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Study of the collisional lifetime of hydroxyl (2Σ+, ν′ = 0) radicals in flames by time-resolved laser-induced fluorescence
COMBUSTION AND FLAME 40:65-70 (1981)
65
Study of the Coilisional Lifetime of Hydroxyl (2E+, v' = 0) Radicals in Flames by Time-Resolved Laser-Induced Fluorescence D. STEPOWSKI and M. J. COTTEREAU Laboratoire de Thermodynamique, L.A. au C.N.R.S. n ° 230, Fucult~des Sciences et des Techniques de Rouen, Universitlde Rouen, B.P. 67 76130 Mont-Saint-Aignan, France
Collisional lifetimes of OH (2~ +, ~., = 0) radicals excited by tunable laser pulses have been measured in three different low pressure C 3H 8 - O 2. Profiles of the quenching rate. in the reaction and burnt gas zones are given. The pressure dependence has also been studied, as well as the effects of rotational redistribution. It is concluded that a mean constant quenching rate can be used in a given flame.
INTRODUCTION
essential for turbulent flame studies where the flame front may wander.
Measurement of local and instantaneous number density of species by laser-induced fluorescence has appeared recently as a new and attractive tool in combustion research [1, 2]. This highly sensi-
This paper reports a direct lifetime measurement of OH excited to a particular rotational level of the (2~+, v' = 0) state by a tuned laser pulse. Experiments were performed at many points in the burnt gas and reaction zone, and for several
rive method is very promising for measurements of low concentrations [3] when fluorescence trapping is negligible. Unfortunately, the fluorescence intensity depends on the quenching due to collisional deexcitation. The methods that have been proposed [4, 5, 6] to avoid this dependence are still very difficult to achieve under strong quenching conditions such as are met in high pressure media; therefore, knowledge of the quenching is still required for single-shot measurements. Published studies of the quenching of excited hydroxyl have been made with conventional exciting sources which allow only indirect measurement via fluorescence efficiency [7, 8, 9]. Despite the relatively large quenching cross sections found, particularly for H20 , quenching is assumed to be mainly governed by collisions without chemical reaction. This assumption becomes questionable in the reaction zone of a flame where physical quenching by many other species, including radicals, is unknown. Understanding such quenching is Copyright © 1981 by The Combustion Institute Published by Elsevier North Holland, Inc. 52 Vanderbilt Avenue, New York, NY 10017
stoichiometries.
THEORY To remind the reader of the quenching dependence of the fluorescence intensity, let us briefly give a simple two-level calculation here. Let N l be the population of the lower level. An exciting pulse of spectral energy density Uv increases the population N 2 of the upper level according to
dN2 -NxB12Uv-N2B21Uv dt - ( A 2 1 + Q21)N2'
(1)
where B12 and B 21 are the probability coefficients for stimulated absorption and emission; A 2 l and Q21 are the probability coefficients of radia-
0010-2180/81/010065+06502.50
66
D. STEPOWSKI and M. J. COTTEREAU
rive and collisional spontaneous relaxation. Writing N 1 + N 2 -- No, one obtains
No~12U~
N2(t) =
(B12 +B21)Uv +A21 + Q21 × [1--e--t(Bx2+B21)Uv+A21+O21]t]. (2) After an excitation of duration At, Uv drops to zero and N z can be seen from Eq. (1) thereafter to decrease according to e-(A21+Q21)t: The photon flux collected in the solid angle ~ from a volume V is A 21 ~2 V
¢(t) = - -
N 2 (t).
Under steady-state saturation conditions, Uv >> 4zr(Bl 2 + B21) is independent of the quenching, However, this condition is very difficult to achieve in real flames. The near saturation method pioneered by Baronavski [5] with a stationary flame is not applicable to single-shot measurements, We have proposed an alternative [6] to this saturation approach: If the pulse duration is very much shorter than quenching, radiative, and pumping times, the expansion of the exponential in Eq. (2) can be approximated by the first term and the population N2 increases linearly leading to a maximum fluorescence flux 4~M = A21 ~2VNoB 12 UuAt/47r, independent of the quenching rate which nevertheless can be obtained from the time-resolved fluorescence decay. This result, which is in fact due to the neglect of the negative terms in Eq. (1) solved with constant NI, remains valid for a multilevel system: N 2 is then the sum E N2j, of the upper levels populated from the laser excitation and consecutive rotational redistribution; N 1 is the population of the particular rotational lower level involved in the transition, The expression then, is,
(4)
dt so that I Z N 2 j ' Iraax =N1Bx2UvAt.
(6)
A2/' is nearly constant [1 1] but Q2/' may depend on the ]' rotational level involved. Measurement of the whole fluorescence decay, if exponential, gives a mean quenching rate Q according to ~ t ) = dPmaxe_(A+Q)t"
EXPERIMENTAL
(.421 + Q21)/(B12 +B21), and ¢ =A21V~BI2No/
-N1B~2U.,
dY~N2f - - ~(A2j, + Q2j,~2j ~, dt
(3)
4n
d~, N2j
It is noticeable that this result is independent of the rotational redistribution rate which appears to be rather complex [10]. After the excitation
(5)
For obvious reasons we have chosen to study first the quenching rate of the OH radical in a low pressure flame (15 torr < P < 80 torr) to obtain a quenching time longer than our pulse duration (4 ns). Since the greatest absorption occurs in the 2 ~+(v' = 0) +- 2 II(v" = 0) transition with ] < 15 [12], we have excited a line of this band, usually the Q17 line (~ = 308,9734 nm). The tunable exciting radiation that is focused in the flame is derived from the second harmonic of a dye laser pulse generated in a KDP crystal at a repetition rate of 20 Hz. An intracavity Fabry-Perotetalon ensures a laser spectral width (AX ~ 2.10 -12 m) closely matching the absorption line in the low pressure flame. Flum:escence light emanating in a direction at 90 ° from the exciting radiation is collected with F/5 optics, focused on the entrance slit of a monochromator and detected by a photomultiplier connected to an oscilloscope or to a boxcar analyzer. The horizontal flat flame is set up in a low pressure enclosure on a cylindrical porous burner (¢ = 2 cm) supplied with a premixed oxygen-propane flow. The enclosure is equipped with windows for excitation and a fluorescence light and with a port for introducing a thermocouple. RESULTS AND DISCUSSION When the laser is tuned on a line, the fluorescence signal is adequately reproduced on the oscilloscope, triggered in a single sweep mode but measurements are made more reliable by averaging ten
QUENCHING OF EXCITED HYDROXYL
67
Ltx3~ T
0
50ns
lOOns
150ns
Fig. 1. Example of a fluorescence signal as averaged by the boxcar over ten shots.
shots with the boxcar analyzer. This fluorescence radiation is detected at the same wavelength with a spectral width of 3 nm corresponding to the band 0, 0 of interest. Fig. 1 shows such a boxcaraveraged signal where the straight line derived
from logarithm conversion confirms the exponential shape of fluorescence decay and thus allows the derivation of a mean quenching time. According to Crosley and Lengel [10], redistribution of populations among the various rota-
O (~")T J J f J
•
f
10
/
f
/
•
•
•
/
5
0
0
I
|
|
=,=
20
40
60
P (torr)
Fig. 2. Quenching rate versus flame pressure.
68
D. STEPOWSKI and M. J. COTTEREAU
tional levels of the upper state is strongly dependent on the particular level directly pumped (and on its spin). Then, excitations from various lower levels give rise to different distributions on the upper level. Since the quenching toward the ground state could vary with the rotational level, the mean quenching and then the fluorescence efflciency may depend on the line chosen for pumping. When different absorption lines ( P l l , P12,
results. Fig. 2 shows how the quenching rate increases linearly with the pressure; beyond 50 torr the observed deviation from linear dependence is due to the exciting pulse duration itself. A 106 s -1 is too low to appear on this plot. Extrapolation to 1 atm leads to a quenching rate Q 109 s - l which is close to the quenching found by Eckbreth for CH and CN in flames [13] but smaller by two orders of magnitude than the
P13; Q13, QI4, Q15, Q16, Q17, Q21, Q24, Q25;
quenchingfound by Baronavskiand McDonaldfor
R22 ) of the 2E(u' = 0) ~ 21I(o" = 0) band of OH were excited, no noticeable variation of the fluorescence decay time was found. It can be deduced that relative measurements of fluorescence intensities (not dependent on quenching) give true relative populations of the lower levels successively excited, and thus the true rotational temperature, if fluorescence trapping is negligible, All the following experiments were performed on the Q17 line. The pressure influence on the fluorescence decay time in the burnt gas zone of the stoichiometric oxygen-propane flame was studied, This experiment was performed at 10 mm above the burner where the gradients are very weak for each flame studied so that change of the flame position with pressure does not affect the
C2 [5]. On Fig. 3, we have plotted the quenching rate versus the height of the exciting spot above the burner plane for several oxygen-propane flames of different stoichiometries. The quenching rate remains nearly constant through the flame in spite of temperature variations in the zone studied, as shown in Fig. 4. These temperatures have been measured with a coated Pt-Pt Rh 10% thermocouple and corrected for radiative losses. Such a relative independence of quenching on temperature can also be deduced from the results of Fig. 2 where the flame temperature decreases from 2,000°K to 1,700°K as heat losses at the burner increase with the pressure. Assuming the major species concentrations [ H 2 0 ] , [CO2], and [02] to be close to their
O (10~ 4)
3
|
~ •
.
; -
_•
! -"
J
3 J2
w
I
2
I I I
1
i i luminous zone I
0
;
2
3
4
;
;
i
;
Z(mm)
Fig. 3. Quenching rate along flames of different stoichiometries: Molar ratio C3H8/O 2 = O; flame 1, 0 = 0.20; flame 2, ~ = 0.11; flame 3, ¢ = 0.25; precision: A Q / Q ~ 10%.
QUENCHING OF EXCITED HYDROXYL
69
5oo
1
2
3
4
5
6
7
8
Z(mm)
Fig. 4. Temperature profiles for three flames studied.
equilibrium values in the burnt gas of flames 1 and 2, one can calculate a mean quenching rate from the specific cross sections given in Ref. [7] or [8] (Table 1). The values thus calculated are compared with our results in Table 2. The calculation for flame 3 was made with a CO cross section taken equal to that of CO 2 (~16 A2). If the uncertainties of the procedure allow such a deduction, this table shows that data o f Ref. [8] fit our results better. The result found for the rich flame, 3, suggests that the cross section o f CO is not very different from that of C O 2 . In the reaction zone where drastic concentration changes occur, as shown in Fig. 5, the quenching is surprisingly near constant. This result con-
TABLE 1 Specific Quenching Cross Sections H20
02
CO2
34 A 2 37 A 2
7 A2 10 A 2
16 A 2 -
TABLE 2 Mean Quenching Rate Flame 1 From Ref. [7] FromRef.[8] Our results
Flame 2
2.8 107 s- 1 2.1 107 s- 1 3.0 107s - 1 2.3 107s - 1 3.1 107 s- 1 2.5 107 s- 1
Flame 3 2.8 107 s- 1 3.0 107s - 1 3.0 107 s- 1
firms first that the quenching is not very sensitive to temperature. It suggests secondly that water, the concentration o f which rises quickly in the reaction zone, remains the most efficient quencher. Its mole fraction determines for a large part the global quenching rate which thus changes with the stoichiometry. Quenching by radical species (including chemical quenching), which might have been expected to be very strong, appears to be on average equal or slightly higher than the quenching by H 2 0 so as to compensate for the decrease o f the temperature. CONCLUSIONS
Ref. [7l Ref. [8]
The results of our experiments performed in low pressure flames indicate that the quenching rate of
70
D. STEPOWSKI and M. J. COTTEREAU
[OH]' (m -~ ]
5 . 1 0 2"1
1 2 3
0
,
.
•
•
1 2 3 4 5 6 7 8 Z(mm) Fig. 5. OH concentration profiles for the three flames studied; these profiles have been obtained by the laser-induced fluorescence method described in Ref. [6]. OH is nearly constant through the reaction and burnt gas zone of a given flame. It may be thought that this result remains valid for higher pressure flames where direct measurement is not achievable. Thereafter reliable relative measurements o f local and instantaneous number density could be derived from the fluorescence intensities in atmospheric flames. Accordingly, local rotational temperatures could be obtained by varying the lower level pumped (if no trapping occurs). Furthermore, extrapolation of the measured low pressure quenching rate gives an estimate o f the quenching rate in a higher pressure flame. In this way the fluorescence efficiency can be determined and an absolute value of [OH] can be estimated from the laser-induced fluorescence measurement. Hopefully, for flames with other fuels, similar results will be obtained; a mean quenching cross section derived from data of Refs. [7 and 8] will give a good approximation to the fluorescence efficiency if direct measurement with a low pressure flame is not available.
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
REFERENCES 1. Penny, C. M., Morey, W. W., St. Peters, R. L., Silverstein, S. D., Lapp, M., and White, D. R., Study of resonance light scattering for remote optical probing.
NASA C.R. 132363, Sept. 1973, by General Electric Corporate System and Development, P. O. Box 8, New York 12301. Demtroder, W., High resolution spectroscopy with lasers. North-Holland Publishing. Physical Reports (SectionCofPhysicalLetters) 7(5):223-277 (1973). Daily, J. W., Applied Optics 15(4):955-960 (April 1976). Daily, J. W., Applied Optics 16(3):568-571 (March 1977). Baronavski, A. P., and McDonald, J. R., Theoret. J. Chem. Phys. 66(7) (April 1977). Stepowski, D., and Cottereau, M. J., Applied Optics 18 (February 1979(. Carrington, T., Theoret. J. Chem. Phys. 30(4): 1087-1095 (April 1959). Hooymayers, H. P., and Alkemade, C. Th. J., J. Quant. Spectrosc. Radiat. Transfer. 7:495-504 (1967). Bennet, R. G., and Dalby, F. W., Theoret. J. Chem. Phys. 40(5):1414-1416 (March 1964). Lengel, R. K., Crosley, D. R., Theoret. J. Chem. Phys. 67(5):2085-2101 (September 1977). German, K. R., Theoret. J. Chem. Phys. 62(7): 2584-2587 (1975). Dieke, G. M., and Crosswhite, H. M., J. Quant. Spectrosc. Radiat. Transfer. 2:97-199 (1962). Eckbreth, A. C., Bonczyk, P. A., Shirley, J. A., Experimental investigations ofsatured laser fluorescence and CARS spectroscopic techniques for practical combustion diagnostics. Technical reports (Feb. 27, 1978), United Technologies Research Center. East Hartford, Connecticut 06108.
Received 2 January 1979; revised 8 March 1979
65
Study of the Coilisional Lifetime of Hydroxyl (2E+, v' = 0) Radicals in Flames by Time-Resolved Laser-Induced Fluorescence D. STEPOWSKI and M. J. COTTEREAU Laboratoire de Thermodynamique, L.A. au C.N.R.S. n ° 230, Fucult~des Sciences et des Techniques de Rouen, Universitlde Rouen, B.P. 67 76130 Mont-Saint-Aignan, France
Collisional lifetimes of OH (2~ +, ~., = 0) radicals excited by tunable laser pulses have been measured in three different low pressure C 3H 8 - O 2. Profiles of the quenching rate. in the reaction and burnt gas zones are given. The pressure dependence has also been studied, as well as the effects of rotational redistribution. It is concluded that a mean constant quenching rate can be used in a given flame.
INTRODUCTION
essential for turbulent flame studies where the flame front may wander.
Measurement of local and instantaneous number density of species by laser-induced fluorescence has appeared recently as a new and attractive tool in combustion research [1, 2]. This highly sensi-
This paper reports a direct lifetime measurement of OH excited to a particular rotational level of the (2~+, v' = 0) state by a tuned laser pulse. Experiments were performed at many points in the burnt gas and reaction zone, and for several
rive method is very promising for measurements of low concentrations [3] when fluorescence trapping is negligible. Unfortunately, the fluorescence intensity depends on the quenching due to collisional deexcitation. The methods that have been proposed [4, 5, 6] to avoid this dependence are still very difficult to achieve under strong quenching conditions such as are met in high pressure media; therefore, knowledge of the quenching is still required for single-shot measurements. Published studies of the quenching of excited hydroxyl have been made with conventional exciting sources which allow only indirect measurement via fluorescence efficiency [7, 8, 9]. Despite the relatively large quenching cross sections found, particularly for H20 , quenching is assumed to be mainly governed by collisions without chemical reaction. This assumption becomes questionable in the reaction zone of a flame where physical quenching by many other species, including radicals, is unknown. Understanding such quenching is Copyright © 1981 by The Combustion Institute Published by Elsevier North Holland, Inc. 52 Vanderbilt Avenue, New York, NY 10017
stoichiometries.
THEORY To remind the reader of the quenching dependence of the fluorescence intensity, let us briefly give a simple two-level calculation here. Let N l be the population of the lower level. An exciting pulse of spectral energy density Uv increases the population N 2 of the upper level according to
dN2 -NxB12Uv-N2B21Uv dt - ( A 2 1 + Q21)N2'
(1)
where B12 and B 21 are the probability coefficients for stimulated absorption and emission; A 2 l and Q21 are the probability coefficients of radia-
0010-2180/81/010065+06502.50
66
D. STEPOWSKI and M. J. COTTEREAU
rive and collisional spontaneous relaxation. Writing N 1 + N 2 -- No, one obtains
No~12U~
N2(t) =
(B12 +B21)Uv +A21 + Q21 × [1--e--t(Bx2+B21)Uv+A21+O21]t]. (2) After an excitation of duration At, Uv drops to zero and N z can be seen from Eq. (1) thereafter to decrease according to e-(A21+Q21)t: The photon flux collected in the solid angle ~ from a volume V is A 21 ~2 V
¢(t) = - -
N 2 (t).
Under steady-state saturation conditions, Uv >> 4zr(Bl 2 + B21) is independent of the quenching, However, this condition is very difficult to achieve in real flames. The near saturation method pioneered by Baronavski [5] with a stationary flame is not applicable to single-shot measurements, We have proposed an alternative [6] to this saturation approach: If the pulse duration is very much shorter than quenching, radiative, and pumping times, the expansion of the exponential in Eq. (2) can be approximated by the first term and the population N2 increases linearly leading to a maximum fluorescence flux 4~M = A21 ~2VNoB 12 UuAt/47r, independent of the quenching rate which nevertheless can be obtained from the time-resolved fluorescence decay. This result, which is in fact due to the neglect of the negative terms in Eq. (1) solved with constant NI, remains valid for a multilevel system: N 2 is then the sum E N2j, of the upper levels populated from the laser excitation and consecutive rotational redistribution; N 1 is the population of the particular rotational lower level involved in the transition, The expression then, is,
(4)
dt so that I Z N 2 j ' Iraax =N1Bx2UvAt.
(6)
A2/' is nearly constant [1 1] but Q2/' may depend on the ]' rotational level involved. Measurement of the whole fluorescence decay, if exponential, gives a mean quenching rate Q according to ~ t ) = dPmaxe_(A+Q)t"
EXPERIMENTAL
(.421 + Q21)/(B12 +B21), and ¢ =A21V~BI2No/
-N1B~2U.,
dY~N2f - - ~(A2j, + Q2j,~2j ~, dt
(3)
4n
d~, N2j
It is noticeable that this result is independent of the rotational redistribution rate which appears to be rather complex [10]. After the excitation
(5)
For obvious reasons we have chosen to study first the quenching rate of the OH radical in a low pressure flame (15 torr < P < 80 torr) to obtain a quenching time longer than our pulse duration (4 ns). Since the greatest absorption occurs in the 2 ~+(v' = 0) +- 2 II(v" = 0) transition with ] < 15 [12], we have excited a line of this band, usually the Q17 line (~ = 308,9734 nm). The tunable exciting radiation that is focused in the flame is derived from the second harmonic of a dye laser pulse generated in a KDP crystal at a repetition rate of 20 Hz. An intracavity Fabry-Perotetalon ensures a laser spectral width (AX ~ 2.10 -12 m) closely matching the absorption line in the low pressure flame. Flum:escence light emanating in a direction at 90 ° from the exciting radiation is collected with F/5 optics, focused on the entrance slit of a monochromator and detected by a photomultiplier connected to an oscilloscope or to a boxcar analyzer. The horizontal flat flame is set up in a low pressure enclosure on a cylindrical porous burner (¢ = 2 cm) supplied with a premixed oxygen-propane flow. The enclosure is equipped with windows for excitation and a fluorescence light and with a port for introducing a thermocouple. RESULTS AND DISCUSSION When the laser is tuned on a line, the fluorescence signal is adequately reproduced on the oscilloscope, triggered in a single sweep mode but measurements are made more reliable by averaging ten
QUENCHING OF EXCITED HYDROXYL
67
Ltx3~ T
0
50ns
lOOns
150ns
Fig. 1. Example of a fluorescence signal as averaged by the boxcar over ten shots.
shots with the boxcar analyzer. This fluorescence radiation is detected at the same wavelength with a spectral width of 3 nm corresponding to the band 0, 0 of interest. Fig. 1 shows such a boxcaraveraged signal where the straight line derived
from logarithm conversion confirms the exponential shape of fluorescence decay and thus allows the derivation of a mean quenching time. According to Crosley and Lengel [10], redistribution of populations among the various rota-
O (~")T J J f J
•
f
10
/
f
/
•
•
•
/
5
0
0
I
|
|
=,=
20
40
60
P (torr)
Fig. 2. Quenching rate versus flame pressure.
68
D. STEPOWSKI and M. J. COTTEREAU
tional levels of the upper state is strongly dependent on the particular level directly pumped (and on its spin). Then, excitations from various lower levels give rise to different distributions on the upper level. Since the quenching toward the ground state could vary with the rotational level, the mean quenching and then the fluorescence efflciency may depend on the line chosen for pumping. When different absorption lines ( P l l , P12,
results. Fig. 2 shows how the quenching rate increases linearly with the pressure; beyond 50 torr the observed deviation from linear dependence is due to the exciting pulse duration itself. A 106 s -1 is too low to appear on this plot. Extrapolation to 1 atm leads to a quenching rate Q 109 s - l which is close to the quenching found by Eckbreth for CH and CN in flames [13] but smaller by two orders of magnitude than the
P13; Q13, QI4, Q15, Q16, Q17, Q21, Q24, Q25;
quenchingfound by Baronavskiand McDonaldfor
R22 ) of the 2E(u' = 0) ~ 21I(o" = 0) band of OH were excited, no noticeable variation of the fluorescence decay time was found. It can be deduced that relative measurements of fluorescence intensities (not dependent on quenching) give true relative populations of the lower levels successively excited, and thus the true rotational temperature, if fluorescence trapping is negligible, All the following experiments were performed on the Q17 line. The pressure influence on the fluorescence decay time in the burnt gas zone of the stoichiometric oxygen-propane flame was studied, This experiment was performed at 10 mm above the burner where the gradients are very weak for each flame studied so that change of the flame position with pressure does not affect the
C2 [5]. On Fig. 3, we have plotted the quenching rate versus the height of the exciting spot above the burner plane for several oxygen-propane flames of different stoichiometries. The quenching rate remains nearly constant through the flame in spite of temperature variations in the zone studied, as shown in Fig. 4. These temperatures have been measured with a coated Pt-Pt Rh 10% thermocouple and corrected for radiative losses. Such a relative independence of quenching on temperature can also be deduced from the results of Fig. 2 where the flame temperature decreases from 2,000°K to 1,700°K as heat losses at the burner increase with the pressure. Assuming the major species concentrations [ H 2 0 ] , [CO2], and [02] to be close to their
O (10~ 4)
3
|
~ •
.
; -
_•
! -"
J
3 J2
w
I
2
I I I
1
i i luminous zone I
0
;
2
3
4
;
;
i
;
Z(mm)
Fig. 3. Quenching rate along flames of different stoichiometries: Molar ratio C3H8/O 2 = O; flame 1, 0 = 0.20; flame 2, ~ = 0.11; flame 3, ¢ = 0.25; precision: A Q / Q ~ 10%.
QUENCHING OF EXCITED HYDROXYL
69
5oo
1
2
3
4
5
6
7
8
Z(mm)
Fig. 4. Temperature profiles for three flames studied.
equilibrium values in the burnt gas of flames 1 and 2, one can calculate a mean quenching rate from the specific cross sections given in Ref. [7] or [8] (Table 1). The values thus calculated are compared with our results in Table 2. The calculation for flame 3 was made with a CO cross section taken equal to that of CO 2 (~16 A2). If the uncertainties of the procedure allow such a deduction, this table shows that data o f Ref. [8] fit our results better. The result found for the rich flame, 3, suggests that the cross section o f CO is not very different from that of C O 2 . In the reaction zone where drastic concentration changes occur, as shown in Fig. 5, the quenching is surprisingly near constant. This result con-
TABLE 1 Specific Quenching Cross Sections H20
02
CO2
34 A 2 37 A 2
7 A2 10 A 2
16 A 2 -
TABLE 2 Mean Quenching Rate Flame 1 From Ref. [7] FromRef.[8] Our results
Flame 2
2.8 107 s- 1 2.1 107 s- 1 3.0 107s - 1 2.3 107s - 1 3.1 107 s- 1 2.5 107 s- 1
Flame 3 2.8 107 s- 1 3.0 107s - 1 3.0 107 s- 1
firms first that the quenching is not very sensitive to temperature. It suggests secondly that water, the concentration o f which rises quickly in the reaction zone, remains the most efficient quencher. Its mole fraction determines for a large part the global quenching rate which thus changes with the stoichiometry. Quenching by radical species (including chemical quenching), which might have been expected to be very strong, appears to be on average equal or slightly higher than the quenching by H 2 0 so as to compensate for the decrease o f the temperature. CONCLUSIONS
Ref. [7l Ref. [8]
The results of our experiments performed in low pressure flames indicate that the quenching rate of
70
D. STEPOWSKI and M. J. COTTEREAU
[OH]' (m -~ ]
5 . 1 0 2"1
1 2 3
0
,
.
•
•
1 2 3 4 5 6 7 8 Z(mm) Fig. 5. OH concentration profiles for the three flames studied; these profiles have been obtained by the laser-induced fluorescence method described in Ref. [6]. OH is nearly constant through the reaction and burnt gas zone of a given flame. It may be thought that this result remains valid for higher pressure flames where direct measurement is not achievable. Thereafter reliable relative measurements o f local and instantaneous number density could be derived from the fluorescence intensities in atmospheric flames. Accordingly, local rotational temperatures could be obtained by varying the lower level pumped (if no trapping occurs). Furthermore, extrapolation of the measured low pressure quenching rate gives an estimate o f the quenching rate in a higher pressure flame. In this way the fluorescence efficiency can be determined and an absolute value of [OH] can be estimated from the laser-induced fluorescence measurement. Hopefully, for flames with other fuels, similar results will be obtained; a mean quenching cross section derived from data of Refs. [7 and 8] will give a good approximation to the fluorescence efficiency if direct measurement with a low pressure flame is not available.
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
REFERENCES 1. Penny, C. M., Morey, W. W., St. Peters, R. L., Silverstein, S. D., Lapp, M., and White, D. R., Study of resonance light scattering for remote optical probing.
NASA C.R. 132363, Sept. 1973, by General Electric Corporate System and Development, P. O. Box 8, New York 12301. Demtroder, W., High resolution spectroscopy with lasers. North-Holland Publishing. Physical Reports (SectionCofPhysicalLetters) 7(5):223-277 (1973). Daily, J. W., Applied Optics 15(4):955-960 (April 1976). Daily, J. W., Applied Optics 16(3):568-571 (March 1977). Baronavski, A. P., and McDonald, J. R., Theoret. J. Chem. Phys. 66(7) (April 1977). Stepowski, D., and Cottereau, M. J., Applied Optics 18 (February 1979(. Carrington, T., Theoret. J. Chem. Phys. 30(4): 1087-1095 (April 1959). Hooymayers, H. P., and Alkemade, C. Th. J., J. Quant. Spectrosc. Radiat. Transfer. 7:495-504 (1967). Bennet, R. G., and Dalby, F. W., Theoret. J. Chem. Phys. 40(5):1414-1416 (March 1964). Lengel, R. K., Crosley, D. R., Theoret. J. Chem. Phys. 67(5):2085-2101 (September 1977). German, K. R., Theoret. J. Chem. Phys. 62(7): 2584-2587 (1975). Dieke, G. M., and Crosswhite, H. M., J. Quant. Spectrosc. Radiat. Transfer. 2:97-199 (1962). Eckbreth, A. C., Bonczyk, P. A., Shirley, J. A., Experimental investigations ofsatured laser fluorescence and CARS spectroscopic techniques for practical combustion diagnostics. Technical reports (Feb. 27, 1978), United Technologies Research Center. East Hartford, Connecticut 06108.
Received 2 January 1979; revised 8 March 1979