Fuel Combustion Study Using Nano Particles

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a. REPORT

Exploratory Study to Determine the Ignition Properties and Combustion

Chemistry of Explosives Using Shock Tube Methods

14. ABSTRACT

16. SECURITY CLASSIFICATION OF:

We investigated the potential of a new experimental method for combustion studies of solid nano-particle energetic materials.

This new method is based on the recent development in our laboratory of an aerosol shock tube and its combined use with

laser extinction (for characterization of particle size and loading) and absorption diagnostics (for characterization of vapor

species). We accomplished three main objectives in this study: 1) we demonstrated the ability to load significant amounts of

solid materials into our aerosol carrier and deliver this aerosol into our shock tube; 2) we demonstrated laser-based diagnosticmeasurements of several key aspects of the ignition process of solid/liquid fuel systems; and 3) we characterized the effect of

U

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01-02-2008

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U.S. Army Research Office

P.O. Box 12211

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15. SUBJECT TERMS

Aerosol Shock Tube, nano-aluminum, ignition delay times

D. F. Davidson, R. K. Hanson

Stanford University

Office of Sponsored Research

Board of Trustees of the Leland Stanford Junior University

Stanford, CA 94305 -

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Ronald Hanson

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Exploratory Study to Determine the Ignition Properties and Combustion Chemistry of Explosives Using Shock Tube

Methods

Report Title

ABSTRACT

We investigated the potential of a new experimental method for combustion studies of solid nano-particle energetic materials. This new

method is based on the recent development in our laboratory of an aerosol shock tube and its combined use with laser extinction (for

characterization of particle size and loading) and absorption diagnostics (for characterization of vapor species). We accomplished three main

objectives in this study: 1) we demonstrated the ability to load significant amounts of solid materials into our aerosol carrier and deliver this

aerosol into our shock tube; 2) we demonstrated laser-based diagnostic measurements of several key aspects of the ignition process of

solid/liquid fuel systems; and 3) we characterized the effect of the addition of aluminum nano-particles on n-dodecane ignition under

conditions previously not studied.

(a) Papers published in peer-reviewed journals (N/A for none)

List of papers submitted or published that acknowledge ARO support during this reporting

period. List the papers, including journal references, in the following categories:

(b) Papers published in non-peer-reviewed journals or in conference proceedings (N/A for none)

0.00Number of Papers published in peer-reviewed journals:

Number of Papers published in non peer-reviewed journals:

(c) Presentations

0.00

Number of Presentations: 0.00

Non Peer-Reviewed Conference Proceeding publications (other than abstracts):

Number of Non Peer-Reviewed Conference Proceeding publications (other than abstracts): 0

Peer-Reviewed Conference Proceeding publications (other than abstracts):

D. E. Jackson, D. F. Davidson, R. K. Hanson, Application of an Aerosol Shock Tube for the Study of Nano-Energetic Material

Decomposition Processes, abstract submitted for Fundamental Combustion Processes Session of 44th AIAA/ASME/SAE/ASEE Joint

Propulsion Conference and Exhibit, 2007.

(d) Manuscripts

Number of Peer-Reviewed Conference Proceeding publications (other than abstracts): 0

Number of Manuscripts: 1.00


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Graduate Students

PERCENT_SUPPORTEDNAME

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Daniel Haylett 0.10

0.35FTE Equivalent:

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National Academy MemberPERCENT_SUPPORTEDNAME

Ronald Hanson 0.02 Yes

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scholarships or fellowships for further studies in science, mathematics, engineering or technology fields:

0.00

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Student MetricsThis section only applies to graduating undergraduates supported by this agreement in this reporting period

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David Davidson 0.10No0.10FTE Equivalent:

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Sub Contractors (DD882)

Inventions (DD882)


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U. S. Army Research OfficeP.O. Box 12211

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The views, opinions and/or findings contained in this report are those of the author(s) and should not be construed as an officialDepartment of the Army position, policy or decision, unless so designated by other documentation.

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Enclosure 1

1/31/2008 Final 4/1/2007-12/31/2007

Exploratory Study to Determine the Ignition Properties and CombustionChemistry of Explosives Using Shock Tube Methods

D. F. Davidson, R. K. Hanson

W911NF-07-1-0123

Stanford University, Office of Sponsored Research320 Panama Street, Stanford CA, 94305

We investigated the potential of a new experimental method for combustion studies of solid nano-particle energetic materials. Thisnew method is based on the recent development in our laboratory of an aerosol shock tube and its combined use with laserextinction (for characterization of particle size and loading) and absorption diagnostics (for characterization of vapor species). Weaccomplished three main objectives in this study: 1) we demonstrated the ability to load significant amounts of solid materials intoour aerosol carrier and deliver this aerosol into our shock tube; 2) we demonstrated laser-based diagnostic measurements of severalkey aspects of the ignition process of solid/liquid fuel systems; and 3) we characterized the effect of the addition of aluminumnano-particles on n-dodecane ignition under conditions previously not studied.

15Aerosol Shock Tube, nano-aluminum, ignition delay times


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FinalTechnicalReport

ShortTermInnovativeResearchProgram

AROGrant(STIR) W911NF0710123

ExploratoryStudytoDeterminetheIgnitionProperties

andCombustionChemistryofExplosives

UsingShockTubeMethods

Prepared

for:

DEPARTMENTOFTHEARMY

Dr.RalphAnthenienJr.,ProgramManager

FortheperiodApril1,2007December31,2007

Submittedby:

D.F.DavidsonandR.K.Hanson(PI)

MechanicalEngineering

Department

StanfordUniversity,Stanford,CA94305

[email protected]

[email protected]

January31,2008

HIGH TEMPERATURE GASDYNAMICS LABORATORYMechanical Engineering Department

Stanford University, Stanford CA 94305


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ContentsAbstract............................................................................................................................... 3Project

Description ............................................................................................................. 4

Introduction .................................................................................................................... 4AnInnovativeSolution:AerosolShockTube.................................................................. 4ResearchGoals................................................................................................................ 5

ExperimentalDescription ................................................................................................... 6AerosolGeneration......................................................................................................... 6LaserDiagnosticsMeasurements ................................................................................... 8BaselineIgnitionMeasurementswithNeatnDodecane............................................... 8EffectofNanoAluminumonnDodecaneIgnition ...................................................... 10IgnitionDelayTimeMeasurements.............................................................................. 12

Discussionof

Results

and

Conclusions ............................................................................. 13PotentialApplicationsofTheAerosolShockTubeMethod ............................................. 13

CurrentPublicationsBaseonThisStudy .......................................................................... 14References ........................................................................................................................ 14


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STIR:ExploratoryStudytoDeterminetheIgnitionPropertiesandCombustionChemistryofExplosivesUsingShockTubeMethods

D.F.Davidson,R.K.Hanson

MechanicalEngineeringDepartment

StanfordUniversity,Stanford,CA,94305

AbstractWeinvestigatedthepotentialofanewexperimentalmethodforcombustionstudiesof

solid nanoparticle energetic materials. This new method is based on the recent

developmentinourlaboratoryofanaerosolshocktubeanditscombinedusewithlaser

extinction(forcharacterizationofparticlesizeand loading)andabsorptiondiagnostics

(forcharacterizationofvaporspecies). Weaccomplishedthreemainobjectives inthis

study:1)wedemonstratedtheabilitytoloadsignificantamountsofsolidmaterialsinto

ouraerosol carrieranddeliver this aerosol intoour shock tube;2)wedemonstrated

laserbased

diagnostic

measurements

of

several

key

aspects

of

the

ignition

process

of

solid/liquidfuelsystems;and3)wecharacterizedtheeffectoftheadditionofaluminum

nanoparticlesonndodecaneignitionunderconditionspreviouslynotstudied.

These resultsprovide theDoDwith ademonstrationofanewmethod to investigate

solid/liquid fuelorexplosivemixtures, inparticular the influenceofnanoparticleson

combustion processes. We believe that this technique can be used for the study of

nanoparticleadditivestoliquidfuelsortononreactivecarrierliquids,aswellasforgel

like materials that can be converted into aerosol using ultrasonic spray nozzle

technology. Ourapproachpermitstheapplicationofanarrayofadvancedstateofthe

artlaser

diagnostic

systems,

developed

originally

for

gas

phase

chemistry

studies,

to

research problems of solid fuels. This present exploratory research program has

successfully demonstrated that these shock tube methods can be used to obtain

criticallyneeded informationaboutrapidlythermalizedexplosives. Wethusanticipate

thatouraerosolshocktubewillprovetobeanimportanttoolinfutureDoDresearchon

advancedpropellants,includinggelledandnanopropellants.


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ProjectDescriptionIntroduction

Amajorprobleminunderstandingthemechanismscontrollingtheexplosionsof

energeticmaterialsisthatthereiscurrentlynooptimalwayofprobingthedetailsofthe

firstdecompositionstepsofthetransitionfromsolidtogaseousphaseinthesesystems,

particularlyunderextremeconditions. Anaccurateassessmentofthenascentspecies

enteringthegasphaseduringthistransitionwouldprovideimportantkinetictargetsfor

constrainingandrefiningtheparticlecombustionmodels[1]. Thesetypesoftransient

species measurements have proven to be very difficult, though some progress along

these lineshasbeenmadeusing STMBMSmethods (simultaneous thermogravimetric

modulated beam mass spectrometry) by Rich Behrens and coworkers at Sandia

National Laboratories in the 1990s [2]. In those experiments the authors identified

major decomposition pathways for some explosive materials when the solids were

heated slowly. However, other methods which probe the dynamics of the interface

betweenthe

solid

and

the

gaseous

phase

are

needed,

particularly

those

that

can

operateundertheextremeconditionsoftemperatureandpressureandtherapidtime

framesfoundduringactualexplosions.

AnInnovativeSolution:AerosolShockTube

Webelievethatthesephenomenacanbestudiedusingshocktubemethodscombined

withlaserdiagnosticmethods. Weproposedtoexploretheuseofarecentlydeveloped

aerosolshocktubeandanexistingarrayof laserextinctionandabsorptiondiagnostics

forstudiesofsolid/liquidmaterials, inparticularnanoparticles/fuelmixtures,and the

transition from solid to gaseous phase in explosives. This new shock tube facility at

Stanforduses

aerosols

to

handle

liquid

and,

now

potentially,

solid

fuels

[3].

A

schematic

representationoftheoperationofthisaerosolshocktubeisshowninFig.1.

ReflectedShock Wave

T5, P5

IncidentShock Wave

EvaporatedAerosol Mixture T2, P2

Shock Tube Filled with Spatially-Uniform Aerosol Test Gas Mixture

Aerosol Mixture Evaporated Behind Incident Shock Wave

Test Gas Mixture Compressed to Reflected Shock Conditions

HeliumDriver

Aerosol Test GasDriven Mixture

T1, P1

Fig.1.Schematicofaerosolshocktubetechnique.


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Theaerosolshockworksbyallowingtheintroductionofnebulizergeneratedmistintoa

shocktube inaspatiallyuniformlymanner. Anyfluidorfluid/particlesuspensionthat

canbeconvertedintoamistbyanebulizercanbeusedasareactant. (Thisreactantfills

theshocktubeattheinitialtemperatureandpressure,i.e.T1,andP1. Ashockwaveis

generated intheshocktubebyrupturingadiaphragm inthedriversectionwithhigh

pressurehelium

gas

and

this

incident

shock

wave

propagates

down

the

shock

tube.

Dropletsformedbythenebulizercompletelyevaporatebehindtheincidentshockwave

at theelevatedconditions (T2,P2)and theevaporated fluid (vapor) rapidlydiffuses to

formauniformdistribution. Thereflectedshockwave(fromtheendwall)thenpasses

backthroughthetestmixture,compressingandheatingittothetargetedtemperature

andpressureconditions forcombustion(T5,P5). Opticalaccesstotheviewingsection

permitstheprobingoftheheatedtestgasmixturewithlaserdiagnosticsandallowsthe

measurement of species concentration time histories for species, such as OH, CH3,

benzyl,NO2,H2O,andothers,duringthecompleteignitionprocess.

ResearchGoals

Whatisofdirectandimmediateinteresttothestudyofsolidexplosiveschemistryisthe

applicationoftheaerosolshocktubetechniquetostudytheeffectofnanoparticleson

fuel mixtures. Only limited quantitative chemical kinetic data are available on the

influence of nanoparticles on kerosenetype fuels; most previous studies have been

concernedwithlargeraluminumparticles(microntomillimetersize).

Bazynetal. [4] investigatedtheburningofnanoaluminumparticles inshock tubes in

CO2/N2andO2/N2. Theycharacterized theburningtimeasa functionof temperature

andpressureandfoundthatnanoaluminumreactedatlowertemperaturesthan10um

particlesundersimilarconditions. At8atmandattemperatureslowerthan1400K,80

nmaluminum

particles

exhibited

light

emission

characteristic

of

burning

for

over

500

s.

Muelleretal.[5]investigatedaluminum/RP1slurrieswith60%byweightaluminumby

feedingtheslurryintoanonpremixedmethaneflame. Theyobservedrigidslurryshells

toformwhosecompositionwasafunctionofdiameter.

WongandTurns[6]investigatedaluminum/JP10slurrydropletsofdiameter500to110

microns(with4micronAlparticles)whichweresuspendedonmicrofibers. Theslurry

dropletswere largeenough that their ignitionmodel included several steps including

burnoff of the JP10, agglomeration/heating of the remaining aluminum, and finally

ignitionofthealuminumparticle. Ignitiontimeswereoftheorderof0.4seconds600

micronslurrydropletsinCO/10%O2/N2flamesat1800K.

Belonietal.[7]investigateddecanebasedslurrieswithmetallicadditives. Theyuseda

lifted laminarflameslurryburnerandstudiedAl,Al0.7Li0.3and2B+Tipowders. TheAl

particleswere15micronsindiameter. Theyobservedthatthealuminumparticlesdid

notburnintheflames.

In this study we have investigated the influence of aluminum nanoparticles on the

ignitiondelaytimesofasinglecomponentkerosene/jetfuelsurrogatendodecane. This


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was accomplished in three steps. 1) Wedemonstrated the ability to load significant

amounts of solid materials into our aerosol carrier and deliver this aerosol into our

shock tube;2)wedemonstrated laserbaseddiagnosticmeasurementsof severalkey

aspectsoftheignitionprocessofsolid/liquidfuelsystems;and3)wecharacterizedthe

effectoftheadditionofaluminumnanoparticlesonndodecaneignitiondelaytimes.

Oneof

the

key

accomplishments

of

the

present

study

was

to

demonstrate

that

we

can

suspend very finely ground solid energetic material particles (in these experiments

aluminum nanoparticles) in a solvent that acts as a carrier for these particles. This

solventcouldeitherbenonreactive(e.g.water)orpartofthefuelchemistry(inthese

experimentsndodecane). Ineithercase,this liquidwouldcarrytheparticlesfromthe

nebulizerintotheshocktube,andthenevaporatecompletelybehindtheincidentshock.

Whatwouldbe leftwouldbe thenanoparticlesofenergeticmaterials inagasphase

carrier(e.g.airandwatervapororairandgaseousfuelasinthecurrentexperiments),

uniformlydistributedspatiallyandshockheatedtotemperaturesandpressureswhere

we could study the chemistry. Using this method enables experiments that directly

investigatetheearlydecompositionchemistryofthesematerialsanddetectionofthe

initialgasphasedecompositionproducts.

ExperimentalDescriptionThecurrentexperimentswereperformed intheaerosolshocktubefacilityatStanford

University. Thedescriptionofthisworkisinthreesections: theoperationofthespray

nozzle,thelaserdiagnosticscheme,andtheignitiontimemeasurements.

AerosolGeneration

Wefoundthattheadditionofsolidnanoparticlestothe liquidaerosolsolventcarrier

significantly

reduced

the

aerosol

output

of

our

existing

disk

nebulizers.

To

maintain

higher aerosol loading levels, the disk nebulizers were replaced with a SonoTek

Ultrasonicspraynozzle. Aphotographoftheoperatingnozzle isshown inFig.2.This

devicecannebulizesuspensionswithupto40wt.%nanoparticles. Thisnozzleoperates

at 120 kHz and produces aerosol with number mean diameter of approximately 18

microns. ArepresentativesizedistributionisshowninFig.3.

Figure2. Sonotekultrasonicspraynozzleinoperation.


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Figure3.Volumesizedistributionplotforandodecaneaerosolproducedusingthe120

kHzSonotekspraynozzleatamassflowrateof200ml/hrandapowerof1.0watt.

TheSonotek

ultrasonic

spray

nozzle

was

installed

in

the

loading

chamber

located

immediatelyadjacenttothetestsectionofouraerosolshocktubefacilityinplaceofthe

disknebulizerassembly. SeeFigs4aand4b.

Figure4a(left).Schematicofsprayjetinstallationwithcoaxialbath(carrier)gas.

Figure4b(right).Photoofsprayjetinstalledonaerosolshocktube.

Thelargeraerosoldroplets(largernumbermeandiameter)producedusingthesprayjet

(thanwith the disk nebulizers) experience faster dropout rates due to gravity. We

characterized thesedropout ratesusing laserextinctionmeasurements (described in

thenext section.) A representative in situ life timemeasurement is shown in Fig.5.Basedonthesemeasurements,weanticipatedthatsuccessfulshockwaveexperiments

wouldrequireveryshort(lessthan10seconds)delaysbetweenfillingandbreakingthe


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diaphragm. We thus modified our experimental protocol to enable this type of

operation.

In the future we plan to upgrade this ultrasonic nozzle. Discussions with the

manufactureroftheSonoteknozzle indicate thatahigher frequencymodel (180kHz)

which will have a number mean droplet diameter of 10 microns, may soon become

available.

Theuse

of

this

nozzle

would

improve

the

aerosol

loading

levels,

as

well

as

reducetheamountofsettlinganddropout.

Figure5. Dodecaneaerosolsettlingrate. Miescatteringwasmeasuredusing670nm

diode laser extinction at preshock filling conditions similar to those of typical shock

waveexperiments.

LaserDiagnosticsMeasurements

Figure6showsthelaserdiagnosticsystemsusedintheshockwaveexperiments. Three

lasersystemsareused. Twodiode lasersat660and670nm (referredtoasthenon

resonant lasersystems)areusedtomeasuredropletevaporationandparticle loading.

OneHeNegaslaserat3.39microns(referredtoastheresonantlasersystem)isusedto

measurehydrocarbon (ndodecane in the currentexperiments)absorption. The3.39

micron absorption measurement is also used to measure fuel loading after droplet

evaporationbehindtheincidentshockwaves. Detailsoftheuseoftheselasersystems

toprobeshockwaveexperimentscanbefoundinRef.[3].

BaselineIgnitionMeasurementswithNeatnDodecane

Representativedatatracesfrombaselineignitionmeasurementswithneatndodecane

usingthenewjetnebulizerareshown inFig.7. The660nmextinctionmeasurement

location is 2 cm from the shock tube endwall; the 670 nm and the 3.39 micron

measurementlocationis5cmfromtheendwall.


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First,examiningthepressuretrace inFig.7,weseethatthere isastep inpressureat

2250swhichcorrespondstothearrivaloftheincidentshockwaveatthelocationof

theKistlerPZTsensor2cm fromtheendwall. Thenextset inpressureat~2340s

corresponds to the arrival of the reflected shockwave returning from the endwall.

Near3000s,thepressurebeginstoriseandatabout3220s,ignitionoccurscausing

thepressure

transducer

to

begin

to

ring.

Figure6. Schematicoflaserdiagnosticsetupforaerosolshocktube.

Next, examining thenonresonant 670nmdiode laser extinction signal and the 3.39

micronlasersignalswhicharebothlocated5cmfromtheshocktubeendwall,wesee

thatwhentheincidentshockwavearrivesatthislocationthepreshockextinction(670

nm)andpreshockextinctionandabsorption(3.39micron)signals(fromthepreshock

ndodecaneaerosolvapor/O2/argonmixture)jumpasaresultofthecompressionand

heatingby

the

incident

shock

wave.

The

3.39

micron

signal

rapidly

achieves

aplateau

level indicativeof complete evaporationof thendodecane,while the670nm signal

decaystozeroconfirmingthecompleteabsenceofdroplets. At2480s,thereflected

shockwavepassesthe5cmlocationandfurthercompressingthendodecanevaporas

indicatedbythestepinthe3.39micronabsorptionsignal;thisshockalsomomentarily

disruptsthe670nmdiodelaserbeamowingtobeamsteering.Near3220s,the3.39

micronsignalgoestozeroasthefuel(ndodecane)isconsumedatignition. Coincident


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with the ignition is a rise in CH* emission which is an indicator of the rapid radical

productionthatoccursduringignition.

Figure7. Representativedatatracesforndodecaneignition. Preshockconditions:T1=

295K,P1=201torr;incidentshockspeed:Vs=757.93m/s,fuelloadingXfuel=0.0021in

21%O2inargon;reflectedshockconditions:T5=1187K,P5=7.1atm,ignitiontime=793

s.

EffectofNanoAluminumonnDodecaneIgnition

Figure8showssimilardatatracesforashocktubeignitionexperimentwith5%or20%

by weight nanoaluminum particles added to the ndodecane aerosol. Several

significant differences are seen between this set of measurements and the neat n

dodecanemeasurements. ThealuminumnanoparticlesweresuppliedbySigmaAldrich

Chemicals: Aluminum nanopowder,99.5%,part#653608. These aluminumparticles

have an average diameter of 100 nm and a thin Al2O3 coating with a core of pure

aluminum.

Pressure:Thepressuretrace(blue) inFig.8showsamuch largerstepandmuchmore

suddenrise

in

pressure

at

ignition.

(The

pressure

signal

actually

goes

off

scale.)

This

is

coincidentwithashorterignitiontime,evenwhennormalizing(usingP1and

1)forthe

differenceinfuelconcentrationandpressureofthetwoexamples. Wehavefoundno

previous direct measurement of the influence of aluminum nanoparticles on the

ignitiontimesofkerosenetypefuels.

NonResonantLasers:Bothnonresonant(tohydrocarbonspecies)lasersignals(660and

670nm)showcontinuedconstantextinctionbehindthe incidentshockwaveafterthe


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11

complete evaporation of the liquid aerosol carrier, confirming the presence (and

allowingmeasurement)ofthenanopropellantloading. Thiscanbeseeninthe670nm

signal(black)from2180to2350sandinthe660nmsignal(pink)from2180to2250s.

Forboththesetwomeasurementsthereisalsoasignificantstepinextinctionafterthe

arrivalofthereflectedshockwaveat2350sforthe670nmsignalandat2250sfor

the660

nm

signal,

consistent

with

the

increase

in

density

across

this

wave.

Figure 8. Representative ignition experiment using ndodecane with 20% nano

aluminumby

weight.

Initial

conditions:

T1

=296

K,

P1

=223.4

torr;

incident

shock

speed

Vs=781m/s, fuelconcentrationXfuel=0.00550021 in21%O2 inargon;reflectedshock

waveconditions:T5=1197K,P5=9.53atm,ignitiontime=193s.

Theextinctioninthe660nmsignaldecaysafterthepassageofthereflectedshockwave

untiljustbeforeignitionwhenitrapidlydecays. Similarbehaviorisseeninthe670nm

signal at the 5 cm location, except that the blast wave from the ignition that occurs

upstream(atthe2cm locationandclosertotheendwall)rapidlysweepsbythe5cm

location and causes the rapid decrease in the 670 nm almost immediately after the

ignitioneventatthe2cmlocation. Aftertheignitionevent,bothofthe660and670nm

signalscontinue

to

show

absorption/extinction.

This

absorption/extinction

is

consistent

with absorption features of gaseous aluminum oxides near these wavelengths or

possiblywiththeappearanceofaluminumoxideparticles.

3.39 um Laser: After the passage of the incident shock wave, the 3.39 m laser

experiences both extinction from the aerosol droplets ane bare aluminum nano

particles,andabsorptionfromthendodecanevapor. AcalculationoftheexpectedMie

scattering of 100 nm nanoparticles at 3.39 microns indicates that only about 1%


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extinctionwillresultfromthenanoparticlesaftertheaerosolhasevaporatedandthus

themeasuredabsorptionafterevaporationiseffectivelydueentirelytofuelvapor.

The3.39micronlasersignal(blue)showsamorerapiddecaybehindthereflectedshock

wave than is seen in the neat ndodecane experiment, possibly owing to the

involvementofthealuminumnanoparticleswiththendodecaneoxidationchemistry.

Afterignition,

there

is

no

apparent

absorption,

consistent

with

acomplete

consumption

ofhydrocarbonspeciesandnotabsorptionbyaluminumoxides.

Emission:Thisrapiddecay inthe660nmsignalatthe2cm location iscoincidentwith

the strong rise seen in the 431 nm emission signal (green). This emission extends

throughthe ignitioneventwithaFWHMofapproximately80s. Noemissionpeak is

seenintheexperimentwithnoaluminumnanoparticles.

IgnitionDelayTimeMeasurements

Figure9showsasummaryoftheignitiondelaytimemeasurementsofneatndodecane

and

n

dodecane

with

aluminum

nano

particles.

In

this

figure

previous

measurements

in

ourlaboratoryofndodecaneignitiondelaytimesareshownasstandarddisknebulizer

C12H26[3]. Measurementsofneatndodecaneusingthenewjetnebulizerareshown

as Sonotekjet nebulizer C12H26. Measurements with 5% or 20% added aluminum

nanoparticles(byweight)arealsoidentified.

Figure9. Ignitiondelaytimemeasurementsofndodecaneandndodecane/NanoAl.


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At temperatures below 1175 K, there appears to be no significant difference in the

ignition times of ndodecane and ndodecane/nanoAl mixtures. Nano aluminum

shortenstheignitiontimesattemperaturesabove1175K. Thisisparticularlyevident

forthethreedatapointsnear1250Kwherethe ignitiontimesaresignificantlyshorter

than the ignition delay times of the neat ndodecane experiments at the same

temperatures.

DiscussionofResultsandConclusionsThepresentexperimentsextends significantly the rangeofcombustionconditions for

which mixtures of aluminum and kerosenelike fuels have been studied. Several

observationscanbemadeincomparisonwithpreviouswork.

Bazynetal.observedslowburning inpurealuminumnanoparticles(burningtimesof

greater than 500s at temperatures below 1400 K). Based on the same criteria of

burningtimesusedbyBazynetal. (emissionmeasurements)wefindburningtimesof

lessthan100snear1200Kinourndodecane/nanoaluminumignitionexperiments.

WongandTurnsobservedlongignitiontimes(oforder0.1to1second)forlargeslurry

droplets (oforder1000microns). The slurries investigated in thepresent studyhave

initialdropletdiametersoforder1020micronswith100nmaluminumnanoparticles.

Our laser absorption and extinction measurements indicate that these droplets fully

evaporate in timesoforder100sbehind incidentshockwavesandwhenheated to

~1200K,ignitedin100s.

The combustion regime studied in the current experiments is different than that

previously studied. Mueller et al., Beloni et al., and Wong and Turns studied larger

dropletsinflamegeometrieswheretheignitionprocessisacombinationofevaporation,

agglomeration,particle

heating

and

particle

flame

kinetics.

This

ignition

process

in

the

current study, we believe, is that of a dry aluminum nanoparticle in a high

temperaturegasphasebathofndodecane/21%oxygen/balanceargon,similartothat

ofBazynetal.butinanhydrocarbonfuelenvironment.

Usingtheaerosolshocktubemethod,thechemicalkineticsofnanoparticlesinreactive

fuel mixtures can be investigated without interference from other physicalchemical

processes. Thisisdonebyseparatelyevaporatingtheaerosoldropletsandthenheating

themtoignitiontemperatures.Thisavoidstheoverlapofevaporationphenomenaand

particleagglomerationprocessesthatcanoccurwithlargerdroplets. Severalexamples

oftheseexperimentsaredescribedinthenextsection.

PotentialApplicationsoftheAerosolShockTubeMethodThemainthrustofthisinitialeffortwastoestablishfeasibilityofusingtheaerosolshock

tube to study solid energetic materials. With this demonstrated, there are several

immediate and obvious problems currently of interest to the energetic materials

communitythatwebelievemeritinvestigation.


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1) RDX decomposition pathways and rates. Large uncertainties exist in the

decomposition chemistry of RDX [8]. Measurements of NO2 concentration time

histories(usinglaserabsorptionat472nm)couldprovideneededaccuratequantitative

measurementsofthedecompositionofRDXandrelatedcompounds.

2)Rapid testingofmixturesofenergeticmaterialsand reactive solvents. Ithas long

beenknown

that

the

decomposition

rate

of

energetic

materials

can

be

influenced

by

the addition of various agents. A comparison of the magnitudes of the effect on

decompositionrateofanenergeticmaterialbyvarioussolvents,bothreactiveandnon

reactive,canbemade inastraightforwardmannerusingtheaerosol loadingmethod.

Informationfromthesestudiesmayguidethedevelopmentofnewenergeticmaterials.

3) Relationship of ignition delay time to material properties. Ignition delay time

measurementsmayprovideasimpleindicatorofenergeticmaterialproperties[9]. An

initialsurveyoftheignitiondelaytimesofseveralestablishedenergeticmaterialswould

allowthisrelationshiptobestudied.

4)Extension

of

the

method

to

investigate

gel

propellant

components.

While

we

have

notyetdevelopedamethodtoconvertfullygelledpropellantmixturesintoaerosolsfor

shock tube loading, it should be immediately possible to study the combustion

propertiesofgelpropellantcomponentsdissolvedinsolventsorslurries.

CurrentPublicationsBaseonThisStudyD.E.Jackson,D.F.Davidson,R.K.Hanson,ApplicationofanAerosolShockTubeforthe

Study of NanoEnergetic Material Decomposition Processes, abstract submitted for

Fundamental Combustion Processes Session of 44th

AIAA/ASME/SAE/ASEE Joint

PropulsionConferenceandExhibit,2007.

References[1] T. R. Botcher, C. A. Wight, Explosive Thermal Decomposition of RDX, Journal of

PhysicalChemistry98:54415444(1994).

[2]R.Behrens,ThermalDecompositionofEnergeticMaterials:TemporalBehaviorsof

the Rates of Formation of the Gaseous Pyrolysis Products for CondensedPhase

Decomposition of Oxtahydrotetranitrotetraocine, Journal of Physical Chemistry 94:

67066718(1990).

[3]D.F.Davidson,D.C.Haylett,R.K.Hanson,DevelopmentofanAerosolShockTube

forKineticStudiesofLowVaporPressureFuels,CombustionandFlame,inpress.

[4]

T.

Bazyn,

H.

Krier,

N.

Glumac,

Combustion

or

Nanoaluminum

at

Elevated

Pressure

andTemperatureBehindReflectedShockWaves,CombustionandFlame145:703713

(2006).

[5] D. C. Mueller, S. R. Turns, Some Aspects of Secondary Atomization of

Aluminum/HydrocarbonSlurryPropellants,JournalofPropulsionandPower9:345351

(1993).

[6]S.C.Wong,S.R.Turns, IgnitionofAluminumSlurryDroplets,Combustion,Science

andTechnology52:221242(1987).


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[7] E. Beloni, V. K. Hoffman, E. L. Dreizin, Combustion of DecaneBased Slurries with

MetallicFuelAdditives,45th

AIAAAerospaceScienceMeetingandExhibitJanuary2007

RenoNV.

[8] M. M. Kuklja, Chemical Decomposition of Solid Cyclotrimethylene Trmitramine,

ComputationalNanoscience,ISBN0970827539(2001).

[9]W.

Anderson,

ARL,

personal

communication

2007.

Fuel Combustion Study Using Nano Particles

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