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Use of laser-induced fluorescence of OH to study the perturbation of a flame by a probe
Eighteenth Symposium (International) on Combustion
The Combustion Institute, 1981
U S E OF L A S E R . I N D U C E D F L U O R E S C E N C E OF O H TO S T U D Y T H E P E R T U R B A T I O N OF A F L A M E BY A P R O B E D. STEPOWSKI, D. PUECHBERTY, AND M. J. C O T T E R E A U Universit~ de Rouen, Facult~ des Sciences et Techniques Laboratoire de Thermodynamique (L.A. au C.N.R.S. N'~230) 76130 Mont-Saint-Aignan, France
The paper presents the first use of laser-induced fluorescence (LIF) of OH to overcome a problem irresolvable otherwise (owing to the spatial resolution needed). Previous work had shown a discrepancy between OH concentration profiles in a low pressure flame measured by mass spectroscopy and optical absorption, especially in the preheated zone. The first part of this work is devoted to a remake of the measurement by absorption of a pulsed dye laser beam tuned on a transition of OH, and to a LIF measurement of the IOnl profile. They both confirm the previous conventional absorption measurements. A detailed study by LIF of IOHI perturbation by the probe is then presented. It leads to the following conclusions: The quartz cone of a mass spectrometer sampling probe reduces the OH concentration in the preheating zone of the low pressure 02 - - C3H ~ flame investigated. The extension of the perturbation is comparable to the probe penetration into the reactive zone of the flame. Therefore, the effect is attributed to a perturbation of the diffusion field by the probe. The OH concentration measured by mass spectrometry is the actual value in the sample. As predicted by the literature, the extension of the sampling zone is two diameters of the probe nozzle.
Introduction In recent years molecular beam sampling and subsequent mass-spectrometric analysis of the sample have been used to determine stable and unstable species concentration profiles in flames often burning at low pressure.'2' .3 In spite of the number of works devoted to the study of the various perturbing effects of probing (in general a'5 and in the particular case of molecular beam sampling ~'7) this question is far from being closed. Some years ago our team worked on this subject, especially to examine the credibility of the unstable species profiles determined by mass spectrometry which are obviously the more sensitive to the probe effects and also to the quantitative analysis procedure of the sample. We compared s the OH concentration profiles in a low pressure flame measured by molecular beam mass spectrometry and by absorption spectroscopy (which is a non-perturbing method). Agreement between the two profiles was good except in the preheating zone of the flame for which the large
discrepancy was attributed to a probe effect localized in its vicinity. Unfortunately, no direct proof of this assumption could be made since the absorption method does not allow point resolution. Use of the new method of local measurement of IOHI by laser-induced fluorescence (LIF), 9'm with the particular procedure developed by us", gives us the opportunity to perform the first direct application of LIF to the study of a given unsolved problem. As the work has to be done on OH, it is worth emphasizing that up to now no local method of measurement, except LIF, was utilizable.
I. Definition of the Problem The experimental arrangement (Fig. 1) is the one used in the work reported in Ref. 8; a detailed description can be found in Ref. 12. The vertical fiat flame (horizontal gas flow) is formed in a low pressure enclosure (25 torr) on a cylindrical porous burner (diameter = 7 cm) supplied with a premixed
1567
1568
C O M B U S T I O N DIAGNOSTICS
incident laser beam
--~ section
molecular
beamAv
~
'
,
/~ b u r n e r
-
Elf C ,.
fll
L I F meas. ~.
sj
,to pump
I
I L.to0uo0
absorption m e a s u r e m e n t FIG. 1. Disposition of the different measurement devices around the flat flame set up in the low pressure enclosure by a cylindrical translatable porous burner. One can see the molecular beam sampling probe and the optics for absorption measurement on the left hand side, the optical arrangement for fluorescence measurement on the right hand side.
p r o p a n e / o x y g e n flow (13% Call s, 87% O~). The molecular beam system consists of three differentially p u m p e d stages, the first one equipped with a conical thin walled deactivated quartz sampling probe, 65 mm high, with an external angle of 40 ~ and a 0.1 m m diameter orifice. Optical measurement through this flame was performed by analyzing the absorption of the U.V. light emitted by a conventional continuous source with a 1.5 m grating spectrometer. The spatial resolution was about 0.5 mm. For each point the rotational temperature determined from the Boltzmann diagram was very close to the thermocouple temperature (from 1300 K in the fresh gases to 2000 K in the burnt gases). Figure 2 shows the OH mole fraction profiles given by the two methods. Curve a is the massspectrometry result; since visual observation of the flame and the thermocouple measurements showed no distortion of the flame front (due to an eventual thermal effect of the probe) no correction was made to the distance scale. Curve b is the classical absorption result; use of a reasonable value of )coo = 10 -3 for the oscillator strength of the (0,0) 2~ ~-- 211 transition of O H gave a good fit in the b u r n t gas zone, The differences between these two curves are of two kinds. The most important fact is the weaker value of IOHI found by mass spectrometric measurement in the preheating zone, but there is also a change in the slope of the curves. In view of
[o.] 4.10 =1m-a
p3
2
0
i 2 dl ,~ 5 distance from the burner surface (mm)
Fl(;. 2. Hydroxyl concentration profiles as measured by three techniques. Curve a, points marked 9 : mass spectrometry result. Curve b, points marked A: absorption spectroscopy with a continuous U.V. source. Curve c, points marked *: laser absorption spectroscopy.
STUDY THE PETURBATION OF A FLAME BY A PROBE the good agreement found in the burnt gases zone (in absolute number density value), it may be thought that the two procedures are correct, thus the sampling and the mass spectrometer calibration seem valuable. Therefore, the low IOHIvalues measured in the preheating and reacting zones by mass spectrometry would be the actual values in the gases sampled there. These values are not the same as those measured by absorption because here the composition of the gases in the vicinity of the probe has been perturbed by the probe itself. The spatial extension of this perturbation is weak and the absorption measurement which is integrated over 7 centimeters does not take it into account, therefore optical absorption measurement gives the unperturbed IOHI value. The main purpose of the work reported here is to study the perturbation in order to verify this hypothetical statement. The difference between the two slopes of the curves could eventually be explained by the poor spatial resolution of the absorption measurement. Our first task was to confirm the absorption profile by using the laser absorption technique with its associated spectral and spatial resolution.
II. Laser Absorption Measurements
1569
the experimental arrangement. The laser beam is focused in the flame by F/100 optics providing an actual spatial resolution of 0.15 mm. This value is the maximum diameter of the laser beam through the flat flame. The beam is then filtered by a passband monochromator to eliminate the wide band super-radiant light. The transmitted light is detected by a photomultiplier connected to a boxcar analyzer averaging the gated signal over ten shots.
B. Calculation The parameter directly provided by experiment when the laser is turned to the frequency vo is the spectral absorptivity
A(vo) =
I I(v - Vo) dv - S I(v - Vo) e - ~ d v
5 I(v - %) dv
(1)
where I(v) is the spectral intensity distribution of the laser and K, is the absorption coefficient which is assumed to be constant along the optical path L through the fiat flame. The useful quantity is the line area
I nAv~176 tl I(v - Vo) e-K'L dv [
In this technique spectral discrimination is achieved in the dye laser cavity before absorption, rather than with a spectrometer after absorption as in the classical scheme. Consequently great energy is centered around the absorption line and the signal is enhanced so that spatial resolution can be improved by diaphragming down the beam. In addition, the use of a pulsed laser associated with gated detection and integration increases the SNR by a factor 108.
By exchanging the order of integration for the double integral, one can easily find the relation
A. Apparatus
For the optically thin case where K,L < < 1 the expansion of the exponential can be approximated by the first term and we obtain:
The incident radiation, tunable around 309 nm is derived from the second harmonic of a dye laser pulse (4 ns duration) generated in a KDP crystal. The dye oscillator is pumped by a nitrogen laser at a repetition rate of 20 Hz. Frequency narrowing is accomplished by means of a Littrow grating at one end and a Fabry-Perot etalon inside the cavity. This etalon has been chosen to assure a laser spectral width matching the absorption line in the low pressure enclosure (Ah --~ 2.10 -1~ m). In any case, we will show that a precise knowledge of the laser spectral function is not necessary, as long as the rotational structure of the absorption spectrum is resolved. The spectral sweeping is achieved by varying the pressure and therefore the index of refraction in the enclosure containing both the Favry-Perot etalon and the grating. Figure i shows
S I(v - v o) dv
[ dv o (2)
I I,,e A(Vo)dVo= I 'i,,o (1 -- e -~'e) dv
A(vo)dv o = L I
I
Kodv
line
(3)
(4)
line
The population N r of the particular rotational level involved is given by the well known relation 13 e2L
L S Kflv = -
4%mc
Nj.Frr,
(5)
where the line oscillator strength F r r is related to the band oscillator strength f,. by the relation Sj ,j ~foo F,.,. -
2J"+ i
(6)
1570
COMBUSTION DIAGNOSTICS
using the Honl-London factor S r r normalized according to the rules advocated by Tatum. 14 The total population NOin the fundamental state is related to the population Nj, by a Boltzmann distribution:
NO =
- -
Nj.
~
=
ZRJ" eERj./kT
(7)
2J"+ 1
where Zar is the partition fuction of the rotation and Ear the rotational energy. If the medium is not optically thin, the relation between S A(v) dv and S Kflv is no longer exactly linear and a correction must be made according to the curve of growth first computed by Van der Held. ~
Q~7
C. Results We made our measurements on the Q~7 line (h = 308.9734 nm) of the 2E(v' = 0) ~ 2II(v" = 0) transition of OH. As the population of the J" = 7 lower level of this transition is close to the maximum of the distribution function ~6 in the temperature range 1000 K - 2000 K, therefore this population is almost independent of the temperature in this range. The choice of this strong absorption line also allows precise measurement in the preheating zone where IOHI is weak. Nevertheless, in the burnt gases zone this absorption remains low enough (K~L < 1) to use the Van der Held curve with a good precision (in our low pressure flame the line shape is pure Doppler). Figure 3 shows an example of absorption in the burnt gases zone: The second absorption line is the Q23 line and its presence is useful for calibrating the wavelength sweeping scale. The absolute IOHI profile obtained (still using the value f~, = 10 -3) by the laser absorption technique is in good agreement (Fig. 2) with the one we had obtained by classical absorption method; the slight difference at the burner can be explained by the improvement of the spatial resolution.
III. Laser-Induced Fluorescence Measurements
A. Experimental The incident optical arrangement is the same as for the absorption measurement (Fig. 1), but the focusing optics aperture is increased. For each set of measurements, the axis of this incident beam is located at a given distance from the tip of the cone and the flame front is explored by translating the burner. The fluorescent light due to radiative relaxation of the excited species is collected by F / 5 optics
308.973
308.986
wavelength (rim)
Fro. 3. Absorption of the laser beam in the burnt gas zone of the flame versus wavelength. Calculations are made from the Q~7 line; the line on the right hand side is a superposition of several unsolved lines.
and focused on the orifice of a photomultiplier. A pass-band filter is interposed to remove the light emitted by the flame itself. This device makes it possible to detect the fluorescence from a small volume formed by the focused beam section (~b < 0.1 mm) and the height of the detected zone (0.2 mm). The photomultiplier is connected to an oscilloscope and a boxcar analyzer. The measurements were performed according to our time resolved LIF technique. The main problem of LIF is the quenching dependance of the fluorescence because of collisional de-excitation of the excited species. Saturation of the transition is usually proposed to avoid this dependance.17"s We have developed'~ an alternative to this approach, based upon short laser excitation. This technique is particularly suitable for low pressure measurements where the quenching time is much longer than the pulse duration. In this case, after a short excitation of duration At the photon flux collected in the solid angle f~ from a volume V is ~(t) =
N o B U~ At V fl A a4~r
exp
[
I
-(A+Q)
t
(8)
STUDY THE PETURBATION OF A FLAME BY A PROBE where B, A and Q are the probability coefficients for absorption, radiative relaxation and collisional relaxation, respectively. U, is the mean spectral energy density of the exciting pulse. Let us note that qbis not sensitive to the focused zone area (which is difficult to measured), but only to the length of the excited detected zone. a, the ratio of the total population to the population of the particular level being pumped usually depends on the temperature, but as pointed out before, for the Q~7 line a is nearly constant in the temperature range investigated. Moreover, by centering the gate (1 ns duration) of the boxcar analyzer on the top of the fluorescence decay signal (25 ns duration) we obtain:
1571
[o.] 4~ lO~'ms
3
2
9 (t = 0) ~ 4~r No -
BUo At V I I A
(9)
which does not depend on Q.
1
B. OH Profile by LIF
inci
3
4
distance from the burner surface
We first used this technique to the measurement of the IOHI profile in the flame zone outside of the influence of the probe. To obtain an accurate profile, it was necessary to reduce both the absorption of the exciting light before the measurement point and florescence trapping. To shorten the optical path, the point measurements (V = 0.02 mm 3) were made at the point A (Fig. 4) rather than at the center of the flame. The result is given in Fig. 5 and compared with the profile provided by laser absortion. For each curve, the reproductibility and the relative uncertainties are better than 10%. Agreement between the two curves in absolute value may be estimated to be better than 50%; these curves have been fitted in the burnt gases zone.
T
2
~
beam
edge of the homogeneous flat flame
FIG. 4. Scheme of the different optical paths involved in laser-induced fluorescence measurements. Fluorescence signals from point A provide accurate concentration values, f l u o r e s c e n c e signals from zone B provided only relative value (because of fluorescence trapping).
(ram)
FIG. 5. Hydroxyl concentration profiles measured by laser techniques with the Q~7 line. Solid line: laser absorption measurement. Dashed line: laser induced florescence measurement.
C. Exploration of the Probe Perturbation The vertical incident beam intersects the sampling cone axis at a distance d from the top. Exploration of the fluorescent image of this exciting beam by shifting the detector height provides a IOHIradial profile. For each distance d, several radial profiles at different depths in the flame front are obtained by translating the burner. Under this experimental situation (Zone B of Fig. 4), absorption of the exciting light before the measurement point and fluorescence trapping are no longer negligible, but the induced signal distortion is the same for all the investigated points of the small extended perturbation zone. Consequently each radial profile is accurately described in relative value. Figure 6 shows such a set of radial profiles. With regard to the sampling problem, the most interesting results are those obtained on the probe nozzle axis. For every radial profile the IOHI minimum is located on this axis. Direct comparison between these profiles is not achievable because incident beam absorption and fluorescence trapping are not the same for different profiles. For each profile, we have measured the ratio I , , / I M of the minimum to the maximum of fluorescence intensity. In each case, this ratio was equal to the corresponding ratio IOHL/IOHIM. Results are shown in Fig. 7 where these ratios are plotted as a function of the distance d for different positions of the burner.
1572
C O M B U S T I O N DIAGNOSTICS Thus, each curve corresponds to a given position of the optical measurement point in the flame front. From Fig. 2, we have deduced the ratios of IOHI measured by mass spectrometry to the IOHI measured by absorption spectroscopy at the same four positions in the flame front, and we have reported the values on the corresponding curves on Fig. 7. They lie on a vertical line corresponding to d = 0.2 mm, which may be thought to be the actual mean distance of sampling by the probe. This experimental determination is in good agreement with computed values found in the literature t9 which gives a sampling zone length of two orifice diameters from the orifice (d~ orifice is 0.1 mm).
probenozzleaxis I I Ia
[oHl.
D. Discussion
r(mm) FIc. 6. Example of a set of relative tOI-II radial profiles in front of the probe measured by laser induced fluorescence for different positions of the probe in the flame. Curve a: burner-LIF measurement point distance = x = 4 mm (probe tip in burnt gas). Curve b: x = 2.7 mm (probe tip in reactive zone). Curve c: x = 0.6 mm (probe tip in preheating zone). Each time, the exciting laser beam intersects the sampling cone axis at a distance d = 0.3 mm from the cone top.
Results from point fluorescence measurements performed in the vicinity of the probe confirm the assumption we had made from the study of Fig, 2. There is an important local probe effect only in the preheated zone, it is weaker in the reactive zone and vanishes in the burnt gas zone. We have demonstrated that the low IOHI value measured by mass spectrometry in this zone is the actual IOHI in the gases sampled there as perturbed by the probe. From IOHI radial profiles we can point out that, as the cone penetrates deeper and deeper into the reactive zone the amplitude of the perturbation increases. Likewise its spatial extension increases from (3.5 mm high x 0.6 mm depth) at 2,7 mm from the b u r n e r to (9 mm high x 1.5 m m depth)
[OH]. 1
/- ' ~a
b
9
~
"
l
iO
a: b: c: d.
.,... T
0
[~
,
,
_,,,.
x=4mm x= 2.7ram x=l.3mm x =0.6ram
d (mini
--
0.5
1
1.5
FIG. 7. Perturbation of ]OH] in the axis of the sampling cone. The figure gives the m i n i m u m of ]OH] normalized by the corresponding unperturbed (maximum value versus the distance d between the probe tip and the exciting beam. Each curve corresponds to a given value of the distance x between the burner and the L I F measurement point. See the text for the point marked ~
STUDY T H E PETURBATION O F A F L A M E BY A PROBE at the burner, as can be deduced from fig. 6 and fig. 7. These orders of magnitude seem to be in agreement with the first results of work pursued at the laboratory 2~ to modelize the perturbation. Let us now try to explain the data. We remind the reader that no visible nor appreciable thermic perturbation of the flame was found, and that the sample probe does no severely perturb the stable species concentrationsf ~ This is not true for OH in the region of the flame where its presence is due to backward diffusion, i.e. in the preheated zone and in the beginning of the reactive zone. The probe and the boundary layers situated on it act like a sink for this species and disturb the backward diffusion field so that the sample gas contains less O H than the unperturbed gas. In the regions where the diffusion field is in the flow direction (second part of the reactive zone) or in the post-flame gases where there is no appreciable concentration gradient, this sink effect of the probe does not affect the flow ahead of it. As the molecular beam system is adapted to prevent recombination of radicals inside the probe the measurement of OH in these regions is valid.
Conclusion The primary result of this work lies in the successful use of LIF to solve an experimental problem. Thanks to its very precise spatial resolution, a perturbation of only 5 mm or less in diameter was analyzed through a homogeneous flame of 7 cm in diameter. However the results of the study are interesting in their own right and lead to the following conclusions: (i) The quartz cone of a mass spectrometer sampiing probe reduces the OH concentration in the preheating zone of the flame investigated. The extension of the perturbation is comparable to the probe penetration into the reactive zone of the flame. Thus, the effect may be attributed to a perturbation of the diffusion field by the probe. (ii) The OH concentration measured by mass spectroscopy is the actual value in the sample. (iii) As predicted by the literature, the extension of the sampling zone is two diameters of the probe orifice. In addition, a direct comparison of IOHIprofiles measured by laser absorption and LIF has been performed leading, to a good agreement.
Acknowledgements We are grateful to I.F.P. for providing a bursary to one of us (D.S.).
1573
REFERENCES 1. B1ORD1,J. C., LAZZARA,C. P., ANDPAPP,J. F.: Fourteenth Symposium (International) on Combustion, p. 367, The Combustion Institute, 1973. 2. VANDDOREN,J., PEETERS, J., AND VAN TIGGELEN, P. J.: Fifteenth Symposium (International) on Combustion, p. 745, The Combustion Institute, 1974. 3. PEETESS,J., ANDMAHNEN, G.: Fourteenth Symposium (International) on Combustion, p. 133, The Combustion Institute, 1973. 4. TINE', G.: "Gas sampling and chemical analysis in combustion processes". Pergamon Press, 1961. 5. FmswsoM, R. M.: WESTENRERG,A.A.: Flame structure, McGraw-Hill series in Advanced Chemistry, 1965. 6. HAYHURST,A. N., ANDKITTLESON,D. B.: Combustion and Flame, vol. 28, n ~ 2, p. 137, 1977. 7. B1ORDI,J. C., LAZZARA,C. P., ANDPAPP,J. F.: Combustion and Flame, vol. 23, p. 73, 1974. 8. REVET,J. M., PUECHBERTY,D., ANDCOTTEREAU,M. J.: Combustion and Flame, vol. 33, p. 5, 1978. 9. DAILY, J. W.: Applied Optics, vol. 15, p. 955, 1976. 10. WANG,C. C. ANDDAVISJR., L. 1.: Phys. Rev. Lett., vol. 32, p. 349, 1974. 11. STEPOWSKI, D. AND COTTEREAU, M. J.: Applied Optics, vol. 18, p. 354, 1979. 12. REVET, J. M.: "Analyse par spectrom6trie de masse de flamme bassepression. Comparaison avec la spectroscopie d'absorption sur le radical OH". Thbse de Doctorat du 3brae cycle, 18 octobre 1977, Universit6 de Rouen, Facult6 des Sciences, 76130 Mont-Saint-Aignan. 13. THORNE, A. T.: "Spectrophysics", p. 307. Chapman and Hall Ltd 1974. 14. TATUM,J. B.: Ap. J. Suppl. vol. 14, p. 21 (1967). 15. VAN DES HELD, E.F.M.: Z. Physik, n ~ 70, p. 508, 1931. 16. DIEKE, G. M., AND CROSSWHITE, H. M.: J. Quant. Spectrosc. Radiat. Transfer, vol. 2, p. 92, 1962. 17. DAILY, J. W.: Applied Optics, vol. 16, p. 568, 1977. 18. BARONAVSKI,A. P., AND McDONALD, J. R.: Applied Optics, vol. 16, p. 1897, 1977. 19. MILNE, T. A. AND GREENE, F. T.: Tenth Symposium (International) on Combustion, p. 153, The Combustion Institute, 1965. 20. GAS(), A.: Thbse de Doctorat du 3bme cycle, to be published in 1980. Facult6 des Sciences de Rouen, Laboratoire de Thermodynamique, 76130 Mont-Saint-Aignan. 21. BIORDI,J. C., LAZZARA,C. P., ANDPAPP,J. F.: Combustion and Flame, vol. 23, p. 73, 1974.
The Combustion Institute, 1981
U S E OF L A S E R . I N D U C E D F L U O R E S C E N C E OF O H TO S T U D Y T H E P E R T U R B A T I O N OF A F L A M E BY A P R O B E D. STEPOWSKI, D. PUECHBERTY, AND M. J. C O T T E R E A U Universit~ de Rouen, Facult~ des Sciences et Techniques Laboratoire de Thermodynamique (L.A. au C.N.R.S. N'~230) 76130 Mont-Saint-Aignan, France
The paper presents the first use of laser-induced fluorescence (LIF) of OH to overcome a problem irresolvable otherwise (owing to the spatial resolution needed). Previous work had shown a discrepancy between OH concentration profiles in a low pressure flame measured by mass spectroscopy and optical absorption, especially in the preheated zone. The first part of this work is devoted to a remake of the measurement by absorption of a pulsed dye laser beam tuned on a transition of OH, and to a LIF measurement of the IOnl profile. They both confirm the previous conventional absorption measurements. A detailed study by LIF of IOHI perturbation by the probe is then presented. It leads to the following conclusions: The quartz cone of a mass spectrometer sampling probe reduces the OH concentration in the preheating zone of the low pressure 02 - - C3H ~ flame investigated. The extension of the perturbation is comparable to the probe penetration into the reactive zone of the flame. Therefore, the effect is attributed to a perturbation of the diffusion field by the probe. The OH concentration measured by mass spectrometry is the actual value in the sample. As predicted by the literature, the extension of the sampling zone is two diameters of the probe nozzle.
Introduction In recent years molecular beam sampling and subsequent mass-spectrometric analysis of the sample have been used to determine stable and unstable species concentration profiles in flames often burning at low pressure.'2' .3 In spite of the number of works devoted to the study of the various perturbing effects of probing (in general a'5 and in the particular case of molecular beam sampling ~'7) this question is far from being closed. Some years ago our team worked on this subject, especially to examine the credibility of the unstable species profiles determined by mass spectrometry which are obviously the more sensitive to the probe effects and also to the quantitative analysis procedure of the sample. We compared s the OH concentration profiles in a low pressure flame measured by molecular beam mass spectrometry and by absorption spectroscopy (which is a non-perturbing method). Agreement between the two profiles was good except in the preheating zone of the flame for which the large
discrepancy was attributed to a probe effect localized in its vicinity. Unfortunately, no direct proof of this assumption could be made since the absorption method does not allow point resolution. Use of the new method of local measurement of IOHI by laser-induced fluorescence (LIF), 9'm with the particular procedure developed by us", gives us the opportunity to perform the first direct application of LIF to the study of a given unsolved problem. As the work has to be done on OH, it is worth emphasizing that up to now no local method of measurement, except LIF, was utilizable.
I. Definition of the Problem The experimental arrangement (Fig. 1) is the one used in the work reported in Ref. 8; a detailed description can be found in Ref. 12. The vertical fiat flame (horizontal gas flow) is formed in a low pressure enclosure (25 torr) on a cylindrical porous burner (diameter = 7 cm) supplied with a premixed
1567
1568
C O M B U S T I O N DIAGNOSTICS
incident laser beam
--~ section
molecular
beamAv
~
'
,
/~ b u r n e r
-
Elf C ,.
fll
L I F meas. ~.
sj
,to pump
I
I L.to0uo0
absorption m e a s u r e m e n t FIG. 1. Disposition of the different measurement devices around the flat flame set up in the low pressure enclosure by a cylindrical translatable porous burner. One can see the molecular beam sampling probe and the optics for absorption measurement on the left hand side, the optical arrangement for fluorescence measurement on the right hand side.
p r o p a n e / o x y g e n flow (13% Call s, 87% O~). The molecular beam system consists of three differentially p u m p e d stages, the first one equipped with a conical thin walled deactivated quartz sampling probe, 65 mm high, with an external angle of 40 ~ and a 0.1 m m diameter orifice. Optical measurement through this flame was performed by analyzing the absorption of the U.V. light emitted by a conventional continuous source with a 1.5 m grating spectrometer. The spatial resolution was about 0.5 mm. For each point the rotational temperature determined from the Boltzmann diagram was very close to the thermocouple temperature (from 1300 K in the fresh gases to 2000 K in the burnt gases). Figure 2 shows the OH mole fraction profiles given by the two methods. Curve a is the massspectrometry result; since visual observation of the flame and the thermocouple measurements showed no distortion of the flame front (due to an eventual thermal effect of the probe) no correction was made to the distance scale. Curve b is the classical absorption result; use of a reasonable value of )coo = 10 -3 for the oscillator strength of the (0,0) 2~ ~-- 211 transition of O H gave a good fit in the b u r n t gas zone, The differences between these two curves are of two kinds. The most important fact is the weaker value of IOHI found by mass spectrometric measurement in the preheating zone, but there is also a change in the slope of the curves. In view of
[o.] 4.10 =1m-a
p3
2
0
i 2 dl ,~ 5 distance from the burner surface (mm)
Fl(;. 2. Hydroxyl concentration profiles as measured by three techniques. Curve a, points marked 9 : mass spectrometry result. Curve b, points marked A: absorption spectroscopy with a continuous U.V. source. Curve c, points marked *: laser absorption spectroscopy.
STUDY THE PETURBATION OF A FLAME BY A PROBE the good agreement found in the burnt gases zone (in absolute number density value), it may be thought that the two procedures are correct, thus the sampling and the mass spectrometer calibration seem valuable. Therefore, the low IOHIvalues measured in the preheating and reacting zones by mass spectrometry would be the actual values in the gases sampled there. These values are not the same as those measured by absorption because here the composition of the gases in the vicinity of the probe has been perturbed by the probe itself. The spatial extension of this perturbation is weak and the absorption measurement which is integrated over 7 centimeters does not take it into account, therefore optical absorption measurement gives the unperturbed IOHI value. The main purpose of the work reported here is to study the perturbation in order to verify this hypothetical statement. The difference between the two slopes of the curves could eventually be explained by the poor spatial resolution of the absorption measurement. Our first task was to confirm the absorption profile by using the laser absorption technique with its associated spectral and spatial resolution.
II. Laser Absorption Measurements
1569
the experimental arrangement. The laser beam is focused in the flame by F/100 optics providing an actual spatial resolution of 0.15 mm. This value is the maximum diameter of the laser beam through the flat flame. The beam is then filtered by a passband monochromator to eliminate the wide band super-radiant light. The transmitted light is detected by a photomultiplier connected to a boxcar analyzer averaging the gated signal over ten shots.
B. Calculation The parameter directly provided by experiment when the laser is turned to the frequency vo is the spectral absorptivity
A(vo) =
I I(v - Vo) dv - S I(v - Vo) e - ~ d v
5 I(v - %) dv
(1)
where I(v) is the spectral intensity distribution of the laser and K, is the absorption coefficient which is assumed to be constant along the optical path L through the fiat flame. The useful quantity is the line area
I nAv~176 tl I(v - Vo) e-K'L dv [
In this technique spectral discrimination is achieved in the dye laser cavity before absorption, rather than with a spectrometer after absorption as in the classical scheme. Consequently great energy is centered around the absorption line and the signal is enhanced so that spatial resolution can be improved by diaphragming down the beam. In addition, the use of a pulsed laser associated with gated detection and integration increases the SNR by a factor 108.
By exchanging the order of integration for the double integral, one can easily find the relation
A. Apparatus
For the optically thin case where K,L < < 1 the expansion of the exponential can be approximated by the first term and we obtain:
The incident radiation, tunable around 309 nm is derived from the second harmonic of a dye laser pulse (4 ns duration) generated in a KDP crystal. The dye oscillator is pumped by a nitrogen laser at a repetition rate of 20 Hz. Frequency narrowing is accomplished by means of a Littrow grating at one end and a Fabry-Perot etalon inside the cavity. This etalon has been chosen to assure a laser spectral width matching the absorption line in the low pressure enclosure (Ah --~ 2.10 -1~ m). In any case, we will show that a precise knowledge of the laser spectral function is not necessary, as long as the rotational structure of the absorption spectrum is resolved. The spectral sweeping is achieved by varying the pressure and therefore the index of refraction in the enclosure containing both the Favry-Perot etalon and the grating. Figure i shows
S I(v - v o) dv
[ dv o (2)
I I,,e A(Vo)dVo= I 'i,,o (1 -- e -~'e) dv
A(vo)dv o = L I
I
Kodv
line
(3)
(4)
line
The population N r of the particular rotational level involved is given by the well known relation 13 e2L
L S Kflv = -
4%mc
Nj.Frr,
(5)
where the line oscillator strength F r r is related to the band oscillator strength f,. by the relation Sj ,j ~foo F,.,. -
2J"+ i
(6)
1570
COMBUSTION DIAGNOSTICS
using the Honl-London factor S r r normalized according to the rules advocated by Tatum. 14 The total population NOin the fundamental state is related to the population Nj, by a Boltzmann distribution:
NO =
- -
Nj.
~
=
ZRJ" eERj./kT
(7)
2J"+ 1
where Zar is the partition fuction of the rotation and Ear the rotational energy. If the medium is not optically thin, the relation between S A(v) dv and S Kflv is no longer exactly linear and a correction must be made according to the curve of growth first computed by Van der Held. ~
Q~7
C. Results We made our measurements on the Q~7 line (h = 308.9734 nm) of the 2E(v' = 0) ~ 2II(v" = 0) transition of OH. As the population of the J" = 7 lower level of this transition is close to the maximum of the distribution function ~6 in the temperature range 1000 K - 2000 K, therefore this population is almost independent of the temperature in this range. The choice of this strong absorption line also allows precise measurement in the preheating zone where IOHI is weak. Nevertheless, in the burnt gases zone this absorption remains low enough (K~L < 1) to use the Van der Held curve with a good precision (in our low pressure flame the line shape is pure Doppler). Figure 3 shows an example of absorption in the burnt gases zone: The second absorption line is the Q23 line and its presence is useful for calibrating the wavelength sweeping scale. The absolute IOHI profile obtained (still using the value f~, = 10 -3) by the laser absorption technique is in good agreement (Fig. 2) with the one we had obtained by classical absorption method; the slight difference at the burner can be explained by the improvement of the spatial resolution.
III. Laser-Induced Fluorescence Measurements
A. Experimental The incident optical arrangement is the same as for the absorption measurement (Fig. 1), but the focusing optics aperture is increased. For each set of measurements, the axis of this incident beam is located at a given distance from the tip of the cone and the flame front is explored by translating the burner. The fluorescent light due to radiative relaxation of the excited species is collected by F / 5 optics
308.973
308.986
wavelength (rim)
Fro. 3. Absorption of the laser beam in the burnt gas zone of the flame versus wavelength. Calculations are made from the Q~7 line; the line on the right hand side is a superposition of several unsolved lines.
and focused on the orifice of a photomultiplier. A pass-band filter is interposed to remove the light emitted by the flame itself. This device makes it possible to detect the fluorescence from a small volume formed by the focused beam section (~b < 0.1 mm) and the height of the detected zone (0.2 mm). The photomultiplier is connected to an oscilloscope and a boxcar analyzer. The measurements were performed according to our time resolved LIF technique. The main problem of LIF is the quenching dependance of the fluorescence because of collisional de-excitation of the excited species. Saturation of the transition is usually proposed to avoid this dependance.17"s We have developed'~ an alternative to this approach, based upon short laser excitation. This technique is particularly suitable for low pressure measurements where the quenching time is much longer than the pulse duration. In this case, after a short excitation of duration At the photon flux collected in the solid angle f~ from a volume V is ~(t) =
N o B U~ At V fl A a4~r
exp
[
I
-(A+Q)
t
(8)
STUDY THE PETURBATION OF A FLAME BY A PROBE where B, A and Q are the probability coefficients for absorption, radiative relaxation and collisional relaxation, respectively. U, is the mean spectral energy density of the exciting pulse. Let us note that qbis not sensitive to the focused zone area (which is difficult to measured), but only to the length of the excited detected zone. a, the ratio of the total population to the population of the particular level being pumped usually depends on the temperature, but as pointed out before, for the Q~7 line a is nearly constant in the temperature range investigated. Moreover, by centering the gate (1 ns duration) of the boxcar analyzer on the top of the fluorescence decay signal (25 ns duration) we obtain:
1571
[o.] 4~ lO~'ms
3
2
9 (t = 0) ~ 4~r No -
BUo At V I I A
(9)
which does not depend on Q.
1
B. OH Profile by LIF
inci
3
4
distance from the burner surface
We first used this technique to the measurement of the IOHI profile in the flame zone outside of the influence of the probe. To obtain an accurate profile, it was necessary to reduce both the absorption of the exciting light before the measurement point and florescence trapping. To shorten the optical path, the point measurements (V = 0.02 mm 3) were made at the point A (Fig. 4) rather than at the center of the flame. The result is given in Fig. 5 and compared with the profile provided by laser absortion. For each curve, the reproductibility and the relative uncertainties are better than 10%. Agreement between the two curves in absolute value may be estimated to be better than 50%; these curves have been fitted in the burnt gases zone.
T
2
~
beam
edge of the homogeneous flat flame
FIG. 4. Scheme of the different optical paths involved in laser-induced fluorescence measurements. Fluorescence signals from point A provide accurate concentration values, f l u o r e s c e n c e signals from zone B provided only relative value (because of fluorescence trapping).
(ram)
FIG. 5. Hydroxyl concentration profiles measured by laser techniques with the Q~7 line. Solid line: laser absorption measurement. Dashed line: laser induced florescence measurement.
C. Exploration of the Probe Perturbation The vertical incident beam intersects the sampling cone axis at a distance d from the top. Exploration of the fluorescent image of this exciting beam by shifting the detector height provides a IOHIradial profile. For each distance d, several radial profiles at different depths in the flame front are obtained by translating the burner. Under this experimental situation (Zone B of Fig. 4), absorption of the exciting light before the measurement point and fluorescence trapping are no longer negligible, but the induced signal distortion is the same for all the investigated points of the small extended perturbation zone. Consequently each radial profile is accurately described in relative value. Figure 6 shows such a set of radial profiles. With regard to the sampling problem, the most interesting results are those obtained on the probe nozzle axis. For every radial profile the IOHI minimum is located on this axis. Direct comparison between these profiles is not achievable because incident beam absorption and fluorescence trapping are not the same for different profiles. For each profile, we have measured the ratio I , , / I M of the minimum to the maximum of fluorescence intensity. In each case, this ratio was equal to the corresponding ratio IOHL/IOHIM. Results are shown in Fig. 7 where these ratios are plotted as a function of the distance d for different positions of the burner.
1572
C O M B U S T I O N DIAGNOSTICS Thus, each curve corresponds to a given position of the optical measurement point in the flame front. From Fig. 2, we have deduced the ratios of IOHI measured by mass spectrometry to the IOHI measured by absorption spectroscopy at the same four positions in the flame front, and we have reported the values on the corresponding curves on Fig. 7. They lie on a vertical line corresponding to d = 0.2 mm, which may be thought to be the actual mean distance of sampling by the probe. This experimental determination is in good agreement with computed values found in the literature t9 which gives a sampling zone length of two orifice diameters from the orifice (d~ orifice is 0.1 mm).
probenozzleaxis I I Ia
[oHl.
D. Discussion
r(mm) FIc. 6. Example of a set of relative tOI-II radial profiles in front of the probe measured by laser induced fluorescence for different positions of the probe in the flame. Curve a: burner-LIF measurement point distance = x = 4 mm (probe tip in burnt gas). Curve b: x = 2.7 mm (probe tip in reactive zone). Curve c: x = 0.6 mm (probe tip in preheating zone). Each time, the exciting laser beam intersects the sampling cone axis at a distance d = 0.3 mm from the cone top.
Results from point fluorescence measurements performed in the vicinity of the probe confirm the assumption we had made from the study of Fig, 2. There is an important local probe effect only in the preheated zone, it is weaker in the reactive zone and vanishes in the burnt gas zone. We have demonstrated that the low IOHI value measured by mass spectrometry in this zone is the actual IOHI in the gases sampled there as perturbed by the probe. From IOHI radial profiles we can point out that, as the cone penetrates deeper and deeper into the reactive zone the amplitude of the perturbation increases. Likewise its spatial extension increases from (3.5 mm high x 0.6 mm depth) at 2,7 mm from the b u r n e r to (9 mm high x 1.5 m m depth)
[OH]. 1
/- ' ~a
b
9
~
"
l
iO
a: b: c: d.
.,... T
0
[~
,
,
_,,,.
x=4mm x= 2.7ram x=l.3mm x =0.6ram
d (mini
--
0.5
1
1.5
FIG. 7. Perturbation of ]OH] in the axis of the sampling cone. The figure gives the m i n i m u m of ]OH] normalized by the corresponding unperturbed (maximum value versus the distance d between the probe tip and the exciting beam. Each curve corresponds to a given value of the distance x between the burner and the L I F measurement point. See the text for the point marked ~
STUDY T H E PETURBATION O F A F L A M E BY A PROBE at the burner, as can be deduced from fig. 6 and fig. 7. These orders of magnitude seem to be in agreement with the first results of work pursued at the laboratory 2~ to modelize the perturbation. Let us now try to explain the data. We remind the reader that no visible nor appreciable thermic perturbation of the flame was found, and that the sample probe does no severely perturb the stable species concentrationsf ~ This is not true for OH in the region of the flame where its presence is due to backward diffusion, i.e. in the preheated zone and in the beginning of the reactive zone. The probe and the boundary layers situated on it act like a sink for this species and disturb the backward diffusion field so that the sample gas contains less O H than the unperturbed gas. In the regions where the diffusion field is in the flow direction (second part of the reactive zone) or in the post-flame gases where there is no appreciable concentration gradient, this sink effect of the probe does not affect the flow ahead of it. As the molecular beam system is adapted to prevent recombination of radicals inside the probe the measurement of OH in these regions is valid.
Conclusion The primary result of this work lies in the successful use of LIF to solve an experimental problem. Thanks to its very precise spatial resolution, a perturbation of only 5 mm or less in diameter was analyzed through a homogeneous flame of 7 cm in diameter. However the results of the study are interesting in their own right and lead to the following conclusions: (i) The quartz cone of a mass spectrometer sampiing probe reduces the OH concentration in the preheating zone of the flame investigated. The extension of the perturbation is comparable to the probe penetration into the reactive zone of the flame. Thus, the effect may be attributed to a perturbation of the diffusion field by the probe. (ii) The OH concentration measured by mass spectroscopy is the actual value in the sample. (iii) As predicted by the literature, the extension of the sampling zone is two diameters of the probe orifice. In addition, a direct comparison of IOHIprofiles measured by laser absorption and LIF has been performed leading, to a good agreement.
Acknowledgements We are grateful to I.F.P. for providing a bursary to one of us (D.S.).
1573
REFERENCES 1. B1ORD1,J. C., LAZZARA,C. P., ANDPAPP,J. F.: Fourteenth Symposium (International) on Combustion, p. 367, The Combustion Institute, 1973. 2. VANDDOREN,J., PEETERS, J., AND VAN TIGGELEN, P. J.: Fifteenth Symposium (International) on Combustion, p. 745, The Combustion Institute, 1974. 3. PEETESS,J., ANDMAHNEN, G.: Fourteenth Symposium (International) on Combustion, p. 133, The Combustion Institute, 1973. 4. TINE', G.: "Gas sampling and chemical analysis in combustion processes". Pergamon Press, 1961. 5. FmswsoM, R. M.: WESTENRERG,A.A.: Flame structure, McGraw-Hill series in Advanced Chemistry, 1965. 6. HAYHURST,A. N., ANDKITTLESON,D. B.: Combustion and Flame, vol. 28, n ~ 2, p. 137, 1977. 7. B1ORDI,J. C., LAZZARA,C. P., ANDPAPP,J. F.: Combustion and Flame, vol. 23, p. 73, 1974. 8. REVET,J. M., PUECHBERTY,D., ANDCOTTEREAU,M. J.: Combustion and Flame, vol. 33, p. 5, 1978. 9. DAILY, J. W.: Applied Optics, vol. 15, p. 955, 1976. 10. WANG,C. C. ANDDAVISJR., L. 1.: Phys. Rev. Lett., vol. 32, p. 349, 1974. 11. STEPOWSKI, D. AND COTTEREAU, M. J.: Applied Optics, vol. 18, p. 354, 1979. 12. REVET, J. M.: "Analyse par spectrom6trie de masse de flamme bassepression. Comparaison avec la spectroscopie d'absorption sur le radical OH". Thbse de Doctorat du 3brae cycle, 18 octobre 1977, Universit6 de Rouen, Facult6 des Sciences, 76130 Mont-Saint-Aignan. 13. THORNE, A. T.: "Spectrophysics", p. 307. Chapman and Hall Ltd 1974. 14. TATUM,J. B.: Ap. J. Suppl. vol. 14, p. 21 (1967). 15. VAN DES HELD, E.F.M.: Z. Physik, n ~ 70, p. 508, 1931. 16. DIEKE, G. M., AND CROSSWHITE, H. M.: J. Quant. Spectrosc. Radiat. Transfer, vol. 2, p. 92, 1962. 17. DAILY, J. W.: Applied Optics, vol. 16, p. 568, 1977. 18. BARONAVSKI,A. P., AND McDONALD, J. R.: Applied Optics, vol. 16, p. 1897, 1977. 19. MILNE, T. A. AND GREENE, F. T.: Tenth Symposium (International) on Combustion, p. 153, The Combustion Institute, 1965. 20. GAS(), A.: Thbse de Doctorat du 3bme cycle, to be published in 1980. Facult6 des Sciences de Rouen, Laboratoire de Thermodynamique, 76130 Mont-Saint-Aignan. 21. BIORDI,J. C., LAZZARA,C. P., ANDPAPP,J. F.: Combustion and Flame, vol. 23, p. 73, 1974.