Paul D. Ronney, Ming-Shin Wu and Howard G. Pearlman

1. Structure Of Flame Balls At Low Lewis-number (SOFBALL): Results from the STS-83 & STS-94 Space Flight Experiments Paul D. Ronney, Ming-Shin Wu and Howard G. Pearlman Department of Aerospace and Mechanical Engineering University of Southern California, Los Angeles, CA 90089 Karen J. Weiland Microgravity Science Division NASA Lewis Research Center, Cleveland, OH 44135 Supported by NASA-Lewis

2. FLAME BALLS - history • Zeldovich, 1944: purely diffusion-controlled stationary spherical flames with steady radius (r*) possible since diffusion equations Ñ2T & Ñ2Y = 0 (T = temperature, Y = fuel mass fraction) have solutions for unbounded domain in spherical geometry: • Analogous to steady solution for spherical fuel droplet burning • Convection velocity V º 0 everywhere • Dynamics dominated by 1/r tail (with r3 volume effects!) - unlike propagating flame where T ~ e-r • BUT CAN THEY REALLY EXIST??? Most people don’t believe it at first • Buckmaster, 1985; Joulin, 1985: adiabatic flame balls are unstable - maybe they can’t exist???

3. Flame balls - history - continued • Ronney (1990): seemingly stable, stationary flame balls accidentally discovered in drop-tower experiment • MAYBE THEY ARE STABLE??? • Confirmed in parabolic aircraft flights (Ronney et al., 1993) • Only seen at microgravity (µg) (needed for spherical symmetry) • Only seen at low Le, near extinction limits • Buckmaster, Joulin & collaborators: window of stable conditions near extinction limits when radiative loss present & Le is low (richer mixtures: 3-d instability, splitting balls, propagating cellular flame front). Why should leaner mixtures (lower temperatures) have more radiative loss??? • Encouraging results, but stability & properties compromised by short duration of µg in drop towers, low quality of µg in aircraft - SKEPTICS NOT CONVINCED • NEED SPACE EXPERIMENT: long duration, high quality µg

4. Practical value of flame ball studies • Improved understanding of lean combustion • Benefit of lean combustion to efficiency & emission reduction in engines well known, but experience shows lean mixtures lead to misfire & rough operation • Need better models of weak combustion - determine ultimate limits of lean operation • Current H2- O2 chemical kinetic models disagree on flame ball properties, but all give similar SL away from limits - need better chemical models of weakly burning flames • H2-O2 essential building block of hydrocarbon-air chemistry • Stationary spherical flame - simplest interaction of chemistry & transport - test combustion models • Motivated > 20 theoretical papers to date • Spacecraft fire safety - flame balls exist in mixtures outside one-g extinction limits

5. Implementation of space experiment • Structure Of Flame Balls At Low Lewis-number (SOFBALL) experiment • Space Shuttle missions MSL-1 (April 4 - 8, 1997) & MSL-1R (July 1 - 16, 1997), • Combustion Module-1 (CM-1) facility • Test strategy • 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) • Only mixtures near limit, few flame balls, little or no splitting SOFBALL logo • 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 - 2 views, intensified video • Temperature - fine-wire thermocouples, 6 locations • Radiometers (4), chamber pressure, acceleration (3 axes) • Gas chromatograph

6. Summary of results • STS-83 - April 4 - 8, 1997 • Shortened mission, 2 tests performed, both successful • STS-94 - July 1 - 16, 1997 • Limited opportunity to make changes from STS-83 • Full mission, 17 tests performed, 16 successful, 10 reburn attempts, 8 successful • STABLE FLAME BALLS OBSERVED FOR ENTIRE EXPERIMENT DURATION (500 s) IN MOST CASES! • 1 to 9 flame balls • First premixed gas combustion experiment in space • Weakest flames ever burned (≈ 1 Watt/ball) (birthday candle ≈ 50 Watts) First 2 flame ball tests in space. It worked the first time!

7. How many flame balls do you get? Why does the number of flame balls usually increase with increasing fuel concentration? There is a 3-d instability of flame balls that occurs only for mixtures away from the limits (see slide: Flame Ball History: continued). The spark ignition source creates some excess enthalpy (above that of the ambient mixture.) This decreases the “effective” heat loss magnitude from the stable mixtures (near the limit) into the unstable region, causing splitting of flame balls. Eventually this excess enthalpy is smeared out by conduction and lost by radiation. The higher the initial fuel concentration, the more the ignition source decreases the effective heat loss into the unstable region and the longer the flame balls stay there, so more flame balls are formed before dropping back into the stable region. Of course, if the mixture is too rich, it never drops down into the stable region and the result is a continually expanding cellular front rather than stable flame balls. False-color flame ball images

8. Results - surprises - #1 of 4 • Very little buoyancy-induced drift - 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, even at 1 µg, due to this drift • Space experiments: Gr = O(10-1) - creeping flow - apparently need to use viscous relation: • With representative property values • Similar to recent prediction (Joulin et al., submitted) • Not yet verified experimentally • Much lower drift speeds with viscous formula - possibly hours before flame balls would drift into walls • Also - fuel consumption rates (1 - 2 Watts/ball) would allow several hours of burning time in some tests with only 1 ball!

9. Results - surprises - #2 of 4 • When more than one flame ball was produced, the 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 • Which is more important? For adiabatic flame balls, the two effects exactly cancel, according to a recent analysis (Buckmaster & Ronney, 1998). For non-adiabatic flame balls, fuel effect wins because thermal effect disappears at large spacings due to radiative loss - and all stable flame balls must be strongly influenced by radiative loss

10. Theory of flame ball mutual repulsion due to competition for fuel - comparison with experiments

11. Results - surprises - #3 of 4 • 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 Without free drift With free drift

12. Results - surprises - #3.5 of 4 • 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 Results - surprises - #4 of 4 • 2 missions, 26 burn tests, 1 atm & 3 atm, N2, CO2, SF6 diluents, 20x range of thermal diffusivity, 2600x range of Planck mean absorption length, 1 to 9 flame balls, yet Every single flame ball, without exception, produced between 1.0 and 1.8 Watts of radiant power !!!!! WHY???

13. Comparison with computation • Computational model (Wu et al., 1998a, 1998b) • 1-d, spherical, unsteady code (Rogg) • Detailed chemistry, transport, radiation • Isothermal, fixed composition at outer boundary • Study evolution over time to steady state or extinction • Comparision of radius, radiant emission & limit composition • Fair-poor (radius), good (radiation & limit) for H2-air (see plots at right - compare predicted r*vis to experimental data points) • Very poor for H2-O2-CO2, H2-O2-SF6 (next page)

14. Comparison of predicted & measured ball radii • Predictions sensitive to chemical mechanism, especially rate of H + O2 + H2O  HO2 + H2O • Different models of H2-O2 chemistry yield very different predictions of flame ball properties, even though all predict burning velocities of planar H2-air flames accurately! • Likelihood of radiation 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) (H2O radiation still included; optically thin & can pass through CO2 & SF6) • Need improved radiation models, including reabsorption effects

15. Conclusions • SOFBALL - dominant factors in flame balls: • (1) Far-field (1/r tail, r3 volume effects, r2/a time constant) Flame ball: a tiny dog wagged by an enormous tail • (2) Radiative heat loss • (3) Radiative reabsorption effects in CO2, SF6 • (4) Branching vs. recombination of H + O2 - since stable flame balls can occur only near extinction limits, they are like a “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 are quickly spoiled by space experiments • “Data feeding frenzy” by some investigators during STS-94 • Caution when interpreting accelerometer data - frequency range, averaging, integrated vs. peak

16. Further information • Web sites • USC (SOFBALL science): www-rcf.usc.edu/~ronney • NASA-Lewis (SOFBALL engineering): http://zeta.lerc.nasa.gov/cm1/SOFT.htm • NASA-Marshall (MSL-1 mission): http://liftoff.msfc.nasa.gov/shuttle/msl • Publications • Wu, M.-S., Liu, J. B. and Ronney, P. D., “Numerical Simulation of Diluent Effects on Flame Ball Structure and Dynamics,” this Symposium • Ju, Y, Masuya, G. and Ronney, P. D., “Effects of Radiative Emission and Absorption on the Propagation and Extinction of Premixed Gas Flames” this Symposium • Buckmaster, J. D. and Ronney, P. D., “Flame Ball Drift in the Presence of a Total Diffusive Heat Flux,” this Symposium. • Ronney, P. D., “Understanding Combustion Processes Through Microgravity Research,” this Symposium, (plenary lecture). • Ronney, P. D., “Premixed Laminar and Turbulent Flames at Microgravity,” to appear in Space Forum (1998). • Abid, M., Wu, M. S., Liu, J. B., Ronney, P. D., Ueki, M., K. Maruta, K., Kobayashi , H., Niioka, T. and VanZandt, D. M., “Experimental and Numerical Study of Flame Ball IR and UV Emissions,” to appear in Combustion and Flame (1998). • Wu, M. S., Ronney, P. D., Colantonio, R., VanZandt, D., “Detailed Numerical Simulation of Flame Ball Structure and Dynamics,” to appear in Combustion and Flame (1998). • Ronney, P. D., Wu, M. S., Pearlman, H. G. and Weiland, K. J., “Experimental Study of Flame Balls in Space: Preliminary Results from STS-83,” AIAA Journal, Vol. 36, pp. 1361-1368 (1998). • Ronney, P. D., Whaling, K. N., Abbud-Madrid, A., Gatto, J. L., Pisowicz, V. L., "Stationary Premixed Flames in Spherical and Cylindrical Geometries," AIAA Journal, Vol. 32, pp. 569-577 (1994). • Ronney, P. D., "Near-Limit Flame Structures at Low Lewis Number," Combustion and Flame, Vol. 82, pp. 1-14 (1990).

17. Suggestions for space flight investigators 1. Learn your lessons in drop towers and aircraft. Experience has shown time and again that preconceived expectations rarely survive the first test. Without exhaustive ground-based µg studies, one cannot hope to learn what might happen in space, what is worth pursuing and what will work to measure it. 2. Space experiments are not just extensions of ground-based microgravity experiments. A few seconds of low gravity won’t prepare you for what happens on the time scale of hundreds or thousands of seconds. Also, from a practical standpoint, space experiments will take on a life of their own because of the number of people involved, the time required, and the costs. You won’t have the flexibility and instant gratification of the two-grad-students-and-a-postdoc type of project. 3. Build, then design. Rarely is one smart enough (or lucky enough) to have thought of all the important aspects of the hardware. There is no point in planning out your entire experimental apparatus in great detail, just to learn as soon as you build it that it can’t possibly work the way you thought. Breadboard quick, simple crude versions of the subsystems and do your smoke-testing on those. 4. KIFS (Keep It Flexible Stupid). We go to space to do basic research. If we were sure what we would find, there wouldn’t be much point in going to space. Don’t expect to predict your results. So the rule of thumb is don’t hard-code any experiment parameters, no matter how unlikely it seems they would need to be changed. They will need to be changed. Make your instruments auto-ranging whenever possible. Make all gain settings, power levels, etc. adjustable somehow. KIFS is an especially difficult rule to enforce because most of the flexibility is obtained through software, and software can’t be written until the hardware design is frozen. Thus, software scheduling slides quickly to the right. But insist upon flexibility - it is your first (and maybe only) line of defense against unexpected in-flight results. 5. If it’s not verified on the flight hardware, it’s not verified. Take two identical pieces of equipment. Tell one it’s going to fly in space. Tell the other it will go through the same vibrations, handling, etc. but will not fly in space. Guess which one will break? Humans aren’t the only things that get nervous about flying in space. Just because the engineering model works, doesn’t mean the flight unit will. Insist on verifying the flight hardware - after making your mistakes on the breadboard unit and/or engineering model. 6. If this system works, and that system works, the two systems will work together - NOT! Make sure that all of your hardware is tested together as a unit before believing it will work. In one case, a piece of hardware (not CM-1) didn’t work because two thermocouple wires were wired into a connect one way on the sample, and with the opposite polarity on the instrument. The sample and instrument had never been tested together. This is easy to overlook because experiment hardware usually has to be delivered to the Cape early, so the schedule for sample preparation slides to the right.

18. Suggestions for space flight investigators • 7. If the test data hasn’t been evaluated, the testing isn’t complete Too frequently, extensive testing is performed on flight hardware, then the test data is not scrutinized to determined whether the hardwared passed or failed the test. Other than chemotherapy, few things in life are as unpleasant as reading test reports, but until you have proven that your system can pass a test, not just complete it, you’re not ready to fly. • 8. Hammer out your agreements in advance If you’re sharing a facility with other PIs, you’ll undoubtedly have conflicts about how to allocate scarce resoures like facility time, crew time, power, etc. Obviously, for nominal activities you’ll agree upon the allocation in advance. But what if the mission is cut short by a day (or 12, in the case of STS-83)? Agree on advance on what to do with limited resources. • 9. Prepare your in-flight and post-flight data analysis plans BEFORE you fly OK, you just ran your first test. Yippee! You’re a flight PI! Now, what are you going to do with that data to (1) make sure the equipment is working, (2) determine if your test parameters chosen pre-flight are optimal or not, and (3) determine if any weird effects (e.g., due to vernier thruster firings) are occurring? If your science team hasn’t figured out how to access the data and how to analyze it by now, you’re not going to get it analyzed during the mission and so your experiments will be run “open loop” without feedback from results. Also, reporting of findings is demanded during the mission (even “instant” interpretation takes too long for mission management and the press). Establish capabilities (e.g. image analysis and digital data plotting routines) long before flight. Have in your mind what the plotted data should look like, then be prepared to respond to other possible behaviors. Also, remember that you as the PI retain exclusive data rights for only a year, so you may not have time to ponder and debate with your students and colleagues. • 10. Fill you hip pocket Frequently crews, being composed mostly of Type A people, complete tasks ahead of schedule and are only too anxious to do more. Also, they may give up scheduled break periods. Sometimes missions are extended by an extra day. Have extra crew activities ready in case you find yourself offered an extra 30 minutes, 2 hours, 1 day, etc. The same applies for shortened missions. What if you find yourself with only half the power or time you had planned on? How can you maximize the science return using the remaining resources? • 11. Prepare for media attention. The public will want to know why your experiment is important and why it is worth spending their tax dollars on your pet project. Prepare very short (10 second) sound-bite type answers to these questions. You don’t need the same level of detail, justification and rigor that your scientific colleagues demand. Take media training, if offered.