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Simultaneous vibrational and pure rotational coherent anti-Stokes Raman spectroscopy for temperature and multispecies concentration measurements demonstrated in sooting flames Christian Brackmann, Joakim Bood, Per-Erik Bengtsson, Thomas Seeger, Martin Schenk, and Alfred Leipertz

The potential of measuring temperature and multiple species concentrations 共N2, O2, CO兲 by use of combined vibrational coherent anti-Stokes Raman spectroscopy 共CARS兲 and pure rotational CARS has been investigated. This was achieved with only one Nd:YAG laser and one dye laser together with a single spectrograph and CCD camera. From measurements in premixed sooting C2H4-air flames it was possible to evaluate temperatures from both vibrational CARS and rotational CARS spectra, O2 concentration from the rotational CARS spectra, and CO concentration from the vibrational CARS spectra. Quantitative results from premixed sooting C2H4–air flames are presented, and the uncertainties in the results as well as the possibility of extending the combined CARS technique for probing of additional species are discussed. © 2002 Optical Society of America OCIS codes: 190.1900, 280.1740, 300.6320.

1. Introduction

Laser techniques are powerful nonintrusive tools for the characterization of combustion processes that most often yield information with high spatial as well as high temporal resolution. Of the various diagnostic techniques, spontaneous Raman scattering and coherent anti-Stokes Raman scattering 共CARS兲 are the methods of choice for simultaneous measurements of flame temperature and major species concentrations 共see, e.g., Refs. 1 and 2兲. With spontaneous Raman scattering it is possible to measure nearly all the major species of a reacting system simultaneously,3– 6 which makes it useful in clean flames. Meier et al. have investigated the potential of simultaneous temperature and multispecies concentration measurements in diffusion flames by comC. Brackmann 共[email protected]兲, J. Bood, and P-E. Bengtsson are with the Division of Combustion Physics, Lund Institute of Technology, P.O. Box 118, Lund S-221 00, Sweden. T. Seeger, M. Schenk, and A. Leipertz are with the Lehrstuhl fu¨r Technische Thermodynamik, Universita¨t Erlangen-Nu¨rnberg, Am Weichselgarten 8, D-91058 Erlangen-Tennenlohe, Germany. Received 12 February 2001; revised manuscript received 23 July 2001. 0003-6935兾02兾030564-09$15.00兾0 © 2002 Optical Society of America 564

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bining the Raman and the Rayleigh techniques7 and Nooren et al.8 by using a combination of the Raman, Rayleigh, and laser-induced fluorescence techniques. In these clean flames the temperature determination was performed by Rayleigh scattering measurements. It was also found that additional laserinduced fluorescence measurements are necessary to obtain reliable information about CO concentration.8 The potential of the Raman technique applied to fuelrich and sooting flames was investigated by KohseHo¨inghaus9 who, by using a polarization scheme and averaging over a large number of laser pulses, found that it was possible to monitor Raman spectra from a fuel-rich laminar premixed flame. It was also found that background levels were too high for a temperature determination by use of the ratio of the Stokes and anti-Stokes signals, especially near the flame front. Nevertheless, this proved that it is difficult to use these techniques in many practical systems because of the luminosity of the environment and that there is still a strong need for improvement in existing techniques as well as a need to develop laser techniques for application to these fuel-rich and sooting flames. For these systems, especially for a timeresolved investigation of sooting flames and soot formation, the CARS technique is preferable.10 The dual-broadband pure rotational CARS tech-

nique11,12 has been developed for temperature and multiple species concentration determination, and the possibility for simultaneous temperature and N2– O2, or N2–O2–CO2, or N2–C2H2 concentration measurements have already been investigated.13–18 The main advantage of this technique is that the CARS signals of nearly all the molecules are generated within the same frequency region. Even so, there are molecules such as CH4 that do not give any resonant rotational CARS signal contribution. Other molecules, such as CO, have a low rotational Raman cross section resulting in a weak signal for typical flame conditions, and H2, which has a too-high rotational constant to give signal contributions spectrally located in the Raman shift region of importance for rotational CARS. Another drawback is the decrease in temperature accuracy with an increase in temperature.14,19 –21 Alternatively, the vibrational CARS technique could be used. Molecules with similar Raman shifts, such as, for example, N2 and CO or O2 and CO2, can be probed with a single dye laser. For Raman resonances further apart, for example, N2 and CO2, a multicolor approach is required.22–25 By use of such techniques, it is possible to probe two groups of two molecules each. However, there are limitations in that more than one dye laser is required, more than one detection system is needed, and the precision of the concentration measurement is not as good as for rotational CARS 共see, e.g., Ref. 26兲. It is also well known that the temperature accuracy of rotational CARS is significantly better than for vibrational CARS at low temperatures.14,19 –21 Thus, there is a logical step for the development of a combination of the dual-broadband pure rotational CARS technique and the vibrational CARS technique. With this arrangement, there is the possibility to benefit from the merits of both techniques, i.e., the multispecies capabilities of both techniques, the high accuracy of temperature measurements for rotational CARS at low temperatures and for vibrational CARS at high temperatures. In addition rotational CARS has been demonstrated to be preferable at high pressures27 and in sooting flames.10 Combined rotational and vibrational CARS have previously been performed by use of three lasers for the simultaneous measurements of temperature and pressure.28,29 Also, combined rotational and vibrational CARS by use of a single dye laser has been used to measure CH4 and O2 concentrations by taking the ratio between the different integrated signal peaks.30 To prevent systematic errors in that setup there is a need for calibration measurements in connection with the measurement itself. Therefore the present experimental setup has been modified and used in combination with contour-fit evaluation procedures for simultaneous N2–O2–CO concentration measurements and temperature measurements in sooting premixed C2H4–air flames. We demonstrate and discuss the capability of this technique for different flame conditions.

Fig. 1. Double-folded BOXCARS phase-matching scheme for the generation of nearly superimposed vibrational and dualbroadband pure rotational CARS signal beams. The red laser beams are illustrated in dark gray, the green laser beams in light gray, and the CARS signals in medium gray.

2. Experimental

The laser systems used for the experiments have been described elsewhere.10 The dye laser was operated in the broadband mode, and a dye mixture of Rhodamine 610 and Rhodamine 640 was used. We adjusted the spectral position of the dye profile by changing the amount of dyes in the mixture, an adjustment that was made to fulfill two requirements: to maximize the pulse energy for the CO vibrational CARS process and to maintain a reasonable singleshot intensity 共a few hundred counts兲 on the N2 Q-branch vibrational CARS signal. The green laser beam at 532 nm from the Nd:YAG laser and the red laser beam from the dye laser were both split into two beams each, resulting in a total of four laser beams. Typical values of the pulse energies of the beams were 30 mJ each for the green beams and 18 mJ each for the red beams. The four beams were directed to a focusing lens of f ⫽ 500 mm and used in a double-folded BOXCARS setup to generate both vibrational and rotational CARS signals simultaneously.30 As illustrated in Fig. 1, the generated CARS signal beams propagated together nearly superimposed from the measurement point with a spatial separation between the centers of the CARS signal beams of approximately 1 mm. The signals were recollimated by a second lens of f ⫽ 500 mm, directed to a spectrograph by dichroic mirrors, and focused onto the entrance slit of the spectrograph by an f ⫽ 100-mm spherical lens. The spectrograph had a focal length of f ⫽ 1.0 m and was a Czerny–Turner type. The signals were spectrally resolved by a grating of 600 grooves兾mm and a blaze of 2.5 ␮m. The rotational CARS signal in the fourth order of the grating was directed to the CCD chip with an ordinary mirror of the spectrograph, as illustrated in Fig. 2. To detect the fourth order of the vibrational CARS signal simultaneously, this signal was reflected from an additional mirror placed between the two ordinary mirrors of the spectrograph. To separate the two signals from each other, they were directed to the chip of the detector at different heights. The detector was a backilluminated unintensified 20 January 2002 兾 Vol. 41, No. 3 兾 APPLIED OPTICS

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Fig. 2. Spectrograph setup and CCD camera picture with spectrally resolved dual-broadband pure rotational CARS signal 共upper兲 and vibrational CARS signal 共lower兲.

Princeton Instruments CCD camera with a chip consisting of 1100 ⫻ 330 pixels. The chip area was divided into five separate areas, where the pixels were binned vertically in each. A mechanical shutter was used to suppress background illumination. Measurements were performed in premixed laminar C2H4–air flames on a sintered stainless-steel porous-plug McKenna burner. Accumulated spectra generated by 100 laser pulses each were registered at different heights above the burner surface. The flame spectra were compensated for the spectral profile of the broadband dye laser by division with an accumulated spectrum from CARS generation in a nonresonant gas, in our case argon. In the present experimental setup, the signals from vibrational CARS and rotational CARS are generated by two common laser beams, an additional red beam for rotational CARS and an additional green beam for vibrational CARS. However, an inherent consequence of the present setup is that an additional vibrational CARS signal is generated with the same laser beams that generate dual-broadband rotational CARS.11,20,30 Unfortunately this signal also propagates in the same direction as the dual-broadband rotational CARS signal. The Q-branch vibrational CARS process, the dual-broadband rotational CARS process, and the extra vibrational CARS process are illustrated in the energy level diagrams of Fig. 3. The signal generated by this extra process appears as a broadband signal at the base of the rotational CARS spectra and can be seen in the rotational CARS spec566


Fig. 3. Energy level diagrams: 共a兲 Q-branch vibrational CARS, 共b兲 dual-broadband pure rotational CARS, 共c兲 vibrational CARS process generated by the same laser beams as rotational CARS.

tra in Fig. 4 and in the upper spectrum of Fig. 5. Thus the additional vibrational CARS process interferes with the rotational CARS spectra and thereby introduces errors in the evaluated results. Since the interfering signal is generated in a process that includes the vibrational Raman resonances of nitrogen and carbon monoxide it does not appear in the nonresonant rotational CARS spectra. The possibility to extract temperature and concentration of CO and O2 from the CARS signals is illustrated by the spectra in Fig. 4. Each pair of

Fig. 4. Vibrational and pure rotational CARS spectra recorded in a premixed C2H4–air flame. Each pair of spectra was simultaneously detected at a different height above the burner, corresponding to different positions in a reaction zone: 共a兲 highest position, that is, the one closest to the product zone; 共b兲 middle position; 共c兲 lowest position, that is, the one closest to the reactant side.

vibrational CARS spectrum 共left兲 and rotational CARS spectrum 共right兲 was recorded at different heights in a premixed C2H4–air flame. In the spectra recorded at the position closest to the burner 关Fig. 4共c兲兴 both spectra show that the temperature is low, and in the rotational CARS spectrum spectral lines from both N2 and O2 are visible. In the spectra recorded at a position located higher in the flame 关Fig. 4共b兲兴 the oxygen contribution to the rotational CARS spectrum decreased and in the vibrational CARS spectrum it is possible to observe a signal generated by CO at around 2140 cm⫺1. Both spectra show that the temperature has increased, and it is especially obvious from the signal increase on the so-called hot band of N2 at ⬃2300 cm⫺1 in the vibrational CARS spectrum. In the spectra recorded at a position located even higher in the flame 关Fig. 4共a兲兴 both spectra indicate an even further increase in temperature, the O2 lines are no longer visible in the rotational CARS spectrum, and in the vibrational CARS spectrum the signal generated by CO has increased. An extra feature in the vibrational CARS spectrum recorded at this height is the H2 rotational line 共 J ⫽ 9 3 J ⫽ 11兲 positioned at 2130.1 cm⫺1 that interferes with the CO fundamental band. This peak was observed previously in vibrational CARS experiments on N2 and CO in flames.31

3. Evaluation Methods A.

Rotational Coherent Anti-Stokes Raman Spectroscopy

Experimental rotational CARS spectra were evaluated by a least-squares fit to a library of theoretical spectra that corresponds to different temperatures and N2–O2 concentrations. The computer code for the generation of rotational CARS spectra together with the evaluation procedure is described in Ref. 32. The parameters varied in the evaluation were the temperature, the linear dispersion, the Raman shift that corresponds to a reference pixel, and the nonresonant susceptibility. The rotational CARS spectral lines are narrow and a small change in the linear dispersion or the Raman shift corresponding to a reference pixel can have a significant influence on the fit, so these parameters were fitted for each spectrum. The reason for fitting the nonresonant susceptibility is that this parameter is not well estimated in rich and especially sooting flames.33 The slit function of the detection apparatus was described by a Voigt profile and its best-fit parameters were determined from a room-temperature N2 spectrum. We evaluated the O2 concentration 共relative to that of N2兲 by fitting libraries corresponding to different N2–O2 concentration combinations to the experimental spectrum to determine the best-fit spectrum. The values 20 January 2002 兾 Vol. 41, No. 3 兾 APPLIED OPTICS

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Gaussian function with a certain width and to fit the amplitude to the experimental spectrum. This curve was then subtracted from the experimental spectrum. This evaluation approach has the same limitations as the previously mentioned approach. The temperature accuracy of the rotational CARS technique was investigated previously,32 and the results of calibration measurements without the broadband disturbance on pure N2 in an oven resulted in an uncertainty in the evaluated temperature of better than 2.5% from 295 up to 1500 K. The temperature uncertainty increases for temperatures above 1500 K, which is most likely due to insufficient accuracy of the Raman linewidths used in the model. The accuracy of O2 concentration measured by rotational CARS was investigated previously13 and was found to have relative errors in the evaluated O2 concentrations to within 10% for O2–N2 mixtures that contained 12% and 20% O2. In our study, relative O2 concentrations of less than approximately 10% could be evaluated only from a curve fit after the use of either of the subtraction methods. B. Vibrational Coherent Anti-Stokes Raman Spectroscopy Fig. 5. Two illustrations of a rotational CARS spectrum recorded in a premixed C2H4–air flame of equivalence ratio ␾ ⫽ 2.3. At these measurement conditions the CARS spectrum covers a larger spectral range than the detector, and therefore the spectra are cut at approximately 280 cm⫺1. The interfering vibrational CARS signal can be seen in the upper spectrum but has been subtracted in the lower illustration.

of temperature and O2 concentration were taken from this best-fit spectrum and thus evaluated simultaneously. As discussed in connection with Fig. 3, an extra vibrational CARS signal interferes with the rotational CARS signal and contributes to a broadband background in the rotational CARS spectra. To be able to evaluate accurate temperatures and concentrations from the rotational CARS spectra, we developed and investigated two methods to correct for this disturbance. One method was to determine the minima between the rotational peaks of the spectrum and to connect these by straight lines. The obtained piecewise linear curve was then subtracted from the spectrum. Figure 5 illustrates a rotational CARS spectrum corrected by this method. Such a subtraction clearly influences the spectral contribution of the nonresonant susceptibility to the rotational CARS spectrum. By fitting the temperature and also the nonresonant susceptibility, an analysis of theoretical spectra showed that the evaluated temperature from such a procedure was accurate by better than 10 K. It is apparent that the evaluated nonresonant susceptibility after this treatment of experimental data has no physical meaning. The second approach was to model the broadband vibrational CARS signal as a 568


Calculations of the N2 and CO vibrational CARS spectra were made with an in-house-developed computer code; see, e.g., Ref. 34. To take line narrowing into account, the theoretical vibrational CARS spectra were calculated by use of the modified exponential gap law.35 The parameter set for N2 is already well known; see, e.g., Ref. 36. The CO parameters were taken from Roblin et al.37 First the nonresonant background susceptibility and the temperature were fitted only from the N2 part of the spectrum to avoid a local minimum in the fit. These values were then used as initial values for evaluation of all three parameters: the temperature, the nonresonant background, and the CO concentration relative to N2. All three parameters were fitted by use of a contour-fit procedure and not by fitting the peak intensities to measured intensities of additional calibration measurements.38 Figure 6 shows a typical measured vibrational CARS spectrum from a premixed C2H4– air flame 共⌽ ⫽ 2.7兲; the lower graph in the figure illustrates the difference between this spectrum and a calculated spectrum corresponding to the best contour fit. In the calculated spectrum the small additional H2 signal was not taken into account. Nevertheless it could be seen that this small and narrow H2 rotational line has no significant influence on the CO concentration. The CO concentration achieved by equilibrium calculations is 23.8% and is in good agreement with the measured value of 23.1%. The vibrational CARS temperature accuracy for measurements performed in a high-temperature oven has already been found to be 2%.14 The accuracy of the CO concentration is assumed to be better than 5% of the measured value.39

Fig. 6. Experimental vibrational CARS spectrum recorded in a premixed C2H4–air flame 共⌽ ⫽ 2.7兲. The difference between the experimental spectrum and the theoretical spectrum that corresponds to the best fit is given in the plot below the spectrum.

4. Results and Discussion

The combined vibrational and pure rotational CARS technique was applied to sooting premixed flat C2H4– air flames burning on a porous-plug McKenna burner. Figure 7 shows the results from evaluated temperatures in a flame with an equivalence ratio of ␾ ⫽ 2.3. In this flame the soot nucleation occurs at around 4 mm above the burner. When the height increases the soot particle size as well as the soot volume fraction increase. The maximum temperature is expected to be at a position slightly below where the soot nucleation starts and then to decrease gradually with increasing height that is due mainly to radiation losses. Two temperature profiles are shown in Fig. 7. One profile was evaluated from vibrational CARS spectra and the other from rotational CARS spectra. Vibrational CARS is known as an accurate technique in this temperature region14 and is expected to give reliable temperatures below the sooting region of the flame. Above 4 mm, in the sooting region of the flame, an absorption interference in the fundamental band of the N2 vibrational CARS signal from C2 radicals produced by laser-heated particles is expected to increase the evaluated temperature.33 This inter-

Fig. 7. Temperature profiles in a premixed C2H4–air flame with an equivalence ratio of ␾ ⫽ 2.3.

Fig. 8. Experimental 共———兲 and calculated 共– – – –兲 vibrational CARS spectra for 共a兲 CO and 共b兲 N2 recorded at the same time in a premixed C2H4–air flame. The corresponding N2 and CO temperatures were achieved separately by a contour fit.

ference from laser-produced soot is also expected to be more severe for an increase in heights. Evaluation of the uncorrected rotational CARS spectra, i.e., spectra with the additional broadband vibrational CARS background, resulted in a lesssmooth profile than the temperature profile from Q-branch vibrational CARS, probably because of the effects of the additional background and the lower accuracy for rotational CARS at these temperatures. After correction by use of either of the two methods

Fig. 9. Temperature, O2 concentration, and CO concentration profiles in a premixed C2H4–air flame with an equivalence ratio of ␾ ⫽ 2.7. 20 January 2002 兾 Vol. 41, No. 3 兾 APPLIED OPTICS

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Fig. 10. Vibrational and pure rotational CARS spectra recorded in a premixed CO–air flame, which demonstrates the potential for multispecies detection of this technique. In addition to the species identified in the spectra from C2H4–air flames, lines from CO2 can also be observed at the lower Raman shifts in the rotational CARS spectrum from this flame.

described above, the evaluated temperatures were 50 –100 K higher, and the profile from one of the correction methods is illustrated in Fig. 7. A comparison between the evaluated temperatures from the two correction methods showed that the results differ at most by 15 K. Generally, the temperature difference between the profiles from vibrational CARS and corrected rotational CARS is acceptable below the sooting region of the flame, whereas the vibrational CARS temperatures evaluated in the sooting region are probably too high because of the C2 absorption interference discussed previously in this section. The measured vibrational CARS spectra could also be used to compare the N2 vibrational CARS temperature with the CO vibrational CARS temperature by use of the same spectrum. As an example a comparison between the evaluated temperatures is shown in Fig. 8. The spectrum used for this comparison was taken in the nonsooting region of the flame at a height of 3.1 mm above the burner surface. The vibrational CO temperature was not significantly influenced by the H2 signal and is in good agreement with the vibrational N2 temperature. Quantitative results of temperatures and species concentrations have also been evaluated for an C2H4– air flame of equivalence ratio ␾ ⫽ 2.7. This flame is lifted higher from the burner surface and is more suitable for measurements that follow the process from the reactants to the products. The results are illustrated in Fig. 9, where we demonstrate the capability of this combined technique to probe different events in the combustion process. The shapes of the profiles represent what are to be expected for a premixed flame such as this. The temperature increases with an increase in flame height, and the values were evaluated from rotational CARS spectra for lower temperatures and from vibrational CARS spectra for higher temperatures up to approximately 7 mm above the burner surface. In the vibrational CARS spectra a noticeable influence of C2 interference caused by soot could be seen at a height approx570


imately 9 mm above the burner surface and higher. The CO concentration also increases with an increase in flame height, and the O2 concentration decreases. The concentrations are presented as a relative concentration to N2 i.e., the relative O2 concentration is the concentration of O2 in relation to the total concentration of O2 and N2. The combined vibrational and pure rotational CARS technique presented here was also demonstrated in CO–air flames, and a pair of simultaneously recorded spectra are shown in Fig. 10. Both CO and N2 give rise to vibrational CARS spectra of high quality, and in the rotational CARS spectra, contributions from both CO and CO2 can be observed. 5. Summary

By using one Nd:YAG laser, one dye laser, and one CCD camera for a combined dual-broadband pure rotational CARS and vibrational CARS setup, we successfully demonstrated simultaneous temperature and multispecies concentration measurements. The technique makes use of the high-temperature accuracy for rotational CARS at low temperatures and for vibrational CARS at high temperatures. The new possibilities and the advantages of this combined technique have been shown by measurements in sooting premixed C2H4–air flames. To obtain accurate results the rotational CARS spectra had to be corrected because of the interference from an additional vibrational CARS signal. Besides N2, O2, and CO, the detection of CO2 in a CO–air flame has also been demonstrated. This project was supported by the Training and Mobility of Researchers Programme, “Access to large scale facilities,” contract ERBFMGECT950020 共DG12兲. The financial support from the Foundation of Strategic Research 共SSF兲 through the Centre of Combustion Science and Technology 共CECOST兲, and from the Swedish Natural Science Research Council 共NFR兲 is gratefully acknowledged.

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