Paul D. RonneyPaul D. RonneyUniv. of Southern California, Los Angeles, USAUniv. of Southern California, Los Angeles, USA

http://ronney.usc.edu/sofballhttp://ronney.usc.edu/sofball

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National Central UniversityNational Central University

Jhong-Li, TaiwanJhong-Li, Taiwan

October 4, 2005October 4, 2005

OUTLINEOUTLINE

About USC & PDRAbout USC & PDR MotivationMotivation Time scalesTime scales Flame ballsFlame balls SummarySummary

University of Southern CaliforniaUniversity of Southern California

Established 125 years ago Established 125 years ago this week!this week! ……jointly by a Catholic, a Protestant and a Jew - USC has jointly by a Catholic, a Protestant and a Jew - USC has

always been a multi-ethnic, multi-cultural, coeducational always been a multi-ethnic, multi-cultural, coeducational universityuniversity

Today: 32,000 students, 3000 facultyToday: 32,000 students, 3000 faculty 2 main campuses: University Park and Health Sciences2 main campuses: University Park and Health Sciences USC Trojans football team ranked #1 in USA last 2 yearsUSC Trojans football team ranked #1 in USA last 2 years

USC Viterbi School of EngineeringUSC Viterbi School of Engineering

Naming gift by Andrew & Erma ViterbiNaming gift by Andrew & Erma Viterbi Andrew Viterbi: co-founder of Qualcomm, co-inventor of CDMAAndrew Viterbi: co-founder of Qualcomm, co-inventor of CDMA 1900 undergraduates, 3300 graduate students, 165 faculty, 30 1900 undergraduates, 3300 graduate students, 165 faculty, 30

degree optionsdegree options $135 million external research funding$135 million external research funding Distance Education Network (DEN): 900 students in 28 M.S. Distance Education Network (DEN): 900 students in 28 M.S.

degree programs; degree programs; 1171 MS degrees awarded in 200571 MS degrees awarded in 2005 More info: More info: http://viterbi.usc.eduhttp://viterbi.usc.edu

Paul RonneyPaul Ronney B.S. Mechanical Engineering, UC BerkeleyB.S. Mechanical Engineering, UC Berkeley M.S. Aeronautics, CaltechM.S. Aeronautics, Caltech Ph.D. in Aeronautics & Astronautics, MITPh.D. in Aeronautics & Astronautics, MIT Postdocs: NASA Glenn, Cleveland; US Naval Research Lab, Postdocs: NASA Glenn, Cleveland; US Naval Research Lab,

Washington DCWashington DC Assistant Professor, Princeton UniversityAssistant Professor, Princeton University Associate/Full Professor, USCAssociate/Full Professor, USC Research interestsResearch interests

Microscale combustion and power generation Microscale combustion and power generation (10/4, INER; 10/5 NCKU)(10/4, INER; 10/5 NCKU)

Microgravity combustion and fluid mechanics Microgravity combustion and fluid mechanics (10/4, NCU)(10/4, NCU) Turbulent combustion Turbulent combustion (10/7, NTHU)(10/7, NTHU) Internal combustion enginesInternal combustion engines Ignition, flammability, extinction limits of flames Ignition, flammability, extinction limits of flames (10/3, NCU)(10/3, NCU) Flame spread over solid fuel bedsFlame spread over solid fuel beds Biophysics and biofilms Biophysics and biofilms (10/6, NCKU)(10/6, NCKU)

Paul RonneyPaul Ronney

MOTIVATIONMOTIVATION

Gravity influences combustion throughGravity influences combustion through Buoyant convectionBuoyant convection Sedimentation in multi-phase systemsSedimentation in multi-phase systems

Many experimental & theoretical studies of µg combustionMany experimental & theoretical studies of µg combustion ApplicationsApplications

Spacecraft fire safetySpacecraft fire safety Better understanding of combustion at earth gravityBetter understanding of combustion at earth gravity

Time scales (hydrocarbon-air, 1 atm)Time scales (hydrocarbon-air, 1 atm)

ConclusionsConclusions Buoyancy unimportant for near-stoichiometric flamesBuoyancy unimportant for near-stoichiometric flames

(t(tinv inv & t& tvisvis >> t >> tchemchem)) Buoyancy strongly influences near-limit flames at 1gBuoyancy strongly influences near-limit flames at 1g

(t(tinv inv & t& tvisvis < t < tchemchem)) Radiation effects unimportant at 1g (tRadiation effects unimportant at 1g (tvisvis << t << tradrad; t; tinvinv << t << tradrad)) Radiation effects dominate flames with low SRadiation effects dominate flames with low SLL

(t(tradrad ≈ t ≈ tchemchem), but only observable at µg), but only observable at µg

TTTiiimmmeee ssscccaaallleee SSStttoooiiiccchhh... FFFlllaaammmeee

(((SSSLLL === 444000 cccmmm///sss)))

LLLiiimmmiiittt ffflllaaammmeee

(((SSSLLL === 222 cccmmm///sss)))

CCChhheeemmmiiissstttrrryyy (((tttccchhheeemmm))) 000...000000000999444 ssseeeccc 000...222555 ssseeeccc

BBBuuuoooyyyaaannnttt,,, iiinnnvvviiisssccciiiddd (((ttt iiinnnvvv))) 000...000777111 ssseeeccc 000...000777111 ssseeeccc

BBBuuuoooyyyaaannnttt,,, vvviiissscccooouuusss (((tttvvviiisss))) 000...000111222 ssseeeccc 000...000111000 ssseeeccc

RRRaaadddiiiaaatttiiiooonnn (((tttrrraaaddd))) 000...111333 ssseeeccc 000...444111 ssseeeccc

µg methodsµg methods

Drop towers - short duration Drop towers - short duration (1 - 10 sec) (≈ t(1 - 10 sec) (≈ tradrad), high ), high

quality (10quality (10-5-5ggoo)) Aircraft - longer duration (25 Aircraft - longer duration (25

sec), low quality sec), low quality (10(10-2-2ggoo - 10 - 10-3-3ggoo))

Sounding rockets - still Sounding rockets - still longer duration (5 min), fair longer duration (5 min), fair quality (10quality (10-3-3ggoo - 10 - 10-6-6ggoo))

Orbiting spacecraft - longest Orbiting spacecraft - longest duration (16 days), best duration (16 days), best quality (10quality (10-5-5ggoo - 10 - 10-6-6ggoo))

““FLAME BALLS”FLAME BALLS”

Zeldovich, 1944: stationary spherical Zeldovich, 1944: stationary spherical flames possibleflames possible

22T & T & 22C = 0 have solutions for C = 0 have solutions for unboundedunbounded domain in spherical domain in spherical geometrygeometry

T(r) = CT(r) = C11 + C + C22/r/r - bounded as r - bounded as r ∞ ∞ Not possible forNot possible for

Cylinder Cylinder (T = C(T = C11 + C + C22ln(r))ln(r)) PlanePlane (T = C(T = C11+C+C22r)r)

Mass conservation requires UMass conservation requires U0 everywhere (0 everywhere (no no convectionconvection) – only ) – only diffusivediffusive transport transport

Perfectly valid steady solution to the governing equations Perfectly valid steady solution to the governing equations for energy & mass conservationfor energy & mass conservation for any combustible for any combustible mixturemixture, , but unstable for virtually all mixtures except…but unstable for virtually all mixtures except…

““FLAME BALLS”FLAME BALLS”

T ~ 1/r - unlike propagating flame where T ~ T ~ 1/r - unlike propagating flame where T ~ ee-r-r

- dominated by 1/r tail (with r- dominated by 1/r tail (with r33 volume effects!) volume effects!)Flame ball: a tiny dog wagged by an enormous tailFlame ball: a tiny dog wagged by an enormous tail

Temperature

Fuel concentration

T ~ 1/r

Reaction zone

Interior filledwith combustion

products

Fuel & oxygen diffuse inward

Heat & products

diffuse outward

C ~ 1-1/r

T*

T∞

0

0.2

0.4

0.6

0.8

1

1.2

0.1 1 10 100Radius / Radius of flame

Propagating flame(δ/r

f=1/10)

Flame ball

Flame balls - historyFlame balls - history

Zeldovich, 1944; Joulin, 1985; Buckmaster, 1985: adiabatic Zeldovich, 1944; Joulin, 1985; Buckmaster, 1985: adiabatic flame balls are flame balls are unstableunstable

Ronney (1990): seemingly Ronney (1990): seemingly stablestable, , stationarystationary flame balls flame balls accidentallyaccidentally discovered in very lean H discovered in very lean H22-air mixtures in drop--air mixtures in drop-tower experiment tower experiment

Farther from limit - expanding cellular flamesFarther from limit - expanding cellular flames

Far from limitFar from limit Close to limitClose to limit

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Flame balls - historyFlame balls - history

Only seen in mixtures having very low Lewis numberOnly seen in mixtures having very low Lewis number

Flame ball: Lewis # effect is so drastic that flame temp. can Flame ball: Lewis # effect is so drastic that flame temp. can greatly exceed adiabatic (planar flame) temp. (Tgreatly exceed adiabatic (planar flame) temp. (Tadad))

Flame balls - historyFlame balls - history

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

KC135 µg aircraft testKC135 µg aircraft test

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Flame balls - historyFlame balls - history

Buckmaster, Joulin, Buckmaster, Joulin, et alet al.: window of .: window of stable stable conditions conditions withwith (1) radiative loss near-limit, (2) low gravity & (3) low (1) radiative loss near-limit, (2) low gravity & (3) low Lewis number (2 of 3 is no go!)Lewis number (2 of 3 is no go!)

Predictions consistent with experimental observationsPredictions consistent with experimental observations

0 0.05 0.1 0.15 0.20

5

10

15

Dimensionless heat loss (Q)

Unstable to 3-d disturbances

Equation of curve:

R-2ln(R) = Q

Unstable to 1-ddisturbances

Stable

€

Impact of heat loss~Heat loss

Heat release~

Tflame2

e-E/RTflame ⇑ as T flame (thus fuel %) ⇓

Flame balls - practical importanceFlame balls - practical importance

Improved understanding of lean combustionImproved understanding of lean combustion Spacecraft fire safety - flame balls exist in mixtures outside Spacecraft fire safety - flame balls exist in mixtures outside

one-g extinction limitsone-g extinction limits Stationary spherical flame - simplest interaction of Stationary spherical flame - simplest interaction of

chemistry & transport - test combustion modelschemistry & transport - test combustion models Motivated > 30 theoretical papers to dateMotivated > 30 theoretical papers to date The flame ball is to combustion research as the fruit fly is to The flame ball is to combustion research as the fruit fly is to

genetics researchgenetics research

Practical importancePractical importance

Space ExperimentsSpace Experiments

Need space experiment - long duration, high quality µgNeed space experiment - long duration, high quality µg Structure Of Flame Balls At Low Lewis-number (SOFBALL)Structure Of Flame Balls At Low Lewis-number (SOFBALL) Combustion Module facilityCombustion Module facility 3 Space Shuttle missions 3 Space Shuttle missions

STS-83 (April 4 - 8, 1997)STS-83 (April 4 - 8, 1997) STS-94 (July 1 - 16, 1997)STS-94 (July 1 - 16, 1997) STS-107 (Jan 16 - Feb 1, 2003)STS-107 (Jan 16 - Feb 1, 2003)

Space experiments - mixturesSpace experiments - mixtures

STS-83 & STS-94 (1997) - 4 mixture typesSTS-83 & STS-94 (1997) - 4 mixture types 1 atm H1 atm H22-air (Le ≈ 0.3)-air (Le ≈ 0.3) 1 atm H1 atm H22-O-O22-CO-CO22 (Le ≈ 0.2) (Le ≈ 0.2) 1 atm H1 atm H22-O-O22-SF-SF66 (Le ≈ 0.06) (Le ≈ 0.06) 3 atm H3 atm H22-O-O22-SF-SF66 (Le ≈ 0.06) (Le ≈ 0.06) None of the mixtures tested in space will burn at earth gravity, None of the mixtures tested in space will burn at earth gravity,

nor will they burn as plane flamesnor will they burn as plane flames STS-107 (2003) - 3 new mixture typesSTS-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 apparatusExperimental apparatus

Combustion vessel - cylinder, 32 cm i.d. x 32 cm lengthCombustion vessel - cylinder, 32 cm i.d. x 32 cm length 15 individual premixed gas bottles15 individual premixed gas bottles Ignition system - spark with variable gap & energyIgnition system - spark with variable gap & energy Imaging - 3 views, intensified videoImaging - 3 views, intensified video Temperature - fine-wire thermocouples, 6 locationsTemperature - fine-wire thermocouples, 6 locations Radiometers (4), chamber pressure, acceleration (3 axes)Radiometers (4), chamber pressure, acceleration (3 axes) Gas chromatographGas chromatograph

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Experimental apparatusExperimental apparatus

Flame balls in spaceFlame balls in space

SOFBALL-1 (1997): flame balls SOFBALL-1 (1997): flame balls stable for > 500 seconds (!)stable for > 500 seconds (!)

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are needed to see this picture. 4.0% H4.0% H22-air, 223 sec elapsed time-air, 223 sec elapsed time

4.9% H4.9% H22- 9.8% O- 9.8% O22 - 85.3% CO - 85.3% CO22, 500 sec, 500 sec

6.6% H6.6% H22- 13.2% O- 13.2% O22 - 79.2% SF - 79.2% SF66, 500 sec, 500 sec

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Surprise #1 - steadiness of flame ballsSurprise #1 - steadiness of flame balls Flame balls survived much longer than expected without drifting into chamber wallsFlame balls survived much longer than expected without drifting into chamber walls Aircraft µg data indicated drift velocity (V) ≈ (grAircraft µg data indicated drift velocity (V) ≈ (gr**))

1/21/2 Gr = O(10Gr = O(1033) - V) ≈ (gr) - V) ≈ (gr**))1/21/2 - like - like inviscidinviscid bubble rise bubble rise In space, flame balls should drift into chamber walls after ≈ 10 min at 1 µgIn space, flame balls should drift into chamber walls after ≈ 10 min at 1 µg

Space experiments: Gr = O(10Space experiments: Gr = O(10-1-1) - creeping flow - apparently need to use ) - creeping flow - apparently need to use viscousviscous relation: relation:

Similar to recent prediction (Joulin Similar to recent prediction (Joulin et al.et al., submitted), submitted) Much lower drift speeds with viscous formula - possibly Much lower drift speeds with viscous formula - possibly hourshours before flame balls would drift into walls before flame balls would drift into walls

Also - fuel consumption rates (1 - 2 Watts/ball) could allow several Also - fuel consumption rates (1 - 2 Watts/ball) could allow several hourshours of burn time of burn time

€

V =13

gr*2

νρb

ρo

−1⎛

⎝⎜

⎞

⎠⎟μo +μb

μo +1.5μb

⇒ V ≈2.4gr*

2

ν

Surprise #2 - flame ball driftSurprise #2 - flame ball drift Flame balls always drifted apart at a continually decreasing rateFlame balls always drifted apart at a continually decreasing rate Flame balls interact by Flame balls interact by

(A) warming each other - attractive(A) warming each other - attractive(B) depleting each other’s fuel - repulsive(B) depleting each other’s fuel - repulsive

Analysis (Buckmaster & Ronney, 1998)Analysis (Buckmaster & Ronney, 1998) AdiabaticAdiabatic flame balls, two effects flame balls, two effects exactly cancelexactly cancel Non-adiabaticNon-adiabatic flame balls, fuel effect wins - thermal effect disappears at large spacings due to radiative loss flame balls, fuel effect wins - thermal effect disappears at large spacings due to radiative loss

Higher fuelconcentration

Lower fuelconcentration

Fuel concentrationprofile

Affected ball Adjacent ball

DRIFTDIRECTION

Flame ball driftFlame ball drift

1

10

10 100 1000Time (seconds)

Space experiments

4.9% H2 - 9.8% O

2 - 85.3% CO

2

MSL-1/STS-833 flame balls

Theory (Buckmaster & Ronney, 1998)

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Surprise #3: g-jitter effects on flame ballsSurprise #3: g-jitter effects on flame balls

Radiometer data drastically affected by impulses caused by small VRCS thrusters used to control Orbiter attitudeRadiometer data drastically affected by impulses caused by small VRCS thrusters used to control Orbiter attitude Temperature data moderately affectedTemperature data moderately affected Vibrations (zero integrated impulse) - no effectVibrations (zero integrated impulse) - no effect

Flame balls & their surrounding hot gas fields are very sensitive accelerometers!Flame balls & their surrounding hot gas fields are very sensitive accelerometers! Requested & received “free drift” (no thruster firings) during most subsequent tests with superb resultsRequested & received “free drift” (no thruster firings) during most subsequent tests with superb results

G-jitter effects on flame ballsG-jitter effects on flame balls

Without free driftWithout free drift With free driftWith free drift

-0.1

-0.05

0

0.05

0.1

0.15

0.2

-20

0

20

40

60

80

0 100 200 300 400 500

Time from ignition (seconds)

VCRS activitiesBeginning of test

-0.1

-0.05

0

0.05

0.1

0.15

-20

0

20

40

60

80

100

15:33:20 15:35:00 15:36:40 15:38:20 15:40:00GMT

Beginning of test

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G-jitter effects on flame balls - continuedG-jitter effects on flame balls - continued

Flame balls seem to respond more strongly than Flame balls seem to respond more strongly than ballisticallyballistically to acceleration impulses, I.e. change in to acceleration impulses, I.e. change in ball velocity ≈ 2 ∫gball velocity ≈ 2 ∫g dtdt

Consistent with “added mass” effect - maximum Consistent with “added mass” effect - maximum possible acceleration of spherical bubble is 2gpossible acceleration of spherical bubble is 2g

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Zel’dovich’s personal watch was flown on STS-94Zel’dovich’s personal watch was flown on STS-94

Astronaut Janice Voss with Zel’dovich’s watchAstronaut Janice Voss with Zel’dovich’s watch

Changes from SOFBALL-1 to SOFBALL-2Changes from SOFBALL-1 to SOFBALL-2

SpaceHab vs. SpaceLab moduleSpaceHab vs. SpaceLab module Higher energy ignition system - Higher energy ignition system -

ignite weaker mixtures nearer ignite weaker mixtures nearer flammability limitflammability limit

Much longer test times (up to Much longer test times (up to 10,000 sec)10,000 sec)

Free drift provided for usable Free drift provided for usable radiometer dataradiometer data

3rd intensified camera with 3rd intensified camera with narrower field of view - improved narrower field of view - improved resolution of flame ball imagingresolution of flame ball imaging

Extensive ground commanding Extensive ground commanding capabilities added - reduce crew capabilities added - reduce crew time scheduling issuestime scheduling issues

SOFBALL-2 objectives based on SOFBALL-1 resultsSOFBALL-2 objectives based on SOFBALL-1 results

Can flame balls last much longer than the 500 sec Can flame balls last much longer than the 500 sec maximum test time on SOFBALL-1 if free drift (no thruster maximum test time on SOFBALL-1 if free drift (no thruster firings) can be maintained for the entire test?firings) can be maintained for the entire test? Answer: not usually - some type of flame ball motion, not Answer: not usually - some type of flame ball motion, not

related to microgravity disturbances, causes flame balls to related to microgravity disturbances, causes flame balls to drift to walls within ≈ 1500 seconds -drift to walls within ≈ 1500 seconds - but there was an but there was an exceptionexception

We have no idea what caused this motion - working We have no idea what caused this motion - working hypothesis is a radiation-induced migration of flame ballhypothesis is a radiation-induced migration of flame ball

The shorter-than-expected test times meant enough time for The shorter-than-expected test times meant enough time for multiple reburns of each mixture within the flight timelinemultiple reburns of each mixture within the flight timeline

Example videos made from individual framesExample videos made from individual frames

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Test point 14a (3.45% HTest point 14a (3.45% H22

in air, 3 atm), 1200 sec in air, 3 atm), 1200 sec total burn timetotal burn time

Test point 6c (6.2% HTest point 6c (6.2% H22 - 12.4% O - 12.4% O22

- balance SF- balance SF66, 3 atm), 1500 sec , 3 atm), 1500 sec

total burn timetotal burn time

SOFBALL-2 objectives based on SOFBALL-1 resultsSOFBALL-2 objectives based on SOFBALL-1 results

Do the flame balls in methane fuel (CHDo the flame balls in methane fuel (CH44-O-O22-SF-SF66 ) )

behave differently from those in hydrogen fuel (e.g. behave differently from those in hydrogen fuel (e.g. HH22-O-O22-SF-SF66) ?) ? Answer: Yes! They frequently drifted in corkscrew Answer: Yes! They frequently drifted in corkscrew

patterns!patterns! We have no idea why.We have no idea why.

9.9% CH9.9% CH44 - 19.8% O - 19.8% O22 - 70.3% SF - 70.3% SF66

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Summary of results - all flightsSummary of results - all flights

SOFBALL hardware performed almost flawlessly on all SOFBALL hardware performed almost flawlessly on all missionsmissions

63 successful tests in 33 different mixtures 63 successful tests in 33 different mixtures 33 flame balls on STS-107 were named by the crew)33 flame balls on STS-107 were named by the crew) Free drift: microgravity levels were excellent (average Free drift: microgravity levels were excellent (average

accelerations less than 1 micro-g for most tests)accelerations less than 1 micro-g for most tests) Despite the loss of Columbia on STS-107, much data was Despite the loss of Columbia on STS-107, much data was

obtained via downlink during missionobtained via downlink during mission ≈ ≈ 90% of thermocouple, radiometer & chamber pressure90% of thermocouple, radiometer & chamber pressure ≈ ≈ 90% of gas chromatograph data90% of gas chromatograph data ≈ ≈ 65% (24/37) of runs has some digital video frames (not 65% (24/37) of runs has some digital video frames (not

always a complete record to the end of the test) - video data always a complete record to the end of the test) - video data need to locate flame balls in 3D for interpretation of need to locate flame balls in 3D for interpretation of thermocouple and radiometer datathermocouple and radiometer data

AccomplishmentsAccomplishments

First premixed combustion experiment in space First premixed combustion experiment in space Weakest flames ever burned, either in space or on the Weakest flames ever burned, either in space or on the

ground (≈ 0.5 Watts) (Birthday candle ≈ 50 watts)ground (≈ 0.5 Watts) (Birthday candle ≈ 50 watts) Leanest flames ever burned, either in space or on the Leanest flames ever burned, either in space or on the

ground (3.2 % Hground (3.2 % H22 in air; equivalence ratio 0.078) (leanest in air; equivalence ratio 0.078) (leanest

mixture that will burn in your car engine: equivalence ratio mixture that will burn in your car engine: equivalence ratio ≈ 0.7)≈ 0.7)

Longest-lived flame ever burned in space (81 minutes)Longest-lived flame ever burned in space (81 minutes)

ConclusionsConclusions

SOFBALL - dominant factors in flame balls:SOFBALL - dominant factors in flame balls: Far-field (1/r tail, rFar-field (1/r tail, r33 volume effects, r volume effects, r22// time constant) time constant) Radiative heat lossRadiative heat loss Radiative reabsorption effects in CORadiative reabsorption effects in CO22, SF, SF66

Branching vs. recombination of H + OBranching vs. recombination of H + O22 - flame balls like - flame balls like

“Wheatstone bridge” for near-limit chemistry“Wheatstone bridge” for near-limit chemistry General comments about space experimentsGeneral comments about space experiments

Space experiments are Space experiments are notnot just extensions of ground-based µg just extensions of ground-based µg experimentsexperiments

Expect surprises and be adaptableExpect surprises and be adaptable µg investigators quickly spoiled by space experimentsµg investigators quickly spoiled by space experiments

““Data feeding frenzy” during STS-94Data feeding frenzy” during STS-94 Caution when interpreting accelerometer data - frequency Caution when interpreting accelerometer data - frequency

range, averaging, integrated vs. peakrange, averaging, integrated vs. peak

Thanks to…Thanks to…

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

Thanks Dave, Ilan, KC and Mike!Thanks Dave, Ilan, KC and Mike!

……and the rest!and the rest!

And ‘The Boss’!And ‘The Boss’!