View µg test 1g test 40% O2 in CO2 @ 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 AME 514 - October 21, 2004

  24. 40% O2-CO2 @ 4atm 40% O2-N2 @ 4atm Polyphenolic fuel - sooty! Movies - µg flame spread 40% O2-CO2 @ 1 atm Polyurethane fuel - not sooty! AME 514 - October 21, 2004

  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 AME 514 - October 21, 2004

  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 AME 514 - October 21, 2004

  27. Flame Spread vs. Pressure • For N2, Sf (µg) << 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) > Sf (1g) • Low P: less reabsorption, more loss, Sf (µg) < Sf (1g) AME 514 - October 21, 2004

  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) AME 514 - October 21, 2004

  29. Flame spread rate vs. thickness • Sf is independent of thickness (t) when t > 2 mm (thermally-thick behavior) • Thermally-thin behavior at t < 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! AME 514 - October 21, 2004

  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 AME 514 - October 21, 2004

  31. Re-radiation (CO2 & SF6) Radiometer configurations (each set) Flame Radiation fromflame Back-side radiometer Views through hole - measures incidentgas radiation only Front-side radiometers (2) (A) Views hole - outwardgasradiation only (B) Offset horizontally from hole - outwardgas + solidradiation Hole Fuel bed AME 514 - October 21, 2004

  32. Radiation (CO2 diluent, µg) Blue: gas-phase radiative loss only Red: gas+surface 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 > gas-phase; peak is later (after flame passage) • Substantial radiative flux to fuel bed - accelerates spread AME 514 - October 21, 2004

  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: gas+surface radiative loss Green: gas-phase radiation to surface AME 514 - October 21, 2004

  34. Radiation (CO2 diluent, 1g) Red: gas-phase radiative loss only Green: gas+surface radiative loss Blue: gas-phase radiation to surface • Gas-phase loss < µg case due to thinner front (less volume) • Negligible re-radiation to surface • Surface radiative loss similar to µg AME 514 - October 21, 2004

  35. Radiation (N2 diluent, 1g) Red: gas-phase radiative loss only Green: gas+surface radiative loss Blue: gas-phase radiation to surface • Gas-phase loss < µg case due to thinner front (less volume) • Negligible re-radiation to surface • Surface radiative loss similar to µg AME 514 - October 21, 2004

  36. Thermocouple data • Penetration depth tp < 2 mm (estimate ≈ 0.07 mm) • Vaporization temperature Tv ≈ 600K AME 514 - October 21, 2004

  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 << 1 AME 514 - October 21, 2004

  38. Olson et al. 1998 AME 514 - October 21, 2004

  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) AME 514 - October 21, 2004

  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 AME 514 - October 21, 2004

  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 AME 514 - October 21, 2004

  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 AME 514 - October 21, 2004

  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 AME 514 - October 21, 2004

  44. Perspective on space flight training AME 514 - October 21, 2004

  45. References • Altenkirch, R.A., Tang, L., Sacksteder, K., Bhattacharjee, S., Delichatsios, M.A. (1998). Proc. Combust. Inst. 27:2515. • 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. 27:2525. • 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. 26:1335-1343. • Wichman, I. S. (1992). Prog. Energy Combust. Sci. 18, 553. • Williams, F.A. (1976). Proc. Combust. Inst. 16:1281. • 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. 27:2815. AME 514 - October 21, 2004

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