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Paul D. Ronney Dept. of Aerospace & Mechanical Engineering University of Southern California Los Angeles, CA 90089-

Understanding combustion processes through microgravity research - Recent progress and future challenges. Paul D. Ronney Dept. of Aerospace & Mechanical Engineering University of Southern California Los Angeles, CA 90089-1453 USA http://carambola.usc.edu Supported by NASA-Glenn.

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Paul D. Ronney Dept. of Aerospace & Mechanical Engineering University of Southern California Los Angeles, CA 90089-

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  1. Understanding combustion processes through microgravity research - Recent progress and future challenges Paul D. Ronney Dept. of Aerospace & Mechanical Engineering University of Southern California Los Angeles, CA 90089-1453 USA http://carambola.usc.edu Supported by NASA-Glenn

  2. OUTLINE • Motivation • Time scales • Examples • Premixed-gas flames • Nonpremixed gas flames • Condensed-phase combustion • Summary • Challenges for future work University of Southern California - Department of Aerospace & Mechanical Engineering

  3. MOTIVATION • Gravity influences combustion through • Buoyant convection • Sedimentation in multi-phase systems • Many experimental & theoretical studies of µg combustion • Applications • Spacecraft fire safety • Better understanding of combustion at earth gravity University of Southern California - Department of Aerospace & Mechanical Engineering

  4. TIME SCALES - PREMIXED-GAS FLAMES • Chemical time scale tchem ≈ d/SL≈ (a/SL)/SL≈ a/SL2 (a = thermal diffusivity, SL = laminar flame speed) • Buoyant transport time scale t ~ d/V; V ≈ (gd(Dr/r))1/2 ≈ (gd)1/2 (g = gravity, d = characteristic dimension) Inviscid: tinv ≈ d/(gd)1/2 ≈ (d/g)1/2 Viscous: d ≈ n/V Þ tvis ≈ (n/g2)1/3 (n = viscosity) • Conduction time scale tcond ≈Tf/(dT/dt) ≈ d2/16a • Radiation time scale trad ≈Tf/(dT/dt) ≈ Tf/(L/rCp) Optically thin radiation: L = 4sap(Tf4 – T∞4) (ap = Planck mean absorption coefficient) Þ trad ~ P/sap(Tf4 – T∞4) ~ P0, P = pressure University of Southern California - Department of Aerospace & Mechanical Engineering

  5. Time scales (hydrocarbon-air, 1 atm) • Conclusions • Buoyancy unimportant for near-stoichiometric flames (tinv & tvis >> tchem) • Buoyancy strongly influences near-limit flames at 1g (tinv & tvis < tchem) • Radiation effects unimportant at 1g (tvis << trad; tinv << trad) • Radiation effects dominate flames with low SL (trad ≈ tchem), but only observable at µg • Radiation > conduction only for d > 3 cm • Re ~ Vd/n ~ (gd3/n2)1/2Þ turbulent flow at 1g for d > 10 cm University of Southern California - Department of Aerospace & Mechanical Engineering

  6. Premixed-gas flames – flammability limits • Too lean or too rich mixtures won’t burn - flammability limits • No limits without losses – no purely chemical criterion (Mukunda et al., 1990; Giovangigli & Smooke, 1992) • Models of limits due to losses - most important prediction: burning velocity at the limit (SL,lim) • Heat loss to walls: tchem ~ tcondÞ SL,lim ≈ 40a/d (Pelim = 40) (Spalding, 1957; Jarosinsky, 1983) • Upward propagation: rise speed at limit ~ (gd)1/2 (Levy, 1965); causes stretch extinction (Buckmaster & Mikolaitis, 1982): tchem ~ tinvÞ SL,lim ~ (ga2/d)1/4 • Downward propagation – sinking layer of cooling gases near wall outruns & “suffocates” flame (Jarosinsky et al., 1982) tchem ~ tvisÞ SL,lim ≈ 1.3(ga)1/3 University of Southern California - Department of Aerospace & Mechanical Engineering

  7. Premixed-gas flames – flammability limits Downward propagation Upward propagation Wang & Ronney, 1993 University of Southern California - Department of Aerospace & Mechanical Engineering

  8. Flammability limits – losses - continued… • Big tube, no gravity – what causes limits? • Radiation heat loss (Joulin & Clavin, Buckmaster, 1976) (no reabsorption) • Doesn’t radiative loss decrease for weaker mixtures, since temperature is lower? NO! • Predicted SL,lim consistent with µg experiments (Ronney, 1988; Abbud-Madrid & Ronney, 1990) University of Southern California - Department of Aerospace & Mechanical Engineering

  9. Reabsorption effects on premixed flames • Is radiation always a loss mechanism? • Reabsorption may be important when aP-1 < d • Blackbody particles - extend limits & increases SL (Eudier & Joulin, 1988; Abbud-Madrid & Ronney, 1993) • Gases – some radiation still escapes (Ju, Masuya, Ronney 1998) • Absorption spectra of products different from reactants • Spectra broader at high T than low T • Dramatic difference in SL & limits compared to optically thin University of Southern California - Department of Aerospace & Mechanical Engineering

  10. Premixed-gas flames - stretched flames • Nonuniform flow, unsteady/curved flames: “flame stretch” (A = flame area) • Strong stretch (S-1 ≈ tchem) extinguishes flames • Moderate stretch strengthens flames for Le < 1 University of Southern California - Department of Aerospace & Mechanical Engineering

  11. Premixed-gases - Self-Extinguishing Flames • Spherical expanding flames, Le < 1: stretch allows flames to exist in mixtures below radiative limit until rf too large & curvature benefit too weak • Dual limit: radiation at large rf, curvature-induced stretch at small rf (ignition limit) University of Southern California - Department of Aerospace & Mechanical Engineering

  12. Premixed-gas flames - stretched flames • Counterflow configuration (Tohoku group) • S = dU/dy – flame located where U = SL • Increased stretch pushes flame closer to stagnation plane - decreased volume of radiant products • Similar Le effects as curved flames • Results: Dual limits, flammability extension even for Le >1, multiple solutions (which ones are stable?) • Dual limits & Le effects seen in µg experiments, but evidence for multivalued behavior inconclusive University of Southern California - Department of Aerospace & Mechanical Engineering

  13. “FLAME BALLS” • Zeldovich, 1944: stationary spherical flames possible: Ñ2T & Ñ2C = 0 have solutions for unbounded domain in spherical geometry - T ~ C1 + C2/r - bounded as r  ∞ • Not possible for • Cylinder (T ~ C1 + C2ln(r)) • Plane (T ~ C1+C2r) • Mass conservation requires U º 0 everywhere (no convection) – only diffusive transport University of Southern California - Department of Aerospace & Mechanical Engineering

  14. “FLAME BALLS” • T ~ 1/r - unlike propagating flame where T ~ e-r - dominated by 1/r tail (with r3 volume effects!) • Flame ball: a tiny dog wagged by an enormous tail University of Southern California - Department of Aerospace & Mechanical Engineering

  15. Flame balls - history • Zeldovich, 1944; Joulin, 1985; Buckmaster, 1985: adiabatic flame balls are unstable • Ronney (1990): seemingly stable, stationary flame balls accidentally discovered H2-air mixtures in drop-tower experiment - but test duration very short • Confirmed in parabolic aircraft flights (Ronney et al., 1994) but g-jitter problematic Drop tower test Aircraft test University of Southern California - Department of Aerospace & Mechanical Engineering

  16. Flame balls - history • Buckmaster, Joulin & collaborators: window of stable conditions with radiative loss & low Lewis (Glenn?) number Buckmaster, Joulin, Ronney (1990) University of Southern California - Department of Aerospace & Mechanical Engineering

  17. Flame balls in space • STS-83 & STS-94, 1997 • Stable for > 500 seconds (!) • Very long evolution time scales ~ (br*)2/a ≈ 100 s • Weakest flames ever burned (1 – 2 Watts/ball) 4.0% H2-air, 223 sec elapsed time 4.9% H2- 9.8% O2 - 85.3% CO2, 500 sec 6.6% H2- 13.2% O2 - 79.2% SF6, 500 sec University of Southern California - Department of Aerospace & Mechanical Engineering

  18. Flame ball drift • Flame balls always drifted apart at a continually decreasing rate • Flame balls interact by • (A) warming each other - attractive • (B) depleting each other’s fuel - repulsive • Analysis (Buckmaster & Ronney, 1998) • Adiabatic flame balls, two effects exactly cancel • Non-adiabatic flame balls, fuel effect wins - thermal effect disappears at large spacings due to radiative loss University of Southern California - Department of Aerospace & Mechanical Engineering

  19. Flame ball drift University of Southern California - Department of Aerospace & Mechanical Engineering

  20. G-jitter effects on flame balls • Radiometer data drastically affected by impulses caused by small VRCS thrusters used to control Orbiter attitude • Temperature data moderately affected • Vibrations (zero integrated impulse) - no effect • Flame balls & their surrounding hot gas fields are very sensitive accelerometers! • Requested & received “free drift” (no thruster firings) during most subsequent tests with superb results University of Southern California - Department of Aerospace & Mechanical Engineering

  21. G-jitter effects on flame balls Without free drift With free drift University of Southern California - Department of Aerospace & Mechanical Engineering

  22. G-jitter effects on flame balls - continued • Flame balls seem to respond more strongly than ballistically to acceleration impulses, I.e. change in ball velocity ≈ 2 ∫gdt • Consistent with “added mass” effect - maximum possible acceleration of spherical bubble is 2g University of Southern California - Department of Aerospace & Mechanical Engineering

  23. Comparison of predicted & measured radii • Detailed numerical modeling (Yale, USC) • Unsatisfactory agreement with experiment - even with chemical models that correctly predict planar H2-air burning velocities! • Results sensitive to H + O2 + H2O ® HO2 + H2O - not important for planar flames away from limits H2-air mixtures, 1 atm University of Southern California - Department of Aerospace & Mechanical Engineering

  24. Chemical rate discrepancies Competition between branching & recombination depends not only on [M] ~ P, but also Chaperon efficiencies, esp. H2O University of Southern California - Department of Aerospace & Mechanical Engineering

  25. Reabsorption effects in flame balls • Probable reabsorption in mixtures diluted with CO2 & SF6 • Not included in radiation model but • Lplanck,CO2 ≈ 3.5 cm at 300K; • Lplanck, SF6 ≈ 0.26 cm at 300K • Reabsorption decreases heat loss, widens flammability limits • Agreement much better when CO2 & SF6 radiation ignored! (limit of zero absorption length for CO2 & SF6) H2-O2-CO2 mixtures (H2:O2 = 1:2) University of Southern California - Department of Aerospace & Mechanical Engineering

  26. Nonpremixed-gas flames - gas-jet flames • Counterflow flames • Nonpremixed flames – less freedom of movement – flame must lie where stoichiometric flux ratio maintained • Radiating gas volume ~ flame thickness ~ (a/S)1/2 • Computations & µg experiments – simple C-shaped dual-limit response • Conductive loss to burners at low S? (Smin)-1 ≈ tcond CH4-N2 vs. air (Maruta et al. 1998) University of Southern California - Department of Aerospace & Mechanical Engineering

  27. EXAMPLES - Condensed-phase - droplets • Spherically-symmetric model (Godsave, Spalding 1953) • Steady burning possible - similar to flame balls • (large radii: transport diffusion-dominated) • Mass burning rate = (π/4)rdddK; K = (8l/rdCP) ln(1+B) • Flame diameter df = dd ln(1+B) / ln(1+f) • Regressing droplet: ddo2 - dd(t)2 = Kt if quasi-steady • 1st µg experiment - Kumagai (1957) - K(µg) < K(1g) University of Southern California - Department of Aerospace & Mechanical Engineering

  28. EXAMPLES - Condensed-phase - droplets • Dual-limit behavior • Residence-time limited (small dd): tdrop = df2/a ≤ tchem • Heat loss (large dd): tdrop ≥ trad • Radiative limit at large dd confirmed by µg experiments Marchese, Dryer, Nayagam (1999) University of Southern California - Department of Aerospace & Mechanical Engineering

  29. Candle flames • Similar to quasi-steady droplet but near-field not spherical • Space experiments (Dietrich et al., 1994, 1997) • Nearly hemispherical at µg • Steady for many minutes - probably > df 2/a • Eventual extinguishment - probably due to O2 depletion 1g µg University of Southern California - Department of Aerospace & Mechanical Engineering

  30. Candle flames - oscillations • Oscillations before extinguishment, except for small df • Near-limit oscillations of spherical flames? (Matalon) • Edge-flame instability? (Buckmaster) • Both models require high Le & near-extinction conditions • Some evidence in droplets also (Nayagam et al., 1998) University of Southern California - Department of Aerospace & Mechanical Engineering

  31. Flame spread over solid fuel beds • deRis (1968); Delichatsios (1986): flame spread rate (Sf) with opposing flow U, infinite-rate kinetics (mixing limited) (thin fuel) - independent of U (thick fuel) - Sf ~ U1 • Diffusive transport time scale (tdiff) ≈ d/U ≈ a/U2 • Heat loss parameter H ~ tdiff/trad = a/U2trad University of Southern California - Department of Aerospace & Mechanical Engineering

  32. Condensed-phase combustion - flame spread • Sf lower at µg: U = Sf << U(1g)  higher H • Infinite-rate kinetics limit not achieved at 21% O2 ! • Dual-limit behavior • Residence-time limited (large U): tdiff ≤ tchem • Heat loss (small U): tdiff ≥ trad • Confirmed by thin-fuel µg experiments (Olson et al., 1990) • Most robust U ≈ 10 cm/s - less than 1g buoyant flow! University of Southern California - Department of Aerospace & Mechanical Engineering

  33. Flame spread - continued • Radiation not all lost if ambient atmosphere absorbs; Honda & Ronney, 1998: • O2-N2, O2-He, O2-Ar: Sf(1g) > Sf(µg) due to radiative loss • O2-CO2, O2-SF6: Sf(1g) < Sf(µg) due to reabsorption • International Space Station uses CO2 fire extinguishers! University of Southern California - Department of Aerospace & Mechanical Engineering

  34. Flame spread - continued • All fronts thicker at µg (d ≈ a/U) • With reabsorption, difference between 1g and µg is larger 19% O2 in N2 (optically thin) 42% O2 in SF6 (optically thick) University of Southern California - Department of Aerospace & Mechanical Engineering

  35. Flame spread - thick fuels • Steady Sf not possible at µg since Sf ~ U - chicken & egg problem • Instead Sf ~ t-1/2, decreases until extinction (Altenkirch et al., 1996) • Strongly radiating flames in O2-CO2 or O2-SF6 could spread steadily at µg (egg = radiation) • Evidence seen recently (Honda & Ronney, 2000?) using foam fuels (smaller lsrsCP,s faster Sf) University of Southern California - Department of Aerospace & Mechanical Engineering

  36. 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 = agas/DO2 ≈ 1 • U  0: heat transport through solid; Leeff = asolid/DO2 << 1  Air 6.5 cm/s University of Southern California - Department of Aerospace & Mechanical Engineering

  37. Summary - what have we learned? • 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 University of Southern California - Department of Aerospace & Mechanical Engineering

  38. 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 University of Southern California - Department of Aerospace & Mechanical Engineering

  39. Thanks to….. Angel Abbud-Madrid, Tom Avedisian, Yousef Bahadori, John Buckmaster, Mun Choi, Dan Dietrich, Ed Dreizin, Fred Dryer, Said Elgobashi, Gerard Faeth, Osamu Fujita, Guy Joulin, Yiguang Ju, Kaoru Maruta, Moshe Matalon, John Moore, Vedha Nayagam, Takashi Niioka, Sandra Olson, Howard Ross, Kurt Sacksteder, Dennis Stocker, Peter Sunderland, Gregory Sivashinsky, James Tien, Arvind Varma, Karen Weiland, Forman Williams, Ming-Shin Wu ….. and especially NASA-Glenn!!! University of Southern California - Department of Aerospace & Mechanical Engineering

  40. Droplet combustion - continued • Large droplets not quasi-steady • Extinction occurs at large dd, but dd decreases during burn - quasi-steady extinction not observable • K & df/dd not constant - depend on ddo & time • Large time scale for diffusion of radiative products to far-field & O2 from far-field • Soot accumulation dependent on ddo • Absorption of H2O from products by fuel University of Southern California - Department of Aerospace & Mechanical Engineering

  41. 0 sec 0.2 sec 0.3 sec 0.4 sec 0.5 sec 0.6 sec 0.7 sec 0.8 sec Soot formation in µg droplet combustion n-heptane in air (Lee et al., 1998) University of Southern California - Department of Aerospace & Mechanical Engineering

  42. Nonpremixed-gas flames - gas-jet flames • Flame height (Lf) and residence time (tjet) determined by equating diffusion time (d2/D) to convection time (Lf/U) • Mass conservation: U(0)d(0)2 ~ U(Lf)d(Lf)2 (round jet); U(0)d(0) ~ U(Lf)d(Lf) (slot jet) • Buoyant flow: U(Lf) ~ (gLf)1/2; nonbuoyant: U(Lf) = U(0) University of Southern California - Department of Aerospace & Mechanical Engineering

  43. Gas-jet flames - results • Lf ≈ same at 1g or µg for round jet Sunderland et al. (1998) - CH4/air University of Southern California - Department of Aerospace & Mechanical Engineering

  44. Flame lengths at 1g and µg • tjet larger at µg than 1g for round jet • Larger µg flame width ~ (Dtjet)1/2 - greater difference at low Re due to axial diffusion & buoyancy effects • Greater radiative loss fraction at µg (≈ 50% vs. 8%) Sunderland et al. (1998) - CH4/air University of Southern California - Department of Aerospace & Mechanical Engineering

  45. Turbulent flame lengths at 1g and µg • Turbulent flames: D ~ u’LI; u’ ~ Uo; LI ~ do • Lf ~ do (independent of Re) • Differences between 1g & µg seen even at high Re - buoyancy effects depend on entire plume Bahadori et al. (1997) - C3H8/air University of Southern California - Department of Aerospace & Mechanical Engineering

  46. Sooting gas-jet flames at 1g and µg 1g µg n-butane in air, 10mm diameter jet, Re = 42 - Fujita et al., 1997 • Typically greater at µg due to larger tjet - outweighs lower T • Smoke points seen at µg - WHY??? • tjet ~ Uo1/2 for buoyant flames BUT... • tjet independent of Uo for nonbuoyant flames ! • Axial diffusion effects negligible at Re > 50 • Thermophoresis effects - concentrates soot in annulus University of Southern California - Department of Aerospace & Mechanical Engineering

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