Flame Balls: Understanding Combustion in Microgravity | Paul D. Ronney & USC

1. Fire in space: results from STS-107 / Columbia's final mission Paul D. Ronney Univ. of Southern California, Los Angeles, USA http://ronney.usc.edu/sofball National Central University Jhong-Li, Taiwan October 4, 2005

2. OUTLINE • About USC & PDR • Motivation • Time scales • Flame balls • Summary

3. University of Southern California • Established 125 years ago this week! • …jointly by a Catholic, a Protestant and a Jew - USC has always been a multi-ethnic, multi-cultural, coeducational university • Today: 32,000 students, 3000 faculty • 2 main campuses: University Park and Health Sciences • USC Trojans football team ranked #1 in USA last 2 years

4. USC Viterbi School of Engineering • Naming gift by Andrew & Erma Viterbi • Andrew Viterbi: co-founder of Qualcomm, co-inventor of CDMA • 1900 undergraduates, 3300 graduate students, 165 faculty, 30 degree options • $135 million external research funding • Distance Education Network (DEN): 900 students in 28 M.S. degree programs; 171 MS degrees awarded in 2005 • More info: http://viterbi.usc.edu

5. Paul Ronney • B.S. Mechanical Engineering, UC Berkeley • M.S. Aeronautics, Caltech • Ph.D. in Aeronautics & Astronautics, MIT • Postdocs: NASA Glenn, Cleveland; US Naval Research Lab, Washington DC • Assistant Professor, Princeton University • Associate/Full Professor, USC • Research interests • Microscale combustion and power generation (10/4, INER; 10/5 NCKU) • Microgravity combustion and fluid mechanics (10/4, NCU) • Turbulent combustion (10/7, NTHU) • Internal combustion engines • Ignition, flammability, extinction limits of flames (10/3, NCU) • Flame spread over solid fuel beds • Biophysics and biofilms (10/6, NCKU)

6. Paul Ronney

7. 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

8. 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

9. µg methods • Drop towers - short duration (1 - 10 sec) (≈ trad), high quality (10-5go) • Aircraft - longer duration (25 sec), low quality (10-2go - 10-3go) • Sounding rockets - still longer duration (5 min), fair quality (10-3go - 10-6go) • Orbiting spacecraft - longest duration (16 days), best quality (10-5go - 10-6go)

10. “FLAME BALLS” • Zeldovich, 1944: stationary spherical flames possible • 2T & 2C = 0 have solutions for unbounded domain in spherical geometry • T(r) = C1 + C2/r- bounded as r  ∞ • Not possible for • Cylinder (T = C1 + C2ln(r)) • Plane(T = C1+C2r) • Mass conservation requires Uº0 everywhere (no convection) – only diffusive transport • Perfectly valid steady solution to the governing equations for energy & mass conservationfor any combustible mixture, but unstable for virtually all mixtures except…

11. “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

12. Flame balls - history • Zeldovich, 1944; Joulin, 1985; Buckmaster, 1985: adiabatic flame balls are unstable • Ronney (1990): seemingly stable, stationary flame balls accidentally discovered in very lean H2-air mixtures in drop-tower experiment • Farther from limit - expanding cellular flames Far from limit Close to limit

13. Flame balls - history • Only seen in mixtures having very low Lewis number • Flame ball: Lewis # effect is so drastic that flame temp. can greatly exceed adiabatic (planar flame) temp. (Tad)

14. Flame balls - history • Results confirmed in parabolic aircraft flights (Ronney et al., 1994) but g-jitter problematic KC135 µg aircraft test

15. Flame balls - history • Buckmaster, Joulin, et al.: window of stable conditions with(1) radiative loss near-limit, (2) low gravity & (3) low Lewis number (2 of 3 is no go!) • Predictions consistent with experimental observations

16. Flame balls - practical importance • Improved understanding of lean combustion • Spacecraft fire safety - flame balls exist in mixtures outside one-g extinction limits • Stationary spherical flame - simplest interaction of chemistry & transport - test combustion models • Motivated > 30 theoretical papers to date • The flame ball is to combustion research as the fruit fly is to genetics research

17. Practical importance

18. Space Experiments • Need space experiment - long duration, high quality µg • Structure Of Flame Balls At Low Lewis-number (SOFBALL) • Combustion Module facility • 3 Space Shuttle missions • STS-83 (April 4 - 8, 1997) • STS-94 (July 1 - 16, 1997) • STS-107 (Jan 16 - Feb 1, 2003)

19. Space experiments - mixtures • STS-83 & STS-94 (1997) - 4 mixture types • 1 atm H2-air (Le ≈ 0.3) • 1 atm H2-O2-CO2 (Le ≈ 0.2) • 1 atm H2-O2-SF6 (Le ≈ 0.06) • 3 atm H2-O2-SF6 (Le ≈ 0.06) • None of the mixtures tested in space will burn at earth gravity, nor will they burn as plane flames • STS-107 (2003) - 3 new mixture types • High pressure H2-air - different chemistry • CH4-O2-SF6 test points - different chemistry • H2-O2-CO2-He test points - higher Lewis number (but still < 1) - more likely to exhibit oscillating flame balls

20. Experimental apparatus • Combustion vessel - cylinder, 32 cm i.d. x 32 cm length • 15 individual premixed gas bottles • Ignition system - spark with variable gap & energy • Imaging - 3 views, intensified video • Temperature - fine-wire thermocouples, 6 locations • Radiometers (4), chamber pressure, acceleration (3 axes) • Gas chromatograph

21. Experimental apparatus

22. Flame balls in space • SOFBALL-1 (1997): flame balls stable for > 500 seconds (!) 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

23. Surprise #1 - steadiness of flame balls • Flame balls survived much longer than expected without drifting into chamber walls • Aircraft µg data indicated drift velocity (V) ≈ (gr*)1/2 • Gr = O(103) - V) ≈ (gr*)1/2 - like inviscid bubble rise • In space, flame balls should drift into chamber walls after ≈ 10 min at 1 µg • Space experiments: Gr = O(10-1) - creeping flow - apparently need to use viscous relation: • Similar to recent prediction (Joulin et al., submitted) • Much lower drift speeds with viscous formula - possibly hours before flame balls would drift into walls • Also - fuel consumption rates (1 - 2 Watts/ball) could allow several hours of burn time

24. Surprise #2 - 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

25. Flame ball drift

26. Surprise #3: 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

27. G-jitter effects on flame balls Without free drift With free drift

28. 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

29. Zel’dovich’s personal watch was flown on STS-94

30. Astronaut Janice Voss with Zel’dovich’s watch

31. Changes from SOFBALL-1 to SOFBALL-2 • SpaceHab vs. SpaceLab module • Higher energy ignition system - ignite weaker mixtures nearer flammability limit • Much longer test times (up to 10,000 sec) • Free drift provided for usable radiometer data • 3rd intensified camera with narrower field of view - improved resolution of flame ball imaging • Extensive ground commanding capabilities added - reduce crew time scheduling issues

32. SOFBALL-2 objectives based on SOFBALL-1 results • Can flame balls last much longer than the 500 sec maximum test time on SOFBALL-1 if free drift (no thruster firings) can be maintained for the entire test? • Answer: not usually - some type of flame ball motion, not related to microgravity disturbances, causes flame balls to drift to walls within ≈ 1500 seconds -but there was an exception • We have no idea what caused this motion - working hypothesis is a radiation-induced migration of flame ball • The shorter-than-expected test times meant enough time for multiple reburns of each mixture within the flight timeline

33. Example videos made from individual frames Test point 14a (3.45% H2 in air, 3 atm), 1200 sec total burn time Test point 6c (6.2% H2 - 12.4% O2 - balance SF6, 3 atm), 1500 sec total burn time

34. SOFBALL-2 objectives based on SOFBALL-1 results • Do the flame balls in methane fuel (CH4-O2-SF6 ) behave differently from those in hydrogen fuel (e.g. H2-O2-SF6) ? • Answer: Yes! They frequently drifted in corkscrew patterns!We have no idea why. 9.9% CH4 - 19.8% O2 - 70.3% SF6

35. Summary of results - all flights • SOFBALL hardware performed almost flawlessly on all missions • 63 successful tests in 33 different mixtures • 33 flame balls on STS-107 were named by the crew) • Free drift: microgravity levels were excellent (average accelerations less than 1 micro-g for most tests) • Despite the loss of Columbia on STS-107, much data was obtained via downlink during mission • ≈ 90% of thermocouple, radiometer & chamber pressure • ≈ 90% of gas chromatograph data • ≈ 65% (24/37) of runs has some digital video frames (not always a complete record to the end of the test) - video data need to locate flame balls in 3D for interpretation of thermocouple and radiometer data

36. Accomplishments • First premixed combustion experiment in space • Weakest flames ever burned, either in space or on the ground (≈ 0.5 Watts) (Birthday candle ≈ 50 watts) • Leanest flames ever burned, either in space or on the ground (3.2 % H2 in air; equivalence ratio 0.078) (leanest mixture that will burn in your car engine: equivalence ratio ≈ 0.7) • Longest-lived flame ever burned in space (81 minutes)

37. Conclusions • SOFBALL - dominant factors in flame balls: • Far-field (1/r tail, r3 volume effects, r2/a time constant) • Radiative heat loss • Radiative reabsorption effects in CO2, SF6 • Branching vs. recombination of H + O2 - flame balls like “Wheatstone bridge” for near-limit chemistry • General comments about space experiments • Space experiments are not just extensions of ground-based µg experiments • Expect surprises and be adaptable • µg investigators quickly spoiled by space experiments “Data feeding frenzy” during STS-94 • Caution when interpreting accelerometer data - frequency range, averaging, integrated vs. peak

38. Thanks to… • National Central University • Prof. Shenqyang Shy • Combustion Institute (Bernard Lewis Lectureship) • NASA (research support)

39. Thanks Dave, Ilan, KC and Mike!

40. …and the rest!

41. And ‘The Boss’!