Transcript AME 436 Energy and Propulsion
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
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
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 i i i m e s c a l l l e S t t t o i i i c h .
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F l l l a m e ( ( ( S L = 4 0 c m / / / s ) ) ) L i i i m i i i t t t f f f l l l a m e ( ( ( S L = 2 c m / / / s ) ) ) C h e m i i i s t t t r r y ( ( ( t t t c h e m ) ) ) B u o y a n t t t , , , i i i n v i i i s c i i i d ( ( ( t t t i i i n v ) ) ) B u o y a n t t t , , , v i i i s c o u s ( ( ( t t t v i i i s ) ) ) R a d i i i a t t t i i i o n ( ( ( t t t r r r a d ) ) ) 0 .
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0 0 0 9 4 s e c 0 .
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0 7 1 s e c 0 .
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0 1 2 s e c 0 .
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1 3 s e c 0 .
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2 5 s e c 0 .
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0 7 1 s e c 0 .
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0 1 0 s e c 0 .
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4 1 s e c
Conclusions
Buoyancy unimportant for near-stoichiometric flames
(t inv & t vis >> t chem ) Buoyancy strongly influences near-limit flames at 1g
(t inv & t vis < t chem ) Radiation effects unimportant at 1g (t vis << t rad ; t Radiation effects dominate flames with low S L (t rad ≈ t chem ), but only observable at µg inv << t rad )
µg methods
Drop towers - short duration (1 - 10 sec) (≈ t rad ), high quality (10 -5 g o ) Aircraft - longer duration (25 sec), low quality (10 -2 g o - 10 -3 g o ) Sounding rockets still longer duration (5 min), fair quality (10 -3 g o - 10 -6 g o ) Orbiting spacecraft - longest duration (16 quality (10 -5 g o days), - 10 -6 g o ) best
“FLAME BALLS”
Zeldovich, 1944:
unbounded
stationary spherical flames possible
2 T &
2 C = 0 have solutions for domain in spherical geometry T(r) = C 1 + C 2 /r - bounded as r
∞ Not possible for
Cylinder (T = C 1
Plane Mass + C 2 ln(r)) (T = C 1 +C 2 r) conservation convection) requires U º 0 everywhere (no – only diffusive transport Perfectly valid steady solution to the governing equations for energy & mass conservation mixture , for any combustible 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 r
3
volume effects!)
Flame ball: a tiny dog wagged by an enormous tail
T * Temperature T • Interior filled with combustion products C ~ 1-1/r Fuel concentration T ~ 1/r Reaction zone Fuel & oxygen diffuse inward Heat & products diffuse outward 1.2
1 0.8
0.6
0.4
0.2
0 0.1
Flame ball Propagating flame (
/r f = 1/10) 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 H 2 -air mixtures in drop tower 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 Le
Thermal diffusivity of the bulk mixture (
) 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. (T ad ) T flame ball
T ambient
T ad
T ambient Le > T ad for Le < 1
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!) Impact of heat loss ~
Heat loss Heat release ~ 2 T flame
e
-E/RT flame
as T flame (thus fuel % )
Predictions consistent with experimental observations 15 Uns table to 3 -d disturbances 10 Stable Equation of curve : R -2 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
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
1 atm H 2 air (Le ≈ 0.3)
1 atm H 2 -O 2 -CO 2 (Le ≈ 0.2)
1 atm H 2 -O 2 -SF 6 (Le ≈ 0.06)
3 atm H 2 -O 2 -SF 6 (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 H 2 -air - different chemistry
CH 4 -O 2 -SF 6 test points - different chemistry
H 2 -O 2 -CO 2 -He test points - higher Lewis number (but still < 1) more likely to exhibit oscillating flame balls
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
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% H 2 - 9.8% O 2 - 85.3% CO 2 , 500 sec 4.0% H 2 -air, 223 sec elapsed time
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6.6% H 2 - 13.2% O 2 - 79.2% SF 6 , 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(10 3 ) 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:
V
1 3
gr
* 2
b o
1
o
o
1.5
b
b
V
2.4
gr
* 2
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 Higher fuel concentration
DRIFT DIRECTION
Affected ball Lower fuel concentration Adjacent ball
Flame ball drift
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10 4.9% H 2 - 9.8% O 2 - 85.3% CO MSL-1/STS-83 3 flame balls 2 Space experiments 1 Theory (Buckmaster & Ronney, 1998) 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
0.1
Beginning of test VCRS activities 0.05
-0.05
0 -0.1
0 100 200 300 400 T ime from ignition (seconds)
Without free drift
500 80 60 40
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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
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4 -1 2 STS-94/MSL-1R, TP 13AR 7.0% H 2 - 14.0% O 2 - 79.0% SF 6 3 atm total pressure 1 flame ball 1 Impulse 0 Flam e ball velocity 0 100 200 300 Time from ignition (s) 400 3 2 1 500 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% H 2 in air, 3 atm), 1200 sec total burn time Test point 6c (6.2% H 2 - 12.4% O 2 - balance SF 6 , 3 atm), 1500 sec total burn time
SOFBALL-2 objectives based on SOFBALL-1 results
Do the flame balls in methane fuel (CH 4 -O 2 -SF 6 behave differently from those in hydrogen fuel (e.g.
) H 2 -O 2 -SF 6 ) ?
Answer: patterns!
Yes!
They frequently drifted in corkscrew We have no idea why.
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9.9% CH 4 - 19.8% O 2 - 70.3% SF 6
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 % H 2 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:
Far-field (1/r tail, r 3 volume effects, r 2 /
time constant)
Radiative heat loss
Radiative reabsorption effects in CO 2 , SF 6
Branching vs. recombination of H + O 2 - 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…
National Central University Prof. Shenqyang Shy Combustion Institute (Bernard Lewis Lectureship) NASA (research support)