Paul D. Ronney Univ. of Southern California, Los Angeles, USA http://ronney.usc.edu/sofball National Central University Jhong-Li, Taiwan QuickTime and aSorenson Video decompressorare needed to see this picture. 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 g combustion Applications Spacecraft fire safety Better understanding of combustion at earth gravity Time scales (hydrocarbon-air, 1 atm) T Tiim mee ssccaallee S Stto oiicch h.. F Fllaam mee
((S SLL == 4400 ccm m//ss)) L Liim miitt ffllaam mee ((S SLL == 22 ccm m//ss)) C Ch heem miissttrryy ((ttcchheemm)) B Bu
uo oyyaan ntt,, iin nvviisscciid d ((ttiinnvv)) 00..0000009944 sseecc 00..007711 sseecc 00..2255 sseecc 00..007711 sseecc B Bu uo oyyaan ntt,, vviisscco ou
uss ((ttvviiss)) R Raad diiaattiio on n ((ttrraadd)) 00..001122 sseecc 00..1133 sseecc 00..001100 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 g g 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 U0 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* C ~ 1-1/r Temperature Fuel concentration T ~ 1/r
1.2 1 0.8 Flameball 0.6 Propagating flame (/rrf=1/r10) T Interior filled with combustion products Reaction zone
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 and aVideo decompressorare needed to see this picture. QuickTime and aVideo decompressorare needed to seeQuickTime this picture. Far from limit Close to limit 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) Flame balls - history Results confirmed in parabolic aircraft flights (Ronney et al., 1994) but g-jitter problematic QuickTime and a Video decompressor are needed to see this picture. KC135 g 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 Heatloss Tflame Impactofheatloss ~ ~ -E/RTflame as T flame (thus fuel %) Heatrelease e Predictions consistent with experimental observations 15 Unstable to 3-d disturbances 10 Stable
Equation of curve: -2 R ln(R) = Q 5 Unstable 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 g 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 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
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 QuickTime and a Motion JPEG A decompressor are needed to see this picture. Flame balls in space SOFBALL-1 (1997): flame balls
stable for > 500 seconds (!) QuickTime and a Video decompressor are needed to see this picture. QuickTime and a Video decompressor are needed to see this picture. 4.9% H2- 9.8% O2 - 85.3% CO2, 500 sec 4.0% H2-air, 223 sec elapsed time QuickTime and a Video decompressor are needed to see this picture. 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 g data indicated drift velocity (V) (gr **)1/2 Gr = O(1033) - V) (gr**)1/2 1/2 - like inviscid bubble rise In space, flame balls should drift into chamber walls after 10 min at 1 g Space experiments: Gr = O(10-1-1) - creeping flow - apparently need to use viscous relation: Similar to recent prediction (Joulin et al., 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 2
1 gr* V = 3 2 r b mo + mb gr* V 2.4 1 r o mo +1.5mb 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 Fuel concentration profile Lower fuel concentration Higher fuel concentration DRIFT DIRECTION Affected ball
Adjacent ball Flame ball drift 10 4.9% H - 9.8% O 2 2 - 85.3% CO 2 MSL-1/STS-83 3 flame balls Space experiments
QuickTime and a Video decompressor are needed to see this picture. Theory (Buckmaster & Ronney, 1998) 1 10 100 Time (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 QuickTime and a Video decompressor are needed to see this picture. 0.2 QuickTime and a Video decompressor are needed to see this picture. 80
Beginning of test 0.15 100 Beginning of test VCRS activities 0.15 60 0.1 40
0.05 80 0.1 60 0.05 40 20 0 0 20
0 -0.05 -0.1 -20 0 100 200 300 400 Time from ignition (seconds)
500 -0.05 -0.1 0 15:33:20 15:35:00 Without free drift With free drift 15:36:40 GMT 15:38:20
15:40:00 -20 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 QuickTime and a Video decompressor are needed to see this picture. Zeldovichs personal watch was flown on STS-94
Astronaut Janice Voss with Zeldovichs 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 QuickTime and aCinepak decompressorare needed to see this picture. Test point 14a (3.45% H2 in air, 3 atm), 1200 sec
total burn time QuickTime and aCinepak decompressorare needed to see this picture. 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 behave differently from H2-O2-SF6) ? Answer: patterns! methane fuel (CH4-O2-SF6 ) those in hydrogen fuel (e.g.
Yes! They frequently drifted in corkscrew We have no idea why. QuickTime and aCinepak decompressorare needed to see this picture. 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:
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 g experiments Expect surprises and be adaptable g 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) Thanks Dave, Ilan, KC and Mike! and the rest! And The Boss!
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