NASA Technical Reports Server (NTRS) 20000070850: Buoyancy and Pressure Effects on Bulk Metal-Oxygen Reactions

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Copyright © 1997, American Institute of Aeronautics and Astronautics, Inc.
AIAA-98-0570
BUOYANCY AND PRESSURE ENTECTS ON BULK METAIrOXYGEN REACTIONS
A. Abbud-Madrid*, C.McKmght _,l_ C. Brancht, andL W. Daily _
Center for Combestion andEnvironmental Research
Department of Mechanical Engineering
University of Colorado atBoulder
Boulder, CO 80309-0427
AbsU_ InUodu_
The combuslion behavior of metal-oxygen Reactions between metals and gaseous species
reactions in aweakly buoyant environment is studied are important in a nmnbcr of pracqical systems. These
to understa_ the rate-controlling mechanisms in the include: rocket propellants with metal particles added
homogeneous and L_e_o_neous comb_ of bulk for increaseA thrust and suppression of combustion
metals. CylJ_Iric_l titanium and magnesium _smbility, the self-propagsdng high-temperature
specimens are ignited in p_en at pressures synthesis processes (SHS) to produce new high
ranging fi'om 0.l to 4.0 atm. Reduced gravity is temperature material& the combustion of metal fuels
_tained from an airca_t flying parabolic trajectories. for plan¢m7 mission_, and vapor-depo_tion processes.
A weakiy buoyant environment is generated at low In spi_ of the many pmc_:al appficafions, the
presmrm under normal gravity and also at 1aim ender combustion of metals remains an incomp_y
reduced gravity (0.01 g). The similarity between _es¢ un_ phenomenon due to the in_mction of the
two _tal condilions comes flora the p2g complex proces_ involved, indudJ.ug low-
buoyancy scale extracted fzom the Gmsl_ number. _e =Lrface oxidation, oxidizer diffusion,
Lower propagation rates of the molten interface on he,_ogcn¢ous reactionsp,roduct condensation,high-
titanium r_mplm are found at progressively lower emissivity of produc_s, natural convection, and multi-
pressures at I g. In the reduced-gravity experiments at phase thermodynamics and reaction kinetics.
1atm, the propagation velocity is almost halfthevalue As has been the case _O.th many other
obtained at 1 g. These rates are compa_ to combustion phenomena, conducting metal combustion
theoretical results flora heat conduction analys_ with a _t_ in the absence of gravity may provide
diffu_on/convoction coutzullcd reaction. The close further unders_tnd_ng of the complex physical and
agreement found between experimental and theoretical Ommical mechanisms involve_ A miaoOavi_
values indicate the importance of mtural _- environment reduces the dism,-'bing buoyanteffectsthat
enhanced oxygen u'auspon on combus_on rates. For frequently obscure weaker forces inch as molecular
magnesium, progressively longer burning times are _on. At the mine time thi_ type of eavironment
exped_ccdat lowerpxmsmcsaud lg. Undermiuccd permits detailed, longer-duration observatlom in larse-
gravity condiu'om at I aim, a burning time twice as scale cx-pcrm_ which aze usually inaccessible duc to
long_ in Igi_ exh_ited. However, in this case, the the rapid development of buoyancy.4_iven
validitoyfthep2gbuoyancymale remai= unte_ due disturbances. The useof mbatmospheric pressur_ can
to the inability to obtmn steady gas-phase bundng of alsoreducethe effect of natural couvect_ aod allow
the magnesium sample at 0.1 aim. Nevertheless, largerspatialandtemporal scalesforflamemxfie_.
longe_ burning times and larger flame standoff" Gravity and pressure effects on buoyancy
distances at low premmes and atlow gravity points to a induced convection ar_ linked through the
diffusion/convection conu_Ued reaction. number (Gr) which evalnates the rdative importance
of molecularand buoyant transport.The Grashof
number is defined as:
o)
* _ Associate, Member AIAA
t Gradmte Student
_: Professor, Member A/AA
wherc Ap is the characteristic density difference, pis
§ Professor, Associate Fellow AIAA
the mean density, g is the acceleration of g_vity, L is
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Aeronautics andA.mmautics, Inc. All rightsreserved.
This isa preprintorreprint ofa paperintended forpresentation ataconference.
Because changes may bemade before formal publication, this ismade available withthe
understanding that itwill not becited or reproduced without the permission of the author.
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AIAA-98-0570
thecharacteristiclengthscale,andvisthekinematic (4° half angle) with broadband radiation 000-1100
viscosi_,._ the_r_,ity_) and.pr_u_ (p) nm). An asphedclensfocuses the beamto providea
dependencefromF.q.(D resultsin thefS buoyancy 2-MW/m_powerdensityonthe top surfaceofa4-ram-
scale_. The use of lowpressuresat normal gravityto diameterand 4-ram-high metal specimen thatdts on
reducethe effect ofbuoyancyhasbeenemployed inthe analuminaholder. A4.5-L,stainless steel,cylindrical
studyofseverafllameconfiguratiToM.ns combustion vessel hees_ the leas, metal
Inthecaseofmetalsthep2&buoyancyscale and alunfiua holder. Optical access for the movie
canbeusedtoassesstheiupot'tanoafsgasdie.ionas camera and spectrograph is provided th,'eeSh two
a burning-ratecontroningmechanism. Several fmed-s_ica _le windows,whileathirdwindowisused
mechanismshavebeen identifsieads beingrate- fursamplereplammcnt.TiandMg metal
controlllantgmetalburningtemperaturienscludinag): (99.95%puritya)reburnedin an 02 envirunment
oxygendi_mion/unrvectitooathesamplesurfaceb,) (99.6%rain.a)tpremuretvaryingfrom0.Ito4 arm.
physicaalndch_ adsorptioofnoxygenn_lecules Themrfa_ tempetatu_ is measuredwitha0.125-ram
andatomsattheoxygen-oxide_, c)_on diameter,type R thennaca_le attachedto the outer
of ionsth.,oeghtheoxidelayer,andd) reactionatthe walloftherumple.
oxide-metalboundary.Thepressurediepmdencvyade_
foreac,hofthe_ processes. Conductingexpedments at
vaziom gravity levels while keeping the pn_Jze
constanthas theadvantageofassessing onlytheroleof
oxygen _n and convection on the bundng rate.
Onthe otherhand,expedme_ atva:ines pressuresat
normal gravity may beused,to correlate theburning
rates _ under both sets of _
condition¢ WJdlemanyinvcstiBation¢s9 have_udied
theerectofInCmueonmetalmmbus_a onlya few
stndieshaveevaluatedtheinfluenceofOavity onthe
bundnSbehavi_ ofmeta_ l°.
Byconduceage_rhne_ at_ced sravity J tins _.
a_l conslaat preuure and at vado_ _ at
normal gravity, thepresentinvc_gation provid_ new
into the Ix_bl¢ role of exygan
di_mion/convecfiaosna tate-contzolling mechani_
in thebuntingof bulkmettlsin pure oxygen.
Titanium(Tl) andmagnesium(Mg) metalsamchosen
to xeptesmt the two diferent ¢ombe_ion modes
observedin metals:hetcxogenm_multi-phasesurface
combe_on (as in Ti) andhomogeneous,gas-phase lq_reI.Diagr_noftheexperimentasl_
combmeoa(astnMS). _a_ _tace _,
p_asaeon rates,bw_g _ and A hlgh_ 16-ram movie _unera previdm
measurements areobtained ander ammal and zeduced _ and flame vi_ the images arealso
gravity. Rede_ed Oavity is obtai_ fiem anai._r_ usedformeasmementofpropagationratesmulbun_
flying parabolic U-ajectode_ V'uma/evidence of timee. With a 7.5" shatter and Weeds up to $00
phenomemb providedbyhigh-q_d cinematography. _n_s/s, eqx)su_timesm short as 1/20,000 sare
obtaiaed. _ addte_ to via_te Ught iw_, eme-
and q_u_-xemlved spectralinksmation on gas-phase
rcactnts and productsis _ with an inn_ing
The appata_ and _ pmccdu_ spectrographanda 1024-dement diode arraydetectm'.
used in this im,e_gafion have been thoroughly TheesI_.meat isconuonedme:elyby a
previouslySe.11,hea_ on],/a briefdescription compute¢a,digital/analogdataa_lui_ion born'd,and
w/ll begivon here. A sc.hema_cof the ex_ m _ codewdum in graphicttpro_nam_ng
$3,stemi| slu_MtinIrlg.I. soamre. The _du_.gravtty expemnem were
The ignition source _ of a 1000.W conductedonboardthe NASA.Lewis DC-9 Research
xenon lamp that generates a highly collimated beam Aircraft in Cleveland,Ohio. Up to 20 s of reduced
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AIAA-98-0570
gravity (_'-0.01g) wereavailablein asingleparabolic moltenmass---alsoconfirmedbythe_heri_M shapeof
maneuver. A minimumoften testswereoonductedfor the reacting mass in the high-speed photooaphs.
each recta/at thevariouspressuresandgravityleveLs Fmthenn_re, acalcolafion ofthe Bondnumber(Bo =
toensureexperimentarlepmt_ity. gravitationalfozue/surfacetension force) under these
conditions resultsinvaluesbelow0.1. Forlargevalues
ResultsandDiscu_on oftheBondnumber(Bo>1), accelerationcanoc¢orby
lateral drippingwhenthe_vila_onal forceovercome_
the surface tension force holding the molten mass
The role of oxygendiE_on/conwctioa on the
together.
burring rotes ofTi andMg is evainated bytwosets of
experiments. The resets form the reduced-gravity
3.0 ¸
testsataconstantpressureared/rcnssedfirstfollowed
theresultsfrom thetestsatva_ouspmssm_at
normal gravity. A comparison of the observations y. =16.2mm/s P=la_
fz_. _ two_ts isa_d by._ e_g 20,
__a_. ,5
_ 1.5
ReducedGrav_
o 1.0
0.5
ForTi the critical temperature(T,,_) of the
0.0
sampl_---t_ tempermm_ atwhichtheheatg_
0._ 0.04 0.08 0.12 0.16 0.20
by oxidat/on tint overcomes heat losses, leading to
ignhinn---occu_ at a point on the top rim of the TB_E(seconds)
cy]/nderataround1750K. Aflm_ard, amoltenmass
cons/s_ng of a _ of meCd and ox/des torts Figure 2. Propagationofthereactingmolten
traveliag ac_ou the epper _m'ace of the speci_,_ massinTisamplesundernormalandreduced
Under normal _ _ gr_4ty c_i_, gravityat Iaun.
dowawaxdpropagatioaofasphericalmastfol]o_ in a
smooth, none:q)Imlve fashlon until n:_,_g the Following a theoretical approachsimilar to
aluminabase, No evidence of emissioa flmnpseons the coe used for gaseous flame propagation, several
species is foend from the vi_'ole images mkmby the studies_,n ha_ &ta_,d an expression of the form
h/gh-speed eamera. Thelackofanydgammm vapor. V.--(wm) to calculatethe propagadonvelocity Valong
phase reactionsdiningthis stageof thecombustion metal cylinden u.ndex'goinghetemgeneous surface
process (further verified by sl_,'troscopic burningwith arate of reaction w. Considedng the
nzammmmts), points m the existence of a diffmiea and convection of 02 to the sampleas the
predominantly heterogeneous xmction withcondensed rate-limiting step, the reaction rate becomes
products in the liquid and solid surfaces of the plg_'tional tO "lP"((_r)TM, Wh_4C_ _ is _ Gg'd&_f
_ectmen. mnnber.ThepropagationvelocitythendependsonGr
Figure2shows a_'aph oftheverticaldistance asF-(Gr)t_, sothatY.-(g)yr. EvaluatingVJ_for Ih¢
traveledbythemoltensm'fitcem afunctionof_umefor (] g) and low gravity(0.01 g) cases, a
typicalrumplesat 1S andtowg. Thepropegation themetiml ratio of 1.78 is obtained. The close
velocitim inf_red fi_m the dope of thefiuedlines are agreementbetweenexpcrin_otal (1.86) andtheos_icel
16.2 mm/s and 8.7 mm/s under uormal and low (1.78) ratios of propagation velocittm indIcatm the
gravity. Similar velod_ies were obtainedforall the importance of the influence of natural_onvecfion-
samples tested (with standarddcvi_ons of 0.6mm/s enhanced oxygentransportoncombustionrates.
and 0.3 mm/s for the l-g and the low-g cas_ This result disagrees with findings f_om
teq_-evdy). studiesthatsuggestoth_ rate-_aurol_g mechanisms
Evaluating the ratioof propaga6on velocities for metals exhibiting heterogeneous combustion.
at nmmal (V,) and low (V_ gravity gives a value of Amoag these proposed mechani.m_ are: a) the
1.86. A_eady Im_paga_on_ diningthisperiod incorporafiouof oxyge_intotheoxidela_, physical
at 1g sugse_ that the propagation velocityhas not and chemical adsorption of c_gen on the oxide
beea alteredbytheinfluenceofthagradty fmc,eoathe _L,'fi_t', andc)themLd,_ ofoxygen withthe ngUd
3
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AIAA-98-0570
iathemoitcubalI x4. The firsttwo mechanisms may times and a qualitative _iscussion of important
control the burning rate in the presence of a phenomenaisgiven.
continuous, coherent oxide layerwhich separatesthe Fromthe visible images andemissionspecWa
mctat from the oxygen. Based on the high-speed measurements,the structureofthe luminoesflamethat
movie images recorded in the present investigation, engulfs the sample afterthe passage of the ignition
non-coherent, irregularpatches of liquid oxides are wave is in general agreement with the extended
clean'y_en flozing andmoving on the molten ball. reactionzonemodelof Glassmanctal_s. Accordingto
Even though no reliable informatian is currently this model, an inner region of Mg-02 vapor-phase
available on adsot_on ratesof oxygen throughliquid reactianisfollowedbyM80 condensationt;hesolid
metals and oxides,adsozpdonratesin theliquidstate MgO parlides eventually pile up in thebrisht whi_e
should be significantly higher than in solids since flamefrontcapturedintheh/gh-speedphotographs,
liquid oxides are not cmlsidered effec_c dlf_on Duringfully devdopedcomb'_on, buoying-
barrier_is. generatecdonvection currenl_are responsibleforthe
A poor mixing of metal and oxygen would main differencoebsserved in the two gravity cases
occurin thecase ofslowc_ulation of nmterialin the investigated. At 1g, highconvectioncurrentsforming
molten ball. IntheaforementionedburningTisamples an elongated vertical plume enhance burning by
strong circulationcurrentsareobservedin the molten increasing 02 flux to the reaction zone and by
Thisd.,calafion Lscaesed, forthemostpart, removingoxideprodectsthat mayconstituteabarrier
by surface tension driven flow in a region with a to 02diffusion. The proximityof the resultingflame
_ gnd/eat--au additionatsmart front to the metal sample h an indication of fast
cantfibution may also come from unstable density bumi_ nte_ In comparison, at low gr_V
gradients in the normalgravitycase. An estimate of condition& the _ redu_don of com_-tion---
the maguitude of this g_ce tension driven threefold, av:ot_g to the (Gr)TM _d
flow b pos_le byevaluating theMarangonlnumber, the in.eased re_ce toO: _on byunnbus_n
M'a,which considerstherelativeimportanceofsurface products_sh the oxygentrae._rt. Thusin the
tension and viscousForce& A detailedevaluation_1of reduced-gravity case a broader, outerblue zone (of
this numberbasedon experimental measurementsand stagnantoxideparticlesin the abse_ee of convection
thermophysi_ dataofliquidTigives avalueof-_a- _.m_ts) and an incr_! flamc standoff"distan_
6931. Since aManmgoni nttmbe_ abovethe critical from themetalcore is detectedin thel_gh-slz_
valuoMa,,_,, 80ismiuiredfor_ onsetofasurface- photo_'q_sascompar_ to the1-_cme. Theflame
fzont diameterincreasesto __c_,mm,_41te
calculatedvalueindicatu a strongflow circulation oxygenfluxandmaintainthestoichiomeuy.Burning
iadde themoltenmau. Thiscizcula_onallowsforan timesvaxywidelydependingentheanmberofjm and
efficient adsorl_n ef oxygen and a thorough and explosieas that acceleratecomberdon. Neve_de_,
rapid mixing of reactants. Thus, a_ording to the the averagebun_g time at low gravity 0.9 s) is
results of the low-gravity expcrim_ts, the ratc- almosttwicetheaveragevaleeat 1g(2.2s). Asimilar
controlling ._.h_ni_., in the combustion of bulk Ti etfe_t of low gravity on burning rates of gas-phase
samples appear to be the _n/convcction of x_-fions havebeenob_._d in manyother
oxygentothesamplesurface. flameconfigurations_.
Presser_e
In the _ of Mg. a critical temperanue of
a) Titanium
1050Kis ebttiaed. Following thefirstflashgenerated
bythe Mg-O_vapor-pha_ hoffi_genams _'_ion, an Theeffectof pressureon propagationratesof
ignition wave runs throughthe sample drivenby the Tibulkrumplesisevaluated byexperiment_ condncted
difference between the flameten_rautre (near3430 at normal gravity at pressuresin the 0.1 to 4.0 atm
K, the vaporization.decompesidon point of MgO_7) range. Thetestat 0.1 a_mis intended toreplicatethe
and the _ of theunreactedmetal, which is weaklybuoyantcondition expedenc_l in the reduced
near the Mg boiling point (1366 K). Owing to the gravity aircraft at 0.01 $. The similarity of
imgetarlyshaped(acon._ee_ ofmetalmete_g expedme_ conditionscom_fromthep_zbueyancy
before ignition), porous,solidoxidelayersurrounding scaldeescn'oeidntheIntroduction.
the sample, noattemptwas made to calculatesurface The resaI_sfrom the seriesof pressure
regression rate& Instead, an eva/uation of burning experimentsareshown inaplotofpropagationratevs.
4
AmuicaulustitouftA__onauficansdAstra_utics
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AIAA-98-0570
02pressure(Fig. 3). Thetrendline inthe figureshows formedresulting in theinabilityofthelamptoheatup
a linear variation of the propagation rate with the the sampleaboveits conduction and radiation losses.
squarerootof the oxygenpressure. Thisresultagrees After severalsecondsof heating, asteadytemlgrature
with px_-viouspropagation rate-pressurerelationships lessthan a100Klowerthan the criticaltcml_raturcis
obtained by several re._arch groups with various recordedby the thermoo0uple attached to the metal
metals_4. Thedecreasingtrend inpropagatingratesat, surface.
progressively lower pressurmappearsto indicate thc
existence ofadi_us/on/convec_on controlledn=ction. b) Magncsium
The close agreement of the mean propagation
Thesamesetofexperiments atpressuresfrom
velocities at 0.01 g and 1atm(8.7 ram/s)and at 0.1
0.1 to 4.0 arm were conducted with Mg. For the
mmatnormal gravityU3 sum/s)scan toconfirmthis
geasonsexplained previously, the burningtime of the
hypothesis. Care musk however, be _ in
Mgsamplesismeasuredinsteadoftheburningrateof
assigning the control of thc burningratem asinslc
anirrc_arly shapedmoltenspecimc.. The
mechanism at these low premuge_ According to
times followthe sametrc.ndwithpressureasobserved
Hiranoct,,1.i_,othermechanismssuchasphysicaland
in the propagation rates of Ti samples. Shorter
chemical adsorption of oxygenatomsinto the molten
burning times characterized by sharply etongated
oxide also follow a square mot dependency with
flamesoccuratprogressivelyhigherprcssurc_ Loug=r
pressure. Althoughacontinuous,coherentoxidelayer
burning times and broader flames are cx_'bit_ at
summnding the molten ball is not observed in the
subatmospheric premm_ wlgre buoyancy induced
burning Ti samples, several rate.controlling conve_on is minimized. These observations dearly
mechanisms maybe compctin8atlowsubaUnosphcrlc show the diffusion/convection controlled reaction of
plcr,sur_.
thegas-phasehomogeneouscombustionofMg
A comparisonof tlg burningbehaviorof Mg
sampk_ inthe normal gravity tests at O.l armv¢_'us
thetestsperformedat0.01g at1armwasnotposm'blc.
Althoughaccording to thep2g relationship the same
buoyantconditions can be simulated under the two
iO,
_tal condi_ons, the distortingc_ct ofgravity
on the shapc of the molten metal sample at normal
gravitycannotbe_iminat_ Thecylindricalshapeor"
the Mg sample is rapidly destroyed after melting
I"
leading to atotal collapsc of the sample at 1g. In
|. addition,no combustion event is detectedat 0.1 arm
Instead,themetalisviolently evaporated when, atthe
boiling tempemUneof Mg, the force generatedbyits
D
4 $ vaporpressureexceeds the stngtm-al inte4grityof the
ozPRESSURE{ram) surface oxi& coating leading to its rupture. A
nonexplosive surface rcaction on the finely divided
l_un 3. Propagationratmofrig reacting condcnsi=g metal particles appears to take place
moltenmass inTisamplesatnormalgravity
at?r_ard leaving smaU oxi& panicles scaum_
undervarious oxygenpressures. around the combu_on cham_r. This dust is a
combina_n of Mg, MS,O, and son_ undeW.,mined
Tests at lower-dum_.l armpress-oreswere
intcgmediateoxides. This behaviormay be due to a
atu_pc=d with no ¢vid¢_:_c_acombustioncvcm. It
switch froma homogeneous gas-phase reaction to a
is bedieved that an igniU'on limit _ than a
lzO_guzous susfacorcac_on in the particles
flammabil_ limit is xeacl_ at tlgse low pmmues.
atsufficientlylowpressure_ Theposu'bilk'yofMg-O_
Due to the x_luced Qonvectienof oxygen, the smface
hetemgemous reactionsoccurringin d_ oxidesurface
oxidation of the solid metal prior to ignition my
at lowtemperamrmand pressures has been proposed
switch from a kinetic- to a diffusion-controlled
byMarkst¢/nin astudyof the reaction ofsmall metal
reaction Atalow rateofheatgenerationbyoxidation
pamclcscxtcmallyheatedatvc_ towpressure.
ascompmedm thehighra_ ofthe variousheatlosses,
a _ temperature is never geach_ Instead, an
increasingly thicker, refractiveoxide layer is slowly
5
AmericanInstituteofAeronauticsandAstronautics
Copyright © 1997, American Institute of Aeronautics and Astronautics, Inc.
AIAA-98-0570
Coadmions oxygen reactions at a constant pressure. This
technique climinatcs the uncertainty introduced bythe
The combustion behavior of metal-oxygen
influence of other possible pressure-dependent rate-
ttactiom in a weakly buoyant ¢nvironmtnt is midied
controlling mechanisms in studies of weakly buoyant
to understand the rate_ontrclling mechanisms in the
metal flamesat low pressures.
homogeneous and heterog_eous combustion of bulk
metals. The ¢xp=imzntal setup consists of a 1000-W
Acknowledgments
xenon lamp that inadiat=_ the top mrfa= of cylimlfiml
_a_imn and __ _ 4mm in diameter This work is mpported by the National
and 4 mm in length, in a quiescent, pure-oxygen A_'omutics and Space Administm_on under Grant
¢m4mnment at pressuresranging from O.t to4.0 aim. NASA-NAG3-1685. The authors _-ateful_
Rcdm:_ gravity is obtained from an aircraft flying acknowkdg¢ the t¢chuical supervision of Robert
parabolic trajectories. Qualitative obsmvat_ns, Friedman from the NASA Lewis Rr.scarch Ccnmr.
propagation ratesandbunting timesarc extractedfrom
hish-speed ¢lmmato_dpby. Endssion _tra of gas-
phase reactions are obtained with an imldng
spectrograph/diode array syst¢_ I. Mormmvi, S., Suadedand, P. B., lurng, 1., and
Aweakly buoyant anvimnmeat is generated at Faeth, G. M., Second Internah'onal M_crograviO,
low pressurm under normal gavity (1 g) and also at a Combu_lion Workshop, NASA Conference
presm_ of 1arm undex roduced gravity (0.01 g). The Publication 10113, pp. 107-112, HASA
stmiladty between thc_ two expa'imcntal conditions Eram_h Center, Cleveland, 1993.
comes fl,om the p2& factor extracted from the Grashof
number. _ propagation velocities of the molten 2. Chart, W. and Tien, L, Combust. Sci. and
interface on titanium samples (heterogeneous surface TechnoL, 18, 139-143 (1978).
burning) ate found at progressively lower p_ at
Ig. In the recktcedgravity expm'imants at 1arm, the 3. Ross, H. D., Sotos, _ (3., and Tien, J., CombusL
propagation velocity is almost ball the value obtained ,_i. and Technol., 75, 155-160 (1991).
=xg. _ =xpe¢_ rmn t_ _g trooy=_ sc_=,_
propagation velocity at O.Xarm and I & is relatively 4. Yuan, T., Dutox, D., and Villernmux, E.,
closc to the value flared at 1 mm and 0.01 g. These Combust. Sci. and 7echnoL 92, 69-86 (1993).
ram are comparal to tlzo_cal results from heat
candu_'on amlysm with a _on/cowa_on 5. Hirano, T., Sam, K., Sato, Y., and Sato, L,
omltolled reaction. The clos_ agreement found Combust. Sci. and TechnoL, 32, 137-159 (1983).
expefimeatal and _ values indicate
the _0_onaac= of _ con_x:tioa<=]_oc_ o_ 6. ICLrschfclLd.,, Angewa_dte Chemle, 71, 663-667
mmsport oncombustion rates. (1959).
Pot magnesium sample, (homogcncotm gas-
phase Immiag), pmgrmsiv_ langer burttin$ times are 7. Harrison, P.L. and Yoff¢, A. D., Proc. Pay=/,._oc.,
cxp_ atIow_ prcssur_andIg.Undm-mtmzd London, 261A, 357-370 (1961).
gravity ¢ondttiom at I arm, a burning ti_ twicz as
long as in 1g is ealu'bited. However, in this case the 8. Steinberg, "1"A.., W_on, D. B., andBcnz, F.L, in
validity ofthep=gb=w',/mcysca1¢remainsunt¢s_ due Flammabili O, and Semitivity of Materials in
to the inability to obtain steady gss-plmse bundng of Ox_en-Eariched A_¢o_phere$: SLoth Volume,
the magmsium sample at 0.1 ram; atthis prcssurc tiM: STP 1197 (lano_ D. D. and Stoltzfi_s, J. Iv[.,
mud samp_ simptyc'vaponm withoutignit_0a, Eds.), American Society for Testing andMatcriah,
followed by which appears as noncxp4osiv¢ suffacc 133-145, Phflade]phis, 1993.
reaction on the flncly dividal condeasing mmal
pastic_, lqmanthde_ aclear ucnd ofIong_ bucudng 9. Bt-zustowski, T. A. and Glassmao, I., in
and larger flame mmdoff distances at low Heterogeneous Combustion, AIAA S=ri¢$ in
pressures aud at low gravity Izints to a ProgressinAstnmautics and Aeronautics,Vol.15
difl_on/convection com_lled reaction. An unique (wom=.,_ H. G.,_ L,and G_, L,,_r.,
aspect of this work is the usc of a reduced gravity Eds.),Academic Press,117-158, New York, 1964.
eavitonment to evaluate the role of buoyancy-induced
oxidizer transport in the training rate of bulk metal-
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