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Local temperature measurements in flames by laser raman spectroscopy
C O M B U S T I O N A N D F L A M E 27, 133-136 (1976)
133
Local Temperature Measurements in Flames by Laser Raman Spectroscopy W. STRICKER DFVLR-lnstitut fur Reaktionskinetik, 7000 Stuttgart 80, West Germany
An accurate investigation of the kinetic behaviour of flames requires both the determination of the concentration of the different constituents and the measurement of the temperature. Conventional methods are probe sampling and thermocouple measurements, but both techniques disturb the flow field and the local gas c o m p o s i t i o n . C o m m o n optical methods of absorption and emission spectroscopy do not influence the flame conditions, but yield only average values across the section traversed. However, laser Raman spectroscopy offers the possibility for measurements without any influence on the system and with with high spatial resolution, which is of special interest for investigations of small areas close to or in the reaction zone itself. The present paper describes the application of laser Raman scattering as a tool for temperature measurements in flames. Up to now only a few Raman experiments of this type in flames have been reported, e.g., I1, 2, 3]. The present measurements were carried out in the postflame region of a propane-oxygen-air flame at atmospheric pressure. The flame burnt horizontally. Due to the small diameter of the flame, there occurred high temperature gradients up to 800 K/mm suitable for testing the spatial resolution. The experimental setup with 90° scattering geometry is shown schematically in Fig. 1. The light from an argon ion laser (Spectra Physics Mod. 171-03), operated at a wavelength of 488 nm with a power of about 2 W, was focused into
flame by a lens of short focal length (f = 30 mm). The scattering volume defined by the laser focus inside the flame could be approximated in this case by a vertical cylinder 0.9 mm in length and 12/zm in diameter. The dimensions of the scattering volume inside the flame determined the spatial resolution which can be achieved by this technique. The orientation of the laser beam in the flame has to be chosen suitably with regard to the temperature gradient. The light from the scattering volume was collected by a lens with large aperture (f = 50/0.95) and was imaged onto the entrance slit of a double m o n o c h r o m a t o r (Spex Mod. 14018). The intensity of the scattered light could be enhanced by means of two spherical mirrors, the first of which doubles the intensity of the laser within the flame by reflecting back
L2
L3
DOUBLE MONOCHROMATOR
r--M3
~ --
FLAME
Fig. 1. Schematic diagram of the experimental arrangement: L = lens; M = mirror; PM = photomultiplier.
Copyright © 1976 by The C o m b u s t i o n Institute Published by A m e r i c a n Elsevier Publishing C o m p a n y , Inc.
134
W. STRICKER
the laser b e a m and the second gathers the backscattered R a m a n light. By this procedure, the waist of the laser focus was s o m e w h a t broadened, because exact alignment of the incoming and reflected laser b e a m was very difficult to achieve. The image of the exit slit was f o c u s e d o n t o the slit-shaped c a t h o d e of a photomultiplier (RCA C 31034). The photomultiplier tube was operated at -20 °C to reduce the thermal dark current. The output of the t u b e w a s fed to the p r e a m p l i f i e r and discriminator of a photon counting device (SSR Mod. 1110). A slit-shaped cathode and cooling of the multiplier tube coupled with the photon c o u n t i n g t e c h n i q u e y i e l d e d the o p t i m u m signal-to-noise ratio.
M e a s u r e m e n t s were performed for a prem i x e d p r o p a n e - o x y g e n - a i r f l a m e with an equivalence ratio ~P = 1.0 (2.64 l/h propane, 10.5 1/h oxygen, 12.7 1/h air) at atmospheric pressure. A simple welding torch, 1 m m in d i a m e t e r , was u s e d w i t h o u t shielding and yielded a flame with the desired steep temperature gradients. In the present investigation the R a m a n spectra of N., were evaluated. A typical vibrational R a m a n spectrum of N._, in the postreaction zone of the flame with the transitions 0 ~ 1, 1 ~ 2, and 2 ~ 3 is shown in Fig. 2. For t e m p e r a t u r e m e a s u r e m e n t s both the p u r e r o t a t i o n a l R a m a n s p e c t r u m and the rotation-vibrational R a m a n spectrum of any of
t 0-O3 Z la.I
U.I >
T= 1790 K
,,s n.,
iL
"1
0.7 0.10~"i 0.19-10~
\ 0.1,5,104
23' 00
2350 '
24'00
2450 '
DISPLACEMENT FROM EXCITING LINE
[cm" 1 ] - ~
Fig. 2. Rotation-vibrational Raman spectrum of N2 in the postflame region of a propane-oxygen-air flame with the transitions 0 ~ 1, I ~ 2, and 2 --*3. The figures above the vibrational peaks in the diagram indicate the count rates in counts/sec.
BRIEF COMMUNICATIONS
135
the major consituents present in the flame can be used. The evaluation of the pure rotational spectrum has the advantage of high accuracy, except for complex gas mixtures, where strong overlap of lines may occur. However, this method requires long measuring times of about 20 minutes per temperature point, whereas the measuring times for temperatures based on recording the intensity ratios of the vibrational transitions 0 ~ 1 and 1 --* 2 (see [1]), could be reduced to 2 minutes. Most of the temperatures reported in this work were obtained by this second method. In this case, however, a proper temperature evaluation requires the correction of the peak intensity of the 1 --~ 2 transition ("hot band") by taking into account the contribution of the rotational wing of the Q-branch of the 0 ~ 1 transition (Fig. 2). The applicability of a simple graphical extrapolation procedure was c h e c k e d by evaluating some simultaneously-recorded rotational spectra. Precise graphical e x t r a p o l a t i o n of the
t LLI r~
2500
/'/
IZ. LU fl-
2000 2500
/
1500
2000
/
iI/~\13 /
1500
u
-2. -1.
lO00 I
3000
H=
i
/ Ix,,
j .
0 ,1 ,,2 ,,3 . .
40
mm
/ I
-3 -2-1
I
i
I
i
i
0 +1 +2 .3
RADIUS[ram]
\\\ Fig. 4. Radial temperature distribution in a propaneoxygen-air flame: p = l a t m ; qb = 1.0.
2500
2000
1500 /
' '
[
1000 2
12
22
32
42
52
DISTANCE FROM BURNER SURFACE [ m i n i
Fig. 3. Temperature distribution along the axis of a propane-oxygen-air flame: p = 1 atm; qb = 1.0; o, measured axial temperature values; ~ , maximum temperature values from radial profiles. The dashed line indicates the extrapolation into the reaction zone.
Q-branch was further facilitated by band shape calculations on the assumption of thermal equilibrium under the actual experimental conditions. It was found that both methods, based either on rotational or on vibrational spectra, resulted in temperatures which never differed by more than 3%. The results of our temperature measurements have been plotted in Figs. 3 and 4. The temperature distribution along the axis of a stoichiometric propane-oxygen-air flame is shown in Fig. 3. The curve can be extrapolated into the main reaction zone of the flame-indicated by the dashed vertical line,--and the temperature thus found agreed very well with that calculated for these flame conditions at thermal equilibrium. The triangles in Fig. 3 correspond to the maximum temperatures ob-
136 tained from the radial temperature profiles of the flame under the same conditions. Figure 4 shows the radial temperature distributions at two positions, 20 mm and 40 mm from the burner surface. As can be seen from the diagram, the spatial resolution in this direction is better than 0.5 mm. Small distortions of the radial profiles are due to the asymmetry of the horizontally burning flame. It may be concluded that, with the development of stable lasers with high power output, laser Raman spectroscopy has become a valuable tool for measurin~ temperatures in flames with high spatial resolution. The application of further improved methods will also be of interest in other fields where local temperature measurements are required.
W.STRICKER
The author would like to thank Dr. Th. Just .for helpful discussions. The financial support of the Deutsche Forschungsgemeinschaft is gratefully acknowledged. References 1. Lapp, K., Goldman, L. M., Penney, C. M., Science 175, 1112 (1972). 2. Leonard, D. A., AVCO Everett Res. Lab. Research Note 914 (1972). 3. Setchell, R. E., paper presented at the Western States Section, The Combustion Institute, Pullman, Wash., 1974.
Received 28 January 1976.
133
Local Temperature Measurements in Flames by Laser Raman Spectroscopy W. STRICKER DFVLR-lnstitut fur Reaktionskinetik, 7000 Stuttgart 80, West Germany
An accurate investigation of the kinetic behaviour of flames requires both the determination of the concentration of the different constituents and the measurement of the temperature. Conventional methods are probe sampling and thermocouple measurements, but both techniques disturb the flow field and the local gas c o m p o s i t i o n . C o m m o n optical methods of absorption and emission spectroscopy do not influence the flame conditions, but yield only average values across the section traversed. However, laser Raman spectroscopy offers the possibility for measurements without any influence on the system and with with high spatial resolution, which is of special interest for investigations of small areas close to or in the reaction zone itself. The present paper describes the application of laser Raman scattering as a tool for temperature measurements in flames. Up to now only a few Raman experiments of this type in flames have been reported, e.g., I1, 2, 3]. The present measurements were carried out in the postflame region of a propane-oxygen-air flame at atmospheric pressure. The flame burnt horizontally. Due to the small diameter of the flame, there occurred high temperature gradients up to 800 K/mm suitable for testing the spatial resolution. The experimental setup with 90° scattering geometry is shown schematically in Fig. 1. The light from an argon ion laser (Spectra Physics Mod. 171-03), operated at a wavelength of 488 nm with a power of about 2 W, was focused into
flame by a lens of short focal length (f = 30 mm). The scattering volume defined by the laser focus inside the flame could be approximated in this case by a vertical cylinder 0.9 mm in length and 12/zm in diameter. The dimensions of the scattering volume inside the flame determined the spatial resolution which can be achieved by this technique. The orientation of the laser beam in the flame has to be chosen suitably with regard to the temperature gradient. The light from the scattering volume was collected by a lens with large aperture (f = 50/0.95) and was imaged onto the entrance slit of a double m o n o c h r o m a t o r (Spex Mod. 14018). The intensity of the scattered light could be enhanced by means of two spherical mirrors, the first of which doubles the intensity of the laser within the flame by reflecting back
L2
L3
DOUBLE MONOCHROMATOR
r--M3
~ --
FLAME
Fig. 1. Schematic diagram of the experimental arrangement: L = lens; M = mirror; PM = photomultiplier.
Copyright © 1976 by The C o m b u s t i o n Institute Published by A m e r i c a n Elsevier Publishing C o m p a n y , Inc.
134
W. STRICKER
the laser b e a m and the second gathers the backscattered R a m a n light. By this procedure, the waist of the laser focus was s o m e w h a t broadened, because exact alignment of the incoming and reflected laser b e a m was very difficult to achieve. The image of the exit slit was f o c u s e d o n t o the slit-shaped c a t h o d e of a photomultiplier (RCA C 31034). The photomultiplier tube was operated at -20 °C to reduce the thermal dark current. The output of the t u b e w a s fed to the p r e a m p l i f i e r and discriminator of a photon counting device (SSR Mod. 1110). A slit-shaped cathode and cooling of the multiplier tube coupled with the photon c o u n t i n g t e c h n i q u e y i e l d e d the o p t i m u m signal-to-noise ratio.
M e a s u r e m e n t s were performed for a prem i x e d p r o p a n e - o x y g e n - a i r f l a m e with an equivalence ratio ~P = 1.0 (2.64 l/h propane, 10.5 1/h oxygen, 12.7 1/h air) at atmospheric pressure. A simple welding torch, 1 m m in d i a m e t e r , was u s e d w i t h o u t shielding and yielded a flame with the desired steep temperature gradients. In the present investigation the R a m a n spectra of N., were evaluated. A typical vibrational R a m a n spectrum of N._, in the postreaction zone of the flame with the transitions 0 ~ 1, 1 ~ 2, and 2 ~ 3 is shown in Fig. 2. For t e m p e r a t u r e m e a s u r e m e n t s both the p u r e r o t a t i o n a l R a m a n s p e c t r u m and the rotation-vibrational R a m a n spectrum of any of
t 0-O3 Z la.I
U.I >
T= 1790 K
,,s n.,
iL
"1
0.7 0.10~"i 0.19-10~
\ 0.1,5,104
23' 00
2350 '
24'00
2450 '
DISPLACEMENT FROM EXCITING LINE
[cm" 1 ] - ~
Fig. 2. Rotation-vibrational Raman spectrum of N2 in the postflame region of a propane-oxygen-air flame with the transitions 0 ~ 1, I ~ 2, and 2 --*3. The figures above the vibrational peaks in the diagram indicate the count rates in counts/sec.
BRIEF COMMUNICATIONS
135
the major consituents present in the flame can be used. The evaluation of the pure rotational spectrum has the advantage of high accuracy, except for complex gas mixtures, where strong overlap of lines may occur. However, this method requires long measuring times of about 20 minutes per temperature point, whereas the measuring times for temperatures based on recording the intensity ratios of the vibrational transitions 0 ~ 1 and 1 --* 2 (see [1]), could be reduced to 2 minutes. Most of the temperatures reported in this work were obtained by this second method. In this case, however, a proper temperature evaluation requires the correction of the peak intensity of the 1 --~ 2 transition ("hot band") by taking into account the contribution of the rotational wing of the Q-branch of the 0 ~ 1 transition (Fig. 2). The applicability of a simple graphical extrapolation procedure was c h e c k e d by evaluating some simultaneously-recorded rotational spectra. Precise graphical e x t r a p o l a t i o n of the
t LLI r~
2500
/'/
IZ. LU fl-
2000 2500
/
1500
2000
/
iI/~\13 /
1500
u
-2. -1.
lO00 I
3000
H=
i
/ Ix,,
j .
0 ,1 ,,2 ,,3 . .
40
mm
/ I
-3 -2-1
I
i
I
i
i
0 +1 +2 .3
RADIUS[ram]
\\\ Fig. 4. Radial temperature distribution in a propaneoxygen-air flame: p = l a t m ; qb = 1.0.
2500
2000
1500 /
' '
[
1000 2
12
22
32
42
52
DISTANCE FROM BURNER SURFACE [ m i n i
Fig. 3. Temperature distribution along the axis of a propane-oxygen-air flame: p = 1 atm; qb = 1.0; o, measured axial temperature values; ~ , maximum temperature values from radial profiles. The dashed line indicates the extrapolation into the reaction zone.
Q-branch was further facilitated by band shape calculations on the assumption of thermal equilibrium under the actual experimental conditions. It was found that both methods, based either on rotational or on vibrational spectra, resulted in temperatures which never differed by more than 3%. The results of our temperature measurements have been plotted in Figs. 3 and 4. The temperature distribution along the axis of a stoichiometric propane-oxygen-air flame is shown in Fig. 3. The curve can be extrapolated into the main reaction zone of the flame-indicated by the dashed vertical line,--and the temperature thus found agreed very well with that calculated for these flame conditions at thermal equilibrium. The triangles in Fig. 3 correspond to the maximum temperatures ob-
136 tained from the radial temperature profiles of the flame under the same conditions. Figure 4 shows the radial temperature distributions at two positions, 20 mm and 40 mm from the burner surface. As can be seen from the diagram, the spatial resolution in this direction is better than 0.5 mm. Small distortions of the radial profiles are due to the asymmetry of the horizontally burning flame. It may be concluded that, with the development of stable lasers with high power output, laser Raman spectroscopy has become a valuable tool for measurin~ temperatures in flames with high spatial resolution. The application of further improved methods will also be of interest in other fields where local temperature measurements are required.
W.STRICKER
The author would like to thank Dr. Th. Just .for helpful discussions. The financial support of the Deutsche Forschungsgemeinschaft is gratefully acknowledged. References 1. Lapp, K., Goldman, L. M., Penney, C. M., Science 175, 1112 (1972). 2. Leonard, D. A., AVCO Everett Res. Lab. Research Note 914 (1972). 3. Setchell, R. E., paper presented at the Western States Section, The Combustion Institute, Pullman, Wash., 1974.
Received 28 January 1976.