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NH3 mixtures by excimer-laser photolysis of NH3
COMBUSTION
AND
FLAME
87: 191-202 (1991)
191
Ignition of H,/02/NH,, Hz/Air/NH, and CH,/OJNH, Mixtures by Excimer-Laser Photolysis of NH, MAU-SONG
CHOU
and TMITRI J. ZUKOWSKI
TR W Space & Technology Group, One Space Park, Redondo Beach, CA 90278 Volumetric ignition of H^JO,, Ha/air, and CHJO, mixtures (in an open flow system, initially at 1 atm and room temperature) is achieved via photolysis of a small amount of NH, present in the flow mixtures. The photolysis is accomplished by an ArF excimer laser operated at 193 nm. The ignition appears to be homogenous, since the ignition-delay times measured at several locations are approximately identical. The minimum ignition-energy density is measured to be 137 f 8, 190 f 20, and 380 + 30 ml/cm3 for stoichiometric mixtures of HJOdNH,, HJair/NH3, and CHJOJNH,, respectively, and appears to be insensitive to fuel-equivalence-ratio values between 0.35 and 3.0. The ignition-delay time depends strongly on both the initial NH, concentration and the laser-energy-deposition density. However, the ignition-delay time is not sensitive to the fuel equivalence ratio. The measured minimum ignition-energy density appears to be substantially lower than that predicted by a kinetic modeling calculation. This result implies that the hot H atoms produced by the photolysis of NH, may play an important role in facilitating the ignition.
INTRODUCTION ‘Photochemical ignition provides an excellent opportunity to study the details of many chemical processes in combustion, since energy can be directed homogeneously and within a brief time interval to achieve a well-characterized initial condition. Earlier experiments by Farkas et al. [l] have shown that HJO, mixtures containing a small amount of NH, become explosive at a moderate temperature (- 420°C) under ultraviolet radiation. The reaction appears to proceed much more rapidly under the photolysis of NH, than by other means of photogeneration of H atoms, such as the photosensitized mercury atoms -that dissociate H, into 2 H [2]. It was speculated that NH, may play a role in the ignition chemistry; however, the detailed reaction mechanism remains poorly understood [3]. Recent advances in high-energy ultraviolet lasers have stimulated renewed interest in photo*chemical ignition, since these lasers can generate sufficient radicals by photolysis to initiate combustion in an exceptionally controlled way. Laser-spark has been used to ignite fuel-oxidizer mixtures, by either gaseous breakdown 14, 51 or multiphoton processes [6, 71, within a tightly focused volume. Absorption of laser photons that produce reactive radical species has also been used to initiate combustion without spark formaCopyright 0 1991 by The Combustion Institute Published by Elsevier Science Publishing CO., Inc.
tion. For example, Lavid and Stevens [8] have used a F, laser, operated at 157 nm to photolyze O,, as a mean of igniting H,/O, mixtures, and Lucas et al. [9] have used a KrF laser, operated at 248 nm to photolyze 0,, as a mean of igniting H,/O,, CHJO, 9 and C,H, /O, mixtures. The related subject of ignition under the addition of radicals has been studied theoretically by Guirguis et al. [lo], Sloane [ll-131, and Sloane and Schoene [ 141. Here we study ignition initiated by the photolysis of NH,, through the use of an ArF laser operated at 193 nm, in mixtures of HJOJNH,, HJair/NH,, and CHJOJNH,. We have also performed kinetic modeling calculations of the ignition of H,/O*/NH, by photolysis. Our calculations suggest that highly energetic photofragments, such as hot H atoms, may play an important role in the initiation of combustion.
REVIEW OF RELEVANT PHOTOCHEMISTRY Light of 193 nm wavelength excites NH, into [15, 161, V; = 6 of the AlA’; + *:‘A’, transition and the excited NH, dissociates into NH, and H with nearly unit quantum efficiency [ 17, 181. The molar extinction coefficient for NH, at 193 nm is 3250 L/mol-cm [19]. This value is much higher OOlO-2180/91/$3.50
192
M.-S. CHOU AND T.J. Z U K O W S ~
than that for O 2 [20]. The absorption by O 2 is estimated to be less than about 1% of the absorption by NH 3 under our experimental conditions. The bond dissociation energy for NH3, D0(HNH2), is - 4 4 6 . 4 kJ/mol [21, 22]; hence there is about 171.5 kJ/mol of residual energy left from the 193-nm photons. Biesner et al. [23] have measured the distribution of the translational energy of the H atoms from the photolysis of NH 3 at - 193.6 nm. About 20% of the H atoms produced have translational energy above 52 kcal/mol, which is about the threshold energy required for the reaction of H with 02 [24]. These investigators [23] also derived a branching ratio of 0.33 for the yield of N H 2 ( A ) to N H 2 ( X ). However, a lower branching ratio (0.025) was reported earlier by Donnelly et al. [25] on the basis of the NH 2 emission that follows the 193-nm p h o t o l y s i s . o f NH 3. The N H z ( X ) is produced with a relatively high internal excitation [23, 25]. At higher laser intensity, N H ( A ) can also be produced by a sequential two-photon absorption process, via an internally excited intermediate state of N H 2 ( ) ? ) [26, 27]. The molar extinction coefficient for the absorption of the intermediate NH2()~') at 193 nm was measured to be - 10.4 L mol-~ cm-1 by Hofzumahaus and Stuhl [28]. The internally excited NH2()~" ) from the photolysis of NH 3 appears to be unreactive with 02 or H e [25]. The N H 2 ( A ) from the photolysis of NH 3 has two components: - 85% in a short-lived component, and - 1 5 % in a long-lived component [29]. The former is postulated to be pure NH2(,,t'), while the latter is postulated to be a mixed state of N H z ( A ) and NH2(,~ ). The longlived component is unreactive with 0 2 [25]. Quenching cross sections for the short-lived component are generally quite high, with several collisional partners, including NH3, H2, Ar, and He [26, 30]; hence, the loss of the short-lived component is likely to be by collisional quenching, rather than to reaction with 0 2 . The translationally hot H atoms from the photolysis of NH 3 may enhance reaction with 0 2 [24, 31-34]: H+O 2--*OH+O.
(1)
According to a theoretical study by Miller [24], the reaction cross section has a threshold energy
for the translational energy of - 52 kJ/mol, and increases gradually to a broad maximum of - 0.4 /~2 for translation energies between 105 and 209 kJ/mol. From the measured distribution of the translational energy of H atoms by the photolysis of NH 3 [23] and the theoretical reaction cross section [24], an effective rate constant of - 9 . 4 × 1012 cm3/mol-s for reaction 1 can be derived. A major competing reaction for the H atoms is reaction 2, which may be considered to bechain-breaking reaction, since HO 2 is not very reactive: H + 0 2 + M ~ HO 2 + M.
(2)
The product of the rate constant of reaction 2 [34] and the gas density ( M ) at 1 atm is relatively lower, about 3.6 × 1012 cm3/mol-s. This argument suggests that 20% mole fraction of the H atoms produced by the photolysis of NH 3 may react favorably via reaction 1.
EXPERIMENTAL PROCEDURE The experimental arrangement is shown in Fig. 1. A Lambda Physik Excimer Laser (150 EST), which provides ArF-laser radiation at 193 nm, was used as a photolysis source. This excimerlaser system consists of two laser units. The output from an unstable resonator of the first unit was amplified through the second unit to achieve a high energy output with a relatively low beam divergence. An ultraviolet-grade MgF 2 lens with a 50-cm focal length was used to deliver a laser beam with a rectangular profile of about 2.1 mm in width × 7.1 mm in height at the leading edge, and 1.2 mm in width × 3.3 mm in height at the trailing edge, of a 6-cm-diameter fiat-flame burner (McKenna Products). The spot size of the laser beam was determined by the burn spots on Polaroid film. The focus of the laser beam was far past the burner to avoid any gaseous breakdown in the mixtures. The incident as well as the transmitted laser energies were monitored by the use of quartz-flat beamsplitters and laser-energy-ratiometer probes (Laser Precision Model 7200). The incident laser energy was in the range of 100-290 mJ per pulse. The OH emission was monitored at three locations along the beam path: 2.2, 3.0, and 3.8
IGNITION BY EXCIMER-LASER PHOTOLYSIS OF NH 3
;
I i P
QF
EXCIMERLASER ABSORBER (193 nm) / ~ LASERDETECTOR / / ~'-/
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/
_
_
FLAT-FLAME
BURNER
./ / / ~, ~'\ ~.< / / ~ ~ \ ,'/'~ ~ '~\ \ ,/ / / ~ ~
ArF
-
193
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PMT ~ /
!
OH FILTER I I
;p I I
~ ~ ~ I ~ \\ \ \\ \~ ~c~
........ Mrz~urilSlCM 193 nm NBF
r [ LASER DETECTOR OH FILTER
Fig. 1. Schematic of the experimental arrangement for the ignition of combustible mixtures by ArF excimer laser photolysis of NH 3. M, mirror; QF, quartz flat; NBF, narrow-band filter; and PMT, photomultiplier tube.
cm from the leading edge of the burner. The center location (3.0 cm) was observed with a m o n o c h r o m a t o r and photomultiplier tube, whereas the other two locations were monitored with a bandpass filter (with center at 310 nm and with 10-nm bandwidth) and photomultiplier. The gaseous mixtures were prepared in a stainless-steel manifold and flowed through the flat-flame burner. A coaxial gaseous shroud of N 2 was used to minimize any mixing of the combustible flow with the surrounding air. Matheson anhydrous-grade ammonia (99.99%), hydrogen (99.995%), and 02 (99.5%) were used. The total gas flow was 3.75 L/min for both H2/O2/NH 3 and CHa/O2/NH 3 mixtures, and 8.75 L/min for H 2/air/NH 3 mixtures. Ignition experiments were conducted on the premixed gases with the excimer-laser beam at 1.5 cm above the flat-flame burner. All the data were collected in the open system at 1 atm and room temperature. The majority of the experiments were carried out under bleaching conditions, for which the incident excimer-laser energy was sufficiently high to generate a bleaching wave and photolyze all the NH 3 along the laser-irradiated pathlength. The bleaching condition can be achieved here because the absorption coefficient of NH 3 is relatively high, whereas the typical NH 3 concentration employed here was relatively low. Under bleaching conditions, relatively uniform radical density within the irradiated volume can be pro-
duced in spite of any nonuniformity or hot spots in the incident laser beam. Numerous observations with an optical multichannel analyzer were made to analyze the emission spectrum. The emission during ArF-laser irradiation was found to be composed mainly of the NH band at 336 nm and NH 2 at - 4 5 0 - 7 4 0 nm. No atomic or ionic emission lines indicative of plasma formation are observed. This observation precludes gaseous breakdown as a cause of ignition. RESULTS
H2/O2/NH3 Mixtures The results for the ignition of H2/O2/NH 3 mixtures by the photolysis of NH 3 are summarized in Figs. 2 - 7 . Figure 2 shows typical OH-emission temporal profiles in two different time scales. Figure 2a shows that the OH emission occurs at a certain delay time after the irradiation of the excimer laser. Figure 2b shows that the OH emission reaches a maximum and then decreases to a relatively low steady-state value at later times. The intensity of the early OH emission peak is about an order of magnitude higher than the steady-state value. The OH emission occurs only if the mixture is ignited. An ignition-delay time can therefore be assigned on the basis of the onset of the OH
194
M.-S. CHOU AND T. J. ZUKOWSKI
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Fig. 2. Typical OH emission profiles at two different time scales following ArF excimer laser irradiation: (a) rise of delayed OH emission and (b) rise and fall of the delayed OH emission. The start of the excimer-laser pulse is marked by T = 0. The OH-emission intensity increases in the downward direction.
emission. Figs. 3 - 5 summarize the ignition-delay times, which are measured at three locations: near the leading edge (2.2 cm), at the center (3.0 cm) and near the trailing edge (3.8 cm) above the fiat-flame burner. The ignition-delay times are plotted as functions of incident laser energy for three initial NH 3 mole fractions of 0.65%, 0.83%, and 1.35% in Figs. 3, 4, and 5, respectively. At the lower NH 3 mole fractions of 65 % and 0.83%, all the incident laser energies ( - 1 4 0 - 2 6 0 mJ) are sufficient to generate a bleaching wave through the entire 6-cm path-
length above the burner. At the higher NH 3 mole fraction of 1.35%, the laser energy required ( - 1 5 7 mJ) to generate a bleaching wave is higher, and only a portion of the data in Fig. 5 pertains to bleaching conditions. For incident laser energy above the bleaching threshold, all NH 3 within the irradiated volume should be dissociated and the density of deposited laser energy as well as the radical concentration should remain constant regardless of the increas-
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260
INCIDENT LASER ENERGY, mJ
Fig. 3. Ignition-delay time in H2/O2/NH 3 stoichiometric mixtures with 0.0065 mol fraction of NH 3, as a function of incident laser energy, from the OH emission measurements at three locations: (V) 2.2 cm, (O) 3.0 cm, and (&) 3.8 cm downstream of the burner leading edge.
120
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l
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140
160
180
200
220
240
iNCIDENT LASER ENERGY, mJ
Fig. 4. Ignition-delay time in H2/O2/NH 3 stoichiometric mixtures with 0.0083 mol fraction of NH 3, as a function of incident laser energy, from the OH emission at three locations: (V) 2.2 cm, (O) 3.0 cm, and (&) 3.8 cm downstream of the burner leading edge.
IGNITION BY EXCIMER-LASER PHOTOLYSIS OF NH 3 LASER-ENERGY-DEPOSITION DENSITY. mJ/cm 3
fl2/O2/NH3 500
195
100
200
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300
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1 240
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260
Fig. 5. Ignition-delay time in H2/O2/NH 3 stoichiometric mixtures with 0.0135 mol fraction of NH3, as a function of incident laser energy, from OH emission at two locations: (o) 3.0 cm and (&) 3.8 cm downstream of the burner leading edge. The arrow indicates the minimum incident-laser energy needed to bleach out the NH 3 absorption up to the 3.8-cm location.
0.5
1.0
1.5
NH3 MOLE FRACTION,%
Fig. 6. Ignition-delay time in H 2 / O J N H 3 mixtures as a function of initial NIl3 mole fraction and laser-energy-deposition density, for several fuel equivalence ratios: (&) 0.35, ( T ) 0.5, (©) 1.0, (O) 2.0, and (&) 3.0. The error bar given for the cases with fuel equivalence ratio of 1.0 indicates one standard deviation in the spread of the measured data points. The shaded area indicates approximately the region of no ignition.
ing values of the incident laser energy. Therefore, the ignition-delay time should also remain constant. At sufficient high NH 3 mole fractions, ignition-delay times are indeed nearly independent of incident laser energy, within the uncertainty of our experimental data, as shown in Fig. 4 and also in Fig. 5 for those data with incident laser energy above the bleaching threshold. However, for NH 3 mole fractions below a certain level, as shown in Fig. 3, the ignition-delay time appears to decrease more substantially with increasing incident laser energy. This apparent de-
pendency at a low NH 3 mole fraction may be attributed to the fact that as the radical concentration is reduced to nearly the minimum value needed for ignition, the ignition becomes more sensitive to multiphoton processes. One of the important multiphoton processes is the sequential two-photon-absorption process, by which NH 3 is dissociated into NH and 2 H, as discussed in the review section. The quantum yield for this proc-
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FUEL EQUIVALENCE RATIO Fig. 7. Minimum ignition-energy density and minimum mole fraction of Nil 3 needed for ignition of H2/O2/NH 3 mixtures, for several fuel equivalence ratios.
196 ess is estimated to be - 1.8 × 10 - 3 at 140 mJ and - 3.5 x 10 -3 at 260 rnJ of the incident laser energy. Although these values are only moderate in comparison with the value of nearly unity for the single-photon process, they may become important if the radical concentration is just barely sufficient for ignition. Figures 3 - 5 show that the ignition-delay times at three different locations appear to be of comparable value for those test runs under bleaching conditions. Although there are some variations among the three different locations, the variations appear to be statistically random. Furthermore, they are too small to be significant for flame propagation from a single ignition site. This argument suggests that the ignition is nearly homogeneous and there are probably well distributed ignition sites within the entire laser-irradiated volume. Figure 6 shows the ignition-delay time as a function of initial NH 3 mole fraction, for several values of fuel equivalence ratio. Also shown on the abscissa is the laser-energy-deposition density, a quantity that is derived based on that required to photodissociate all NH 3 density present initially in the mixtures. This derived laserenergy-deposition density agrees well with the measured difference in the incident and transmitted laser energy upon division by the total irradiated volume. The ignition-delay time plotted here is an average for all data with incident laser energy above a bleaching threshold (as shown in Figs. 3-5). Some uncertainty may arise, for those events with low initial NH 3 mole fractions, by use of the average value, owing to the observed variation with the incident laser energy. This uncertainty, however, appears to be relatively unimportant in view of the much steeper variation of the ignition-delay time with the initial NH 3 mole fraction. The error bar given for the cases with a stoichiometric mixture (q~ = 1.0) indicates one standard deviation in the spread of the measured data points. Figure 6 shows that the ignition-delay times are nearly insensitive to the fuel equivalence ratio for values between 0.35 and 3.0. Also shown in Fig. 6 is an approximate threshold value of laser-energy-deposition density or initial NH 3 mole fraction needed for ignition. Below this threshold no ignition occurs. Figure 7 summarizes the minimum ignition-energy density and the minimum NH 3 mole fraction needed for
M.-S. CHOU AND T. J. ZUKOWSKI ignition, for several equivalence ratios. The minimum ignition-energy density is taken to be the minimum laser-energy-deposition density needed for ignition. It is noted that the minimum igni-" tion-energy density is also insensitive to the fuel equivalence ratio for values between 0.35 and 3.0. We have also performed additional experiments with a larger laser beam, of nominal beam size - 2.1 mm × 8.5 mm. The ignition-delay time and the minimum ignition-energy density forthis condition are not significantly different from the corresponding values reported here.
H z/Air/NH 3 Mixtures The results for the ignition of H2/air/NH 3 stoichiometric mixtures by the photolysis of NH 3 are summarized in Fig. 8. The ignition-delay time plotted here is an average for all data with incident laser energies above the bleaching threshold. The error bar indicates one standard deviation. Figure 8 shows that the ignition-delay time de-" pends strongly on the initial NH 3 mole fraction or the laser-energy-deposition density. In comparison with the H2/O2/NH 3 mixtures, the ignitiondelay time is longer for the same laserenergy-deposition density. This result is plausible owing to the increase in the total heat capacity in the presence of excess N 2. However, Fig. 8 shows that the minimum ignition-energy density
DENSITY,mJ/cm 3
LASER-ENERGY-DEPOSITION
100
200 I
1000
300 I
400 I H2/AIR/NH 3 0 :I.G
id
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1110
10
NO IGNITION
I 0.5
I I.O
.
.
.
.
t 1.5
,
NH 3 MOLE FRACTION, %
Fig. 8. Ignition-delay time in H2/air/NH 3 stoichiometric mixtures as a function of initial NH3 mole fraction and laser-energy-deposition density. The error bar indicates one standard deviation in the spread of the measured data points. The shaded area indicates approximately the region of no ignition.
IGNITION BY EXCIMER-LASER PHOTOLYSIS OF NH 3 increases only moderately from - 137 + 8 m J / c m 3 in H2/O2/NH 3 to - 190 _+ 20 rnJ/cm 3 in H2/air/NH 3, at the stoichiometric ratio.
197
energy or species out of the irradiated volume, should yield a lower bound on the ignition-delay time and mixture ignitability. The H - O - N reaction mechanism, which comprises 20 species and 61 reactions, is adopted from a larger H - O - N - C mechanism published by Glarborg et al. [361. All the rate coefficients are adopted without any modification. The experimental condition with an initial NH 3 mole fraction of 0.012 under bleaching conditions is adopted in the following modeling simulation. Table 1 presents the modeling results for three cases of assumed initial radical concentrations. The gas temperature is estimated by taking all the available energy of the photon (remaining after the breaking of the N - H bond) from the photolysis of NH 3 to be utilized in heating the gas mixture, initially at room temperature. This procedure should yield an upper bound for the gas temperature, since some of the residual energies is known to remain in the photolysis products, explicitly, the hot H atoms and internally excited NH 2. In case 1, if all the initial NH 3 molecules are assumed to be photolyzed into H and NH 2 and the excitation in the photolysis products are quenched before reaction proceeds, one predicts no ignition within a time interval of 1 s. The
CH4/O2/NH 3 Mixtures The results for the ignition of CH4/O2/NH 3 stoichiometric mixtures by NH 3 photolysis are summarized in Fig. 9. The ignition-delay time plotted here is an average for all data with incident laser energies above the bleaching threshold. The error bar indicates one standard deviation. Figure 9 shows that the ignition-delay time depends strongly on the initial NH 3 mole fraction or the laser-energy-deposition density. The minimum NH 3 mole fraction needed for ignition is measured to be - 1 . 5 % + 0.1% and the corresponding minimum ignition-energy density is 3 8 0 _ 30 m J / c m 3. These values are higher than those for the H 2//O2/NH3 mixtures.
Kinetic Modeling of H~/O~JNH 3 Ignition Ignition of H2fO2/NH 3 mixtures by NH 3 photolysis is simulated by using a zero-dimensional version of the CHEMKIN code [35]. The zerodimensional modeling, which assumes no loss of
LASER'ENERGY-DEPOSITION DENSITY, m J / c m 3 600
400 500
1
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1000
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1.5
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2.0
2.5
3.0
3.5
4.0
4.5
NH 3 MOLE FRACTION. %
Fig. 9. Ignition-delay-time in CH4/O2fNH 3 stoichiometric mixtures as a function of initial NH 3 mole fraction and laser-energy-deposition density. The error bar indicates one standard deviation in the spread of measured data points. The shaded area indicates approximately the region of no ignition.
198
M.-S. CHOU AND T. J. ZUKOWSKI TABLE
1
Computer Modeling of H 2 / 0 2 / N H 3 Ignition by NH 3 Ph°t°ly sisa Case No. 1
2
3
Gas Composition (mole fraction) He 02 H NH 2 H2 02 NH 2 H O OH H2 O2 NH 2 O OH
0.651 0.325 0.012 0.012 0.651 0.324 0.012 0.0096 0.0024 0.0024 0.643 0.321 0.012 0.012 0.012
Gas Temperature
IgnitionDelay Time
Remarks on Gas Composition
369
> 1s
Photolysis products of H and NH 2 are thermalized before reaction proceeds.
369
> 1s
20% of H reacted via reaction 1.
369
110/~s
All H atoms reacted via reaction 1.
aThis initial NH 3 mole fraction of 0.012 is assumed to photolyze entirely into H + NH 2 under a bleaching condition. Various fractions of H atoms are also assumed to be converted further into O and OH before reaction proceeds.
failure for the mixture to ignite may be explained by the likelihood that thermal H atoms recombine rapidly with 02 to form unreactive HO 2 at such a low temperature (369 K). As discussed in the review section, 20% of the H atoms produced by the NH 3 photolysis [22, 23] should have translational energy above the threshold for reaction 1 [24]. In case 2, 20% of the H atoms are assumed to be converted to OH and O via reaction 1. However, no ignition is predicted within a time interval of 1 s. In case 3, if all the H atoms from the NH 3 photolysis are assumed to be converted into OH and O, one calculates an ignition-delay time of 110/zs. This is still too long in comparison with the observed value of - 2 6 + 9 /~s. Although we have not been able to predict an ignition-delay time in agreement with that observed even under a very optimistic condition, these calculations suggest that the excitation in the photolysis products, such as hot H atoms, may play an important role in enhancing the initiation of the combustion. Figure 10 shows the variation of the calculated ignition-delay times as functions of the fuel equivalence ratio and initial radical concentrations. The initial concentrations of O, OH, and NH 2 are taken to be identical. This implies that all the H atoms from the photolysis of NH 3 are assumed to convert rapidly into O and OH. This may be an oversimplifying assumption, as dis-
cussed in the review section. However, it may be adopted for the purpose of predicting the trend of the dependence of the ignition-delay time. The calculated ignition-delay time appears to decrease drastically with increase in the initial radical concentration. However, it varies only slightly with the fuel equivalence ratio for values in the range of 0.35 to 3.0. The predicted dependence of the I
I
I
I
/
MIXTURES 10-3 NH 2 , O, OH = 0.01
>,.
:5
J
Z
_o i-
10-4 --
0,012
Z
J 0.014
10-5
I 0.5
I 1.0
I I I 1.5 2.0 2.5 FUEL EQUIVALENCE RATIO
3.0
Fig. 10. Calculated ignition-delay time for H2/O 2 mixtures under the addition of NH2, O, and OH radicals, as functions of fuel equivalence ratio, for an initial temperature of 369 K and an initial pressure of 1 atm. The initial mole fractions of NH 2 , O, and OH are taken to be equal, and to be equal to 0.01, 0.012, and 0,014 in the three calculated cases.
IGNITION BY EXCIMER-LASER PHOTOLYSIS OF NH 3 ignition-delay time on the radical concentration and the fuel equivalence ratio appears to be qualitatively consistent with the experimental results. DISCUSSION
Stoichiometric Dependence The observation that the minimum ignitionenergy density and ignition-delay time are insensitive to the fuel equivalence ratio is of interest. Lucas et al. [9] have also shown that the minimum ignition-energy density is insensitive to the equivalence ratio for values between 0.7 and 3.3, for the ignition of H2/air/O 3 by the photolysis of 0 3. Lavid and Stevens [8] have shown that the minimum ignition-energy density has a minimum for an equivalence ratio near 0.6 and increases only moderately at 0.4, 1.0, and 1.6, for the ignition of H2,/O 2 by the photolysis of 0 2. More recently, Arnold et al. [37] have reported that the minimum ignition-energy density is insensitive to the equivalence ratio for the thermal ignition of H2/O2/O 3 with excitation achieved by use of a CO 2 laser operated at 9.552 #m. All the above methods have dealt with homogenous ignition of gas mixtures in a relatively large ignition volume. The insensitivity of the ignition-delay time to the fuel equivalence ratio for the H2/O2/NH3 mixtures is consistent with the kinetic modeling prediction based on homogenous chemical reactions, as shown in Fig. 10. Sloane predicted the insensitivity of the minimum ignition-energy density for CHa/air mixtures under the addition of atomic oxygen [11]. In modeling thermal ignition of H2/O 2, Maas and Warnatz [38] have shown explicitly the effects of ignition volume on the minimum ignition-energy density. The minimum ignition-energy density is quite sensitive to the fuel equivalence ratio for a small ignition volume and becomes relatively insensitive for a relatively large ignition volume with a diameter greater than - 1 mm. Such insensitivity to the fuel equivalence ratio under homogenous laser photolysis or thermal ignition is in remarkable contrast to the results achieved by spark-ignition [3] and laser-inducedbreakdown [4-7] methods, for which the mini mum ignition energy appears to be at a lowest value near the stoichiometric ratio and to increase drastically on both the rich and the lean side
199
of the stoichiometric ratio. Both of the sparkignition and laser-induced-breakdown methods have been used for ignition with an extremely small spot size only. For such a small spot, transport processes may control the mixture ignitability. According to Lewis and von Elbe [3], the criterion for spark ignition is that the energy added to the gas must be sufficient to heat a volume of gas of about the thickness of a steadily propagating flame to the adiabatic flame temperature. If the flame thickness is related to the laminar burning velocity, the ignition-delay time becomes a strong function of laminar burning velocity. The laminar burning velocity attains a maximum near the stoichiometric ratio, and decreases off the stoichiometric ratio. This fact may explain the strong dependence of the minimum ignition energy on the fuel equivalence ratio. Recently, Sloane [13] and Sloane and Schoene [14] have been able to predict correctly the nature of the dependence of the minimum ignition energy on the equivalence ratio by carrying out time-dependent modeling calculations in spherical coordinates, with detailed chemistry. The main cause for the different behavior with varying equivalence ratio among these various ignition methods appears to involve the ignition spot size. Unlike the spark-ignition and laserinduced-breakdown methods, the present method of ignition by laser photolysis is volumetric and not complicated by the transport processes. Hence, both the minimum ignition-energy density and ignition-delay time appear to be relatively insensitive to the equivalence ratio.
Radical Species Effects Table 2 compares the minimum ignition parameters for several ignition processes. The radicalpair densities of NH 2 and H required for ignition under the photolysis of NH 3 are - 1.3 +_ 0.1 × 1017, 1.8 ___0.2 × 1017, and 3.5 ___0.3 x 1017 cm -3 for stoichiometric mixtures of Hz/O2, H J a i r , and CH4//O2, respectively. These are derived from the measured minimum ignitionenergy densities of - 137 +_ 8, 190 ___20, and 380 +_ 30 m J / c m 3, respectively. These radicalpair densities appear to be comparable to those for the photolysis of 0 3 [9] and 0 2 [8]. This comparison implies that the radical pairs of NH 2 and H from the photolysis of NH 3 are as reactive
200
M.-S. CHOU AND T. J. ZUKOWSKI TABLE 2
Comparison of Ignition Conditions for Stoichiometric Fuel/Oxidizer Mixtures by Various Methods Photolysis Fuel/Oxidizer H2/O2 H2/Air CH4/O2
NH3 a H and NH 2 (cm-3)
03 b O atoms (cm-3)
O2 c O atoms (cm -3)
Spark Ignition a (/zJ)
1.3 + 0.1 × 1017 1.8 ___0.2 × 1017 3.5 + 0.3 × 1017
0.96 × 1017
3.0 × 1017 3.9 × 1017
4.2 18.9 4.2
3.2 × 1017
aThis work, at 1 atm. bReference 9, at - 20-200 torr. CReference 8, at - 16-30 torr. dReference 3, at 1 atm.
as the O atoms from the photolysis of 03 or O z. The NH 2 from the photolysis of NH 3 should not be reactive with 02 or H2, as discussed in the review section. If the H atoms from the photolysis of NH 3 were thermalized rapidly, they would tend to recombine with 02 to form unreactive HO2, via reaction 2. Our kinetic modeling calculations have indeed predicted no ignition if the H atoms were thermalized. Our calculations further imply that the hot H atoms, which can react with 02 to form reactive O and OH via reaction 1, may be responsible for observed enhanced ignition. Maas and Warnatz [38] have recently modeled thermal ignition of H 2/02 mixtures and predicted a minimum ignition-energy density of - 4 3 0 m J / c m 3 in a relatively large ignition volume. This value appears to be substantially higher than that required for ignition under the photolysis of NH 3, 03 [9], or 02 [8]. This suggests that the photolysis processes are perhaps more efficient than the thermal. In other words, less energy may be required to generate radicals than to heat the whole gas mixture, to achieve ignition.
Dilution Effects Our results show that the minimum ignitionenergy density increases only moderately, from 137 ___ 8 m J / c m 3 for H2/O 2 mixtures to 190 _ 20 m J / c m 3 for H2/air mixtures, for ignition achieved by the photolysis of NH 3. This observation corresponds to an increase in the required H and NH 2 radical concentration from - 1 . 3 ___ 0.1 × 1017 tO 1.8 __+ 0 . 2 × 1017 c m - 3 , a s shown in Table 2. Such a moderate increase is also
observed in ignition achieved by the photolysis of 02 [8]. In contrast, the minimum ignition energy increases more significantly, from 4.2 to 18.9/,J, in spark ignition. In spark ignition, energy is needed to raise the gas mixture above a critical temperature. The thermal inertia of the diluent gas of N 2 can result in such a drastic increase in the required energy. On the other hand, the photolysis processes, which do not rely upon direct heating of the gas mixture, appear to be relatively insensitive to the diluent gas N 2.
Fuel Species Effects Table 2 shows that the radical-pair densities required for ignition increase substantially from 1.3 + 0.1 × 1017 cm -3 for H2/O2 to - 3.5 + 0.3 x 1017 cm -3 for CH4/O 2 mixtures for ignition achieved by the photolysis of NH 3. This trend is consistent with that observed for ignition achieved by the photolysis of 03 [9]. In contrast, the minimum ignition energy in spark ignition is about the same for both gas mixtures. This result again shows that the ignition achieved by photolysis differs from ignition achieved by spark ignition.
CONCLUSION Volumetric homogeneous ignition of H 2 / O2/NH 3, H2/air/NH 3 and CHn/O2/NH 3 mixtures in an open flow system initially at 1 atm and room temperature is achieved via photolysis of NH 3. The emission spectrum monitored by an optical multichannel analyzer shows no dis-
IGNITION BY E X C I M E R - L A S E R PHOTOLYSIS OF NH 3
cernible atomic or ionic emission, an observation that excludes gaseous breakdown as a cause of ignition. The ignition-delay times based on the 308-nm OH emission at three locations appear to be of comparable value, an observation that suggests the presence of well-distributed ignition sites within the entire irradiated volume. The ignition delay time depends strongly on the initial NH 3 mole fraction and is nearly independent of the f u e l equivalence ratio for values between 0.35 and 3.0 for H z / O 2 / N H 3 mixtures. The minim u m ignition-energy density is also insensitive to the equivalence ratio. Furthermore, the required m i n i m u m ignition-energy density (137 ___ 8 m J / c m 3) appears to be substantially less than that required for ignition by a thermal process ( - 430 mJ/cm3), for a stoichiometric H2/O 2 mixture. Our kinetic modeling calculations show that if the hot H atoms from the photolysis of NH 3 were collisionally quenched rapidly, ignition should not have occurred. This result implies that the excitation energy that remained in the initial products of photolysis may play an important role in enhancing the ignition. However, the detailed mechanism leading to such an enhanced ignition is not yet well understood.
The authors acknowledge Mr. E. Y. Wong (TR IV) and Dr. J. Y. Chen (Sandia National Laboratories) f o r assistance with computer modeling calculations, Mr. E. I4/. Rogala f o r technical assistance in gathering C H 4 / O 2 / N H 3 ignition data, and Drs. F. Fendell (TR IV) and T. Sloane (General Motors) f o r helpful discussions. This work was sponsored by the A i r Force Office o f Scientific Research, A i r Force System Command, USAF, under Contract F49620-87-C-0081. The contract technical monitor was Dr. J. Tishkoff.
REFERENCES 1. Farkas, H. L., Haber, F., and Harteck, P., Z. Elektrochem. 36:711-714 (1930). 2. Taylor, H. S., and SaUey, D. J., Am. Chem. Soc. J. 55:96-109 (1933). 3. Lewis, B., and von Elbe, G., Combustion, Flames and Explosions of Gases, 3rd ed., Academic, New York, 1987. 4. Weinberg, F. J., and Wilson, J. R., Proc. R. Soc. Lond. A 321:41-52 (1971).
201
5. Syage,J. A., Fournier, E. W., Rianda, R., and Cohen, R. B., J. Appl. Phys. 64:1499-1507 (1988). 6. Forch, B. E., and Miziolek, A. W., Combust. Sci. Technol. 52:151-159 (1987). 7. Forch, B. E., and Miziolek, A. W., Opt. Lett. 11:129-131 (1986). 8. Lavid, M., and Stevens, J. G., Combust. Flame 60:195-202 (1985). 9. Lucas, D., Dunn-Rankin,D., Horn, K., and Brown, N. J., Combust. Flame 69:171-184 (1987). 10. Guirguis,R. H., Oppenheim, A. K., Karasalo, I., and Creighton, J. R., Prog. Aeronaut. Astronaut., 76:134-153 (1981). 11. Sloane, T. M., Combust. Sci. Technol. 34:317-330 (1983). 12. Sloane, T. M., Combust. Sci. Technol. 42:131-144 (1985). 13. Sloane, T. M., Combust. Sci. Technol. 63:287-313 (1989). 14. Sloane, T. M., and Schoene, A. Y., Twenty-Second Symposium (International) on Combustion, The CombustionInstitute, Pittsburgh, 1988, pp. 1669-1676. 15. Vaida, V., McCarthy, M. I., Engelking, P. C., Rosmus, P., Werner, H. J., and Botschwina, P., J. Chem. Phys. 86:6669-6676 (1987). 16. Rosmus, P., Botschwina, P., Werner, H. J., Vaida, V., Engelking, P. C., and McCarthy, M. I., J. Chem. Phys. 86:6677-6692 (1987). 17. Okabe, H., Photochemistry of Small Molecules, Wiley, New York, 1978. 18. McCarthy, M. I., Rosmus, P., Werner, H. J., Botschwina, P., and Vaida, V., J. Chem. Phys. 86:6693-6700 (1987). 19. Kenner, R. D., Rohrer, F., and Stuhl, F., J. Chem. Phys. 86:2036-2043 (1987). 20. Thompson, B. A., Harteck, P., and Reeves, R. R., J. Geophys. Res. 68:6431-6436 (1963). 21. Gibson, S. T., Greene, J. P., and Berkowitz, J., J. Chem. Phys. 83:4319-4328 (1985). 22. Biesner, J., Schnieder, L., Schmeer, J., Ahlers, G., Xie, X., Welge, K. H., Ashfold, M. N. R., and Dixon, R. N., J. Chem. Phys. 88:3607-3616 (1988). 23. Biesner, J., Schnieder, L., Ahlers, G., Xie, X., Welge, K. H., Ashfold, M. N. R., and Dixon, R. N., J. Chem. Phys. 91:2901-2911 (1989). 24. Miller,J. A., J. Chem. Phys. 74:5120-5132 (1981). 25. Donnelly,V. M., Baronavski, A. P., and McDonald, J. R., Chem. Phys. 43:271-281 (1979). 26. Kenner, R. D., Rohrer, F., Browarzik, R. K., Kaes, A., and Stuhl, F., Chem. Phys. 118:141-152 (1987). 27. Kenner,R. D., Browarzik, R. K., and Stuhl, F., Chem. Phys. 121:457-471 (1988). 28. Hofzumahaus, A., and Stuhl, F., J. Chem. Phys. 82:5519-5526 (1985). 29. Donnelly,V. M., Baronavski, A. P., and McDonald,J. R., Chem. Phys. 43:283-293 (1979). 30. Halpern,J. B., Hancock, G., Lenzi, M., and Welge, K. H., J. Chem. Phys. 63:4808-4816 (1975).
202
31. 32. 33. 34. 35.
M . - S . C H O U A N D T. J. Z U K O W S K I
Kleinermanns, K., and Wolfrum, J., J. Chem. Phys. 80:1446-1450 (1984). Kleinermanns, K., and Linnebach, E., J. Chem. Phys. 82:5012-5017 (1985). Kleinermanns, K., and Schinke, R., J. Chem. Phys. 80:1440-1445 (1984). Bronikowski, M. J., Zhang, R., Rakestraw, D. J., and Zare, R. N., Chem. Phys. Lett. 156:7-13 (1989). Kee, R. J., Miller, J. A., and Jefferson, T. H., Report No. SAND 80-8003, Sandia National Laboratory, Livermore, CA, 1980.
36. 37.
38.
Glarborg, P., Miller, J. A., and Kee, R. J., Combust. Flame 65:177-202 (1986). Arnold, A., Hemberger, R., Herden, R., Ketterle, W., and Wolfrum, J., Twenty-Third Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, pp. 1783-1788. Maas, U., and Warnatz, J., Combust. Flame 74:53-69 (1988).
Received 5 April 1991; revised 27 July 1991
AND
FLAME
87: 191-202 (1991)
191
Ignition of H,/02/NH,, Hz/Air/NH, and CH,/OJNH, Mixtures by Excimer-Laser Photolysis of NH, MAU-SONG
CHOU
and TMITRI J. ZUKOWSKI
TR W Space & Technology Group, One Space Park, Redondo Beach, CA 90278 Volumetric ignition of H^JO,, Ha/air, and CHJO, mixtures (in an open flow system, initially at 1 atm and room temperature) is achieved via photolysis of a small amount of NH, present in the flow mixtures. The photolysis is accomplished by an ArF excimer laser operated at 193 nm. The ignition appears to be homogenous, since the ignition-delay times measured at several locations are approximately identical. The minimum ignition-energy density is measured to be 137 f 8, 190 f 20, and 380 + 30 ml/cm3 for stoichiometric mixtures of HJOdNH,, HJair/NH3, and CHJOJNH,, respectively, and appears to be insensitive to fuel-equivalence-ratio values between 0.35 and 3.0. The ignition-delay time depends strongly on both the initial NH, concentration and the laser-energy-deposition density. However, the ignition-delay time is not sensitive to the fuel equivalence ratio. The measured minimum ignition-energy density appears to be substantially lower than that predicted by a kinetic modeling calculation. This result implies that the hot H atoms produced by the photolysis of NH, may play an important role in facilitating the ignition.
INTRODUCTION ‘Photochemical ignition provides an excellent opportunity to study the details of many chemical processes in combustion, since energy can be directed homogeneously and within a brief time interval to achieve a well-characterized initial condition. Earlier experiments by Farkas et al. [l] have shown that HJO, mixtures containing a small amount of NH, become explosive at a moderate temperature (- 420°C) under ultraviolet radiation. The reaction appears to proceed much more rapidly under the photolysis of NH, than by other means of photogeneration of H atoms, such as the photosensitized mercury atoms -that dissociate H, into 2 H [2]. It was speculated that NH, may play a role in the ignition chemistry; however, the detailed reaction mechanism remains poorly understood [3]. Recent advances in high-energy ultraviolet lasers have stimulated renewed interest in photo*chemical ignition, since these lasers can generate sufficient radicals by photolysis to initiate combustion in an exceptionally controlled way. Laser-spark has been used to ignite fuel-oxidizer mixtures, by either gaseous breakdown 14, 51 or multiphoton processes [6, 71, within a tightly focused volume. Absorption of laser photons that produce reactive radical species has also been used to initiate combustion without spark formaCopyright 0 1991 by The Combustion Institute Published by Elsevier Science Publishing CO., Inc.
tion. For example, Lavid and Stevens [8] have used a F, laser, operated at 157 nm to photolyze O,, as a mean of igniting H,/O, mixtures, and Lucas et al. [9] have used a KrF laser, operated at 248 nm to photolyze 0,, as a mean of igniting H,/O,, CHJO, 9 and C,H, /O, mixtures. The related subject of ignition under the addition of radicals has been studied theoretically by Guirguis et al. [lo], Sloane [ll-131, and Sloane and Schoene [ 141. Here we study ignition initiated by the photolysis of NH,, through the use of an ArF laser operated at 193 nm, in mixtures of HJOJNH,, HJair/NH,, and CHJOJNH,. We have also performed kinetic modeling calculations of the ignition of H,/O*/NH, by photolysis. Our calculations suggest that highly energetic photofragments, such as hot H atoms, may play an important role in the initiation of combustion.
REVIEW OF RELEVANT PHOTOCHEMISTRY Light of 193 nm wavelength excites NH, into [15, 161, V; = 6 of the AlA’; + *:‘A’, transition and the excited NH, dissociates into NH, and H with nearly unit quantum efficiency [ 17, 181. The molar extinction coefficient for NH, at 193 nm is 3250 L/mol-cm [19]. This value is much higher OOlO-2180/91/$3.50
192
M.-S. CHOU AND T.J. Z U K O W S ~
than that for O 2 [20]. The absorption by O 2 is estimated to be less than about 1% of the absorption by NH 3 under our experimental conditions. The bond dissociation energy for NH3, D0(HNH2), is - 4 4 6 . 4 kJ/mol [21, 22]; hence there is about 171.5 kJ/mol of residual energy left from the 193-nm photons. Biesner et al. [23] have measured the distribution of the translational energy of the H atoms from the photolysis of NH 3 at - 193.6 nm. About 20% of the H atoms produced have translational energy above 52 kcal/mol, which is about the threshold energy required for the reaction of H with 02 [24]. These investigators [23] also derived a branching ratio of 0.33 for the yield of N H 2 ( A ) to N H 2 ( X ). However, a lower branching ratio (0.025) was reported earlier by Donnelly et al. [25] on the basis of the NH 2 emission that follows the 193-nm p h o t o l y s i s . o f NH 3. The N H z ( X ) is produced with a relatively high internal excitation [23, 25]. At higher laser intensity, N H ( A ) can also be produced by a sequential two-photon absorption process, via an internally excited intermediate state of N H 2 ( ) ? ) [26, 27]. The molar extinction coefficient for the absorption of the intermediate NH2()~') at 193 nm was measured to be - 10.4 L mol-~ cm-1 by Hofzumahaus and Stuhl [28]. The internally excited NH2()~" ) from the photolysis of NH 3 appears to be unreactive with 02 or H e [25]. The N H 2 ( A ) from the photolysis of NH 3 has two components: - 85% in a short-lived component, and - 1 5 % in a long-lived component [29]. The former is postulated to be pure NH2(,,t'), while the latter is postulated to be a mixed state of N H z ( A ) and NH2(,~ ). The longlived component is unreactive with 0 2 [25]. Quenching cross sections for the short-lived component are generally quite high, with several collisional partners, including NH3, H2, Ar, and He [26, 30]; hence, the loss of the short-lived component is likely to be by collisional quenching, rather than to reaction with 0 2 . The translationally hot H atoms from the photolysis of NH 3 may enhance reaction with 0 2 [24, 31-34]: H+O 2--*OH+O.
(1)
According to a theoretical study by Miller [24], the reaction cross section has a threshold energy
for the translational energy of - 52 kJ/mol, and increases gradually to a broad maximum of - 0.4 /~2 for translation energies between 105 and 209 kJ/mol. From the measured distribution of the translational energy of H atoms by the photolysis of NH 3 [23] and the theoretical reaction cross section [24], an effective rate constant of - 9 . 4 × 1012 cm3/mol-s for reaction 1 can be derived. A major competing reaction for the H atoms is reaction 2, which may be considered to bechain-breaking reaction, since HO 2 is not very reactive: H + 0 2 + M ~ HO 2 + M.
(2)
The product of the rate constant of reaction 2 [34] and the gas density ( M ) at 1 atm is relatively lower, about 3.6 × 1012 cm3/mol-s. This argument suggests that 20% mole fraction of the H atoms produced by the photolysis of NH 3 may react favorably via reaction 1.
EXPERIMENTAL PROCEDURE The experimental arrangement is shown in Fig. 1. A Lambda Physik Excimer Laser (150 EST), which provides ArF-laser radiation at 193 nm, was used as a photolysis source. This excimerlaser system consists of two laser units. The output from an unstable resonator of the first unit was amplified through the second unit to achieve a high energy output with a relatively low beam divergence. An ultraviolet-grade MgF 2 lens with a 50-cm focal length was used to deliver a laser beam with a rectangular profile of about 2.1 mm in width × 7.1 mm in height at the leading edge, and 1.2 mm in width × 3.3 mm in height at the trailing edge, of a 6-cm-diameter fiat-flame burner (McKenna Products). The spot size of the laser beam was determined by the burn spots on Polaroid film. The focus of the laser beam was far past the burner to avoid any gaseous breakdown in the mixtures. The incident as well as the transmitted laser energies were monitored by the use of quartz-flat beamsplitters and laser-energy-ratiometer probes (Laser Precision Model 7200). The incident laser energy was in the range of 100-290 mJ per pulse. The OH emission was monitored at three locations along the beam path: 2.2, 3.0, and 3.8
IGNITION BY EXCIMER-LASER PHOTOLYSIS OF NH 3
;
I i P
QF
EXCIMERLASER ABSORBER (193 nm) / ~ LASERDETECTOR / / ~'-/
.~:~ / ~ -\,,,~
/
_
_
FLAT-FLAME
BURNER
./ / / ~, ~'\ ~.< / / ~ ~ \ ,'/'~ ~ '~\ \ ,/ / / ~ ~
ArF
-
193
//
PMT ~ /
!
OH FILTER I I
;p I I
~ ~ ~ I ~ \\ \ \\ \~ ~c~
........ Mrz~urilSlCM 193 nm NBF
r [ LASER DETECTOR OH FILTER
Fig. 1. Schematic of the experimental arrangement for the ignition of combustible mixtures by ArF excimer laser photolysis of NH 3. M, mirror; QF, quartz flat; NBF, narrow-band filter; and PMT, photomultiplier tube.
cm from the leading edge of the burner. The center location (3.0 cm) was observed with a m o n o c h r o m a t o r and photomultiplier tube, whereas the other two locations were monitored with a bandpass filter (with center at 310 nm and with 10-nm bandwidth) and photomultiplier. The gaseous mixtures were prepared in a stainless-steel manifold and flowed through the flat-flame burner. A coaxial gaseous shroud of N 2 was used to minimize any mixing of the combustible flow with the surrounding air. Matheson anhydrous-grade ammonia (99.99%), hydrogen (99.995%), and 02 (99.5%) were used. The total gas flow was 3.75 L/min for both H2/O2/NH 3 and CHa/O2/NH 3 mixtures, and 8.75 L/min for H 2/air/NH 3 mixtures. Ignition experiments were conducted on the premixed gases with the excimer-laser beam at 1.5 cm above the flat-flame burner. All the data were collected in the open system at 1 atm and room temperature. The majority of the experiments were carried out under bleaching conditions, for which the incident excimer-laser energy was sufficiently high to generate a bleaching wave and photolyze all the NH 3 along the laser-irradiated pathlength. The bleaching condition can be achieved here because the absorption coefficient of NH 3 is relatively high, whereas the typical NH 3 concentration employed here was relatively low. Under bleaching conditions, relatively uniform radical density within the irradiated volume can be pro-
duced in spite of any nonuniformity or hot spots in the incident laser beam. Numerous observations with an optical multichannel analyzer were made to analyze the emission spectrum. The emission during ArF-laser irradiation was found to be composed mainly of the NH band at 336 nm and NH 2 at - 4 5 0 - 7 4 0 nm. No atomic or ionic emission lines indicative of plasma formation are observed. This observation precludes gaseous breakdown as a cause of ignition. RESULTS
H2/O2/NH3 Mixtures The results for the ignition of H2/O2/NH 3 mixtures by the photolysis of NH 3 are summarized in Figs. 2 - 7 . Figure 2 shows typical OH-emission temporal profiles in two different time scales. Figure 2a shows that the OH emission occurs at a certain delay time after the irradiation of the excimer laser. Figure 2b shows that the OH emission reaches a maximum and then decreases to a relatively low steady-state value at later times. The intensity of the early OH emission peak is about an order of magnitude higher than the steady-state value. The OH emission occurs only if the mixture is ignited. An ignition-delay time can therefore be assigned on the basis of the onset of the OH
194
M.-S. CHOU AND T. J. ZUKOWSKI
T=O
b
T--O
a
i
\
\
/.~_-., ~"
\
,
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,
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500 ps/DIV, 20 mv/DIV
Fig. 2. Typical OH emission profiles at two different time scales following ArF excimer laser irradiation: (a) rise of delayed OH emission and (b) rise and fall of the delayed OH emission. The start of the excimer-laser pulse is marked by T = 0. The OH-emission intensity increases in the downward direction.
emission. Figs. 3 - 5 summarize the ignition-delay times, which are measured at three locations: near the leading edge (2.2 cm), at the center (3.0 cm) and near the trailing edge (3.8 cm) above the fiat-flame burner. The ignition-delay times are plotted as functions of incident laser energy for three initial NH 3 mole fractions of 0.65%, 0.83%, and 1.35% in Figs. 3, 4, and 5, respectively. At the lower NH 3 mole fractions of 65 % and 0.83%, all the incident laser energies ( - 1 4 0 - 2 6 0 mJ) are sufficient to generate a bleaching wave through the entire 6-cm path-
length above the burner. At the higher NH 3 mole fraction of 1.35%, the laser energy required ( - 1 5 7 mJ) to generate a bleaching wave is higher, and only a portion of the data in Fig. 5 pertains to bleaching conditions. For incident laser energy above the bleaching threshold, all NH 3 within the irradiated volume should be dissociated and the density of deposited laser energy as well as the radical concentration should remain constant regardless of the increas-
H2/O2/NH 3 200
• H2/O2/NH 3
500
¢,
~m 100
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%
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• 2.2 cm • 3.0 cm • 3.8 cm
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t 200
I 220
I 240
10
260
INCIDENT LASER ENERGY, mJ
Fig. 3. Ignition-delay time in H2/O2/NH 3 stoichiometric mixtures with 0.0065 mol fraction of NH 3, as a function of incident laser energy, from the OH emission measurements at three locations: (V) 2.2 cm, (O) 3.0 cm, and (&) 3.8 cm downstream of the burner leading edge.
120
I
I
l
L
I
i
140
160
180
200
220
240
iNCIDENT LASER ENERGY, mJ
Fig. 4. Ignition-delay time in H2/O2/NH 3 stoichiometric mixtures with 0.0083 mol fraction of NH 3, as a function of incident laser energy, from the OH emission at three locations: (V) 2.2 cm, (O) 3.0 cm, and (&) 3.8 cm downstream of the burner leading edge.
IGNITION BY EXCIMER-LASER PHOTOLYSIS OF NH 3 LASER-ENERGY-DEPOSITION DENSITY. mJ/cm 3
fl2/O2/NH3 500
195
100
200
ooo t
1.35% moleNH3
300
400 H2/O2/NH 3
V
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100
t-.
50 120
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e 4,- zo
•
l TI I 1 160 fRO 200 INCIDENTLASERENERGY.mJ
I 140
1 240
t
X
260
Fig. 5. Ignition-delay time in H2/O2/NH 3 stoichiometric mixtures with 0.0135 mol fraction of NH3, as a function of incident laser energy, from OH emission at two locations: (o) 3.0 cm and (&) 3.8 cm downstream of the burner leading edge. The arrow indicates the minimum incident-laser energy needed to bleach out the NH 3 absorption up to the 3.8-cm location.
0.5
1.0
1.5
NH3 MOLE FRACTION,%
Fig. 6. Ignition-delay time in H 2 / O J N H 3 mixtures as a function of initial NIl3 mole fraction and laser-energy-deposition density, for several fuel equivalence ratios: (&) 0.35, ( T ) 0.5, (©) 1.0, (O) 2.0, and (&) 3.0. The error bar given for the cases with fuel equivalence ratio of 1.0 indicates one standard deviation in the spread of the measured data points. The shaded area indicates approximately the region of no ignition.
ing values of the incident laser energy. Therefore, the ignition-delay time should also remain constant. At sufficient high NH 3 mole fractions, ignition-delay times are indeed nearly independent of incident laser energy, within the uncertainty of our experimental data, as shown in Fig. 4 and also in Fig. 5 for those data with incident laser energy above the bleaching threshold. However, for NH 3 mole fractions below a certain level, as shown in Fig. 3, the ignition-delay time appears to decrease more substantially with increasing incident laser energy. This apparent de-
pendency at a low NH 3 mole fraction may be attributed to the fact that as the radical concentration is reduced to nearly the minimum value needed for ignition, the ignition becomes more sensitive to multiphoton processes. One of the important multiphoton processes is the sequential two-photon-absorption process, by which NH 3 is dissociated into NH and 2 H, as discussed in the review section. The quantum yield for this proc-
E
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a~
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H2/02/NH3 t~ --r
z
Z
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~
0.2
50-
Z
z_
o
I
i
I
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i
0.5
1.0
1.5
2.0
2.5
0
3.0
FUEL EQUIVALENCE RATIO Fig. 7. Minimum ignition-energy density and minimum mole fraction of Nil 3 needed for ignition of H2/O2/NH 3 mixtures, for several fuel equivalence ratios.
196 ess is estimated to be - 1.8 × 10 - 3 at 140 mJ and - 3.5 x 10 -3 at 260 rnJ of the incident laser energy. Although these values are only moderate in comparison with the value of nearly unity for the single-photon process, they may become important if the radical concentration is just barely sufficient for ignition. Figures 3 - 5 show that the ignition-delay times at three different locations appear to be of comparable value for those test runs under bleaching conditions. Although there are some variations among the three different locations, the variations appear to be statistically random. Furthermore, they are too small to be significant for flame propagation from a single ignition site. This argument suggests that the ignition is nearly homogeneous and there are probably well distributed ignition sites within the entire laser-irradiated volume. Figure 6 shows the ignition-delay time as a function of initial NH 3 mole fraction, for several values of fuel equivalence ratio. Also shown on the abscissa is the laser-energy-deposition density, a quantity that is derived based on that required to photodissociate all NH 3 density present initially in the mixtures. This derived laserenergy-deposition density agrees well with the measured difference in the incident and transmitted laser energy upon division by the total irradiated volume. The ignition-delay time plotted here is an average for all data with incident laser energy above a bleaching threshold (as shown in Figs. 3-5). Some uncertainty may arise, for those events with low initial NH 3 mole fractions, by use of the average value, owing to the observed variation with the incident laser energy. This uncertainty, however, appears to be relatively unimportant in view of the much steeper variation of the ignition-delay time with the initial NH 3 mole fraction. The error bar given for the cases with a stoichiometric mixture (q~ = 1.0) indicates one standard deviation in the spread of the measured data points. Figure 6 shows that the ignition-delay times are nearly insensitive to the fuel equivalence ratio for values between 0.35 and 3.0. Also shown in Fig. 6 is an approximate threshold value of laser-energy-deposition density or initial NH 3 mole fraction needed for ignition. Below this threshold no ignition occurs. Figure 7 summarizes the minimum ignition-energy density and the minimum NH 3 mole fraction needed for
M.-S. CHOU AND T. J. ZUKOWSKI ignition, for several equivalence ratios. The minimum ignition-energy density is taken to be the minimum laser-energy-deposition density needed for ignition. It is noted that the minimum igni-" tion-energy density is also insensitive to the fuel equivalence ratio for values between 0.35 and 3.0. We have also performed additional experiments with a larger laser beam, of nominal beam size - 2.1 mm × 8.5 mm. The ignition-delay time and the minimum ignition-energy density forthis condition are not significantly different from the corresponding values reported here.
H z/Air/NH 3 Mixtures The results for the ignition of H2/air/NH 3 stoichiometric mixtures by the photolysis of NH 3 are summarized in Fig. 8. The ignition-delay time plotted here is an average for all data with incident laser energies above the bleaching threshold. The error bar indicates one standard deviation. Figure 8 shows that the ignition-delay time de-" pends strongly on the initial NH 3 mole fraction or the laser-energy-deposition density. In comparison with the H2/O2/NH 3 mixtures, the ignitiondelay time is longer for the same laserenergy-deposition density. This result is plausible owing to the increase in the total heat capacity in the presence of excess N 2. However, Fig. 8 shows that the minimum ignition-energy density
DENSITY,mJ/cm 3
LASER-ENERGY-DEPOSITION
100
200 I
1000
300 I
400 I H2/AIR/NH 3 0 :I.G
id
"~
1110
10
NO IGNITION
I 0.5
I I.O
.
.
.
.
t 1.5
,
NH 3 MOLE FRACTION, %
Fig. 8. Ignition-delay time in H2/air/NH 3 stoichiometric mixtures as a function of initial NH3 mole fraction and laser-energy-deposition density. The error bar indicates one standard deviation in the spread of the measured data points. The shaded area indicates approximately the region of no ignition.
IGNITION BY EXCIMER-LASER PHOTOLYSIS OF NH 3 increases only moderately from - 137 + 8 m J / c m 3 in H2/O2/NH 3 to - 190 _+ 20 rnJ/cm 3 in H2/air/NH 3, at the stoichiometric ratio.
197
energy or species out of the irradiated volume, should yield a lower bound on the ignition-delay time and mixture ignitability. The H - O - N reaction mechanism, which comprises 20 species and 61 reactions, is adopted from a larger H - O - N - C mechanism published by Glarborg et al. [361. All the rate coefficients are adopted without any modification. The experimental condition with an initial NH 3 mole fraction of 0.012 under bleaching conditions is adopted in the following modeling simulation. Table 1 presents the modeling results for three cases of assumed initial radical concentrations. The gas temperature is estimated by taking all the available energy of the photon (remaining after the breaking of the N - H bond) from the photolysis of NH 3 to be utilized in heating the gas mixture, initially at room temperature. This procedure should yield an upper bound for the gas temperature, since some of the residual energies is known to remain in the photolysis products, explicitly, the hot H atoms and internally excited NH 2. In case 1, if all the initial NH 3 molecules are assumed to be photolyzed into H and NH 2 and the excitation in the photolysis products are quenched before reaction proceeds, one predicts no ignition within a time interval of 1 s. The
CH4/O2/NH 3 Mixtures The results for the ignition of CH4/O2/NH 3 stoichiometric mixtures by NH 3 photolysis are summarized in Fig. 9. The ignition-delay time plotted here is an average for all data with incident laser energies above the bleaching threshold. The error bar indicates one standard deviation. Figure 9 shows that the ignition-delay time depends strongly on the initial NH 3 mole fraction or the laser-energy-deposition density. The minimum NH 3 mole fraction needed for ignition is measured to be - 1 . 5 % + 0.1% and the corresponding minimum ignition-energy density is 3 8 0 _ 30 m J / c m 3. These values are higher than those for the H 2//O2/NH3 mixtures.
Kinetic Modeling of H~/O~JNH 3 Ignition Ignition of H2fO2/NH 3 mixtures by NH 3 photolysis is simulated by using a zero-dimensional version of the CHEMKIN code [35]. The zerodimensional modeling, which assumes no loss of
LASER'ENERGY-DEPOSITION DENSITY, m J / c m 3 600
400 500
1
800
I
I
1000
I
i
I
CH4/O2/NH 3 200 :1.o
uS
=_ e-=
i
100
50
O
m
20 GNITION 10
1.5
i
i
I
i
I
2.0
2.5
3.0
3.5
4.0
4.5
NH 3 MOLE FRACTION. %
Fig. 9. Ignition-delay-time in CH4/O2fNH 3 stoichiometric mixtures as a function of initial NH 3 mole fraction and laser-energy-deposition density. The error bar indicates one standard deviation in the spread of measured data points. The shaded area indicates approximately the region of no ignition.
198
M.-S. CHOU AND T. J. ZUKOWSKI TABLE
1
Computer Modeling of H 2 / 0 2 / N H 3 Ignition by NH 3 Ph°t°ly sisa Case No. 1
2
3
Gas Composition (mole fraction) He 02 H NH 2 H2 02 NH 2 H O OH H2 O2 NH 2 O OH
0.651 0.325 0.012 0.012 0.651 0.324 0.012 0.0096 0.0024 0.0024 0.643 0.321 0.012 0.012 0.012
Gas Temperature
IgnitionDelay Time
Remarks on Gas Composition
369
> 1s
Photolysis products of H and NH 2 are thermalized before reaction proceeds.
369
> 1s
20% of H reacted via reaction 1.
369
110/~s
All H atoms reacted via reaction 1.
aThis initial NH 3 mole fraction of 0.012 is assumed to photolyze entirely into H + NH 2 under a bleaching condition. Various fractions of H atoms are also assumed to be converted further into O and OH before reaction proceeds.
failure for the mixture to ignite may be explained by the likelihood that thermal H atoms recombine rapidly with 02 to form unreactive HO 2 at such a low temperature (369 K). As discussed in the review section, 20% of the H atoms produced by the NH 3 photolysis [22, 23] should have translational energy above the threshold for reaction 1 [24]. In case 2, 20% of the H atoms are assumed to be converted to OH and O via reaction 1. However, no ignition is predicted within a time interval of 1 s. In case 3, if all the H atoms from the NH 3 photolysis are assumed to be converted into OH and O, one calculates an ignition-delay time of 110/zs. This is still too long in comparison with the observed value of - 2 6 + 9 /~s. Although we have not been able to predict an ignition-delay time in agreement with that observed even under a very optimistic condition, these calculations suggest that the excitation in the photolysis products, such as hot H atoms, may play an important role in enhancing the initiation of the combustion. Figure 10 shows the variation of the calculated ignition-delay times as functions of the fuel equivalence ratio and initial radical concentrations. The initial concentrations of O, OH, and NH 2 are taken to be identical. This implies that all the H atoms from the photolysis of NH 3 are assumed to convert rapidly into O and OH. This may be an oversimplifying assumption, as dis-
cussed in the review section. However, it may be adopted for the purpose of predicting the trend of the dependence of the ignition-delay time. The calculated ignition-delay time appears to decrease drastically with increase in the initial radical concentration. However, it varies only slightly with the fuel equivalence ratio for values in the range of 0.35 to 3.0. The predicted dependence of the I
I
I
I
/
MIXTURES 10-3 NH 2 , O, OH = 0.01
>,.
:5
J
Z
_o i-
10-4 --
0,012
Z
J 0.014
10-5
I 0.5
I 1.0
I I I 1.5 2.0 2.5 FUEL EQUIVALENCE RATIO
3.0
Fig. 10. Calculated ignition-delay time for H2/O 2 mixtures under the addition of NH2, O, and OH radicals, as functions of fuel equivalence ratio, for an initial temperature of 369 K and an initial pressure of 1 atm. The initial mole fractions of NH 2 , O, and OH are taken to be equal, and to be equal to 0.01, 0.012, and 0,014 in the three calculated cases.
IGNITION BY EXCIMER-LASER PHOTOLYSIS OF NH 3 ignition-delay time on the radical concentration and the fuel equivalence ratio appears to be qualitatively consistent with the experimental results. DISCUSSION
Stoichiometric Dependence The observation that the minimum ignitionenergy density and ignition-delay time are insensitive to the fuel equivalence ratio is of interest. Lucas et al. [9] have also shown that the minimum ignition-energy density is insensitive to the equivalence ratio for values between 0.7 and 3.3, for the ignition of H2/air/O 3 by the photolysis of 0 3. Lavid and Stevens [8] have shown that the minimum ignition-energy density has a minimum for an equivalence ratio near 0.6 and increases only moderately at 0.4, 1.0, and 1.6, for the ignition of H2,/O 2 by the photolysis of 0 2. More recently, Arnold et al. [37] have reported that the minimum ignition-energy density is insensitive to the equivalence ratio for the thermal ignition of H2/O2/O 3 with excitation achieved by use of a CO 2 laser operated at 9.552 #m. All the above methods have dealt with homogenous ignition of gas mixtures in a relatively large ignition volume. The insensitivity of the ignition-delay time to the fuel equivalence ratio for the H2/O2/NH3 mixtures is consistent with the kinetic modeling prediction based on homogenous chemical reactions, as shown in Fig. 10. Sloane predicted the insensitivity of the minimum ignition-energy density for CHa/air mixtures under the addition of atomic oxygen [11]. In modeling thermal ignition of H2/O 2, Maas and Warnatz [38] have shown explicitly the effects of ignition volume on the minimum ignition-energy density. The minimum ignition-energy density is quite sensitive to the fuel equivalence ratio for a small ignition volume and becomes relatively insensitive for a relatively large ignition volume with a diameter greater than - 1 mm. Such insensitivity to the fuel equivalence ratio under homogenous laser photolysis or thermal ignition is in remarkable contrast to the results achieved by spark-ignition [3] and laser-inducedbreakdown [4-7] methods, for which the mini mum ignition energy appears to be at a lowest value near the stoichiometric ratio and to increase drastically on both the rich and the lean side
199
of the stoichiometric ratio. Both of the sparkignition and laser-induced-breakdown methods have been used for ignition with an extremely small spot size only. For such a small spot, transport processes may control the mixture ignitability. According to Lewis and von Elbe [3], the criterion for spark ignition is that the energy added to the gas must be sufficient to heat a volume of gas of about the thickness of a steadily propagating flame to the adiabatic flame temperature. If the flame thickness is related to the laminar burning velocity, the ignition-delay time becomes a strong function of laminar burning velocity. The laminar burning velocity attains a maximum near the stoichiometric ratio, and decreases off the stoichiometric ratio. This fact may explain the strong dependence of the minimum ignition energy on the fuel equivalence ratio. Recently, Sloane [13] and Sloane and Schoene [14] have been able to predict correctly the nature of the dependence of the minimum ignition energy on the equivalence ratio by carrying out time-dependent modeling calculations in spherical coordinates, with detailed chemistry. The main cause for the different behavior with varying equivalence ratio among these various ignition methods appears to involve the ignition spot size. Unlike the spark-ignition and laserinduced-breakdown methods, the present method of ignition by laser photolysis is volumetric and not complicated by the transport processes. Hence, both the minimum ignition-energy density and ignition-delay time appear to be relatively insensitive to the equivalence ratio.
Radical Species Effects Table 2 compares the minimum ignition parameters for several ignition processes. The radicalpair densities of NH 2 and H required for ignition under the photolysis of NH 3 are - 1.3 +_ 0.1 × 1017, 1.8 ___0.2 × 1017, and 3.5 ___0.3 x 1017 cm -3 for stoichiometric mixtures of Hz/O2, H J a i r , and CH4//O2, respectively. These are derived from the measured minimum ignitionenergy densities of - 137 +_ 8, 190 ___20, and 380 +_ 30 m J / c m 3, respectively. These radicalpair densities appear to be comparable to those for the photolysis of 0 3 [9] and 0 2 [8]. This comparison implies that the radical pairs of NH 2 and H from the photolysis of NH 3 are as reactive
200
M.-S. CHOU AND T. J. ZUKOWSKI TABLE 2
Comparison of Ignition Conditions for Stoichiometric Fuel/Oxidizer Mixtures by Various Methods Photolysis Fuel/Oxidizer H2/O2 H2/Air CH4/O2
NH3 a H and NH 2 (cm-3)
03 b O atoms (cm-3)
O2 c O atoms (cm -3)
Spark Ignition a (/zJ)
1.3 + 0.1 × 1017 1.8 ___0.2 × 1017 3.5 + 0.3 × 1017
0.96 × 1017
3.0 × 1017 3.9 × 1017
4.2 18.9 4.2
3.2 × 1017
aThis work, at 1 atm. bReference 9, at - 20-200 torr. CReference 8, at - 16-30 torr. dReference 3, at 1 atm.
as the O atoms from the photolysis of 03 or O z. The NH 2 from the photolysis of NH 3 should not be reactive with 02 or H2, as discussed in the review section. If the H atoms from the photolysis of NH 3 were thermalized rapidly, they would tend to recombine with 02 to form unreactive HO2, via reaction 2. Our kinetic modeling calculations have indeed predicted no ignition if the H atoms were thermalized. Our calculations further imply that the hot H atoms, which can react with 02 to form reactive O and OH via reaction 1, may be responsible for observed enhanced ignition. Maas and Warnatz [38] have recently modeled thermal ignition of H 2/02 mixtures and predicted a minimum ignition-energy density of - 4 3 0 m J / c m 3 in a relatively large ignition volume. This value appears to be substantially higher than that required for ignition under the photolysis of NH 3, 03 [9], or 02 [8]. This suggests that the photolysis processes are perhaps more efficient than the thermal. In other words, less energy may be required to generate radicals than to heat the whole gas mixture, to achieve ignition.
Dilution Effects Our results show that the minimum ignitionenergy density increases only moderately, from 137 ___ 8 m J / c m 3 for H2/O 2 mixtures to 190 _ 20 m J / c m 3 for H2/air mixtures, for ignition achieved by the photolysis of NH 3. This observation corresponds to an increase in the required H and NH 2 radical concentration from - 1 . 3 ___ 0.1 × 1017 tO 1.8 __+ 0 . 2 × 1017 c m - 3 , a s shown in Table 2. Such a moderate increase is also
observed in ignition achieved by the photolysis of 02 [8]. In contrast, the minimum ignition energy increases more significantly, from 4.2 to 18.9/,J, in spark ignition. In spark ignition, energy is needed to raise the gas mixture above a critical temperature. The thermal inertia of the diluent gas of N 2 can result in such a drastic increase in the required energy. On the other hand, the photolysis processes, which do not rely upon direct heating of the gas mixture, appear to be relatively insensitive to the diluent gas N 2.
Fuel Species Effects Table 2 shows that the radical-pair densities required for ignition increase substantially from 1.3 + 0.1 × 1017 cm -3 for H2/O2 to - 3.5 + 0.3 x 1017 cm -3 for CH4/O 2 mixtures for ignition achieved by the photolysis of NH 3. This trend is consistent with that observed for ignition achieved by the photolysis of 03 [9]. In contrast, the minimum ignition energy in spark ignition is about the same for both gas mixtures. This result again shows that the ignition achieved by photolysis differs from ignition achieved by spark ignition.
CONCLUSION Volumetric homogeneous ignition of H 2 / O2/NH 3, H2/air/NH 3 and CHn/O2/NH 3 mixtures in an open flow system initially at 1 atm and room temperature is achieved via photolysis of NH 3. The emission spectrum monitored by an optical multichannel analyzer shows no dis-
IGNITION BY E X C I M E R - L A S E R PHOTOLYSIS OF NH 3
cernible atomic or ionic emission, an observation that excludes gaseous breakdown as a cause of ignition. The ignition-delay times based on the 308-nm OH emission at three locations appear to be of comparable value, an observation that suggests the presence of well-distributed ignition sites within the entire irradiated volume. The ignition delay time depends strongly on the initial NH 3 mole fraction and is nearly independent of the f u e l equivalence ratio for values between 0.35 and 3.0 for H z / O 2 / N H 3 mixtures. The minim u m ignition-energy density is also insensitive to the equivalence ratio. Furthermore, the required m i n i m u m ignition-energy density (137 ___ 8 m J / c m 3) appears to be substantially less than that required for ignition by a thermal process ( - 430 mJ/cm3), for a stoichiometric H2/O 2 mixture. Our kinetic modeling calculations show that if the hot H atoms from the photolysis of NH 3 were collisionally quenched rapidly, ignition should not have occurred. This result implies that the excitation energy that remained in the initial products of photolysis may play an important role in enhancing the ignition. However, the detailed mechanism leading to such an enhanced ignition is not yet well understood.
The authors acknowledge Mr. E. Y. Wong (TR IV) and Dr. J. Y. Chen (Sandia National Laboratories) f o r assistance with computer modeling calculations, Mr. E. I4/. Rogala f o r technical assistance in gathering C H 4 / O 2 / N H 3 ignition data, and Drs. F. Fendell (TR IV) and T. Sloane (General Motors) f o r helpful discussions. This work was sponsored by the A i r Force Office o f Scientific Research, A i r Force System Command, USAF, under Contract F49620-87-C-0081. The contract technical monitor was Dr. J. Tishkoff.
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Received 5 April 1991; revised 27 July 1991