e

rs, wereflames.(by vol-tivitiesratios ofioxide as–10 for, steady,ith de-

stretchedlovitzlocity

ts theirlocities byhe flamesunteract

Combustion and Flame 143 (2005) 79–96www.elsevier.com/locate/combustflam

Suppression effects of diluents on laminar premixedhydrogen/oxygen/nitrogen flames

L. Qiao∗, C.H. Kim, G.M. Faeth

Department of Aerospace Engineering, The University of Michigan, Ann Arbor, MI 48109, USA

Received 27 September 2004; received in revised form 10 April 2005; accepted 10 May 2005

Available online 2 August 2005

Abstract

Laminar burning velocities and the flame response to stretch, as characterized by Markstein numbedetermined experimentally and computationally for outwardly propagating spherical laminar premixedThe mixtures studied were premixed hydrogen/air/diluent and hydrogen/30% oxygen and 70% nitrogenume)/diluent flames, with the latter condition of interest for pre-external vehicular activity preparation acon board manned spacecraft. Other flame conditions were room temperature (300 K), fuel-equivalence1.0 and 1.8, pressures of 0.5, 0.7, and 1.0 atm, diluents including helium, argon, nitrogen, and carbon dsuppression agents, and diluent concentrations of 0–40% (by volume), which implies oxygen indices of 30present conditions. Predicted flame behavior was obtained from one-dimensional, spherically symmetricand time-dependent numerical simulations with variable-property and multicomponent transport and wtailed hydrogen/oxygen chemical kinetics. Flames studied were sensitive to stretch, yielding unstretched/laminar burning velocity ratios of 0.6–1.25 for conditions well away from quenching conditions (e.g., Karnumbers; Ka� 0.5). Diluents became more effective (provided greater reductions of the laminar burning vefor a given diluent concentration) in the order helium, argon, nitrogen, and carbon dioxide, which reflecincreased capabilities either to quench the reaction zone by increased specific heats or to reduce flame vereduced transport rates. The addition of diluents generally decreased Markstein numbers, which made tmore susceptible to preferential-diffusion instability. This effect increases flame speeds and tends to cothe effect of diluents to reduce laminar burning velocities. 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Keywords: Extinction; Flame-stretch interactions; Hydrogen; Fire extinguishing

mi-onsres

offaceistureionthei-der-ism

1. Introduction

Halons have been very successful as checally active flame suppression agents in applicatiwhere effective and clean control of unwanted fiis needed; see Drysdale[1] and Tuhtar[2]. Unfor-

* Corresponding author. Fax: +1 734 763 0578.E-mail address: lqiao@umich.edu(L. Qiao).

0010-2180/$ – see front matter 2005 The Combustion Institutdoi:10.1016/j.combustflame.2005.05.004

tunately, halons also contribute to the depletionstratospheric ozone that protects the Earth’s surfrom harmful ultraviolet solar radiation. Due to thundesirable environmental effect, halon manufacwas stopped in 1994, except for limited productin some developing countries, under the terms ofMontreal Protocol[3]. Subsequently, many expermental and computational studies have been untaken to gain a better understanding of the mechan

e. Published by Elsevier Inc. All rights reserved.

80 L. Qiao et al. / Combustion and Flame 143 (2005) 79–96

Nomenclature

D Mass diffusivityK Flame stretch, Eq.(1)Ka Karlovitz number,KDu/S2

LL Markstein lengthMa Markstein number,L/δDP Pressurerf Flame radiusSL Laminar burning velocity based on un-

burned gas propertiesS′

L∞ Value of SL at the largest radius ob-served

t TimeT Temperature

Xi Mole fraction of speciesi

Greek symbols

δD Characteristic flame thickness,Du/SLρ Densityϕ Fuel-equivalence ratio

Subscripts

b Burned gasmax Maximum observed valueu Unburned gas∞ Unstretched flame condition

ns

ownun-icalllyenvi-thata-to

up-of

a-ingon

eri-

dys-ghesns

ribeherallyun-se

up-ies.bu-eletm-n-

xedsion

in-s,

dif-as

nt tots ofon-n,ex-im-m-

ofionmi-re-n/heses-

wn,la-

allycon-ener-les

arelexbse-p-

ienten-

ereand

of flame suppression of the chemically active haloand their potential replacements; see Safieh et al.[4],Sheinsohn et al.[5], Walravens et al.[6], Miziolekand Tsang[7], McIlroy and Johnson[8], Linteris andTruett [9], Noto et al.[10,11], Casias and McKinnon[12,13], Linteris et al.[14], Takahashi et al.[15], Sasoet al. [16–18], Kim et al. [19], Faeth et al.[20], andreferences cited therein. These studies have shthat chemically active suppression agents areusually effective because they interrupt the chempathway of fuel oxidation. Unfortunately, chemicaactive agents often generate substances in flameronments that prevent their use in confined spacescontain living organisms. Motivated by this observtion, the objective of the present investigation wasstudy the properties of typical chemically passive spression agents—diluents that avoid the limitationsthe Montreal Protocol[3] and problems of the genertion of substances in flames that are harmful to livorganisms in confined spaces. Effects of diluentslaminar premixed flames were studied both expmentally and computationally.

An important issue concerning the present stuis the justification for considering only the suppresion properties of laminar premixed flames. Althoumost practical flames are turbulent, turbulent flamare difficult to study because experimental conditioare substantially complicated by the need to descand treat a variety of turbulence properties. Anotadvantage of laminar flames is that they generare tractable for detailed numerical simulations,like turbulent flames, enhancing capabilities to ucomputations to supplement information about spression behavior from directly measured propertIn addition, laminar flames are also relevant to turlent flames based on widely accepted laminar flamconcepts of turbulent flames. Finally, due to the coplexities of turbulent flames, it seems unlikely that u

derstanding of the suppression of turbulent premiflames will precede understanding of the suppresof laminar premixed flames.

Another issue concerning the present studyvolves limiting considerations to premixed flameeven though both premixed and nonpremixed (fusion) flames are important in practice. This wdone because premixed flames are most relevaprocesses of flame suppression (e.g., even poinflame attachment in diffusion flames are largely ctrolled by premixed flame phenomena). In additiopremixed flames lend themselves to well-definedperimental and computational conditions that splify the interpretation of both experimental and coputational results.

Other limitations used to control the scopethe present study involved considering combustprocesses involving only hydrogen/oxygen checal kinetics for outwardly propagating spherical pmixed flames. Limiting combustion to hydrogeoxygen chemical kinetics is reasonable because tkinetics are fundamentally important for all combution processes of hydrocarbons in air, are well knoand are sufficiently simple so that numerical simutions involving these reactants are computationtractable. Furthermore, these reactants provideservative suppression properties because they gally are the hardest to extinguish among combustibof practical interest; see Wieland[21]. In addition,outwardly propagating spherical premixed flamesalso attractive because they do not involve compquenching processes near surfaces. Finally, suquent discussion will also show that outwardly proagating spherical flames are particularly convenfor directly measuring and computing the fundamtal properties of various suppression agents.

To fix ideas, two general reactant systems wconsidered during the present study: (1) hydrogen

L. Qiao et al. / Combustion and Flame 143 (2005) 79–96 81

nt aat

en0%h isvityaft;

ebonfects

to

l-

e-rop-ig-d

pi-

in-am-ity

einedlyro-edrs,

s.e

to at arein-thegenleye-wisrs

ns,

nd--f Hnele tore-iorab-

uetor-of

s,re-

up-andichrn-si-the

e Hne

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rk

ednd

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air at pressures of 0.5 and 1.0 atm, to represeconservative combustible mixture for suppressionaltitudes typical of cities on Earth, and (2) hydrogin an atmosphere consisting of 30% oxygen and 7nitrogen (by volume) at a pressure of 0.7 atm, whicthe atmosphere used during external vehicular acti(EVA) preparation of astronauts on board spacecrsee Wieland[21]. Finally, only simple diluents werconsidered (e.g., helium, argon, nitrogen and cardioxide) as suppression agents to emphasize efof specific heats and transport properties knowninfluence diluent performance (see Huggett[22,23])without having to deal with the complexities of muticomponent flame suppression processes.

A difficulty that is encountered when laminar prmixed flames are used to study the suppression perties of diluents is that flame/stretch interactions snificantly affect the laminar burning velocities anstructure of laminar premixed flames[24–29]and therate of propagation of turbulent premixed flames tycal of practical applications[30,31]. To deal with thisproblem, diluent performance during the presentvestigation was determined by using unstretched linar burning velocities to characterize the intensof combustion of the flames and by using Markstnumbers(Ma) to characterize the sensitivity of thflames to effects of stretch. Fortunately, outwarpropagating spherical laminar premixed flames pvide a straightforward determination of unstretchlaminar burning velocities and Markstein numbeas demonstrated by Aung et al.[32,33] and Kwonand Faeth[34] for hydrogen laminar premixed flameAnother finding of these studies is that their flamstructure predictions suggested that mainly H andlesser degree OH radical production and transporimportant aspects of preferential-diffusion/stretchteractions. This is not surprising, however, due towell known proportionality between laminar burninvelocities and H radical concentrations of hydroglaminar premixed flames, first pointed out by Padand Sugden[35] based on the laminar burning vlocity measurements of Jahn that are cited in Leand von Elbe[36] and subsequently noted by othefor hydrogen premixed flames for various conditioe.g., Kim et al.[19], Butler and Hayhurst[37], andreferences cited therein. Another aspect of the fiings of Kim et al.[19] is that changes of flame conditions that tended to reduce the concentrations oand OH radical concentrations in the reaction zoalso tended to make these flames more susceptibpreferential-diffusion instability as measured byduced values of the Markstein number. This behavhas the potential to increase flame speeds (thesolute flame velocity in laboratory coordinates) dto the creation of flame surface area by the distion or wrinkling of the flame surface as a result

the action of preferential-diffusion instability. Thuthe application of flame suppression agents to pmixed flames involves two counteracting effects. Spression agents have the capability to reduce HOH radical concentrations in the reaction zone, whleads to corresponding reductions of laminar buing velocities which tend to reduce flame intenties and thus suppress the flame. Furthermore,same capability of suppression agents to reducand OH radical concentrations in the reaction zoalso enhances the potential for the developmenpreferential-diffusion-induced flame surface instabities that increase flame speeds, which tends tocrease flame intensities and thus reduces effecflame suppression.

2. Experimental methods

2.1. Apparatus

Experimental methods were similar to past woand will be described very briefly; see Aung et al.[32,33] and Kwon and Faeth[34] for more details. Theexperiments were conducted in a spherical windowchamber having an inside diameter of 360 mm aan internal volume of 0.024 m3. Optical access waprovided by two 100-mm-diameter quartz windomounted opposite one another along a horizontalpassing through the center of the chamber. The chber was capable of operation over a pressure raextending from complete vacuum up to a maximof 34 atm.

The reactant mixture was prepared within tchamber by adding gases at appropriate partial psures to reach the total initial pressure of the reacmixture for a test (0.5, 0.7, and 1.0 atm for the prestest range). The reactant gases were mixed usismall metal fan located inside the chamber withfan-induced motion allowed to decay before igniti(5–10 min for mixing and at least 30 min for decagiven these conditions, motion picture shadowgradid not indicate any distortion of the flame surfaor convection of the flame kernel from its positiocentered on the spark kernel. After combustion wcomplete, the chamber was vented to the laboraexhaust system and then purged with dry air tomove condensed water vapor prior to refilling for tnext test.

The combustible mixture was spark-ignited atcenter of the chamber using electrodes extendfrom the top and bottom of the chamber. One eltrode was fixed, whereas the other electrode coulmoved with a micrometer having a positioning accracy of 10 µm. The tips of the electrodes were fitungsten wires having diameters of 250 µm and

82 L. Qiao et al. / Combustion and Flame 143 (2005) 79–96

thenitec-s.ca-ncerge

giesin-

al-are

had-w-

-arcdming

20per

ay-o

rategni-asereA)

av-ontheto-ureb-terh a

ar-nedsid-

ed

ef-ere

an

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en

-umcen-ses.ceial-ss

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,

lengths of 40 mm. The spark gap was varied inrange 0.5–2.0 mm, with the larger gaps used to igflames having relatively small laminar burning veloities that required relatively large ignition energieThe spark energy was supplied by a high-voltagepacitor discharge circuit having a variable capacita(100–7000 pF) and voltage (0–10 kV) and a dischatime of roughly 5 µs. Spark gaps and spark enerwere adjusted by trial so that they were close to mimum ignition energies (5–20 mJ, with the larger vues used for flames having relatively small laminburning velocities) to minimize effects of initial flamacceleration due to excessive spark energies.

2.2. Instrumentation

The flames were observed using high-speed sowgraph motion picture photography. The shadograph system consisted of a 100-W mercury shortlamp (ARC; HSA-150 HP) with the light collimateby a pair of f6 parabolic reflectors having 1220-mfocal lengths. The flame images were recorded usa 16-mm motion picture camera (Hycam; Model K34E) operating at speeds of 4000–8000 picturessecond. Kodak Hawkeye surveillance film on a dlight loading spool (SP-430), with perforations on twsides, was used for the photographs. The framingof the camera was sensed electronically so that ition occurred only when the proper framing rate wreached. The framing rate and the ignition pulse wrecorded using a digital oscilloscope (LeCroy 9400so that the film records could be synchronized.

2.3. Data reduction

Present measurements were limited to flames hing diameters larger than 10 mm, to avoid ignitidisturbances, and smaller than 60 mm, to limitvolume of burned gas to less than 0.5% of thetal chamber volume so that the chamber pressremained constant within 0.7% throughout the oserved period of flame propagation. Laser velocimemeasurements for this test arrangement (but witslightly smaller chamber), due to Kwon et al.[31],indicated that velocities within the unburned gas vied as expected for outwardly propagating unconfispherical flames for the range of flame sizes conered during the present investigation.

Similar to past measurements of laminar premixflame properties[32–34], determinations of flameproperties were limited to conditions whereδD/rf <

2% so that effects of flame curvature and transientfects associated with the thickness of the flame wnegligible, as discussed by Tseng et al.[38]. Next,laminar burning velocities were generally greater th

150 mm/s, so that the intrusion of effects of buoancy due to Earth’s gravity was negligible as shoby Ronney and Wachman[39]; this behavior was confirmed using the present flame photographs whindicated negligible effects of flame distortion, or mtion of the origin of the flame, due to buoyancythe period when present observations were made.thermore, effects of radiative heat losses were sm(less than 1% of the rate of thermal energy releof reaction within the test flames) based on earestimates of these losses for hydrogen flames atilar conditions due to Aung et al.[33] that were car-ried out as discussed by Siegel and Howell[40]. Thisassessment also agrees with an earlier evaluatioeffects of radiation for hydrogen flames at simiconditions due to Dixon-Lewis[41]. Finally, effectsof the spark energy were small compared to theergy release due to combustion in the region whthe flames were observed (the energy release ducombustion was generally greater than 20 timesspark energy for flames having diameters larger t10 mm for present test conditions). Under thesesumptions, Strehlow and Savage[26] showed that thelaminar burning velocity and flame stretch are givby the following quasi-steady expressions:

(1)SL = (ρb/ρu)drf/dt, K = (2/rf)drf/dt.

The density ratio appearing in Eq.(1) was foundfrom McBride et al.[42], assuming adiabatic constant-pressure combustion with chemical equilibriin the combustion product gases and the same contrations of elements in the unburned and burned gaThis is only a convention that follows past practi[32–34], however, because it ignores preferentdiffusion effects that modify local element mafractions and energy transport and causeρb/ρu todiffer from plane adiabatic flame conditions. Thconvention is convenient, however, because agle density ratio relates all flame speeds at a gireactant mixture condition. In addition, this convetion retrieves the correct flame displacement velity, drf/dt , for given unburned mixture conditionand degree of flame stretch. Finally, based on pnumerical simulations of stretched hydrogen pmixed flames, the present assumptions used toρb/ρu are quite reasonable for the conditions cosidered during the present study. In particular, valof ρb/ρu for stretched hydrogen flames agree with10% with those for unstretched (plane) flames whδD/rf < 2%; see Kwon[43].

Final results were obtained by averaging the msurements of four to six tests at each condition. Expimental uncertainties were estimated as describeTseng et al.[38] and references cited therein. Thesulting experimental uncertainties (95% confidenare as follows:SL less than 9%, Ka less than 21%

L. Qiao et al. / Combustion and Flame 143 (2005) 79–96 83

r

inarrizendter-f an-to

-nd

thebe

einet

sern-inssed

dval-ustchatalp-

ws

ofeen

-hasantitzd nu-

n-rn-

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n of

-rthat

-andithup-1.0me

ep-

SL∞ less than 10%, and|Ma| less than 25% fo|Ma| > 1 and less than 25%/|Ma| for |Ma| < 1.

2.4. Data correlation

The measurements were analyzed to find lamflame properties (Markstein numbers to charactethe sensitivity of the flame to effects of stretch aunstretched laminar burning velocities to characize the intensity of combustion and the capability oparticular diluent to reduce flame intensity). As metioned earlier, present considerations were limitedthin flames(δD/rf < 2%) for conditions where effects of ignition disturbances, flame radiation, avariations of element concentrations onρb/ρu weresmall. Then, a convenient relationship betweenlaminar burning velocity and the flame stretch canobtained by combining an early proposal of Markst[25] and the local conditions hypothesis of Kwonal. [31] to yield the expression

(2)SL∞/SL = 1+ MaKa,

where values ofSL and the Karlovitz number, Ka(the dimensionless flame stretch= KδD/SL), werefound from Eq.(1) as already discussed. For thedefinitions,δD is based on the stretched laminar buing velocity and the mass diffusivity of the fuelthe unburned (and in the present case, unsuppregas, as conventions. The decision to use theDu forunsuppressed flames as theDu for the suppresseflames also was made to provide an absolute euation of SL∞ so that the effectiveness of variodiluents could be directly compared. The small strelimit is also of interest to connect present resultsfinite levels of stretch to the conditions of classicasymptotic theories of laminar premixed flame proagation at negligibly small levels of stretch, as follo(see Aung et al.[33]):

(3)SL/SL∞ = 1− Ma∞ Ka∞, |Ka∞ | � 1.

Several other proposals to represent effectsflame stretch on laminar burning velocities have bmade; see Taylor[44], Dowdy et al.[45], Brown etal. [46], Karpov et al.[47], and Bradley et al.[48].The approach used in Eq.(2), however, is particularly convenient because the Markstein numberproven to be relatively constant for particular reactmixture conditions over wide ranges of the Karlovnumber based on both measurements and detailemerical simulations of laminar premixed flames[30–34]. Thus,SL∞ and Ma provide convenient and cocise measurements of laminar premixed flame buing rates and response to stretch, as discusseAung et al.[33]. See Aung et al.[33] for a discussionof other advantages of the present characterizatiopremixed-flame/stretch interactions.

)

2.5. Test conditions

Experimental conditions are summarized inTa-bles 1 and 2. Experimental conditions for H2/air/dilu-ent flames, summarized inTable 1, seek to be representative of human habitation conditions on Eaat various altitudes, as follows: reactant mixturesroom temperature(298±0.5 K), fuel-equivalence ratios of 1.0 and 1.8, pressures of 0.5 and 1.0 atm,diluent concentrations of 0–40% (by volume) whelium, argon, nitrogen, and carbon dioxide as spression agents. The fuel-equivalence ratios ofand 1.8 were chosen due to their relevance to flasuppression with stoichiometric conditions being r

Table 1H2/air/diluents laminar premixed flame test conditionsa

Diluents XD ρu/ρb SL∞ (mm/s) Kamax Ma

p = 1.0 atm,φ = 1.0, Du = 72.9 mm2/s– 0.0 6.89 2140 0.07 0.2He 0.1 6.63 1960 0.06 1.5He 0.2 6.50 1730 0.06 1.3He 0.3 6.19 1430 0.06 1.3He 0.4 5.79 1170 0.09 0.9Ar 0.1 6.63 1770 0.05 1.1Ar 0.2 6.50 1290 0.08 0.5Ar 0.3 6.19 1100 0.14 −0.2Ar 0.4 5.79 760 0.22 −0.3N2 0.1 6.56 1650 0.09 0.0N2 0.2 6.15 1170 0.13 −0.3N2 0.3 5.67 860 0.11 −0.7N2 0.4 5.13 480 0.19 −1.0CO2 0.1 6.23 1330 0.09 0.0CO2 0.2 5.61 770 0.14 −0.5CO2 0.3 4.98 400 0.18 −0.7CO2 0.4 4.36 180 0.40 −0.6

p = 1.0 atm,φ = 1.8, Du = 72.9 mm2/s– 0.0 6.30 2900 0.04 3.5N2 0.1 5.89 2340 0.03 3.2N2 0.2 5.44 1830 0.04 3.1N2 0.3 4.71 1400 0.05 2.8N2 0.4 4.48 830 0.08 2.6CO2 0.1 5.48 2040 0.05 2.7CO2 0.2 4.78 1330 0.05 2.1CO2 0.3 4.18 760 0.09 1.4CO2 0.4 3.65 240 0.12 1.2

p = 0.5 atm,φ = 1.0, Du = 145.8 mm2/s– 0.0 6.89 2020 0.09 1.7N2 0.1 6.54 1600 0.11 1.0N2 0.2 6.14 1320 0.13 0.7N2 0.3 5.67 920 0.22 −0.2N2 0.4 5.13 640 0.36 −0.4CO2 0.1 6.20 1320 0.13 0.5CO2 0.2 5.60 950 0.26 −0.2CO2 0.3 4.98 480 0.29 −0.6CO2 0.4 4.36 270 0.49 −0.6

a Initial mixture temperature of 298± 0.5 K.

84 L. Qiao et al. / Combustion and Flame 143 (2005) 79–96

e

on-i-ties

shuests in-0–k-

e-er-a

ra-o-heseing

f

es

i-ingm-

o-o-ties.-

portre

e etm

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ilar

Table 2H2/30% O2 and 70% N2/diluents laminar premixed flamtest conditionsa

Diluents XD ρu/ρb SL∞ (mm/s) Kamax Ma

p = 0.7, φ = 1.0, Du = 104.1 mm2/s– 0.0 7.45 3350 0.04 2.9He 0.2 7.21 2700 0.04 2.4He 0.4 6.67 1880 0.04 2.1Ar 0.2 7.21 2320 0.06 2.2Ar 0.4 6.67 1380 0.09 0.6N2 0.2 6.92 2100 0.08 0.6N2 0.4 5.97 1150 0.10 −0.3CO2 0.2 6.44 1570 0.07 0.7CO2 0.4 5.06 480 0.21 −0.3

a Initial mixture temperature of 298± 0.5 K.

resentative of flame attachment conditions in npremixed flames, whereasφ = 1.8 represents condtions where the unstretched laminar burning velociof hydrogen/air mixtures reach a maximum[32–34]and conditions that are most difficult to extinguifor premixed flames of these reactants. The valof ρu/ρb in Table 1 were found from McBride eal. [42], as discussed earlier. These measurementvolved unstretched laminar burning velocities of 182900 mm/s, Karlovitz numbers of 0–0.5, and Marstein numbers of−1.0–3.5.

Experimental conditions for H2/30% O2 and 70%N2 (by volume)/diluent flames, summarized inTa-ble 2, involve EVA-preparation conditions for spaccraft, as follows: reactant mixtures at room tempature (298± 0.5 K) and a pressure of 0.7 atm,fuel-equivalence ratio of unity, and diluent concenttions of 0–40% (by volume) with helium, argon, nitrgen, and carbon dioxide as suppression agents. Tmeasurements involved unstretched laminar burnvelocities of 480–3350 mm/s, Karlovitz numbers o0–0.21, and Markstein numbers of−0.3–2.9.

3. Computational methods

3.1. Numerical simulations

Computational methods for the present flamwere similar to those of Aung et al.[32,33]and Kwonand Faeth[34]. The outwardly propagating sphercal laminar premixed flames were simulated usthe unsteady one-dimensional laminar flame coputer code, RUN-1DL, developed by Rogg[49]. Thisalgorithm allows for mixture-averaged multicompnent diffusion, thermal diffusion, variable thermchemical properties, and variable transport properThe CHEMKIN package[50–53]was used as a preprocessor to find the thermochemical and transproperties for RUN-1DL. Transport properties we

found from the transport property database of Keal. [51]. Thermochemical properties were found frothe thermodynamic data base of Kee et al.[50], ex-cept for HO2, where the recommendations of Kimal. [54] were used. Before computing flame propties, all transport and thermodynamic properties wchecked against original sources. Similar to the msurements, effects of radiation were small due torelatively large flame velocities of hydrogen flamfor present conditions and were ignored. Flame pragation was allowed to proceed sufficiently far so teffects of initial conditions were small, similar to thmeasurements. Other limitations used to controlperimental uncertainties, e.g.,δD/rf < 2%, etc., werealso applied to the predictions. The computatiogrid in space and time was varied to ensure numeraccuracy within 1%, estimated by Richardsontrapolation ofSL . Finally, the numerical simulationwere analyzed similar to the measurements, takthe flame position to be the point where gas tempatures were the average of the temperatures ofhot and cold boundaries. Due to the present stringflame thickness limitations, however, the presentsults were not affected significantly by the criteriused to define the flame position.

Separate numerical simulations were carriedfor unstretched (plane) flames using the steady odimensional laminar premixed flame code, PREMdue to Kee et al.[53]. Other properties of these caculations and the limits of numerical accuracy wesimilar to those using the RUN-1DL algorithm. Thcode was mainly used to predict the structure ofstretched flames.

3.2. Chemical kinetic mechanism

Aung et al. [32,33], Kwon and Faeth[34], andKim et al. [19] carried out extensive evaluationsavailable detailed hydrogen/oxygen chemical kinemechanisms proposed by Kim et al.[54], Yetter et al.[55], Mueller et al.[56], Marinov et al.[57], Wangand Rogg[58], and Frenklach et al.[59,60] basedon their measurements of the properties of hydrooutwardly propagating laminar premixed flames. Tchemical kinetic mechanism of Mueller et al.[56]was found to provide the best comparison betwmeasurements and predictions for hydrogen flainvolving nitrogen, argon, and helium as suppressagents, fuel-equivalence ratios of 0.6–4.5, pressof 0.3–3.0 atm, and volumetric oxygen concentratioin the nonfuel gases of 0.21–0.36. These conditiwere generally similar to present flame conditiotherefore, the numerical simulations of flamesported here were limited to the Mueller et al.[56] hy-drogen/oxygen chemical kinetic mechanism. Simto the earlier evaluation of Mueller et al.[56] chemi-

L. Qiao et al. / Combustion and Flame 143 (2005) 79–96 85

/Onsfi-

11re-

outeironson-sim-the

retial-

esenrn-

phsiesr infdcend

ven-

beHy-e-nceser-san

ea-anederecethethethethe

anter,nar

ge.

e atob-

ite

ra-n-e,

iusrk

ed

res

usdic-til-

-

the

esezed

tiesof

air

cal kinetic mechanism by Kwon and Faeth[34], thismechanism was simplified because C/H/O and Nchemistry were not important for present conditioand could be deleted from the mechanism. Thenal reduced chemical kinetic mechanism involvedchemical species and 19 reversible reactions. Thisaction mechanism does not provide information abthe elementary reactions involving helium and threaction coefficients; therefore, the same reactiand their reaction coefficients as those used for argsuppressed hydrogen flames were chosen for theulations of the helium-suppressed flames due tothermodynamic similarities of argon and helium.

4. Results and discussion

4.1. Flame stability and evolution

Three kinds of flame surface instabilities weobserved during present experiments: preferendiffusion instability (observed only when Ma< 0),hydrodynamic instability (observed for all valuof Ma), and buoyant instabilities (observed only whlaminar flame speeds or corresponding laminar buing velocities were small). Shadowgraph photograof flame surfaces after distortion by these instabilitfor outwardly propagating spherical flames appeaKim et al. [19] and Kwon et al.[31]. The presence opreferential-diffusion instability could be identifieby irregular (chaotic) distortions of the flame surfarelatively early in the flame propagation process aas noted earlier only when Ma< 0. Fortunately, flamesurfaces remained smooth at small flame radii efor conditions that involved preferential-diffusion instability so that laminar burning velocities couldmeasured for a time even at these conditions.drodynamic instability could be identified by the dvelopment of a somewhat regular cellular disturbapattern on the flame surface, very similar to the obvations of Groff[61]; fortunately, these instabilitiewere observed only for flame diameters larger th60 mm so that they did not affect the present msurements limited to flame diameters smaller th50 mm. Finally, buoyant instabilities were observwhen laminar burning velocities were small and wreadily detected by distortion of the flame surfafrom a spherical shape when vertical planes offlame were observed (which was the case forpresent experimental arrangement). In addition,flame boundary was also deflected upward fromlocation of the spark kernel when effects of buoyinstability were important. As noted earlier, howeveffects of buoyancy were small as long as lamiburning velocities were larger than 150 mm/s, whichinvolved velocities well below the present test ran

Finally, no measurements reported here were madconditions where any of these instabilities wereserved.

4.2. Burning velocity/stretch interactions

Measurements at finite flame radii involve finvalues of flame stretch through Eq.(1); therefore,the laminar burning velocity at the largest flamedius observed still differs from the fundamental ustretched laminar burning velocity of a plane flamSL∞. Thus, values ofSL∞ were found from Eq.(2)by plotting S′

L∞/SL , whereS′L∞ is the value of the

laminar burning velocity at the largest flame radobserved, as a function of Ka, similar to past wo[32–34]. As will be seen subsequently, this yieldlinear plots so that extrapolation to Ka= 0 yieldedS′

L∞/SL∞ and thusSL∞ as summarized inTables 1and 2. GivenSL∞, plots ofSL∞/SL as a function ofKa could be constructed for various reactant mixtuand pressures, as prescribed by Eq.(2). Examplesof plots of this type for hydrogen flames at varioconditions, based on both measurements and pretions, are illustrated inFigs. 1–4; results at other tesconditions were qualitatively similar to the resultslustrated inFigs. 1–4. Finally, it is evident that thevariations ofSL∞/SL as a function of Ka are all linear for the results illustrated inFigs. 1–4. This impliesthat Ma is a constant that can be determined fromconstant slopes of the plots ofSL∞/SL as a functionof Ka for each flame condition that was studied. Thvalues of the Markstein number are also summari

Fig. 1. Measured and predicted laminar burning velocias functions of Karlovitz number and the concentrationnitrogen diluent for premixed stoichiometric hydrogen/flames at NTP.

86 L. Qiao et al. / Combustion and Flame 143 (2005) 79–96

tiesof

o-

tiesof

airtion

e

theeinedent

s asar-ention

son

s at

e of

d inslec-

uredthecer-lier.ofwill

c-

iv-

Fig. 2. Measured and predicted laminar burning velocias functions of Karlovitz number and the concentrationcarbon dioxide diluent for premixed stoichiometric hydrgen/air flames at NTP.

Fig. 3. Measured and predicted laminar burning velocias functions of Karlovitz number and the concentrationnitrogen diluent for premixed stoichiometric hydrogen/flames at room temperature and spacecraft EVA-preparaconditions.

in Tables 1 and 2for all conditions tested during thpresent investigation.

Given the preceding description of the way thatunstretched laminar burning velocities and Markstnumbers of outwardly propagating laminar premixflames were found, it is of interest to compare pres

Fig. 4. Measured and predicted laminar burning velocitiefunctions of Karlovitz number and the concentration of cbon dioxide diluent for premixed stoichiometric hydrogflames at room temperature and spacecraft EVA-preparaconditions.

Table 3H2/air laminar premixed flame property measurementsa

Source ρu/ρb SL∞ (mm/s) Kamax Ma

Present study 6.30 2900 0.04 3.5Kwon and Faeth[34] 6.30 2860 0.06 2.4Aung et al.[32,33] 6.30 2610 0.08 3.7

a Unsuppressed flames havingφ = 1.80 at an initial mix-ture pressure and temperature of 1 atm and 298± 3 K;Du = 72.9 mm2/s.

measurements with earlier results. This comparicould be carried out only for premixed H2/air flamesat a fuel-equivalence ratio of 1.8 with the reactantroom temperature and pressure (NTP or 298± 3 Kand 1 atm) where present test conditions and thosKwon and Faeth[34] and Aung et al.[32,33] over-lap. The results of all three studies are summarizeTable 3. Values of Kamax differ for the three studiebut this occurs due to the somewhat arbitrary setion of the range of flame radii to be used to findSL∞and Ma. On the other hand, the fundamental measquantities,SL∞ and Ma, are seen to agree amongthree studies within the ranges of experimental untainties (95% confidence) that were specified earThis behavior was typical of other comparisonspresent and earlier results that could be made, asbe seen subsequently.

The plots of laminar burning velocity as a funtion of stretch illustrated inFigs. 1–4involve resultsfor hydrogen/air flames at NPT for a fuel-equ

L. Qiao et al. / Combustion and Flame 143 (2005) 79–96 87

x-

el-r-

peni-

le

tousetial-r-

antthe5p-ve

hipll

inn-tch

be-pa-

thes-oflynsr oftial-nsial-nslt ofthislu-inarive-

ea-ac-tic.re-ical

er-em-

ro-ea-re-

itzizeds.ndent

m-

ox-he-e-tra-

fortra-x-

ntial-mesionthee-ch-hens.be-ablys

unc-nd

ro-

alence ratio of unity with nitrogen and carbon dioide as suppression agents (Figs. 1 and 2) and forhydrogen/EVA-preparation atmospheres for a fuequivalence ratio of unity with nitrogen and cabon dioxide as suppression agents (Figs. 3 and 4).Measurements on these plots are indicated by osymbols for stable preferential-differential condtions (Ma � 0) and closed symbols for unstabpreferential-diffusion conditions(Ma < 0). For un-stable conditions, the measurements are limitedvalues of Ka significantly greater than zero becaflame surfaces became wrinkled due to preferendiffusion instability for radius values within the nomal range of measurements.

The first notable observation fromFigs. 1–4isthat effects of flame/stretch interactions are importfor present test conditions; for example, over allpresent results,SL∞/SL varied in the range 0.6–1.2for Ka < 0.49, which does not involve a close aproach to quenching conditions (which would involKa ≈ 1 from Law [28]) where effects of Ka onSLare expected to be large. Next, the linear relationsbetweenSL∞/SL and Ka clearly is satisfied for ameasurements and predictions illustrated inFigs. 1–4which correspondingly implies constant Markstenumbers for each flame condition, providing a covenient and concise way to summarize flame/streinteractions for the present flames. Notably, thishavior has been observed for all outwardly progating flame conditions studied thus far (see[19,20,30–34]and references cited therein). Furthermore,progressive addition of diluents to the flames illutrated inFigs. 1–4causes the slopes of the plotsSL∞/SL as a function of Ka to become progressivemore negative, with the exception of a few conditiohaving large concentrations of diluent. In a numbecases, this implies that stable flames to preferendiffusion/stretch interactions at small concentratioof diluent become unstable flames to preferentdiffusion/stretch interactions at large concentratioof diluent. Due to increased flame speeds as a resuincreased flame surface area caused by wrinkling,behavior clearly tends to counteract the ability of dients to reduce combustion rates by reducing lamburning velocities and tends to reduce the effectness of diluents to some extent.

Finally, the qualitative agreement between msured and predicted burning velocity/stretch intertions, using the hydrogen/oxygen chemical kinemechanism of Mueller et al.[56], is reasonably goodThis is particularly promising because the measuments used to develop the hydrogen/oxygen chemkinetic mechanism of Mueller et al.[56] did not in-volve any direct consideration of flame/stretch intactions. This evaluation of predictions will continuduring subsequent consideration of Markstein nu

bers and unstretched laminar velocities, which pvide more direct and complete comparisons of msurements and predictions than is possible for thesults illustrated inFigs. 1–4.

4.3. Markstein numbers

Markstein numbers are independent of Karlovnumbers for present conditions and are summarin Tables 1 and 2as a function of reactant conditionA portion of these results, involving measured apredicted Markstein numbers as a function of diluconcentrations, are plotted inFigs. 5–7.

Measurements and predictions of Markstein nubers as a function of diluent concentrations for H2/airflames havingφ = 1 at NTP are illustrated inFig. 5,considering helium, argon, nitrogen, and carbon diide as suppression agents. With the exception oflium, values of the Markstein number generally bcome progressively more negative as the concention of diluent increases, with some tendencythis decrease to become small at large concentions of the more effective nitrogen and carbon dioide suppression agents. In these cases, preferediffusion instability is promoted as the flames becomore suppressed. Results for helium as a suppresagent differ from this behavior, however, becauselarge thermal conductivity of the fast-diffusing hlium molecules tends to promote preferential quening of the reaction zone and thus stability of tflames to preferential-diffusion/stretch interactioFinally, the qualitative and quantitative agreementtween predictions and measurements is reasongood inFig. 5, providing potential for the prediction

Fig. 5. Measured and predicted Markstein numbers as ftions of the concentration of helium, argon, nitrogen, acarbon dioxide diluents for premixed stoichiometric hydgen/air flames.

88 L. Qiao et al. / Combustion and Flame 143 (2005) 79–96

unc-ox-el-

al-av-

m-

res-uel-

bil-ed

ctsst-isin-andr, isdi-erthetions

m-for

res-ilar

esres-to

as ates

unc-ndy-VA-

andel-

rn-e-es.ing

lam-n-

ant-be

res-ents

entsthethoseentxcel-the

e asthatthe

oc-in

ox-on,

Fig. 6. Measured and predicted Markstein numbers as ftions of the concentration of nitrogen and carbon diide diluents for premixed hydrogen/air flames at a fuequivalence ratio of 1.8 and NTP.

to help explain the complex effects of preferentidiffusion/stretch interactions that influence the behior of suppressed laminar premixed flames.

Measurements and predictions of Markstein nubers as a function of diluent concentrations for H2/airflames havingφ = 1.8 at NTP are illustrated inFig. 6,considering nitrogen and carbon dioxide as suppsion agents. In this case, the experiments involve frich conditions where H2/air flames are intrinsicallystable based on classical models of flame instaity due to effects of preferential diffusion proposby Manton et al.[24] and Markstein[25], namely,that laminar premixed flames are unstable to effeof preferential diffusion at conditions where the fadiffusing component (H2 in the present instance)deficient (at fuel-lean conditions in the presentstance). The subsequent effect of adding nitrogencarbon dioxide as suppression agents, howevesimilar to results at other conditions where the adtion of a diluent tends to shift the Markstein numbtoward more negative (unstable) values. Finally,comparison between measurements and predicin Fig. 6 is excellent.

Measurements and predictions of Markstein nubers as a function of diluent concentrationsH2/EVA-preparation conditions forφ = 1 and roomtemperature are illustrated inFig. 7, considering he-lium, argon, nitrogen, and carbon dioxide as suppsion agents. These results are qualitatively simto the results for H2/air mixtures at NTP illustratedin Fig. 5: the addition of diluents generally causMarkstein number to decrease, helium as a suppsion agent differs from the rest due to its capabilityquench the reaction zone at stretched conditionsresult of the fast-diffusion and high heat transfer ra

Fig. 7. Measured and predicted Markstein numbers as ftions of the concentration of helium, argon, nitrogen, acarbon dioxide diluents for premixed stoichiometric hdrogen flames at room temperature and spacecraft Epreparation conditions.

of helium, and the agreement between measuredpredicted values of the Markstein numbers is exclent.

4.4. Unstretched laminar burning velocities

In the following, measured values of laminar buing velocities will be limited to stretch-corrected rsults that yield unstretched laminar burning velocitiThe measured values of unstretched laminar burnvelocities are summarized inTables 1 and 2. Plots ofmeasured and predicted values of unstretchedinar burning velocities as functions of the concetrations of various diluents for some typical reactconditions appear inFigs. 8–11(the second independent variable on these figures, oxygen index, willdiscussed later). These results are plotted for H2/airflames havingφ = 1 at NTP inFig. 8, considering he-lium, argon, nitrogen, and carbon dioxide as suppsion agents. In addition to the present measuremand predictions, the measurements of Kim et al.[19]for nitrogen and carbon dioxide as suppression agat these conditions are shown on the plot. Notably,agreement between present measurements andof Kim et al. [19] and the agreement between prespredictions and measurements are seen to be elent. These results indicate that all diluents causeunstretched laminar burning velocities to decreasthe concentrations of diluent are increased andthe suppression effectiveness of diluents (taken asreduction of the unstretched laminar burning velity for a particular diluent concentration) increasesthe order helium, argon, nitrogen, and carbon diide. Both these behaviors can be explained for arg

L. Qiao et al. / Combustion and Flame 143 (2005) 79–96 89

s ofre-

s ofro-.

ettifictra-m-ith

tiesalseere-eationsedo in-thered

onstricres-

s ofre-craft

he-

m-

e isin-

ion-and

Fig. 8. Measured and predicted unstretched (Ka= 0) lami-nar burning velocities as functions of the concentrationhelium, argon, nitrogen, and carbon dioxide diluents for pmixed stoichiometric hydrogen/air at NTP.

Fig. 9. Measured and predicted unstretched (Ka= 0) lami-nar burning velocities as functions of the concentrationnitrogen and carbon dioxide diluents for premixed hydgen/air flames at a fuel-equivalence ratio of 1.8 and NTP

nitrogen, and carbon dioxide according to Hugg[22,23] as a result of the increase of the specheat of the nonfuel gases per unit oxygen concention. This causes a corresponding reduction of teperatures within the reaction zone of the flames wthe associated reduction of laminar burning velocifollowing in accord with classical phenomenologictheories of premixed laminar flame propagation;Law [28]. Helium as a suppression agent is a pdictable exception to this behavior; its specific heffect is identical to that of argon as a suppressagent but this effect is counteracted by its increaheat and mass transfer rate capabilities that tend tcrease unstretched laminar burning velocities forhelium-containing flames to some extent, compa

Fig. 10. Measured and predicted unstretched (Ka= 0) lam-inar burning velocities as functions of the concentratiof nitrogen and carbon dioxide diluents for stoichiomepremixed hydrogen/air flames at room temperature and psures of 0.5 and 1.0 atm.

Fig. 11. Measured and predicted unstretched (Ka= 0) lam-inar burning velocities as functions of the concentrationhelium, argon, nitrogen, and carbon dioxide diluents for pmixed hydrogen flames at room temperature and spaceEVA-preparation conditions.

to argon-containing flames, based on classical pnomenological theories of premixed flames.

The third-body reaction H+ O2 + M = HO2 + Mis important as a chain-terminating reaction. It copetes with the branching reaction H+ O2 = OH + Oat temperatures less than∼900 K[63]. Therefore, forweakly propagating flames where the temperaturlow, this third-body reaction has a dominant effectthe H2–O2 chemistry. It is also found that the thirdbody reactions, especially H+ OH + M = H2O +M, are important for laminar flame speed propagatonly when the pressure is high[64]. Pressures considered here, however, are low, 0.5, 0.7, and 1 atm,

90 L. Qiao et al. / Combustion and Flame 143 (2005) 79–96

en-city

urn-n-

asami-

of

andre-eir

ive-nd

urn-nt

atmded ini-ltsrres

or

en-ityl tothede-tende re-sionm-tsishthis

iga-

urn-n-m-

o-as

s in-car-

een

nsen

1%ssedt-.g.,

at-fore,eingA-the

ilu-hus,ad-n-or-

ofa

hen-gen

10s,a-

-n

ex-

ex-ter-

er,y-

efi-ent

ine insureea-ard,

hert-

ane-ease

the flame, even with 40% CO2 as diluent, is still faraway from the flammability limit. Therefore, in thpresent investigation, the third-body reaction efficiecies are not as important as the impact of heat capaand transport properties of the diluents.

Measured and predicted unstretched laminar bing velocities are plotted as a function of diluent cocentration for H2/air flames havingφ = 1.8 at NTPin Fig. 9, considering nitrogen and carbon dioxidesuppression agents. In general, the unstretched lnar burning velocities atφ = 1.8 in Fig. 9 are largerthan those atφ = 1.0 in Fig. 8, which is well knownbehavior because the laminar burning velocitiesH2/air mixtures at NTP reach a maximum atφ = 1.8[32–34]. Other trends inFig. 9 are similar to thosein Fig. 8: the comparison between measurementspredictions is excellent, and diluents progressivelyduce unstretched laminar burning velocities as thconcentrations increase with suppression effectness additionally increasing in the order nitrogen acarbon dioxide.

Measured and predicted unstretched laminar bing velocities are plotted as a function of dilueconcentration for H2/air flames havingφ = 1.0 atroom temperature and pressures of 0.5 and 1.0in Fig. 10, considering nitrogen and carbon dioxias suppression agents. Similar results are founFigs. 8 and 9. Finally, the effect of pressure on lamnar burning velocities is relatively small for the resuillustrated inFig. 10, which is well-known behaviofor H2/air flames at room temperature and pressuin the range 0.35–1.0 atm; see Aung et al.[33]. EventhoughSL∞ is not strongly affected by pressure fthe range of conditions illustrated inFig. 10, how-ever, it should be recalled that rates of chemicalergy release for a particular laminar burning velocand reactant temperature are directly proportionathe concentration of the reactant mixture and thuspressure. As a result, fire severity decreases withcreasing pressure. In addition, reduced pressuresto increase suppression agent concentrations in thactant mixture for a given mass release of suppresagents, helping to promote extinction. Finally, cheical kinetic considerations near flammability limialso point toward improved capabilities to extingupremixed flames at reduced pressures, althoughproperty was not studied during the present investtion.

Measured and predicted unstretched laminar bing velocities are plotted as a function of diluent cocentration for premixed hydrogen flames at room teperature and EVA-preparation conditions inFig. 11.In this case, all four diluents—helium, argon, nitrgen, and carbon dioxide—have been comparedsuppression agents. The suppression effectivenescreases in the order helium, argon, nitrogen, and

bon dioxide for the reasons that have already bdiscussed in connection withFig. 8. The enrichedoxygen concentration of EVA-preparation conditiocompared to a conventional air environment (oxygconcentrations of 30% by volume compared to 2by volume) causes the unstretched and unsupprelaminar burning velocity in the EVA-preparation amosphere to be larger than that in air at NTP, e3350 mm/s compared to 2140 mm/s. On the otherhand, the reduced pressure of the EVA-preparationmosphere reduces the mass burning rate; therethe fire intensity is only roughly 10% larger in thEVA-preparation atmosphere than the mass burnrate in air at NTP. The reduced pressure of the EVpreparation environment also tends to increaseeffectiveness of particular mass discharges of dents compared to systems operated at NTP. Tpresent results do not clearly establish potentialvantages for either normal or EVA-preparation evironments with respect to fire suppression perfmance.

The oxygen index, which is the concentrationoxygen (in %) by volume in the nonfuel gases, issingle-valued function of diluent concentration for tconditions ofFigs. 8–11and is shown as an indepedent variable on these figures. For present hydroflames, oxygen indices reach values as small aswith no sign of approach to extinction conditionwhereas hydrocarbon flames typically reach flammbility limits for oxygen indices of 12–15. For example, Westbrook[62] has proposed an approximatioto find conditions where laminar premixed flamestinguish at a laminar burning velocity of 50 mm/s; incontrast, present flames at oxygen indices of 10hibit unstretched laminar burning velocities greathan 180 mm/s and are still well away from the extinction conditions suggested by the Westbrook[62]criterion. This behavior is not unexpected, howevdue to the well-known difficulties of suppressing hdrogen flames; see Lewis and von Elbe[36].

The use of EVA-preparation atmospheres dnitely increases oxygen indices at a particular diluconcentration, as illustrated inFig. 11, which is re-flected by the increased laminar burning velocitiesEVA-preparation atmospheres compared to thosair at NTP. On the other hand, the reduced presof the EVA-preparation atmosphere reduces the msure of the fire hazard, taken as the degree of hazby a factor of 1/P 1/2 according to Huggett[22]. Fur-ther study is required, however, to establish whetsuppression is more difficult in EVA-preparation amospheres at room temperatures due to the simultous increase of the oxygen concentration and decrof the pressure.

L. Qiao et al. / Combustion and Flame 143 (2005) 79–96 91

nt

dic-reof

bi-nts;et-

narcal

ednd

ent-m,seical

er-

les

thegthcor-

Fig. 12. Predicted structure of an unstretched (Ka= 0) pre-mixed stoichiometric hydrogen/air flame with no diluepresent at NTP.

4.5. Flame structure

As mentioned earlier, measurements and pretions of unstretched laminar burning velocities wein reasonably good agreement, including effectsvariations of fuel-equivalence ratio, pressure, ament oxygen concentration, and presence of diluetherefore, the predictions were exploited to gain a bter understanding of the effects of diluents on lamiburning velocities. The approach involved numerisimulations of plane (unstretched) H2/air flames inthe presence of various diluents.

Typical predicted structures of plane unstretchH2/air flames at a fuel-equivalence ratio of unity aNTP are illustrated inFigs. 12–16. Results inFig. 12provide the baseline flame structure when no diluis present.Figs. 13–16provide similar results for diluent concentrations of 40% (by volume) for heliuargon, nitrogen, and carbon dioxide, in turn. All theresults are based on the hydrogen/oxygen chemkinetics mechanism of Mueller et al.[56]. In eachfigure, the top graph provides profiles of the tempature and the stable species (H2, O2, H2O) concen-trations, whereas the bottom graph provides profiof radical species (H, OH, O, HO2, and H2O2) con-centrations, all as functions of distance throughflame. It should be noted that the origins of the lenscales in these figures are arbitrary and do not

Fig. 13. Predicted structure of an unstretched (Ka= 0) pre-mixed stoichiometric hydrogen/air flame with a 40% by vol-ume helium diluent at NTP.

Fig. 14. Predicted structure of an unstretched (Ka= 0) pre-mixed stoichiometric hydrogen/air flame with a 40% by vol-ume argon diluent at NTP.

92 L. Qiao et al. / Combustion and Flame 143 (2005) 79–96

ol-

heoor-

xi-singthat

lerO;ef-bleilu-a-

nsnte-nt,in--

zonees-m-nt

ol-

-s-s in

ofmeatic.l Hs inax-

ifi-xi-elyni-nts.n-nton-find-

thatH

am-tialc-

Fig. 15. Predicted structure of an unstretched (Ka= 0) pre-mixed stoichiometric hydrogen/air flame with a 40% by vume nitrogen diluent at NTP.

respond to the central ignition point. In addition, tscales of the concentrations of radical species cdinates become more expanded going fromFig. 12to Fig. 16so that the plots remain readable as mamum radical concentrations decrease with increasuppression agent effectiveness. The results showthe maximum concentrations of the radicals HO2 andH2O2 are roughly two orders of magnitude smalthan the concentrations of the radicals H, OH, andtherefore, the latter tend to dominate reactivefects in the present flames. Comparing the staspecies concentrations for a flame not having a dent present (Fig. 12) to those in flames havingdiluent present (Figs. 13–16), indicates expected reductions of the reactant concentrations (H2 and O2)and product concentrations (H2O) due to the dilutioncaused by the diluents having initial concentratioof 40% (by volume). Another effect that is evideis the preferential diffusion of the fast-diffusing ractant, H2, compared to the slow-diffusing reactaO2, for a plane flame; this can be seen from thecrease of the concentration of O2 near the cold boundary of the flame before the concentration of O2 de-creases once again upon approach to the reactionof the flame. The next major trend is the progrsive reduction of the final flame temperature fro2250 K (for no diluents), to 1750 K (for the diluents He and Ar), to 1600 K (for the diatomic dilue

Fig. 16. Predicted structure of an unstretched (Ka= 0) pre-mixed stoichiometric hydrogen/air flame with a 40% by vume carbon dioxide diluent at NTP.

N2), and finally to 1350 K (for the triatomic diluent CO2). This behavior is solely due to the progresive increase of the specific heat of these diluentthe order He and Ar (the same), N2, and CO2. Onthe other hand, the increased thermal diffusivityHe compared to Ar has no effect on the final flatemperature because these flames are all adiabFor the present stoichiometric flames, the radicagenerally has the largest maximum concentrationthe flames, with OH having somewhat smaller mimum concentrations, e.g., roughly 1/4–1/3 as largeas H, and with the other radicals all having signcantly smaller concentrations. In addition, the mamum concentration of H in the flames progressivdecreases in the order no diluent, helium, argon,trogen, and carbon dioxide as suppression ageSimilarly, but not shown here, the maximum cocentration of H in the flame for a particular reactamixture progressively decreases with increasing ccentrations of suppression agents. Based on theings of Kwon and Faeth[34], for flames having hy-drogen and oxygen as reactants, it is expectedthis reduction of the maximum concentration ofshould cause a corresponding reduction of the linar burning velocity of these flames. The potenfor this behavior will be considered in the next setion.

L. Qiao et al. / Combustion and Flame 143 (2005) 79–96 93

-alsdi-esteH

fi-

ned by

d intthesid-

la-calndumllyac-on

ltsdnd-

ntald ing-

eof

e

the

ax-y-s atl

dbyres-

es--he.7–

tch;ofree

edt al.pro-

n

4.6. Radical behavior

The flame structure results ofFigs. 12–16, andsimilar results presented earlier by Kim et al.[19]and Kwon and Faeth[34], indicate that the H radical has the highest concentrations of all the radicin premixed hydrogen and oxygen flames. In adtion, OH radical concentrations are the next largconcentration after H forφ � 1, ranging up to samorder of magnitude of maximum concentration ofand OH forφ as small as 0.6. This behavior is signicant due to the strong correlation between theSL andthe maximum concentration of H in the reaction zoof premixed hydrogen and oxygen flames observePadley and Sugden[35] and Butler and Hayhurst[37].Based on the flame structure results just discusseconnection withFigs. 12–16, it appeared likely thaa similar correlation would also be observed forpresent suppressed flames; this possibility is conered next.

Similar to past work, the most robust corretion between laminar burning velocities and radiconcentrations for laminar premixed hydrogen aoxygen flames was obtained by using the maximH + OH mole fraction in the flames, which generawas obtained at the condition where the mole frtion of H was a maximum. The resulting correlatiis illustrated inFig. 17, where the value ofSL∞ isplotted as a function of the maximum H+ OH molefraction, computed as just described. All the resuillustrated inFig. 17are for premixed hydrogen anoxygen flames at room temperature and include fiings from Kwon and Faeth[34], Kim et al. [19], andthe present investigation. The ranges of experimeconditions for these investigations are summarizeTable 4. The original correlation of Padley and Suden[35] along these lines is not included inFig. 17because their plotted results were limited toSL asa function of the maximum H mole fraction in thflames. Another difficulty about the measurementsPadley and Sugden[35] is that the extent of flamstretch is unknown for these results.

Clearly, there is a rough correlation betweenSL∞ and the maximum H+ OH mole fraction in

Fig. 17. Laminar burning velocities as functions of the mimum H + OH mole fraction in the reaction zone for hdrogen flames having various concentrations of diluentroom temperature; seeTable 4for the range of experimentaconditions.

the flames for the results illustrated inFig. 17. No-tably, SL∞ and radical mole fractions were variein a number of ways for these results: dilutionvarious concentrations of chemically passive suppsion agents (He, Ar, N2, and CO2), dilution by var-ious concentrations of a chemically active supprsion agent (CF3Br), variation of fuel-equivalence ratios, variation of the concentration of oxygen in tnonfuel gases, modest variations of pressure (01.0 atm), and variation of the degree of flame streseeTable 4for a complete summary of the rangesthe experimental flame conditions. Finally, the degof scatter of the correlation ofSL∞ as a function ofthe maximum H+ OH mole fraction inFig. 17 isparticularly small for the recent studies of unstretchsuppressed laminar premixed flames due to Kim e[19] and the present investigation. These resultsvide a best-fit correlation between theSL∞ and themaximum H+ OH mole fraction, which is shown othe plot, as

(4)SL∞ (mm/s) = 260+ 36,600 (XH + XOH)max,

t.

Table 4Test conditions for hydrogen premixed flame studiesa

Source Diluentsb φ O2/(O2 + N2)(% vol.)

P (atm) Kac (–) XDd (% vol.)

Kwon and Faeth[34] – 0.6–4.5 21–36 1.0 0.0–0.50 0Kim et al. [19] N2, CO2, CF3Br 0.6–1.8 21 1.0 0.0 0–2Present investigation He, Ar, N2, O2 1.0 and 1.8 21 and 30 0.7–1.0 0.0 0–40

a Initial mixture temperature 298± 5 K.b He, Ar, N2, and CO2 are chemically passive suppression agents, whereas CF3Br is a chemically active suppression agenc Ka= 0 denotes unstretched flames.d XD = 0 denotes unsuppressed flames.

94 L. Qiao et al. / Combustion and Flame 143 (2005) 79–96

hed

tra-ingud-heat-

adyrk

xedndei-con-);of

sup-on,en-ntdi-blem-gen

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m-ere

seivel ofn-tionsdH

s of-

sion

forallyrk-theve-thattche-fectsa-us,lam-themere-

sta-ted

nts3-ashiorsn-

y,

m

where this expression considers only the unstretc(plane) laminar premixed flames from Kim et al.[19]and the present investigation.

5. Conclusions

The effects of flame stretch and the concentions and types of diluents on the laminar burnvelocities of hydrogen premixed flames were stied both experimentally and computationally. Texperiments involved unsteady outwardly propaging laminar premixed spherical flames and steplane laminar premixed flames, similar to past woin this laboratory, e.g., Tseng et al.[38], Aung etal. [32,33], and Kwon and Faeth[34]. Experimentaland computational conditions considered premihydrogen/air/diluent and hydrogen/30% oxygen a70% nitrogen (by volume)/diluent flames, with thlatter condition of interest for EVA-preparation activties onboard manned spacecraft. The other flameditions were as follows: room temperature (298 Kfuel-equivalence ratios of 1.0 and 1.8; pressures0.5, 0.7, and 1.0 atm; chemically passive gaseouspression agents (diluents) including helium, argnitrogen, and carbon dioxide; and diluent conctrations of 0–40% (by volume), which is equivaleto oxygen indices of 30–10 for present flame contions. Predicted flame behavior considered variatransport and thermodynamics properties, multicoponent transport, and the detailed hydrogen/oxychemical kinetic mechanism of Mueller et al.[56].The major conclusions of the study are as flows.

(1) Effects of flame/stretch interactions for bomeasurements and predictions of suppressed flacould be correlated based on the local-conditionspothesis according toSL∞/SL = 1 + MaKa to ob-tain a linear relationship betweenSL∞/SL and theKarlovitz number. This behavior implies a constaMarkstein number for given reactant conditions, siilar to earlier findings for unsuppressed flames.

(2) Effects of flame stretch on laminar burning vlocities were substantial, yielding values ofSL∞/SLin the range 0.60–1.25, for Ka< 0.5, which doesnot involve a close approach to quenching conditiwhere Karlovitz numbers typically have values onorder of unity[28]; corresponding Markstein numbewere in the range−1.0 to 3.5.

(3) Measured and predicted unstretched lamburning velocities and Markstein numbers werereasonably good agreement using the hydrogen/gen chemical kinetic mechanism of Mueller et al.[56].

(4) The chemically passive suppression agendiluents increase in effectiveness (based on redtion of the unstretched laminar burning velocity f

a given concentration of diluent (in % by volume))the order helium, argon, nitrogen, and carbon dioxwhich mainly reflects their progressively increasispecific heats and progressively decreasing massthermal transport properties.

(5) Predictions showed that the unstretched lainar burning velocities of the present flames wstrongly correlated with the maximum H+ OH molefraction in the reaction zone for variations of theconcentrations due to effects of chemically passsuppression agents, similar to an early proposaPadley and Sugden[35] for unsuppressed and ustretched hydrogen/air flames, the recent observaof Kwon and Faeth[34] for a variety of unsuppresseand stretched and unstretched flames involving2and O2 as reactants, and the recent observationKim et al. [19] for a variety of stretched and unstretched flames involving H2 and O2 as reactantsthat were subjected to a chemically active suppresagents (Halon 1301).

(6) Finally, there is a consistent tendencythe addition of suppression agents, either chemicactive or chemically passive, to reduce the Mastein number for a given reactant mixture atsame time that the unstretched laminar burninglocity is reduced, causing unsuppressed flamesare stable to effects of preferential-diffusion/streinteractions (positive Markstein numbers) to bcome suppressed flames that are unstable to efof preferential-diffusion/stretch interactions (negtive Markstein numbers) in some instances. Ththe tendency of suppression agents to reduceinar burning velocities (and thus act to reduceseverity of unwanted fires) is counteracted to soextent by the tendency of suppression agents toduce Markstein numbers (and promote flame inbilities that tend to increase the severity of unwanfires).

Acknowledgments

This research was sponsored by NASA GraNCC3-661, NAG3-1878, NAG3-2040, and NAG2404 under the technical management of F. Takahof the NASA Glenn Research Center. The auththank Dr. Elaine Oran at NRL for her help and ecouragement in the revising process of this paper.

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