No flames

Twenty-Second Symposium (International) on Combustion/The Combustion Institute, 1988/pp. 1165-1173

LASER-FLUORESCENCE MEASUREMENTS LOW-PRESSURE H2/O~/NO

OF NITRIC FLAMESt

OXIDE

IN

R. J. CATI'OLICA, J. A. CAVOLOWSKY* AND T. G. MATAGA** Combustion Research Facility Sandia National Laboratories Liver~nore, CA 94550

The concentration profiles of NO in low-pressure (76 Torr) H2/O2/Ar flames to which nitric oxide was added (0.94% and 1.74% in argon), were measured by pulsed, laser-induced fluorescence. Laser excitation of NO in the (1,0) band of the A2~ ---* X2II transition at 214.34 nm was followed by detection, either time-integrated or temporally resolved, of the fluorescence in the (1,4) band at 252.2 nm. The temporally resolved fluorescence measurements were used to determine the collisional de-excitation rates needed to convert time-integrated fluorescence signal into nitric oxide concentration. Five flames were studied with H J O e equivalence ratios of 0.88, 0.98, 1.22, 1.37, and 1.50. In these flames the collisional deexcitation rate decreases rapidly above the burner surface as the density decreases with increasing temperature. A 20% decrease was observed for the lean flames, and a 30% decrease for the rich flames. For the near stoiehiometric flame, the measured de-excitation rate constant was used to determine a collision cross section of 38 ~2 at 1414 K for collisional deexcitation of the A2E (v' = 1) state (including both electronic quenching and vibrational relaxation) by water. Within the precision of the measurement technique (-+ 10%), no significant removal of nitric oxide was observed in these flames.

Introduction The interaction of nitric oxide with hydrogen/oxygen flames has important implications with respect to pollutant formation, ignition chemistry, and the application of laser diagnostic techniques. From a molecular-beam mass spectrometry (MB/MS) study, t there is evidence of substantial removal of nitric oxide in low-pressure hydrogen/oxygen flames. However, a laser-induced fluorescence (LIF) study 2 of similar atmospheric-pressure flames showed the nitric oxide concentration to be constant in the burnedgas region of the flame and proportional to the level of nitric oxide addition. In this latter study, measurements through the flame front were not possible and absolute concentration measurements were not made. In another atmospheric-pressure experiment, 3 under rich-flame conditions, probe-sam-

tThis work is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences. *Visiting Scientist, NRC/NASA Post-Doctoral Fellow, NASA Langley Research Center, Langley, VA. **Association of Western Universities Research Assistant, Department of Mechanical Engineering, University of California, Berkeley, CA.

piing chemiluminescent measurements indicated that nitric oxide concentration was constant in the postflame gases, but reduced by 40% from its initial level. Here, we present additional quantitative measurements of the behavior of nitric oxide in lowpressure hydrogen/oxygen flames that are needed to reconcile the different findings of these studies. Laser-fluorescence spectroscopy has been used previously to detect nitric oxide in flames. Singlepoint measurements 2'4 and two-dimensional meas u r e m e n t s of n i t r i c oxide f l u o r e s c e n c e for concentration 5 and temperature imaging6 have been reported. In these studies, relative fluorescence signals from nitric oxide were converted to concentration measurements either by neglecting collisional quenching or by assuming a model for the quenching process. While this approach may be sufficient in post-flame gases, where the composition and temperature are relatively constant, it is not a satisfactory method for measurements through the reaction zone of the flame, where these two parameters change significantly. Quantitative fluorescence concentration measurements in the reaction zone of a flame require an understanding of the collisional quenching processes and an appropriate calibration method. For the temperature imaging application, it is necessary to assume no removal of nitric oxide by combustion reactions, as well as a model for collisional quenching. In the laser-in-

1165

COMBUSTION-GENERATED NO, AND SO~

1166

duced fluorescence experiments that are reported in this study, both the question of the removal of nitric oxide in hydrogen-oxygen flames and the behavior of the collisional quenching of the fluorescence are examined. Laser-Fluorescence Method The essential physics of laser-indueed fluorescence of a molecule or atom can be described in terms of a simple two-level model representing the ground state, NI, and excited state, Nz, of the electronic transition that is coupled by broadband laser radiation. The total population, No, of the system is constant (No = N1 + N2), and the initial population of the excited state N.z(t=0) = 0. The population of the excited state is related to the population of the ground state by the rate equation: dN2 -

-

dt

= N1B12112(t) - N2BzlI12(t) - (A21 + Q2I)N2 (1)

where Ilz is spectral energy density, B12 and B21 are the probability coefficients for absorption and stimulated emission, and A21 and Qzl are the spontaneous emission rate and quenching rate due to de-excitation by collisions with background gas. Variation in the collisional quenching rate as gas density, temperature, and composition change through the flame complicate the use of Eq. (1) in determining the excited-state population. Two techniques have been employed to eliminate the dependence of the rate equation on quenching, namelyb saturation of the trans~tmn • • 7 and photomnization. However, for low-pressure flame studies, neither of these quenching-mitigation schemes are necessary. I n s t e a d of trying to remove the effects of quenching, the approach taken in this study is to measure the collisional de-excitation rate directly. If the laser pulse falltime is short compared to the characteristic lifetime of the excited state, (A21 + Q21)-1, and the signal detection system can temporally resolve the fluorescence signal, then the quenching rate can be measured directly. This method has been used successfully for both OH and CH concentration measurements in low-pressure flames9'm and for OH concentration measurements in atmospheric-pressure flames. HAz During the laser pulse the fluorescence signal follows the time-dependent solution of the excitedstate rate equation: NoB12112(t)" [1 N2(t) =

-

e -[(Blz+B21)Ilz(t)+Az'+Qzllt]

(B12 + B21)IIz(t) + Azl + Q21

The photon flux, If t, from the fluorescence that is collected in a solid angle fl from the fluorescence volume V with a detection sensitivity -q is: Ift(t) = ~qI'~VAzlNz(t).

(3)

After a time At, laser excitation ceases, 1 1 2 drops to zero, and the photon flux decays exponentially with a time constant (A21 + Q21).-1 This time constant can be measured directly from the decay of the fluorescence signal. If the emission rate for this transition, A21, is known independently, or if Q21 > > A2x, then the quenching rate can be determined from this time constant. If the laser excitation is in the unsaturated limit, i.e., (B12 + B21)Ilz(t) < < A21 + Q21, then No Nx. Also, if (A21 + Q21) * At < 1, the photon flux can be integrated in time to yield a simple relation between the integrated photon signal, Sfl, the concentration of the ground state, No, and the laser pulse energy, Elz: S£1 =

"qf~VA2~NoEl~B12 A21 + Q21

(4)

To evaluate this equation, two fluorescence measurements are required: a time-resolved measurement to determine the term Azx + Q2x, which has been described above, and the integrated fluorescence, Sfl. The integrated fluorescence measurement can be obtained either by integrating the timeresolved signal or by using a gated integrator that directly accumulates the signal. The constants in Eq. (4) that are a function of the details of the experimental setup ('q, II, and V) can be evaluated with a measurement of a known number density No. This simple two-level excitation/emission model is applicable to the complex multi-level system of the nitric oxide molecule, with only a minor redefinition of terms and with the proper choice of laser-excitation and detection scheme. If the laser excitation involves only one rotational level in the ground state of the molecule, the ground-state number density, No, in Eq. (4) is given by the Boltzmann relation describing the population fraction in that level. If the fluorescence is collected from all rotational levels in the vibrational level populated by the laser excitation, then rotational energy transfer within the excited vibrational level has little impact on the relation given in Eq. (4). Fluorescence signal loss due to vibrational energy transfer is a more serious effeet, but can be taken into account by a redefinition of the quenching rate to include this additional de-excitation process. Also, the fluoreseenee emission rate, A21, in the numerator of Eq. (4) must be multiplied by the branching ratio for the observed transition. The laser excitation scheme used for the measurement of nitric oxide consisted of exciting a single rotational level in the X211 (v" = 0) ground state

LASER-FLUORESCENCE MEASUREMENTS to the AZE(v' = 1) state. The Qz(26) transition at 214.34 nm was used for excitation because it is sufficiently separated from adjacent (spin conserving) main-branch transitions to prevent strong multiplelevel excitation from the ground state. This transition is also advantageous because the rotational level involved (J = 25.5) is nearly a constant fraction (---3%) of the total nitric oxide number density from 1000 to 1700 K. There is, however, spin splitting of the energy level in the A2E excited state. This energy level splitting is very small, la and the Qz(26) transition is directly overlapped by the R12(26) satellite transition, which has a factor of 8 weaker transition probability because of the spin-state change that is involved. The appropriate line strengths 14 for these transitions can be added together to define effective transition probabilities. After excitation to the A~E state, collisions with the background gas produce rotational and vibrational energy transfer as well as electronic quenching. For this experiment, fluorescence is detected at 252.2 nm from the entire (v' = 1, v" = 4) band of the A2E ~ X2II transition. This band provides sufficient wavelength separation for the fluorescence detection system to reject scattered laser light and takes advantage of a favorable branching ratio. 15 Vibrational energy transfer, particularly relaxation to v' = 0, introduces an additional loss mechanism to the rate equation formulation in Eq. (1). This effect, as noted before, can be included by redefining Qzx to be the total collisional de-excitation rate including vibrational relaxation.

Experiment The hydrogen/oxygen flames in this study were stabilized on a 6.0-cm-diameter, sintered stainlesssteel burner ~6 located in a vacuum chamber with optical access. The burner was water-cooled with an imbedded cooling coil; the heat loss to the burner was measured from the water flow rate, and the temperature rise was monitored with thermocoupies. The burner was translated under computer control both horizontally and vertically with a precision of -0.01 cm. The hydrogen, oxygen, and argon flow rates were measured with electronic massflow meters calibrated to ---1% accuracy. The chamber pressure was maintained constant at 76 --+ 0.1 Torr, independent of gas flow rate, with a feedback-controlled valve connecting the low-pressure chamber to the vacuum pumping system. The chamber pressure and the five flame conditions studied (see Table I) were chosen to match most of the flames used in the previously reported MB/MS experiments. 1 The nitric oxide was introduced into the flames by replacing the pure argon diluent with argon premixed with two different concentration levels of nitric oxide (0.94% and 1.74%). The el'-

1167

feetive doping levels with these two mixtures are also given in Table I. The laser system for the experiments consisted of a frequency-doubled (532-nm, 220-mJ, 10-Hz) Nd:YAG laser (Quanta-Ray DCR-IA) pumping a dye laser (oscillator plus amplifier), followed by frequency-doubling and Raman shifting in a hydrogen cell. Two different laser dyes were used in this system to generate laser radiation (-0.3 cm - t bandwidth) for nitric oxide fluorescence measurements and the absorption measurement needed to calibrate Eq. (4). For the fluorescence measurements the NO(l,0) Q2(26) + Rlz(26) transition was excited using the frequency doubled output of kiton red, Raman shifted to the 3rd anti-stokes wavelength 214.3 nm (70 I~J). The absorption measurements in NO(0,0) Q2(26) + R12(26) transition used fluorescein 548, frequency doubled and Raman shifted to the 2nd anti-Stokes wavelength 225.6 nm (100 I~J). The energy for each laser pulse was measured with pyroelectric energy meters (Molectron model J-5) and recorded on a computer before and after passing through the vacuum chamber. To measure the laser pulse shape, a vacuum diode (ITr F4018) with a 0.5-ns response time was used. The laser was focused with a 500-mm focal-length uv lens to a beam diameter of 600 i~m at the centerline of the burner. The fluorescence signal was collected and focused onto the entrance slit of a 0.25m monochromator (3-nm bandwidth) with a single 75-ram-diameter, 150-mm focal-length uv lens used at f/4. To prevent vignetting of the light-collection lens by the burner when making measurements near the surface, the optical window and lens were positioned at an angle of 20 degrees above the burner surface. The spatial resolution with this system consisted of a cylindrical measurement volume 600 i.Lm in diameter and 8 mm long. The fluorescence signal from the monochromator was measured using a photomultiplier tube (Hamamatsu R955) with a nonuniform dynode-voltage distribution to obtain a 2-ns rise time. The distance between the point where the laser was monitored to the fluorescence measurement point above the burner provided a 12-ns optical delay. With this delay, the fluorescence signal and the laser pulse shape from the vacuum diode could be recorded simultaneously with a single Tektronix 7912AD transient digitizer at a 1-GHz sampling rate. The fluorescence signal was also integrated directly using a charge preamplifier followed by a sample and hold A / D converter. Both the transient and integrated fluorescence signals were stored on a computer,

Results and Discussion The Q2(26) line in the (1,0) hand of the A2E --~ X2I] transition of nitric oxide was located and identiffed with laser fluorescence excitation spectra taken

1168

COMBUSTION-GENERATED NO~ AND SO~ TABLE 1 Flow conditions for 76-Torr H 2 / O J A r flames stabilized on a 6-cm-diameter, sintered stainless-steel burner.

No. -

(l)

H2

Ar

Total

(Liters/rain at NTP)

-

1 2 3 4 5

02

0.88 0.98 1.22 1.37 1.50

2.31 2.57 3.20 3.61 3,91

1.31 1.31 1.31 1.31 1.31

3.94 3.94 3.94 3.94 3.94

Heat Loss to Burner

NO addition high low

(era/s)

(kcal/min)

(ppm)

44.5 46.1 49.8 52.2 54.2

2.04 2.40 2.88 3.00 2.88

7.55 7.81 8.45 8.86 9.19

in a flame. The laser was scanned, under computer control, through the (1,0) band near the Q2(26) line and the fluorescence detected in the (1,4) band at 252.2 nm. For this measurement the integrated fluorescence signal was normalized by the laser pulse energy. An example of a coarse excitation scan (0.5 cm-1/step) with 20 laser pulses averaged at each laser frequency position is shown in Fig. 1. Only the main-branch transitions are labeled; the accompanying satellite-branch transitions that are coincident are not listed. The Q2(26) line was identified by matching the wave number distribution of adjacent lines in the excitation scan with a calculated spectrum. The Q2(26) line is sufficiently separated from the Ra(15) and the combination of P1(30) and Q](22) lines to maintain single rotational level excitation from ] = 25.5 in the XZI1 ground state. In all of the following fluorescence measurements, the laser frequency was positioned at the peak of the 10

0.8

Gas Velocity

9065 8752 8108 7743 7447

Q2(26) line by maximizing the fluorescence signal. To determine the de-excitation rate for use in Eq. (4), time-resolved measurements of the laser and fluorescence signal were made as a function of distance above the burner surface for all five flames listed in Table I, An example of the average of the time-resolved measurement for 32 laser shots is shown in Fig. 2, which is a semi-log plot of the decay of both the laser and fluorescence signals. The decay rate was measured by a linear least-squares fit to the logarithm of the fluorescence signal 4-10 ns after the peak signal. This limited time range was necessary to eliminate the effects of laser excitation and shot noise. A summary of the behavior of the de-excitation rate through each of the flames that was studied is given in Fig. 3, along with cubic spline fits to the data to be used for interpolation. The data were taken at l-ram intervals from 1 to 11 mm, and at 2-mm intervals from 11 to 41 mm above the burner surface. The de-excitation rate decreases rapidly in the first 10 mm above the burner surface as the density decreases with in-

PI(29).Ql(21), P2(33)

'*,,,,

LASERSIGNAL

•

Q

...~.. 9". F.s..c.Erfc.E. .......

Rl(16),P~(34)

0.6

RI(15) ? Q (26) :t

%

4897 4728 4380 4183 4023

Q2(27)

R2(20)

:"%... ""....

0.2 i

n

0.0 5

10

relative

15

20

frequency

25

30

";'...%.,

35

".=.

( c m "1)

FIG. 1. Fluorescence excitation spectra through the (1,0) band of nitric oxide near the Q2(26) transition at v = 46656 cm -l (k = 214.34 nm) with detection in the (1,4) band at v = 39651 c m x (k = 252,2 nm) taken in an H J O z / A r flame with an equivalence ratio of qb = 0.88 at 10 mm above the burner surface.

12

,; 2'0 ~', lime (1O"g s}

2'8

~2

3'e

O1 40

FIG. 2. Semi-log plot of time-resolved measurements of the decay of the laser pulse and nitric oxide fluorescence taken in an H J O J A r flame with an equivalence ratio of el) = 0.88 at 39 mm above the burner surface.

1169

LASER-FLUORESCENCE MEASUREMENTS

vary more than -+5% around a value of 2.28 × 10 -l° cm 5 s -1 for the range of flame conditions that were studied. For the stoichiometric flame the burnedgas region is composed primarily of HzO and Ar. Compared to water, argon is three orders of magnitude less efficient in quenching nitric oxide in the A2E(v ' = 0) state.iS Neglecting the contribution due to argon or self quenching, the calculated de-excitation rate constant based only on the water concentration would be 6.23 × 10 - l ° cm 3 s -I at a gas temperature of 1414 K. The previously measured value t8 for quenching of nitric oxide in the A2]~(v ' = 0) state by water at 293 K is 7.6 × 10 -1° cm 3 s -1. Interpreting these de-excitation rates constants in terms of collision cross sections, to remove the temperature effect in the relative collisional velocities, yields 38 AZ at 1414 K for water quenching of the A2Y,(v' = 1) state (including vibrational relaxation) compared to 102 ~2 at 293 K for quenching of the A' ]£(v' = 0) state. In Fig. 4 a-c the nitric oxide fluorescence profiles for the qb = 0.88, 0.98, and 1.37 flame conditions are plotted for both levels of nitric oxide addition (0.94% and 1.74% in argon). The integrated fluorescence signal, normalized by the laser energy, from either 30 or 60 laser pulses was averaged (to maintain similar signal-to-noise levels - 2 0 ) at each location and recorded at 1-mm intervals from 1 to 40 mm above the burner surface. For comparison in Fig. 4, the fluorescence signal from the low nitric oxide doping condition was multiplied by 1.85, the ratio of the concentrations of the high and low levels of nitric oxide addition (1.74%/0.94%). All the fluorescence profiles show a rapid decrease in signal in the first 10 mm above the burner surface, following the density change due to the increase of temperature across the reaction zone of the flame, and then a constant level further downstream. The normalized fluorescence signal for the two concentrations of nitric oxide addition are nearly identical

2..0-

[] o L~ + ×

1.8-

~

1.6

~

1.4

1.2

0.88 0.98 1.22 1.37 1.50

0 distance above burner surface (cm)

FIG. 3. Collisional de-excitation rates of nitric oxide fluorescence as a function of distance above the burner surface for H 2 / O J A r flames doped with 1.74% nitric oxide added to argon.

creasing temperature through the flame. A 20% decrease was observed for the lean flames, and a 30% decrease for the rich flames. Since the spontaneous emission rate, Azl, from the v' = 1 level17 is 5.0.106 s -1, the measured de-excitation rates, which are much greater, are therefore completely dominated by collisional processes such as quenching and vibrational relaxation. The higher rate of de-excitation for the rich flames is related to the increased density associated with the lower gas temperature in these flames. From the measured gas temperatures (see Table II) and pressure (76 torr) the number density for each flame can be determined and used to calculate a de-excitation rate constant kNo(v'=l) = Q21/n. In the burned-gas region 40 mm above the burner surface, where the gas composition and temperature are nearly constant, the de-excitation rate constant (based on the total number density) does not

TABLE II Comparison of measured burned-gas flame temperatures from OH laser-induced fluorescence and thermocouple measurements (Ref. 1) in low-pressure H 2 / O J A r flames as a function of equivalence ratio qb.

~P

[adiabatic]

Temperature (K) [this study[ (10 mm)

0.60 0.88 0.98 1.22 1.37 1.50

[Ref. 20, 21]

(40 mm) 1350

2510 2539 2551 2530 2502

1462 1507 1105 1068 1134

[Ref. 1]

1387 1414 1158 1073 1152

± ± ± ± ±

50 50 50 50 50

1680 (~ = 0.66) 1750

1342 1680 1100 (~ = 1.40)

1170

COMBUSTION-GENERATED NO, AND SO~

flame gases and proportional to the level of nitric oxide addition. ~ 1.4 The gas temperature in the flames in the current 1.2 .=_= study was determined from OH rotational temper" ature measurements 19 from laser-fluorescence ext.o ~ citation scans through the band head of the R-branch o° of the AzY~ - X2II(1,0) transition at 281.6 nm, with 0.6 o fluorescence detection in the (1,1) band at 314 nm. ~Temperature measurements were made from 1 to 0.6 40 mm above the burner surface for all five flame conditions. These measurements indicate a rapid 0,4 0 1 2 3 4 temperature increase near the b u r n e r surface, d i s t a n c e a b o v e burner s u r f a c e (cm} peaking between 5 and 10 mm above the burner surface, and a nearly constant temperature further (a) downstream. This result is in agreement with other OH LIF temperature measurements in similar 12 NO addition 0.94% 1.74% 1.6 flames, z°'2] A comparison of the temperatures in the × [N0]'1.85 [3 [NO] 1.4 burned-gas region of these flames is presented in × '~ 10 + Sf1"1.85 £3 Sfl Table II. The adiabatic flame temperatures are also presented and indicate substantial energy loss by 0 8 1.2 ~® the flames in the experiment. The measured heat 8 1.o ®=° loss to the burner (see Table I) accounts for 75+ o 80% of this energy loss. e~ O+ 0.6 ~ The temperature measurements given in Table II 8 = are substantially different from those obtained from 0 2e.6 thermocouple measurements in the previous MB/ MS study, 1 which showed the temperatures rising O0.4 i .... to above 1900 K at 5 mm above the burner surface d i s t a n c e a b o v e burner s u r f a c e (crn) and decreasing as much as 300 K at 38 mm. Al(b) though the thermocouples used in that experiment were coated to inhibit radical recombination, the peak temperatures occurred where recent H-atom 12 E3 NO addition 0.94% 1.74% 1.6 measurements 2~ in similar hydrogen/oxygen flames × × I~o]*t.Bs ~ [No] 1.4 indicate peak H-atom concentrations. Thus, the [2 + Sft'1.85 o Sfl × possibility exists for anomalously high temperatures 1.2 .~ due to H-atom recombination on the thermocouple ~, surface. To examine this possibility we used both 8 ".~ 61.o o= silicon dioxide coated and uncoated Pt/Pt/13% Rh m ®° thermocouples (0.1 mm diameter) to measure flame o.s = ~ temperature. These thermocouples melted in the 8o flames listed in Table I, indicating thermocouple ~ e ~ e ~ e e e ~ e ~ 9 ~ e e e ~ 6 ~ e ~ 9 ~ e ¢ ~ e e e 8 O 2Z 0.8 surface temperatures greater than 2000 K. Using 0.4 similar thermocouples in 20 torr flames at the same 1 2 3 conditions, the thermocouples did survive but indistance above burner surface (cm) dicated temperatures as much as 1000 K higher than OH fluorescence temperature measurements. These (c) results indicate that great care must be taken in the F1c. 4. Relative fluorescence signal and nitric ox- coating and use of thermocouples in hydrogen-oxide concentration profiles for two levels of nitric oxygen flames to prevent radical recombination and ide addition (0.94% and 1.74% in argon) for HJOz/ catalyic oxidation from producing erroneous flame Ar flames with equivalence ratios: (a) 0.88, (b) 0.98, temperatures. and (c) 1.37. The relative nitric oxide fluorescence profiles for each flame were converted to concentration measurements using Eq. (4) with the collisional de-exin each flame. The fluorescence profiles for the = 1.22 and 1.50 flames are similar to those precitation rates from Fig. 3. To calibrate Eq. (4) for sented in Fig. 4. The behavior of the nitric oxide nitric oxide concentration a laser absorption specfluorescence profile in all of these flames is similar tral scan of the Qz(26) and its overlapped R12(26) to the atmospheric-pressure LIF study2 that found satellite line at 225.66 nm in the (0,0) band of the A2~ ~ XzYI transition was made in the • = 0.88 the fluorescence signal to be constant in the post1 2x/ -~

~'~ ~ 10

NO addition 0.94%

x [NO]'1.85 + Sf~'1.85

1.74%

I~1 [NO] o S~l

t.6

LASER-FLUORESCENCE MEASUREMENTS flame 10 mm above the burner surface. This spectrally integrated absorption measurement was analyzed with the curve of growth methodz3 to obtain a reference concentration of (6.8 +-- 0.34.1015 cm 3). The absolute concentration profiles in Figs. 4a-c are significantly different in shape from their respective fluorescence profiles in the first 15 mm above the burner due to the variation in the collisional de-excitation rates. Although the population fraction in Eq. (4) is rigorously a function of temperature, from 1000 to 1700 K the population fraction for the ground state of the Q2(26) line varies less than +3%. Although the gas temperature in the flames [19] does drop below 1000 K near the burner surface (below 2 mm), downstream from this point error in the temperature measurement would have little affect on the absolute nitric oxide number density measurement. In Table II1 a summary of the measured nitric oxide concentration and a comparison with the previous MB/MS results are presented for a measurement location 38 mm downstream from the burner surface. The reference nitric oxide concentrations are higher in the combustion product gas than the original reactants (see Table I) because of the 1517% reduction in mole number in the conversion of hydrogen and oxygen to water. The laser-fluorescence measurements indicate no significant removal nitric oxide in the flames that were studied. The measured nitric oxide mole fraction in the = 1.37 and 1.5 flames are greater than the initial doping levels and are the result of the --- 10 percent measurement uncertainty. This precision is the result of the combination of the base-line error contribution to the absorption calibration, the uncertainty in the de-excitation rate correction to the flu-

1171

orescence signal, and the uncertainty in the temperature measurement. The previous MB/MS measurements for the lean (dp = 0.88) and near stoichiometric (4) = 0,98) flames indicate a 20 to 30 per cent removal of nitric oxide. The MB/MS concentration profiles for these two flame conditions are also very different and display a significant drop in the nitric oxide to a minimum of 50% and 60%, respectively, of the initial doping level at 10 mm above the burner, before recovering to the values given in Table III, This reversible behavior of the nitric oxide concentration is unexplained and qualitatively inconsistent with the LIF measurements, which show that the nitric oxide concentration remains constant in the postflame gases. For the rich flame conditions, the MB/M S measurements indicate a 50-60% nitric oxide removal. Clearly there are significant differences in the observed behavior of the nitric oxide concentration with these two different measurement techniques. At present, no chemical kinetic mechanism has been put forward to explain large removal of nitric oxide in these low-pressure flames. Radical attack (H, O, N) and reduction of NO24 is not a likely mechanism because of the relatively low flame temperatures, particularly in the flame-preheat zone near the b u r n e r . A reaction that has b e e n proposed 1'3 for nitric oxide removal in these flames is the recombination reaction, H + NO + M --~ HNO + M (R1). In an atmospheric pressure H2/O2/Ar flame doped with nitric oxide, Roby 1'25 made NO measurements using a probe-sampling/chemiluminescent technique. He observed a rapid decrease within the first 1 mm above the burner surface to 60 per cent of the initial doping level and a nearly con-

TABLE III Comparison of measured nitric oxide concentrations 38 mm above burner with reference levels, assuming no removal in the flame.

No.

qb

[reference]

nitric oxide concentration [this study]

[Ref. 1]

(ppm)

(ppm; % ref,)

(ppm; % ref.)

1

0.88

10,700 5,780

10,660 (99%) 5,260 (91%)

8,600 (80%) 4,000 (69%)

2

0.98

10,490 5,670

10,330 (91%) 4,970 (87%)

7,500 (71%) 3,300 (58%)

3

1.22

9,600 5,190

9,710 (101%) 4,900 (94%)

5,100 (53%) 2,400 (46%)

4

1.37

9,090 4,970

9,550 (106%) 5,000 (100%)

4,500 (50%) 2,100 (42%)

5

1.50

8,700 4,700

10,030 (114%) 5,010 (107%)

1172

COMBUSTION-GENERATED NO, AND SO,

stant concentration further downstream. This behavior was obtained over a range of doping levels from 450 to 2300 ppm and is consistent with the results found by Morely z in the post-flame region of a similar flame. Roby also modeled his flame results using two different values of the reaction rate of R1, one proposed by Seery and Zabielski 1 which is a factor of ten higher than the value used by Throne, et al, 26 Using the lower value of the recombination rate constant for R1 gave good agreement for the rapid removal of nitric oxide near the burner surface, but it predicted substantial reduction further downstream where no significant removal was observed. Also, with this lower value for the rate constant for R1, no removal of nitric oxide was predicted for the low-pressure flames in the MB/MS study. We have used a similar kinetic mechanism and the Sandia flame code 27 to predict the flame structure of the MB/MS study and Roby's atmospheric pressure flame measurements. Using the lower value of the recombination rate for R1, we predict 19 no significant removal of nitric oxide in the low-pressure flames of Seery and Zabielski. 1 We also predict nitric oxide removal in Roby's atmospheric flame consistent with his flame model results, however, both modeling studies over predict nitric oxide removal in the post-flame gas. The difference between the current LIF measurements and the previous MB/MS study are as yet unexplained. Possible probe sampling effects must always be considered as a potential source of error. The measured (thermoeouple) flame temperatures in the MB/MS experiments were substantially higher than those measured in our experiments. Our modeling indicates, however, that even this difference would not have resulted in significant nitric oxide removal. Another factor to be considered in understanding the behaviour of nitric oxide in these flames is the role played by the burner surface in catalyzing.the recombination process. Since the gas velocities in these flames are low compared to the corresponding flame speeds, these flames are attached to the burner surface. This strong interaction with the burner surface is indicated by the high heat transfer rates to the burner observed in the experiment. In addition, the high diffusivity of H-atoms would be expected to enhance the recombination reaction near the burner surface. In the MB/MS experiments, the burner surface was sintered copper. The LIF study at atmospheric pressure used a burner made of stainless-steel capillary tubes, the probe sampling study at atmospheric pressure using a sintered bronze burner, and in our low-pressure flame study a sintered stainless-steel burner was used. To determine the possible influence of the burner surface on the behaviour of nitric oxide in these flames requires improved temperature and concentration measurements at the burner surface and an ira-

provement in the one-dimensional flame code to provide the capability to introduce surface reactions at the flame boundary at the burner surface.

Acknowledgment The authors would like to thank Jon Meeks for his excellent technical assistance in performing the experiments.

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