Paul D. Ronney
Univ. of Southern California, Los Angeles, USA
http://ronney.usc.edu/sofball
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National Central University
Jhong-Li, Taiwan
October 4, 2005
OUTLINE
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About USC & PDR
Motivation
Time scales
Flame balls
Summary
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
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
Paul Ronney
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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)
Paul Ronney
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
Time scales (hydrocarbon-air, 1 atm)
T
Tiim
mee ssccaallee
C
Ch
heem
miissttrryy ((ttcchheemm))
B
Bu
uo
oyyaan
ntt,, iin
nvviisscciid
d ((ttiinnvv))
B
Bu
uo
oyyaan
ntt,, vviisscco
ou
uss ((ttvviiss))
R
Raad
diiaattiio
on
n ((ttrraadd))
S
Stto
oiicch
h.. F
Fllaam
mee
((S
SLL == 4400 ccm
m//ss))
L
Liim
miitt ffllaam
mee
((S
SLL == 22 ccm
m//ss))
00..0000009944 sseecc
00..007711 sseecc
00..001122 sseecc
00..2255 sseecc
00..007711 sseecc
00..001100 sseecc
00..1133 sseecc
00..4411 sseecc
 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
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)
“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 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…
“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
T*
1.2
C ~ 1-1/r
Temperature
•
Normalized temperature
(T - T ) / (T f - T )
1
Fuel concentration
T ~ 1/r
Interior filled
with combustion
products
Fuel & oxygen
diffuse inward
0.6
•
T•
0.8
Reaction zone
Heat &
products
diffuse outward
Flame ball
Propagating flame
(/r f = 1/10)
0.4
0.2
0
0.1
1
10
Radius / Radius of flame
100
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 droptower experiment
 Farther from limit - expanding cellular flames
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Far from limit
Close to limit
Flame balls - history
 Only seen in mixtures having very low Lewis number
Thermal diffusivity of the bulk mixture (  )
Le 
Mass diffusivity of scarce reactant into the bulk mixture (D)
 Flame ball: Lewis # effect is so drastic that flame temp.
can greatly exceed adiabatic (planar flame) temp. (Tad)
T T
Tflame ball  Tambient  ad ambient > Tad for Le < 1
Le

Flame balls - history
 Results confirmed in parabolic aircraft flights (Ronney et
al., 1994) but g-jitter problematic
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KC135 µg aircraft test
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!)
2
Heat loss
Tflame
Impact of heat loss ~
~ -E/RTflame  as Tflame (thus fuel % ) 
Heat release
e
 Predictions consistent with experimental observations
15
Dimensionless flame ball radius (R)
Uns table to 3 -d disturbances
10
Stable
Equation of curve :
-2
R ln(R) = Q
5
Uns table to 1 -d
disturbances
0
0
0.05
0.1
0.15
Dimensionless heat loss (Q)
0.2
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
Practical importance
Space Experiments
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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)
Space experiments - mixtures
 STS-83 & STS-94 (1997) - 4 mixture types
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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
Experimental apparatus
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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
Experimental apparatus
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Flame balls in space
 SOFBALL-1 (1997): flame balls
stable for > 500 seconds (!)
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4.9% H2- 9.8% O2 - 85.3% CO2, 500 sec
4.0% H2-air, 223 sec elapsed time
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6.6% H2- 13.2% O2 - 79.2% SF6, 500 sec
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:
2
2
1 gr* b  o  b
gr*
V
 V  2.4
 1
3  o o  1.5b

 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
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
Fuel concentration
profile
Lower fuel
concentration
Higher fuel
concentration
DRIFT
DIRECTION
Affected ball
Adjacent ball
Flame ball drift
10
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Radius of separation (cm)
4.9% H
- 9.8% O
2
2
- 85.3% CO
2
MSL-1/STS-83
3 flame balls
Space experiments
Theory (Buckmaster
& Ronney, 1998)
1
10
100
T ime (seconds)
1000
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
G-jitter effects on flame balls
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0.2
0.15
80
Beginning of test
VCRS activities
40
0.05
20
0
0
-0.05
-0.1
-20
0
100
200
300
400
T ime from ignition (seconds)
Without free drift
500
80
0.1
Radiometer voltage (V)
0.1
Acceleration (µg)
60
60
0.05
40
0
20
-0.05
0
-0.1
-20
15:33:20
15:35:00
15:36:40
GMT
15:38:20
15:40:00
With free drift
Acceleration (micro-g)
0.15
Radiometer voltage
100
Beginning of test
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
dt (mm/s)
4
STS-94/MSL-1R, TP 13AR
7.0% H - 14.0% O - 79.0% SF
2
2
2
6
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Impulse
0
2
1
Flame ball velocity
-1
0
100
200
300
Time from ignition (s)
400
500
(mm/s)
Impulse
g
 •
z
1
3
y
Drift velocity, V
3 atm total pressure
1 flame ball
0
Zel’dovich’s personal watch was flown on STS-94
Astronaut Janice Voss with Zel’dovich’s watch
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
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
Example videos made from individual frames
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Test point 14a (3.45% H2
in air, 3 atm), 1200 sec
total burn time
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Test point 6c (6.2% H2 - 12.4% O2
- balance SF6, 3 atm), 1500 sec
total burn time
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.
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9.9% CH4 - 19.8% O2 - 70.3% SF6
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
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)
Conclusions
 SOFBALL - dominant factors in flame balls:
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Far-field (1/r tail, r3 volume effects, r2/ 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
Thanks to…
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National Central University
Prof. Shenqyang Shy
Combustion Institute (Bernard Lewis Lectureship)
NASA (research support)
Thanks Dave, Ilan, KC and Mike!
…and the rest!
And ‘The Boss’!