Paul D. Ronney

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 - PREMIXED-GAS FLAMES

• Chemical time scale
• tchem d/SL (a/SL)/SL a/SL2
• (a thermal diffusivity, SL laminar flame
  speed)
• Buoyant transport time scales
• 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)
• Radiation time scale
• trad Tf/(dT/dt) Tf/(L/rCp)
• Optically thin radiation L 4sap(Tf4 T84)
• (ap Planck mean absorption coefficient)
• Þ trad P/sap(Tf4 T84) P0, P pressure

9
Time scales (hydrocarbon-air, 1 atm)

• Conclusions
• Buoyancy unimportant for near-stoichiometric
  flames
• (tinv tvis gtgt tchem)
• Buoyancy strongly influences near-limit flames at
  1g
• (tinv tvis lt tchem)
• Radiation effects unimportant at 1g (tvis ltlt
  trad tinv ltlt trad)
• Radiation effects dominate flames with low SL
• (trad tchem), but only observable at µg

10
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)

11
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 ? 8
• Not possible for
• Cylinder (T C1 C2ln(r))
• Plane (T C1C2r)

• Mass conservation requires U º 0 everywhere (no
  convection) only diffusive transport
• Perfectly valid steady solution to the governing
  equations for energy mass conservation for any
  combustible mixture, but unstable for virtually
  all mixtures except

12
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

13
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
14
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)

15
Flame balls - history

• Results confirmed in parabolic aircraft flights
  (Ronney et al., 1994) but g-jitter problematic

KC135 µg aircraft test
16
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

17
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 gt 30 theoretical papers to date
• The flame ball is to combustion research as the
  fruit fly is to genetics research

18
Practical importance
19
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)

20
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 lt 1) - more likely to exhibit
  oscillating flame balls

21
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

22
Experimental apparatus
23
Flame balls in space

• SOFBALL-1 (1997) flame balls stable for gt 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
24
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

25
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 others 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

26
Flame ball drift
27
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

28
G-jitter effects on flame balls
Without free drift With free drift
29
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 ?g dt
• Consistent with added mass effect - maximum
  possible acceleration of spherical bubble is 2g

30
Surprise 4 heat release from flame balls

• 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???

31
Zeldovichs personal watch was flown on STS-94
32
Astronaut Janice Voss with Zeldovichs watch
33
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

34
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

35
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
36
Example videos made from individual frames
Wide field of view camera Narrow field of
view camera Test point 9a (3.32 H2 in air, 1
atm), 470 sec total burn time
37
Hypothesized mechanism of flame ball drift

• Reabsorption of emitted radiation is a probably
  significant factor for all flame balls (discussed
  later)
• For most gases, opacity decreases as T increases
• A small increase in T in some radial direction
  will lead to more radiative transfer (longer
  absorption length) in that direction
• Previous work (Buckmaster and Ronney, 1998) shows
  that flame balls will drift up temperature
  gradients
• This drift will decrease/increase the
  convection-diffusion zone thickness in the
  upstream/downstream direction, thereby amplifying
  this gradient and encouraging drift
• Mineav, Kagan, Joulin, Sivashinsky (CTM, 2000)
  propose a mechanism for self-drift but
  predictions suggest it exists only for flame
  balls larger than 3D stability limit

38
SOFBALL-2 objectives based on SOFBALL-1 results

• Can oscillating flame balls be observed in
  long-duration, free-drift conditions?
• Answer Probably - but need to check to see if
  flame ball motion rather than inherent
  oscillations of stationary flame ball caused
  radiometer data to show oscillations

39
SOFBALL-2 objectives based on SOFBALL-1 results

• Are higher Lewis number flame balls (e.g.
  H2-O2-He-CO2, Le  0.8) more likely to oscillate,
  as predicted theoretically?
• Answer No. These flames were extremely stable.
• Test point 11C 8 H2 - 16 O2 - 7.6 CO2 - 68.4
  He

40
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
41
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

42
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)

43
Parting comments

• When the Gods want to punish you they answer your
  prayers. It will take us a long time to analyze
  data mine all of the data obtained on STS-107
  (due to extensive downlinking during the mission)
• Flame balls live by the old stage performer motto
  leave em wanting more Several tests were
  expected to last gt 1 hour, but none did because
  of the mysterious drift, UNTIL
• the very last test 9 flame balls formed
  initially and extinguished one by one until only
  one (Kelly) remained. Unexpectedly, Kelly
  survived 81 minutes, seemingly immune to drift,
  until it was intentionally extinguished due to
  operational limitations (it was still burning at
  the time).
• BUT WHY DIDNT KELLY DRIFT????

44
Orbit 2 flame balls (lead flame ball Kelly)

• 7.5 H2 - 15 O2 - 77.5 SF6, 3 atm

Camera 1 view
Camera 2 (orthogonal) view
First 15 minutes only shown
45
Comparison of predicted measured radii

• 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

46
Comparison of predicted measured radii

• Unsatisfactory agreement with experiment - even
  with chemical models that correctly predict
  planar H2-air burning velocities!

H2-air mixtures, 1 atm
47
Comparison of predicted measured radii

• Results sensitive to H O2 H2O HO2 H2O -
  not important for planar flames away from limits
• Also depend strongly on rate of H O2 OH O,
  but everybody agrees on this rate!

48
Chemical rate discrepancies

• Competition between branching recombination
  depends not only on M P, but also Chaperon
  efficiencies, esp. H2O

49
Reabsorption effects in flame balls

• Not included in radiation model but
• Lplanck,CO2 3.5 cm at 300K Lplanck, SF6
  0.26 cm at 300K
• Decreases heat loss, widens flammability limits
• Agreement much better when CO2 SF6 radiation
  ignored! (limit of zero absorption length for CO2
  SF6)
• Still better with optically thick model (Ju et
  al.)

H2-O2-CO2 mixtures (H2O2 12)
50
Other examples of spherical flames - droplets

• Spherically-symmetric model (Godsave, Spalding
  1953)
• Steady burning possible - similar to flame balls
• (large radii transport diffusion-dominated)
• 1st µg experiment - Kumagai (1957) - burning rate
  (µg) lt burning rate (1g)

51
Other examples of spherical flames - candles

• 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 gt df 2/a
• Eventual extinguishment - probably due to O2
  depletion

1g µg
52
Spherical flames - oscillations

• Oscillations seen before extinguishment
• 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)
• Predicted but not yet seen in flame balls!

53
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

54
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
• 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

55
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
• Most practical flames occur at high pressure
• Buoyancy effects (tchem/tvis) increase with P for
  weak mixtures
• Reabsorption effects increase with P
• Turbulence more problematic
• Few µg studies - mostly droplets
• Chemical models
• µg studies reveal limitations of existing
  reaction rate data
• 3-d effects
• Flame balls - breakup of balls

56
Crew operations
57
Thanks to

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

58
Thanks Dave, Ilan, KC and Mike!
59
and the rest!
60
And The Boss!