nitrogen flames

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

Suppression effects of diluents on laminar premixed hydrogen/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 numbers, were determined experimentally and computationally for outwardly propagating spherical laminar premixed flames. The mixtures studied were premixed hydrogen/air/diluent and hydrogen/30% oxygen and 70% nitrogen (by volume)/diluent flames, with the latter condition of interest for pre-external vehicular activity preparation activities on board manned spacecraft. Other flame conditions were room temperature (300 K), fuel-equivalence ratios of 1.0 and 1.8, pressures of 0.5, 0.7, and 1.0 atm, diluents including helium, argon, nitrogen, and carbon dioxide as suppression agents, and diluent concentrations of 0–40% (by volume), which implies oxygen indices of 30–10 for present conditions. Predicted flame behavior was obtained from one-dimensional, spherically symmetric, steady, and time-dependent numerical simulations with variable-property and multicomponent transport and with detailed hydrogen/oxygen chemical kinetics. Flames studied were sensitive to stretch, yielding unstretched/stretched laminar burning velocity ratios of 0.6–1.25 for conditions well away from quenching conditions (e.g., Karlovitz numbers; Ka
0.5). Diluents became more effective (provided greater reductions of the laminar burning velocity for a given diluent concentration) in the order helium, argon, nitrogen, and carbon dioxide, which reflects their increased capabilities either to quench the reaction zone by increased specific heats or to reduce flame velocities by reduced transport rates. The addition of diluents generally decreased Markstein numbers, which made the flames more susceptible to preferential-diffusion instability. This effect increases flame speeds and tends to counteract the 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

1. Introduction Halons have been very successful as chemically active flame suppression agents in applications where effective and clean control of unwanted fires is needed; see Drysdale [1] and Tuhtar [2]. Unfor* Corresponding author. Fax: +1 734 763 0578.

E-mail address: [email protected] (L. Qiao).

tunately, halons also contribute to the depletion of stratospheric ozone that protects the Earth’s surface from harmful ultraviolet solar radiation. Due to this undesirable environmental effect, halon manufacture was stopped in 1994, except for limited production in some developing countries, under the terms of the Montreal Protocol [3]. Subsequently, many experimental and computational studies have been undertaken to gain a better understanding of the mechanism

0010-2180/$ – see front matter  2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.combustflame.2005.05.004

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Nomenclature D K Ka L Ma P rf SL  SL∞

t T

Mass diffusivity Flame stretch, Eq. (1) Karlovitz number, KDu /SL2 Markstein length Markstein number, L/δD Pressure Flame radius Laminar burning velocity based on unburned gas properties Value of SL at the largest radius observed Time Temperature

of flame suppression of the chemically active halons and their potential replacements; see Safieh et al. [4], Sheinsohn et al. [5], Walravens et al. [6], Miziolek and Tsang [7], McIlroy and Johnson [8], Linteris and Truett [9], Noto et al. [10,11], Casias and McKinnon [12,13], Linteris et al. [14], Takahashi et al. [15], Saso et al. [16–18], Kim et al. [19], Faeth et al. [20], and references cited therein. These studies have shown that chemically active suppression agents are unusually effective because they interrupt the chemical pathway of fuel oxidation. Unfortunately, chemically active agents often generate substances in flame environments that prevent their use in confined spaces that contain living organisms. Motivated by this observation, the objective of the present investigation was to study the properties of typical chemically passive suppression agents—diluents that avoid the limitations of the Montreal Protocol [3] and problems of the generation of substances in flames that are harmful to living organisms in confined spaces. Effects of diluents on laminar premixed flames were studied both experimentally and computationally. An important issue concerning the present study is the justification for considering only the suppression properties of laminar premixed flames. Although most practical flames are turbulent, turbulent flames are difficult to study because experimental conditions are substantially complicated by the need to describe and treat a variety of turbulence properties. Another advantage of laminar flames is that they generally are tractable for detailed numerical simulations, unlike turbulent flames, enhancing capabilities to use computations to supplement information about suppression behavior from directly measured properties. In addition, laminar flames are also relevant to turbulent flames based on widely accepted laminar flamelet concepts of turbulent flames. Finally, due to the complexities of turbulent flames, it seems unlikely that un-

Xi

Mole fraction of species i

Greek symbols δD ρ ϕ

Characteristic flame thickness, Du /SL Density Fuel-equivalence ratio

Subscripts b max u ∞

Burned gas Maximum observed value Unburned gas Unstretched flame condition

derstanding of the suppression of turbulent premixed flames will precede understanding of the suppression of laminar premixed flames. Another issue concerning the present study involves limiting considerations to premixed flames, even though both premixed and nonpremixed (diffusion) flames are important in practice. This was done because premixed flames are most relevant to processes of flame suppression (e.g., even points of flame attachment in diffusion flames are largely controlled by premixed flame phenomena). In addition, premixed flames lend themselves to well-defined experimental and computational conditions that simplify the interpretation of both experimental and computational results. Other limitations used to control the scope of the present study involved considering combustion processes involving only hydrogen/oxygen chemical kinetics for outwardly propagating spherical premixed flames. Limiting combustion to hydrogen/ oxygen chemical kinetics is reasonable because these kinetics are fundamentally important for all combustion processes of hydrocarbons in air, are well known, and are sufficiently simple so that numerical simulations involving these reactants are computationally tractable. Furthermore, these reactants provide conservative suppression properties because they generally are the hardest to extinguish among combustibles of practical interest; see Wieland [21]. In addition, outwardly propagating spherical premixed flames are also attractive because they do not involve complex quenching processes near surfaces. Finally, subsequent discussion will also show that outwardly propagating spherical flames are particularly convenient for directly measuring and computing the fundamental properties of various suppression agents. To fix ideas, two general reactant systems were considered during the present study: (1) hydrogen and

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air at pressures of 0.5 and 1.0 atm, to represent a conservative combustible mixture for suppression at altitudes typical of cities on Earth, and (2) hydrogen in an atmosphere consisting of 30% oxygen and 70% nitrogen (by volume) at a pressure of 0.7 atm, which is the atmosphere used during external vehicular activity (EVA) preparation of astronauts on board spacecraft; see Wieland [21]. Finally, only simple diluents were considered (e.g., helium, argon, nitrogen and carbon dioxide) as suppression agents to emphasize effects of specific heats and transport properties known to influence diluent performance (see Huggett [22,23]) without having to deal with the complexities of multicomponent flame suppression processes. A difficulty that is encountered when laminar premixed flames are used to study the suppression properties of diluents is that flame/stretch interactions significantly affect the laminar burning velocities and structure of laminar premixed flames [24–29] and the rate of propagation of turbulent premixed flames typical of practical applications [30,31]. To deal with this problem, diluent performance during the present investigation was determined by using unstretched laminar burning velocities to characterize the intensity of combustion of the flames and by using Markstein numbers (Ma) to characterize the sensitivity of the flames to effects of stretch. Fortunately, outwardly propagating spherical laminar premixed flames provide a straightforward determination of unstretched laminar burning velocities and Markstein numbers, as demonstrated by Aung et al. [32,33] and Kwon and Faeth [34] for hydrogen laminar premixed flames. Another finding of these studies is that their flame structure predictions suggested that mainly H and to a lesser degree OH radical production and transport are important aspects of preferential-diffusion/stretch interactions. This is not surprising, however, due to the well known proportionality between laminar burning velocities and H radical concentrations of hydrogen laminar premixed flames, first pointed out by Padley and Sugden [35] based on the laminar burning velocity measurements of Jahn that are cited in Lewis and von Elbe [36] and subsequently noted by others for hydrogen premixed flames for various conditions, e.g., Kim et al. [19], Butler and Hayhurst [37], and references cited therein. Another aspect of the findings of Kim et al. [19] is that changes of flame conditions that tended to reduce the concentrations of H and OH radical concentrations in the reaction zone also tended to make these flames more susceptible to preferential-diffusion instability as measured by reduced values of the Markstein number. This behavior has the potential to increase flame speeds (the absolute flame velocity in laboratory coordinates) due to the creation of flame surface area by the distortion or wrinkling of the flame surface as a result of

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the action of preferential-diffusion instability. Thus, the application of flame suppression agents to premixed flames involves two counteracting effects. Suppression agents have the capability to reduce H and OH radical concentrations in the reaction zone, which leads to corresponding reductions of laminar burning velocities which tend to reduce flame intensities and thus suppress the flame. Furthermore, the same capability of suppression agents to reduce H and OH radical concentrations in the reaction zone also enhances the potential for the development of preferential-diffusion-induced flame surface instabilities that increase flame speeds, which tends to increase flame intensities and thus reduces effects of flame suppression.

2. Experimental methods 2.1. Apparatus Experimental methods were similar to past work and will be described very briefly; see Aung et al. [32, 33] and Kwon and Faeth [34] for more details. The experiments were conducted in a spherical windowed chamber having an inside diameter of 360 mm and an internal volume of 0.024 m3 . Optical access was provided by two 100-mm-diameter quartz windows mounted opposite one another along a horizontal line passing through the center of the chamber. The chamber was capable of operation over a pressure range extending from complete vacuum up to a maximum of 34 atm. The reactant mixture was prepared within the chamber by adding gases at appropriate partial pressures to reach the total initial pressure of the reactant mixture for a test (0.5, 0.7, and 1.0 atm for the present test range). The reactant gases were mixed using a small metal fan located inside the chamber with the fan-induced motion allowed to decay before ignition (5–10 min for mixing and at least 30 min for decay); given these conditions, motion picture shadowgraphs did not indicate any distortion of the flame surface or convection of the flame kernel from its position centered on the spark kernel. After combustion was complete, the chamber was vented to the laboratory exhaust system and then purged with dry air to remove condensed water vapor prior to refilling for the next test. The combustible mixture was spark-ignited at the center of the chamber using electrodes extending from the top and bottom of the chamber. One electrode was fixed, whereas the other electrode could be moved with a micrometer having a positioning accuracy of 10 µm. The tips of the electrodes were fine tungsten wires having diameters of 250 µm and free

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lengths of 40 mm. The spark gap was varied in the range 0.5–2.0 mm, with the larger gaps used to ignite flames having relatively small laminar burning velocities that required relatively large ignition energies. The spark energy was supplied by a high-voltage capacitor discharge circuit having a variable capacitance (100–7000 pF) and voltage (0–10 kV) and a discharge time of roughly 5 µs. Spark gaps and spark energies were adjusted by trial so that they were close to minimum ignition energies (5–20 mJ, with the larger values used for flames having relatively small laminar burning velocities) to minimize effects of initial flame acceleration due to excessive spark energies. 2.2. Instrumentation The flames were observed using high-speed shadowgraph motion picture photography. The shadowgraph system consisted of a 100-W mercury short-arc lamp (ARC; HSA-150 HP) with the light collimated by a pair of f6 parabolic reflectors having 1220-mm focal lengths. The flame images were recorded using a 16-mm motion picture camera (Hycam; Model K20 34E) operating at speeds of 4000–8000 pictures per second. Kodak Hawkeye surveillance film on a daylight loading spool (SP-430), with perforations on two sides, was used for the photographs. The framing rate of the camera was sensed electronically so that ignition occurred only when the proper framing rate was reached. The framing rate and the ignition pulse were recorded using a digital oscilloscope (LeCroy 9400A) so that the film records could be synchronized. 2.3. Data reduction Present measurements were limited to flames having diameters larger than 10 mm, to avoid ignition disturbances, and smaller than 60 mm, to limit the volume of burned gas to less than 0.5% of the total chamber volume so that the chamber pressure remained constant within 0.7% throughout the observed period of flame propagation. Laser velocimeter measurements for this test arrangement (but with a slightly smaller chamber), due to Kwon et al. [31], indicated that velocities within the unburned gas varied as expected for outwardly propagating unconfined spherical flames for the range of flame sizes considered during the present investigation. Similar to past measurements of laminar premixed flame properties [32–34], determinations of flame properties were limited to conditions where δD /rf < 2% so that effects of flame curvature and transient effects associated with the thickness of the flame were negligible, as discussed by Tseng et al. [38]. Next, laminar burning velocities were generally greater than

150 mm/s, so that the intrusion of effects of buoyancy due to Earth’s gravity was negligible as shown by Ronney and Wachman [39]; this behavior was confirmed using the present flame photographs which indicated negligible effects of flame distortion, or motion of the origin of the flame, due to buoyancy in the period when present observations were made. Furthermore, effects of radiative heat losses were small (less than 1% of the rate of thermal energy release of reaction within the test flames) based on earlier estimates of these losses for hydrogen flames at similar conditions due to Aung et al. [33] that were carried out as discussed by Siegel and Howell [40]. This assessment also agrees with an earlier evaluation of effects of radiation for hydrogen flames at similar conditions due to Dixon-Lewis [41]. Finally, effects of the spark energy were small compared to the energy release due to combustion in the region where the flames were observed (the energy release due to combustion was generally greater than 20 times the spark energy for flames having diameters larger than 10 mm for present test conditions). Under these assumptions, Strehlow and Savage [26] showed that the laminar burning velocity and flame stretch are given by the following quasi-steady expressions: SL = (ρb /ρu ) drf /dt,

K = (2/rf ) drf /dt.

(1)

The density ratio appearing in Eq. (1) was found from McBride et al. [42], assuming adiabatic constant-pressure combustion with chemical equilibrium in the combustion product gases and the same concentrations of elements in the unburned and burned gases. This is only a convention that follows past practice [32–34], however, because it ignores preferentialdiffusion effects that modify local element mass fractions and energy transport and cause ρb /ρu to differ from plane adiabatic flame conditions. This convention is convenient, however, because a single density ratio relates all flame speeds at a given reactant mixture condition. In addition, this convention retrieves the correct flame displacement velocity, drf /dt, for given unburned mixture conditions and degree of flame stretch. Finally, based on past numerical simulations of stretched hydrogen premixed flames, the present assumptions used to find ρb /ρu are quite reasonable for the conditions considered during the present study. In particular, values of ρb /ρu for stretched hydrogen flames agree within 10% with those for unstretched (plane) flames when δD /rf < 2%; see Kwon [43]. Final results were obtained by averaging the measurements of four to six tests at each condition. Experimental uncertainties were estimated as described by Tseng et al. [38] and references cited therein. The resulting experimental uncertainties (95% confidence) are as follows: SL less than 9%, Ka less than 21%,

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

SL∞ less than 10%, and | Ma | less than 25% for | Ma | > 1 and less than 25%/| Ma | for | Ma | < 1. 2.4. Data correlation The measurements were analyzed to find laminar flame properties (Markstein numbers to characterize the sensitivity of the flame to effects of stretch and unstretched laminar burning velocities to characterize the intensity of combustion and the capability of a particular diluent to reduce flame intensity). As mentioned earlier, present considerations were limited to thin flames (δD /rf < 2%) for conditions where effects of ignition disturbances, flame radiation, and variations of element concentrations on ρb /ρu were small. Then, a convenient relationship between the laminar burning velocity and the flame stretch can be obtained by combining an early proposal of Markstein [25] and the local conditions hypothesis of Kwon et al. [31] to yield the expression SL∞ /SL = 1 + Ma Ka,

(2)

where values of SL and the Karlovitz number, Ka (the dimensionless flame stretch = KδD /SL ), were found from Eq. (1) as already discussed. For these definitions, δD is based on the stretched laminar burning velocity and the mass diffusivity of the fuel in the unburned (and in the present case, unsuppressed) gas, as conventions. The decision to use the Du for unsuppressed flames as the Du for the suppressed flames also was made to provide an absolute evaluation of SL∞ so that the effectiveness of various diluents could be directly compared. The small stretch limit is also of interest to connect present results at finite levels of stretch to the conditions of classical asymptotic theories of laminar premixed flame propagation at negligibly small levels of stretch, as follows (see Aung et al. [33]): SL /SL∞ = 1 − Ma∞ Ka∞ ,

| Ka∞ |  1.

(3)

Several other proposals to represent effects of flame stretch on laminar burning velocities have been made; see Taylor [44], Dowdy et al. [45], Brown et al. [46], Karpov et al. [47], and Bradley et al. [48]. The approach used in Eq. (2), however, is particularly convenient because the Markstein number has proven to be relatively constant for particular reactant mixture conditions over wide ranges of the Karlovitz number based on both measurements and detailed numerical simulations of laminar premixed flames [30– 34]. Thus, SL∞ and Ma provide convenient and concise measurements of laminar premixed flame burning rates and response to stretch, as discussed by Aung et al. [33]. See Aung et al. [33] for a discussion of other advantages of the present characterization of premixed-flame/stretch interactions.

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2.5. Test conditions Experimental conditions are summarized in Tables 1 and 2. Experimental conditions for H2 /air/diluent flames, summarized in Table 1, seek to be representative of human habitation conditions on Earth at various altitudes, as follows: reactant mixtures at room temperature (298 ± 0.5 K), fuel-equivalence ratios of 1.0 and 1.8, pressures of 0.5 and 1.0 atm, and diluent concentrations of 0–40% (by volume) with helium, argon, nitrogen, and carbon dioxide as suppression agents. The fuel-equivalence ratios of 1.0 and 1.8 were chosen due to their relevance to flame suppression with stoichiometric conditions being repTable 1 H2 /air/diluents laminar premixed flame test conditionsa Diluents

XD

ρu /ρb

SL∞ (mm/s)

Kamax

Ma

0.07 0.06 0.06 0.06 0.09 0.05 0.08 0.14 0.22 0.09 0.13 0.11 0.19 0.09 0.14 0.18 0.40

0.2 1.5 1.3 1.3 0.9 1.1 0.5 −0.2 −0.3 0.0 −0.3 −0.7 −1.0 0.0 −0.5 −0.7 −0.6

p = 1.0 atm, φ = 1.8, Du = 72.9 mm2 /s – 0.0 6.30 2900 0.1 5.89 2340 N2 0.2 5.44 1830 N2 N2 0.3 4.71 1400 0.4 4.48 830 N2 CO2 0.1 5.48 2040 0.2 4.78 1330 CO2 CO2 0.3 4.18 760 0.4 3.65 240 CO2

0.04 0.03 0.04 0.05 0.08 0.05 0.05 0.09 0.12

3.5 3.2 3.1 2.8 2.6 2.7 2.1 1.4 1.2

p = 0.5 atm, φ = 1.0, Du = 145.8 mm2 /s – 0.0 6.89 2020 0.1 6.54 1600 N2 N2 0.2 6.14 1320 0.3 5.67 920 N2 N2 0.4 5.13 640 0.1 6.20 1320 CO2 CO2 0.2 5.60 950 CO2 0.3 4.98 480 0.4 4.36 270 CO2

0.09 0.11 0.13 0.22 0.36 0.13 0.26 0.29 0.49

1.7 1.0 0.7 −0.2 −0.4 0.5 −0.2 −0.6 −0.6

p = 1.0 atm, φ = 1.0, Du = 72.9 mm2 /s – He He He He Ar Ar Ar Ar N2 N2 N2 N2 CO2 CO2 CO2 CO2

0.0 0.1 0.2 0.3 0.4 0.1 0.2 0.3 0.4 0.1 0.2 0.3 0.4 0.1 0.2 0.3 0.4

6.89 6.63 6.50 6.19 5.79 6.63 6.50 6.19 5.79 6.56 6.15 5.67 5.13 6.23 5.61 4.98 4.36

2140 1960 1730 1430 1170 1770 1290 1100 760 1650 1170 860 480 1330 770 400 180

a Initial mixture temperature of 298 ± 0.5 K.

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Table 2 H2 /30% O2 and 70% N2 /diluents laminar premixed flame test conditionsa Diluents

XD

ρu /ρb

SL∞ (mm/s)

p = 0.7, φ = 1.0, Du = 104.1 mm2 /s – 0.0 7.45 3350 He 0.2 7.21 2700 He 0.4 6.67 1880 Ar 0.2 7.21 2320 Ar 0.4 6.67 1380 0.2 6.92 2100 N2 N2 0.4 5.97 1150 0.2 6.44 1570 CO2 CO2 0.4 5.06 480

Kamax

Ma

0.04 0.04 0.04 0.06 0.09 0.08 0.10 0.07 0.21

2.9 2.4 2.1 2.2 0.6 0.6 −0.3 0.7 −0.3

a Initial mixture temperature of 298 ± 0.5 K.

resentative of flame attachment conditions in nonpremixed flames, whereas φ = 1.8 represents conditions where the unstretched laminar burning velocities of hydrogen/air mixtures reach a maximum [32–34] and conditions that are most difficult to extinguish for premixed flames of these reactants. The values of ρu /ρb in Table 1 were found from McBride et al. [42], as discussed earlier. These measurements involved unstretched laminar burning velocities of 180– 2900 mm/s, Karlovitz numbers of 0–0.5, and Markstein numbers of −1.0–3.5. Experimental conditions for H2 /30% O2 and 70% N2 (by volume)/diluent flames, summarized in Table 2, involve EVA-preparation conditions for spacecraft, as follows: reactant mixtures at room temperature (298 ± 0.5 K) and a pressure of 0.7 atm, a fuel-equivalence ratio of unity, and diluent concentrations of 0–40% (by volume) with helium, argon, nitrogen, and carbon dioxide as suppression agents. These measurements involved unstretched laminar burning velocities of 480–3350 mm/s, Karlovitz numbers of 0–0.21, and Markstein numbers of −0.3–2.9.

3. Computational methods 3.1. Numerical simulations Computational methods for the present flames were similar to those of Aung et al. [32,33] and Kwon and Faeth [34]. The outwardly propagating spherical laminar premixed flames were simulated using the unsteady one-dimensional laminar flame computer code, RUN-1DL, developed by Rogg [49]. This algorithm allows for mixture-averaged multicomponent diffusion, thermal diffusion, variable thermochemical properties, and variable transport properties. The CHEMKIN package [50–53] was used as a preprocessor to find the thermochemical and transport properties for RUN-1DL. Transport properties were

found from the transport property database of Kee et al. [51]. Thermochemical properties were found from the thermodynamic data base of Kee et al. [50], except for HO2 , where the recommendations of Kim et al. [54] were used. Before computing flame properties, all transport and thermodynamic properties were checked against original sources. Similar to the measurements, effects of radiation were small due to the relatively large flame velocities of hydrogen flames for present conditions and were ignored. Flame propagation was allowed to proceed sufficiently far so that effects of initial conditions were small, similar to the measurements. Other limitations used to control experimental uncertainties, e.g., δD /rf < 2%, etc., were also applied to the predictions. The computational grid in space and time was varied to ensure numerical accuracy within 1%, estimated by Richardson extrapolation of SL . Finally, the numerical simulations were analyzed similar to the measurements, taking the flame position to be the point where gas temperatures were the average of the temperatures of the hot and cold boundaries. Due to the present stringent flame thickness limitations, however, the present results were not affected significantly by the criterion used to define the flame position. Separate numerical simulations were carried out for unstretched (plane) flames using the steady onedimensional laminar premixed flame code, PREMIX, due to Kee et al. [53]. Other properties of these calculations and the limits of numerical accuracy were similar to those using the RUN-1DL algorithm. This code was mainly used to predict the structure of unstretched flames. 3.2. Chemical kinetic mechanism Aung et al. [32,33], Kwon and Faeth [34], and Kim et al. [19] carried out extensive evaluations of available detailed hydrogen/oxygen chemical kinetic mechanisms proposed by Kim et al. [54], Yetter et al. [55], Mueller et al. [56], Marinov et al. [57], Wang and Rogg [58], and Frenklach et al. [59,60] based on their measurements of the properties of hydrogen outwardly propagating laminar premixed flames. The chemical kinetic mechanism of Mueller et al. [56] was found to provide the best comparison between measurements and predictions for hydrogen flames involving nitrogen, argon, and helium as suppression agents, fuel-equivalence ratios of 0.6–4.5, pressures of 0.3–3.0 atm, and volumetric oxygen concentrations in the nonfuel gases of 0.21–0.36. These conditions were generally similar to present flame conditions; therefore, the numerical simulations of flames reported here were limited to the Mueller et al. [56] hydrogen/oxygen chemical kinetic mechanism. Similar to the earlier evaluation of Mueller et al. [56] chemi-

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cal kinetic mechanism by Kwon and Faeth [34], this mechanism was simplified because C/H/O and N/O chemistry were not important for present conditions and could be deleted from the mechanism. The final reduced chemical kinetic mechanism involved 11 chemical species and 19 reversible reactions. This reaction mechanism does not provide information about the elementary reactions involving helium and their reaction coefficients; therefore, the same reactions and their reaction coefficients as those used for argonsuppressed hydrogen flames were chosen for the simulations of the helium-suppressed flames due to the thermodynamic similarities of argon and helium.

4. Results and discussion 4.1. Flame stability and evolution Three kinds of flame surface instabilities were observed during present experiments: preferentialdiffusion instability (observed only when Ma < 0), hydrodynamic instability (observed for all values of Ma), and buoyant instabilities (observed only when laminar flame speeds or corresponding laminar burning velocities were small). Shadowgraph photographs of flame surfaces after distortion by these instabilities for outwardly propagating spherical flames appear in Kim et al. [19] and Kwon et al. [31]. The presence of preferential-diffusion instability could be identified by irregular (chaotic) distortions of the flame surface relatively early in the flame propagation process and as noted earlier only when Ma < 0. Fortunately, flame surfaces remained smooth at small flame radii even for conditions that involved preferential-diffusion instability so that laminar burning velocities could be measured for a time even at these conditions. Hydrodynamic instability could be identified by the development of a somewhat regular cellular disturbance pattern on the flame surface, very similar to the observations of Groff [61]; fortunately, these instabilities were observed only for flame diameters larger than 60 mm so that they did not affect the present measurements limited to flame diameters smaller than 50 mm. Finally, buoyant instabilities were observed when laminar burning velocities were small and were readily detected by distortion of the flame surface from a spherical shape when vertical planes of the flame were observed (which was the case for the present experimental arrangement). In addition, the flame boundary was also deflected upward from the location of the spark kernel when effects of buoyant instability were important. As noted earlier, however, effects of buoyancy were small as long as laminar burning velocities were larger than 150 mm/s, which involved velocities well below the present test range.

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Finally, no measurements reported here were made at conditions where any of these instabilities were observed. 4.2. Burning velocity/stretch interactions Measurements at finite flame radii involve finite values of flame stretch through Eq. (1); therefore, the laminar burning velocity at the largest flame radius observed still differs from the fundamental unstretched laminar burning velocity of a plane flame, SL∞ . Thus, values of SL∞ were found from Eq. (2)  /S , where S  by plotting SL∞ L L∞ is the value of the laminar burning velocity at the largest flame radius observed, as a function of Ka, similar to past work [32–34]. As will be seen subsequently, this yielded linear plots so that extrapolation to Ka = 0 yielded  /S SL∞ L∞ and thus SL∞ as summarized in Tables 1 and 2. Given SL∞ , plots of SL∞ /SL as a function of Ka could be constructed for various reactant mixtures and pressures, as prescribed by Eq. (2). Examples of plots of this type for hydrogen flames at various conditions, based on both measurements and predictions, are illustrated in Figs. 1–4; results at other test conditions were qualitatively similar to the results illustrated in Figs. 1–4. Finally, it is evident that the variations of SL∞ /SL as a function of Ka are all linear for the results illustrated in Figs. 1–4. This implies that Ma is a constant that can be determined from the constant slopes of the plots of SL∞ /SL as a function of Ka for each flame condition that was studied. These values of the Markstein number are also summarized

Fig. 1. Measured and predicted laminar burning velocities as functions of Karlovitz number and the concentration of nitrogen diluent for premixed stoichiometric hydrogen/air flames at NTP.

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Fig. 2. Measured and predicted laminar burning velocities as functions of Karlovitz number and the concentration of carbon dioxide diluent for premixed stoichiometric hydrogen/air flames at NTP.

Fig. 4. Measured and predicted laminar burning velocities as functions of Karlovitz number and the concentration of carbon dioxide diluent for premixed stoichiometric hydrogen flames at room temperature and spacecraft EVA-preparation conditions. Table 3 H2 /air laminar premixed flame property measurementsa Source

ρu /ρb SL∞ (mm/s) Kamax Ma

Present study 6.30 Kwon and Faeth [34] 6.30 Aung et al. [32,33] 6.30

2900 2860 2610

0.04 0.06 0.08

3.5 2.4 3.7

a Unsuppressed flames having φ = 1.80 at an initial mixture pressure and temperature of 1 atm and 298 ± 3 K; Du = 72.9 mm2 /s.

Fig. 3. Measured and predicted laminar burning velocities as functions of Karlovitz number and the concentration of nitrogen diluent for premixed stoichiometric hydrogen/air flames at room temperature and spacecraft EVA-preparation conditions.

in Tables 1 and 2 for all conditions tested during the present investigation. Given the preceding description of the way that the unstretched laminar burning velocities and Markstein numbers of outwardly propagating laminar premixed flames were found, it is of interest to compare present

measurements with earlier results. This comparison could be carried out only for premixed H2 /air flames at a fuel-equivalence ratio of 1.8 with the reactants at room temperature and pressure (NTP or 298 ± 3 K and 1 atm) where present test conditions and those of Kwon and Faeth [34] and Aung et al. [32,33] overlap. The results of all three studies are summarized in Table 3. Values of Kamax differ for the three studies but this occurs due to the somewhat arbitrary selection of the range of flame radii to be used to find SL∞ and Ma. On the other hand, the fundamental measured quantities, SL∞ and Ma, are seen to agree among the three studies within the ranges of experimental uncertainties (95% confidence) that were specified earlier. This behavior was typical of other comparisons of present and earlier results that could be made, as will be seen subsequently. The plots of laminar burning velocity as a function of stretch illustrated in Figs. 1–4 involve results for hydrogen/air flames at NPT for a fuel-equiv-

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alence ratio of unity with nitrogen and carbon dioxide as suppression agents (Figs. 1 and 2) and for hydrogen/EVA-preparation atmospheres for a fuelequivalence ratio of unity with nitrogen and carbon dioxide as suppression agents (Figs. 3 and 4). Measurements on these plots are indicated by open symbols for stable preferential-differential conditions (Ma  0) and closed symbols for unstable preferential-diffusion conditions (Ma < 0). For unstable conditions, the measurements are limited to values of Ka significantly greater than zero because flame surfaces became wrinkled due to preferentialdiffusion instability for radius values within the normal range of measurements. The first notable observation from Figs. 1–4 is that effects of flame/stretch interactions are important for present test conditions; for example, over all the present results, SL∞ /SL varied in the range 0.6–1.25 for Ka < 0.49, which does not involve a close approach to quenching conditions (which would involve Ka ≈ 1 from Law [28]) where effects of Ka on SL are expected to be large. Next, the linear relationship between SL∞ /SL and Ka clearly is satisfied for all measurements and predictions illustrated in Figs. 1–4 which correspondingly implies constant Markstein numbers for each flame condition, providing a convenient and concise way to summarize flame/stretch interactions for the present flames. Notably, this behavior has been observed for all outwardly propagating flame conditions studied thus far (see [19,20, 30–34] and references cited therein). Furthermore, the progressive addition of diluents to the flames illustrated in Figs. 1–4 causes the slopes of the plots of SL∞ /SL as a function of Ka to become progressively more negative, with the exception of a few conditions having large concentrations of diluent. In a number of cases, this implies that stable flames to preferentialdiffusion/stretch interactions at small concentrations of diluent become unstable flames to preferentialdiffusion/stretch interactions at large concentrations of diluent. Due to increased flame speeds as a result of increased flame surface area caused by wrinkling, this behavior clearly tends to counteract the ability of diluents to reduce combustion rates by reducing laminar burning velocities and tends to reduce the effectiveness of diluents to some extent. Finally, the qualitative agreement between measured and predicted burning velocity/stretch interactions, using the hydrogen/oxygen chemical kinetic mechanism of Mueller et al. [56], is reasonably good. This is particularly promising because the measurements used to develop the hydrogen/oxygen chemical kinetic mechanism of Mueller et al. [56] did not involve any direct consideration of flame/stretch interactions. This evaluation of predictions will continue during subsequent consideration of Markstein num-

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bers and unstretched laminar velocities, which provide more direct and complete comparisons of measurements and predictions than is possible for the results illustrated in Figs. 1–4. 4.3. Markstein numbers Markstein numbers are independent of Karlovitz numbers for present conditions and are summarized in Tables 1 and 2 as a function of reactant conditions. A portion of these results, involving measured and predicted Markstein numbers as a function of diluent concentrations, are plotted in Figs. 5–7. Measurements and predictions of Markstein numbers as a function of diluent concentrations for H2 /air flames having φ = 1 at NTP are illustrated in Fig. 5, considering helium, argon, nitrogen, and carbon dioxide as suppression agents. With the exception of helium, values of the Markstein number generally become progressively more negative as the concentration of diluent increases, with some tendency for this decrease to become small at large concentrations of the more effective nitrogen and carbon dioxide suppression agents. In these cases, preferentialdiffusion instability is promoted as the flames become more suppressed. Results for helium as a suppression agent differ from this behavior, however, because the large thermal conductivity of the fast-diffusing helium molecules tends to promote preferential quenching of the reaction zone and thus stability of the flames to preferential-diffusion/stretch interactions. Finally, the qualitative and quantitative agreement between predictions and measurements is reasonably good in Fig. 5, providing potential for the predictions

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

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

to help explain the complex effects of preferentialdiffusion/stretch interactions that influence the behavior of suppressed laminar premixed flames. Measurements and predictions of Markstein numbers as a function of diluent concentrations for H2 /air flames having φ = 1.8 at NTP are illustrated in Fig. 6, considering nitrogen and carbon dioxide as suppression agents. In this case, the experiments involve fuelrich conditions where H2 /air flames are intrinsically stable based on classical models of flame instability due to effects of preferential diffusion proposed by Manton et al. [24] and Markstein [25], namely, that laminar premixed flames are unstable to effects of preferential diffusion at conditions where the fastdiffusing component (H2 in the present instance) is deficient (at fuel-lean conditions in the present instance). The subsequent effect of adding nitrogen and carbon dioxide as suppression agents, however, is similar to results at other conditions where the addition of a diluent tends to shift the Markstein number toward more negative (unstable) values. Finally, the comparison between measurements and predictions in Fig. 6 is excellent. Measurements and predictions of Markstein numbers as a function of diluent concentrations for H2 /EVA-preparation conditions for φ = 1 and room temperature are illustrated in Fig. 7, considering helium, argon, nitrogen, and carbon dioxide as suppression agents. These results are qualitatively similar to the results for H2 /air mixtures at NTP illustrated in Fig. 5: the addition of diluents generally causes Markstein number to decrease, helium as a suppression agent differs from the rest due to its capability to quench the reaction zone at stretched conditions as a result of the fast-diffusion and high heat transfer rates

Fig. 7. Measured and predicted Markstein numbers as functions of the concentration of helium, argon, nitrogen, and carbon dioxide diluents for premixed stoichiometric hydrogen flames at room temperature and spacecraft EVApreparation conditions.

of helium, and the agreement between measured and predicted values of the Markstein numbers is excellent. 4.4. Unstretched laminar burning velocities In the following, measured values of laminar burning velocities will be limited to stretch-corrected results that yield unstretched laminar burning velocities. The measured values of unstretched laminar burning velocities are summarized in Tables 1 and 2. Plots of measured and predicted values of unstretched laminar burning velocities as functions of the concentrations of various diluents for some typical reactant conditions appear in Figs. 8–11 (the second independent variable on these figures, oxygen index, will be discussed later). These results are plotted for H2 /air flames having φ = 1 at NTP in Fig. 8, considering helium, argon, nitrogen, and carbon dioxide as suppression agents. In addition to the present measurements and predictions, the measurements of Kim et al. [19] for nitrogen and carbon dioxide as suppression agents at these conditions are shown on the plot. Notably, the agreement between present measurements and those of Kim et al. [19] and the agreement between present predictions and measurements are seen to be excellent. These results indicate that all diluents cause the unstretched laminar burning velocities to decrease as the concentrations of diluent are increased and that the suppression effectiveness of diluents (taken as the reduction of the unstretched laminar burning velocity for a particular diluent concentration) increases in the order helium, argon, nitrogen, and carbon dioxide. Both these behaviors can be explained for argon,

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Fig. 8. Measured and predicted unstretched (Ka = 0) laminar burning velocities as functions of the concentrations of helium, argon, nitrogen, and carbon dioxide diluents for premixed stoichiometric hydrogen/air at NTP.

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

nitrogen, and carbon dioxide according to Huggett [22,23] as a result of the increase of the specific heat of the nonfuel gases per unit oxygen concentration. This causes a corresponding reduction of temperatures within the reaction zone of the flames with the associated reduction of laminar burning velocities following in accord with classical phenomenological theories of premixed laminar flame propagation; see Law [28]. Helium as a suppression agent is a predictable exception to this behavior; its specific heat effect is identical to that of argon as a suppression agent but this effect is counteracted by its increased heat and mass transfer rate capabilities that tend to increase unstretched laminar burning velocities for the helium-containing flames to some extent, compared

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Fig. 10. Measured and predicted unstretched (Ka = 0) laminar burning velocities as functions of the concentrations of nitrogen and carbon dioxide diluents for stoichiometric premixed hydrogen/air flames at room temperature and pressures of 0.5 and 1.0 atm.

Fig. 11. Measured and predicted unstretched (Ka = 0) laminar burning velocities as functions of the concentrations of helium, argon, nitrogen, and carbon dioxide diluents for premixed hydrogen flames at room temperature and spacecraft EVA-preparation conditions.

to argon-containing flames, based on classical phenomenological theories of premixed flames. The third-body reaction H + O2 + M = HO2 + M is important as a chain-terminating reaction. It competes with the branching reaction H + O2 = OH + O at temperatures less than ∼900 K [63]. Therefore, for weakly propagating flames where the temperature is low, this third-body reaction has a dominant effect in the H2 –O2 chemistry. It is also found that the thirdbody reactions, especially H + OH + M = H2 O + M, are important for laminar flame speed propagation only when the pressure is high [64]. Pressures considered here, however, are low, 0.5, 0.7, and 1 atm, and

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the flame, even with 40% CO2 as diluent, is still far away from the flammability limit. Therefore, in the present investigation, the third-body reaction efficiencies are not as important as the impact of heat capacity and transport properties of the diluents. Measured and predicted unstretched laminar burning velocities are plotted as a function of diluent concentration for H2 /air flames having φ = 1.8 at NTP in Fig. 9, considering nitrogen and carbon dioxide as suppression agents. In general, the unstretched laminar burning velocities at φ = 1.8 in Fig. 9 are larger than those at φ = 1.0 in Fig. 8, which is well known behavior because the laminar burning velocities of H2 /air mixtures at NTP reach a maximum at φ = 1.8 [32–34]. Other trends in Fig. 9 are similar to those in Fig. 8: the comparison between measurements and predictions is excellent, and diluents progressively reduce unstretched laminar burning velocities as their concentrations increase with suppression effectiveness additionally increasing in the order nitrogen and carbon dioxide. Measured and predicted unstretched laminar burning velocities are plotted as a function of diluent concentration for H2 /air flames having φ = 1.0 at room temperature and pressures of 0.5 and 1.0 atm in Fig. 10, considering nitrogen and carbon dioxide as suppression agents. Similar results are found in Figs. 8 and 9. Finally, the effect of pressure on laminar burning velocities is relatively small for the results illustrated in Fig. 10, which is well-known behavior for H2 /air flames at room temperature and pressures in the range 0.35–1.0 atm; see Aung et al. [33]. Even though SL∞ is not strongly affected by pressure for the range of conditions illustrated in Fig. 10, however, it should be recalled that rates of chemical energy release for a particular laminar burning velocity and reactant temperature are directly proportional to the concentration of the reactant mixture and thus the pressure. As a result, fire severity decreases with decreasing pressure. In addition, reduced pressures tend to increase suppression agent concentrations in the reactant mixture for a given mass release of suppression agents, helping to promote extinction. Finally, chemical kinetic considerations near flammability limits also point toward improved capabilities to extinguish premixed flames at reduced pressures, although this property was not studied during the present investigation. Measured and predicted unstretched laminar burning velocities are plotted as a function of diluent concentration for premixed hydrogen flames at room temperature and EVA-preparation conditions in Fig. 11. In this case, all four diluents—helium, argon, nitrogen, and carbon dioxide—have been compared as suppression agents. The suppression effectiveness increases in the order helium, argon, nitrogen, and car-

bon dioxide for the reasons that have already been discussed in connection with Fig. 8. The enriched oxygen concentration of EVA-preparation conditions compared to a conventional air environment (oxygen concentrations of 30% by volume compared to 21% by volume) causes the unstretched and unsuppressed laminar burning velocity in the EVA-preparation atmosphere to be larger than that in air at NTP, e.g., 3350 mm/s compared to 2140 mm/s. On the other hand, the reduced pressure of the EVA-preparation atmosphere reduces the mass burning rate; therefore, the fire intensity is only roughly 10% larger in the EVA-preparation atmosphere than the mass burning rate in air at NTP. The reduced pressure of the EVApreparation environment also tends to increase the effectiveness of particular mass discharges of diluents compared to systems operated at NTP. Thus, present results do not clearly establish potential advantages for either normal or EVA-preparation environments with respect to fire suppression performance. The oxygen index, which is the concentration of oxygen (in %) by volume in the nonfuel gases, is a single-valued function of diluent concentration for the conditions of Figs. 8–11 and is shown as an independent variable on these figures. For present hydrogen flames, oxygen indices reach values as small as 10 with no sign of approach to extinction conditions, whereas hydrocarbon flames typically reach flammability limits for oxygen indices of 12–15. For example, Westbrook [62] has proposed an approximation to find conditions where laminar premixed flames extinguish at a laminar burning velocity of 50 mm/s; in contrast, present flames at oxygen indices of 10 exhibit unstretched laminar burning velocities greater than 180 mm/s and are still well away from the extinction conditions suggested by the Westbrook [62] criterion. This behavior is not unexpected, however, due to the well-known difficulties of suppressing hydrogen flames; see Lewis and von Elbe [36]. The use of EVA-preparation atmospheres definitely increases oxygen indices at a particular diluent concentration, as illustrated in Fig. 11, which is reflected by the increased laminar burning velocities in EVA-preparation atmospheres compared to those in air at NTP. On the other hand, the reduced pressure of the EVA-preparation atmosphere reduces the measure of the fire hazard, taken as the degree of hazard, by a factor of 1/P 1/2 according to Huggett [22]. Further study is required, however, to establish whether suppression is more difficult in EVA-preparation atmospheres at room temperatures due to the simultaneous increase of the oxygen concentration and decrease of the pressure.

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Fig. 12. Predicted structure of an unstretched (Ka = 0) premixed stoichiometric hydrogen/air flame with no diluent present at NTP.

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Fig. 13. Predicted structure of an unstretched (Ka = 0) premixed stoichiometric hydrogen/air flame with a 40% by volume helium diluent at NTP.

4.5. Flame structure As mentioned earlier, measurements and predictions of unstretched laminar burning velocities were in reasonably good agreement, including effects of variations of fuel-equivalence ratio, pressure, ambient oxygen concentration, and presence of diluents; therefore, the predictions were exploited to gain a better understanding of the effects of diluents on laminar burning velocities. The approach involved numerical simulations of plane (unstretched) H2 /air flames in the presence of various diluents. Typical predicted structures of plane unstretched H2 /air flames at a fuel-equivalence ratio of unity and NTP are illustrated in Figs. 12–16. Results in Fig. 12 provide the baseline flame structure when no diluent is present. Figs. 13–16 provide similar results for diluent concentrations of 40% (by volume) for helium, argon, nitrogen, and carbon dioxide, in turn. All these results are based on the hydrogen/oxygen chemical kinetics mechanism of Mueller et al. [56]. In each figure, the top graph provides profiles of the temperature and the stable species (H2 , O2 , H2 O) concentrations, whereas the bottom graph provides profiles of radical species (H, OH, O, HO2 , and H2 O2 ) concentrations, all as functions of distance through the flame. It should be noted that the origins of the length scales in these figures are arbitrary and do not cor-

Fig. 14. Predicted structure of an unstretched (Ka = 0) premixed stoichiometric hydrogen/air flame with a 40% by volume argon diluent at NTP.

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

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

respond to the central ignition point. In addition, the scales of the concentrations of radical species coordinates become more expanded going from Fig. 12 to Fig. 16 so that the plots remain readable as maximum radical concentrations decrease with increasing suppression agent effectiveness. The results show that the maximum concentrations of the radicals HO2 and H2 O2 are roughly two orders of magnitude smaller than the concentrations of the radicals H, OH, and O; therefore, the latter tend to dominate reactive effects in the present flames. Comparing the stable species concentrations for a flame not having a diluent present (Fig. 12) to those in flames having a diluent present (Figs. 13–16), indicates expected reductions of the reactant concentrations (H2 and O2 ) and product concentrations (H2 O) due to the dilution caused by the diluents having initial concentrations of 40% (by volume). Another effect that is evident is the preferential diffusion of the fast-diffusing reactant, H2 , compared to the slow-diffusing reactant, O2 , for a plane flame; this can be seen from the increase of the concentration of O2 near the cold boundary of the flame before the concentration of O2 decreases once again upon approach to the reaction zone of the flame. The next major trend is the progressive reduction of the final flame temperature from 2250 K (for no diluents), to 1750 K (for the diluents He and Ar), to 1600 K (for the diatomic diluent

N2 ), and finally to 1350 K (for the triatomic diluent CO2 ). This behavior is solely due to the progressive increase of the specific heat of these diluents in the order He and Ar (the same), N2 , and CO2 . On the other hand, the increased thermal diffusivity of He compared to Ar has no effect on the final flame temperature because these flames are all adiabatic. For the present stoichiometric flames, the radical H generally has the largest maximum concentrations in the flames, with OH having somewhat smaller maximum concentrations, e.g., roughly 1/4–1/3 as large as H, and with the other radicals all having significantly smaller concentrations. In addition, the maximum concentration of H in the flames progressively decreases in the order no diluent, helium, argon, nitrogen, and carbon dioxide as suppression agents. Similarly, but not shown here, the maximum concentration of H in the flame for a particular reactant mixture progressively decreases with increasing concentrations of suppression agents. Based on the findings of Kwon and Faeth [34], for flames having hydrogen and oxygen as reactants, it is expected that this reduction of the maximum concentration of H should cause a corresponding reduction of the laminar burning velocity of these flames. The potential for this behavior will be considered in the next section.

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4.6. Radical behavior The flame structure results of Figs. 12–16, and similar 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 radicals in premixed hydrogen and oxygen flames. In addition, OH radical concentrations are the next largest concentration after H for φ  1, ranging up to same order of magnitude of maximum concentration of H and OH for φ as small as 0.6. This behavior is significant due to the strong correlation between the SL and the maximum concentration of H in the reaction zone of premixed hydrogen and oxygen flames observed by Padley and Sugden [35] and Butler and Hayhurst [37]. Based on the flame structure results just discussed in connection with Figs. 12–16, it appeared likely that a similar correlation would also be observed for the present suppressed flames; this possibility is considered next. Similar to past work, the most robust correlation between laminar burning velocities and radical concentrations for laminar premixed hydrogen and oxygen flames was obtained by using the maximum H + OH mole fraction in the flames, which generally was obtained at the condition where the mole fraction of H was a maximum. The resulting correlation is illustrated in Fig. 17, where the value of SL∞ is plotted as a function of the maximum H + OH mole fraction, computed as just described. All the results illustrated in Fig. 17 are for premixed hydrogen and oxygen flames at room temperature and include findings from Kwon and Faeth [34], Kim et al. [19], and the present investigation. The ranges of experimental conditions for these investigations are summarized in Table 4. The original correlation of Padley and Sugden [35] along these lines is not included in Fig. 17 because their plotted results were limited to SL as a function of the maximum H mole fraction in the flames. Another difficulty about the measurements of Padley and Sugden [35] is that the extent of flame stretch is unknown for these results. Clearly, there is a rough correlation between the SL∞ and the maximum H + OH mole fraction in

Fig. 17. Laminar burning velocities as functions of the maximum H + OH mole fraction in the reaction zone for hydrogen flames having various concentrations of diluents at room temperature; see Table 4 for the range of experimental conditions.

the flames for the results illustrated in Fig. 17. Notably, SL∞ and radical mole fractions were varied in a number of ways for these results: dilution by various concentrations of chemically passive suppression agents (He, Ar, N2 , and CO2 ), dilution by various concentrations of a chemically active suppression agent (CF3 Br), variation of fuel-equivalence ratios, variation of the concentration of oxygen in the nonfuel gases, modest variations of pressure (0.7– 1.0 atm), and variation of the degree of flame stretch; see Table 4 for a complete summary of the ranges of the experimental flame conditions. Finally, the degree of scatter of the correlation of SL∞ as a function of the maximum H + OH mole fraction in Fig. 17 is particularly small for the recent studies of unstretched suppressed laminar premixed flames due to Kim et al. [19] and the present investigation. These results provide a best-fit correlation between the SL∞ and the maximum H + OH mole fraction, which is shown on the plot, as SL∞ (mm/s) = 260 + 36,600 (XH + XOH )max , (4)

Table 4 Test conditions for hydrogen premixed flame studiesa Source

Diluentsb

φ

O2 /(O2 + N2 ) (% vol.)

P (atm)

Kac (–)

XD d (% vol.)

Kwon and Faeth [34] Kim et al. [19] Present investigation

– N2 , CO2 , CF3 Br He, Ar, N2 , O2

0.6–4.5 0.6–1.8 1.0 and 1.8

21–36 21 21 and 30

1.0 1.0 0.7–1.0

0.0–0.50 0.0 0.0

0 0–2 0–40

a b c d

Initial mixture temperature 298 ± 5 K. He, Ar, N2 , and CO2 are chemically passive suppression agents, whereas CF3 Br is a chemically active suppression agent. Ka = 0 denotes unstretched flames. XD = 0 denotes unsuppressed flames.

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

5. Conclusions The effects of flame stretch and the concentrations and types of diluents on the laminar burning velocities of hydrogen premixed flames were studied both experimentally and computationally. The experiments involved unsteady outwardly propagating laminar premixed spherical flames and steady plane laminar premixed flames, similar to past work in this laboratory, e.g., Tseng et al. [38], Aung et al. [32,33], and Kwon and Faeth [34]. Experimental and computational conditions considered premixed hydrogen/air/diluent and hydrogen/30% oxygen and 70% nitrogen (by volume)/diluent flames, with the latter condition of interest for EVA-preparation activities onboard manned spacecraft. The other flame conditions were as follows: room temperature (298 K); fuel-equivalence ratios of 1.0 and 1.8; pressures of 0.5, 0.7, and 1.0 atm; chemically passive gaseous suppression agents (diluents) including helium, argon, nitrogen, and carbon dioxide; and diluent concentrations of 0–40% (by volume), which is equivalent to oxygen indices of 30–10 for present flame conditions. Predicted flame behavior considered variable transport and thermodynamics properties, multicomponent transport, and the detailed hydrogen/oxygen chemical kinetic mechanism of Mueller et al. [56]. The major conclusions of the study are as follows. (1) Effects of flame/stretch interactions for both measurements and predictions of suppressed flames could be correlated based on the local-conditions hypothesis according to SL∞ /SL = 1 + Ma Ka to obtain a linear relationship between SL∞ /SL and the Karlovitz number. This behavior implies a constant Markstein number for given reactant conditions, similar to earlier findings for unsuppressed flames. (2) Effects of flame stretch on laminar burning velocities were substantial, yielding values of SL∞ /SL in the range 0.60–1.25, for Ka < 0.5, which does not involve a close approach to quenching conditions where Karlovitz numbers typically have values on the order of unity [28]; corresponding Markstein numbers were in the range −1.0 to 3.5. (3) Measured and predicted unstretched laminar burning velocities and Markstein numbers were in reasonably good agreement using the hydrogen/oxygen chemical kinetic mechanism of Mueller et al. [56]. (4) The chemically passive suppression agents– diluents increase in effectiveness (based on reduction of the unstretched laminar burning velocity for

a given concentration of diluent (in % by volume)) in the order helium, argon, nitrogen, and carbon dioxide which mainly reflects their progressively increasing specific heats and progressively decreasing mass and thermal transport properties. (5) Predictions showed that the unstretched laminar burning velocities of the present flames were strongly correlated with the maximum H + OH mole fraction in the reaction zone for variations of these concentrations due to effects of chemically passive suppression agents, similar to an early proposal of Padley and Sugden [35] for unsuppressed and unstretched hydrogen/air flames, the recent observations of Kwon and Faeth [34] for a variety of unsuppressed and stretched and unstretched flames involving H2 and O2 as reactants, and the recent observations of Kim et al. [19] for a variety of stretched and unstretched flames involving H2 and O2 as reactants that were subjected to a chemically active suppression agents (Halon 1301). (6) Finally, there is a consistent tendency for the addition of suppression agents, either chemically active or chemically passive, to reduce the Markstein number for a given reactant mixture at the same time that the unstretched laminar burning velocity is reduced, causing unsuppressed flames that are stable to effects of preferential-diffusion/stretch interactions (positive Markstein numbers) to become suppressed flames that are unstable to effects of preferential-diffusion/stretch interactions (negative Markstein numbers) in some instances. Thus, the tendency of suppression agents to reduce laminar burning velocities (and thus act to reduce the severity of unwanted fires) is counteracted to some extent by the tendency of suppression agents to reduce Markstein numbers (and promote flame instabilities that tend to increase the severity of unwanted fires).

Acknowledgments This research was sponsored by NASA Grants NCC3-661, NAG3-1878, NAG3-2040, and NAG32404 under the technical management of F. Takahashi of the NASA Glenn Research Center. The authors thank Dr. Elaine Oran at NRL for her help and encouragement in the revising process of this paper.

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