University of Technology, Eindhoven, The Netherlands. Romania

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1 Cavity Ring-Down Spectroscopy of CH, CH, HCO and H CO in a premixed flat flame at both atmospheric and sub-atmospheric pressure R. Evertsen *, A. Staicu, J.A. van Oijen, N.J. Dam, L.P.H. de Goey and J.J. ter Meulen Department of Mechanical Engineering, Section Combustion Technology, Eindhoven University of Technology, Eindhoven, The Netherlands National Institute for Laser Physics and Radiation Physics, Laser Department, Bucharest, Romania Department of Applied Physics, University of Nijmegen, Nijmegen, The Netherlands Abstract Density distributions of CH, CH, HCO and H CO have been measured in a premixed CH /air flat flame by Cavity Ring Down Spectroscopy (CRDS). At atmospheric pressure problems are encountered due to the narrow spatial distribution of these species. Rotational flame temperatures have been derived from the spectral line intensities of CH. Additional distributions of the species are recorded in a low-pressure set up at x Pa. The results at both pressures are compared to modeling calculations using GRI-Mech. and. showing a relatively good agreement. Deviations are attributed to the applied reaction mechanisms. Introduction The last couple of years, there has been a rapidly increasing development in the field of combustion with respect to computational methods and reaction mechanisms. However, a quantitative validation of reaction mechanisms based on absolute experimental data is essential to attain the highest levels of accuracy. Several laser-based techniques can be applied to obtain this quantitative information from combustion processes []. Cavity ring-down spectroscopy (CRDS) is a very sensitive absorption technique capable to provide direct absolute data. Its potential has been shown from the detection of several minority species such as CH, HCO and CN in flames [-]. CRDS is based on the intensity decay of a laser pulse injected in an optical cavity in which the flame is placed []. Absolute number densities can be obtained from the absorption of the laser light by the molecule or radical. The /e decay time or cavity ring-down time τ(ν) is given by: Specific Objectives To validate the qualitative and quantitative aspects of numerical calculations, the absolute density distributions of several minority species in a premixed laminar flat flame have been measured using CRDS. Rotational flame temperatures could be derived from the experimental CH-spectrum. To circumvent specific difficulties at atmospheric pressure, a lowpressure set up was built to investigate flames at low pressure as well. The experimental results have been analysed and compared to the numerical simulations using GRI-Mech to evaluate the reaction mechanisms. L L τ ( ν) = lnr( ν) + σ i( ν) Ni ( x) dx + Sj () c i j where L is the distance between both mirrors and R includes the mirror reflectivity and scattering observed when no absorbing species are present; the first summation includes losses due to all absorbing species with non-zero cross sections σ i (ν) at frequency ν and number densities N i in the energy states from which the transitions take place, and the summation over S j includes all other scattering and absorption. The absolute species density can be found if a single transition is induced with a well-known absorption cross section σ by comparing the on- and off-resonance decay times. Fig.. Schematic of the cavity ring-down experimental set up. M, M cavity mirrors; L lens; PH pinhole; F filter; D diaphragm; P quartz diffuser plate. Experimental Details The general experimental setup has been described in more detail elsewhere [6, 7] and is shown schematically in Figure. A Nd:YAG/dye laser combination (Quantel YG78C/TDL-) was used as the source of excitation. Two sets of cavity mirrors (radius of curvature mm) were used: one with a reflectivity >.998 at 6 nm (Laser Optik) and a second set with R >.997 at nm (REO). The distance between the mirrors was mm. The laser beam was spatially filtered by a lens-pinhole-lens

2 system before it was coupled into the cavity. The resulting beam had a waist in the centre of the cavity of about. mm FWHM determined with imaging of the Rayleigh/Mie scattering with a CCD camera mounted perpendicular to the cavity. The flat flame burner [8] was placed in the center of the cavity. The circular burner produced a flat flame on a plate of mm diameter. The plate had a hexagonal pattern of holes of diameter. mm and pitch.6 mm and was efficiently cooled by a water-cooling jacket (6 K). It was justified both numerically and experimentally that for mixtures of hydrocarbon and air a flat reaction layer develops on top of this burner [8- ]. The flow velocity and gas mixture were regulated by carefully calibrated mass flow controllers (MFCs). The flat flame was burner-stabilised with a mixture of CH (99.% purity, Hoekloos) and air with an equivalence ratio of φ =.. The flow velocity of u = cm/s produced a large stand-off distance of the flame from the burner of ~. mm. A CCD camera was used to check the parallel alignment of the burner plate to the laser beam axis. The output was detected with both a gated intensified CCD camera (Princeton Instruments -T) and a fast photomultiplier tube (Thorn-EMI), coupled to a -bit digital oscilloscope (Yokogawa DL). In all results presented below, each data point represents an average over laser pulses. Number densities were derived from the difference between the on- and off-resonance ring down times. Cavity losses due to absorption were small ( τ < %) and ring down times were determined by fitting the first /e-time window of the signal. Densities were obtained by including the Doppler width of the transition and the bandwidth of the laser [-]. The single exponential behaviour of the curves was checked whenever a parameter was changed. The alignment of the cavity was optimised on-line using the CCD image recorded directly behind the second mirror. The distributions of the species were measured at different distances d between the burner and laser beam. In the calculations of the absolute densities an effective absorption path length of 6.8 mm was taken as derived from lateral measurements to the flame axis [7]. The temperature was assumed to be constant over this path length []. Background signals were reduced by a diaphragm of mm placed inside the cavity and a diaphragm, low wavelength cut-off filter (λ = 9 nm) and diffuser mounted in front of the photomultiplier tube. The low-pressure measurements were performed with the burner placed inside a vacuum chamber. The cavity mirrors formed the entrance and exit windows of the set up and were placed in a bellows construction to enable the adjustment of the mirrors while maintaining the pressure inside the chamber. The chamber pressure is controlled using a feedback-loop with an absolute pressure transducer coupled to a control unit regulating a MFC (Bronckhorst HiTec), supplying a N load to the rotary-pump. Pressure stability was better than Pa. The combustion gases pass two water-cooling units before reaching the rotary-pump. The experiments were conducted with a fuel equivalence ratio of φ =. at a flow velocity of u =. cm/s at a pressure of p = x Pa. The cavity output is detected with both a gated intensified CCD camera (LaVision Flamestar) and a fast red- or UV-sensitive photomultiplier tube (Thorn-EMI). In this set up the PMT-signal is coupled to a computer via an 8-bits, Msamples/s digital oscilloscope (Lecroy 96). Results and Discussion The application of a laser-based line-of-sight optical technique to an atmospheric flat flame poses a challenge due to the relatively small dimensions of the flame front. The width of the distributions of the species under study is comparable to the spatial width of the laser beam. To be able to conduct accurate measurements and to reach the desired high spatial resolution, alignment has to be secure and deflection due to temperature effects has to be taken into account [7]. Fortunately, using an iccd camera the optical path and distribution of the laser beam can be monitored and the alignment can be optimised in real time. CH, HCO and CH at atmospheric pressure Previous experiments have shown the feasibility of the CRDS-method to obtain absolute CH density profiles in a burner-stabilised CH /air flat flame at atmospheric pressure[7, ]. The CH radicals were probed via the A (v'=) X Π (v''=) Q and Q (N''=) lines around nm. Deflections of the laser beam were measured and accounted for. In view of the narrow width of the distribution of [CH] a correction had to be made for the spatial laser beam profile. The resulting distribution of [CH] and temperature after deconvolution have been compared to simulations using GRI-Mech. and. [6, 7]. A careful investigation [7] showed that the experimental [CH] profile was found much closer to the burner than expected and the most probable cause for the shift in [CH] position relative to the temperature is the reaction mechanism, which predicted that the computed [CH] peak appears at a higher temperature, and thus farther away from the burner. As a next step, the radical distributions of HCO and CH have been studied [8]. Both radicals have spectral features in a similar wavelength regime and can be detected with the same experimental configuration. Figure shows the distribution of CH from the b B a ~ A (,,) 7 7 (J KaKc ) (,,) 7 7 line at 6. nm, which was selected on its intensity, isolation and its known cross-section with B ik given as 8.6x -8 m J - s - [9]. The systematic error in the absorption cross-section is not included in the figure and is <%. Unfortunately, due to the thermal effects and deflections of the laser beam close to the burner, it

3 [ CH ] ( cm - ) Fig. : Experimental and calculated densities of CH ( a ~ A (,,) J=7 7 ) for a burner-stabilised flame at φ =., u = cm/s at atmospheric pressure: measurements, --- numerical curve GRI-Mech., numerical curve GRI-Mech.. and HCO X ~ A" (,,) J=9 numerical distributions are presented in Figs. and, as well. These figures also show the small differences between the two versions of the reaction mechanism for both the CH and the HCO profiles. The accuracy of the relative positions of the experimental and convoluted numerical profiles is better than. mm. The experimental CH distribution appears higher than the numerical values. However, the absolute experimental concentrations found for both species compare well to the numerically obtained values within the experimental and systematic errors. The measured distribution of [HCO] seems to be positioned a little closer to the burner than in the numerical simulations. However, a definite conclusion can only be drawn when more information on the position and absolute value of the maximum is available..6 [HCO] ( cm - ).. Fig.. Chemiluminescence of CH in a low-pressure CH /air flame at x Pa with φ =. and u = cm/s (a), cm/s (b) and cm/s (c). bs indicates the burner surface Fig.. Experimental and calculated densities of HCO ( X ~ A" (,,) J=9) for a burner-stabilised flame at φ =., u = cm/s at atmospheric pressure: measured data, --- numerical curve GRI-Mech., numerical curve GRI-Mech.. was not possible to measure a distinct maximum in the axial profile of [ CH ]. In Figure, the HCO distribution is shown, taken from the A ~ A' X ~ A" (,9,-,,) P(9) line at 6.9 nm using an absorption cross-section of.x -8 cm - at room temperature with an uncertainty of 6% []. The measurements on HCO showed less experimental noise than the CH spectra. Unfortunately, also for HCO it was not possible to record distributions closer to the burner to obtain the expected maximum of the axial profiles. The experimental data consists of a convolution of the radical density profile and the spatial laser beam profile. Because in the present measurements the maximum in the axial profiles could not be detected, the numerical values were also convoluted with the laser beam profile to enable a direct comparison with the experimental results. The resulting CH (,,) J=7 7 Measurements at Pa: HCO, CH and H CO For a more complete evaluation of the distribution of [ CH ] and [HCO] and to study radicals which are positioned very close to the burner surface, use can be made of a low-pressure set up. A reduction in pressure results in a widening of the flame front, decreasing the molecular density, but enabling a more detailed study of the flame structure. Curvature of the flame was studied by recording the chemiluminescence of CH with an iccd camera through a bandpass filter (λ = ± nm). It was observed that at reduced pressures the flame exhibits curvature of the flame front depending on the speed of the flow. Figure shows the results for three different flow speeds at x Pa and φ =.. At higher flows, the flame stabilises on the edges and a slight cone shape is visible. At low flow speeds, the edges of the flame front curve upward as a result of diffusion []. As CRDS is a line-of-sight technique, the conditions were chosen such that the flame front was optically flat with minimum curvature as is the case in Figure.b. The flow was found to be close to the adiabatic laminar burning velocity []. Measurements on HCO, CH and H CO are presented. Partly, these results have not yet been corrected for the temperature and the laser beam profile. However, from a comparison of the relative experimental data and the numerical values some

4 HCO number density ( cm - ) 8 6 Temperature (K) Cavity loss ( - cm - ) pp pq Wavelength (nm) Fig.. Numerical and experimental HCO ( X ~ A" (,,) J=9) density distribution at x Pa with φ =. and u =. cm/s. ( (---) numerical data and temperature profile (GRI-Mech.). Fig. 7. CRDS spectrum of the H CO vibronic band of à A X A in a CH /air flame at x Pa with φ =. and u =. cm/s at.7 mm above the burner. CH cavity loss ( -8 cm - ) Numerical total CH density ( cm - ) Cavity loss ( - cm - ) H CO number density ( cm - ) Fig. 6. Numerical total CH number density (---; GRI- Mech.) and experimental relative distribution from ~ a A (,,) J=7 7 ( "! # Pa with φ =. and u =. cm/s. Fig. 8. Numerical total H CO number density (---) and experimental relative H CO density distribution from X A J=6 ($ % & "! # Pa with φ =. and u =. cm/s. valuable conclusions can be drawn. Figure shows the absolute HCO number density profile and a comparison to the numerical results using GRI-Mech.. The absolute number densities of both the experimental and the numerical results are in quantitative agreement, though the large systematic error in the absorption cross-section has to be considered. The experimental result presented here is not corrected for the spatial width of the probe beam. It can be expected that such a correction would lead to a higher experimental maximum number density, and a smaller width of the profile. The maximum of the experimental HCO profile is found about.7 mm further away from the burner surface than the numerical profile. The difference might be explained by the experimental burner surface temperature profile, which is assumed the same as the cooling water temperature in the simulation, but which is not known. In addition, the flow speed is near the adiabatic burning velocity with a flame very susceptible to small experimental disturbances or differences in the flow. Figure 6 shows the cavity loss resulting from absorption by CH ((,,) J=7 7 ). The larger width of the density profile can be attributed to a larger width of the probe laser beam profile. The results of the numerical simulation for total CH are also included in the figure. Again the experimental profile is shifted about.6 mm away from the burner as compared to the numerical values. The results for both radicals show that CRDS can be performed at low pressure to measure radical distributions which are positioned close to the burner surface and cannot be measured at atmospheric pressures. For example, Figure 7 shows the CRDS spectrum of the H CO vibronic band of à A X A in a CH /air flame at x Pa. This spectrum could not be recorded at atmospheric pressure. From the intensity of the J=6 line [], a relative density distribution as a function of height could be constructed, as is shown in Figure 8 together with the numerical total H CO density distribution. Finally, some initial measurements on HCN were

5 Cavity loss ( - cm - ) Cavity loss ( - cm - ).9 J=7 J= Wavelength (nm) 6 8 Height above burner (mm ) Fig. 9. Numerical total HCN (---, GRI-Mech.) and experimental J=7 HCN (' ()*,+.- /* -* -687"9 : Pa with φ =. and u = 8.9 cm/s. Insert: Cavity loss spectrum with the partial assignment of the HCN P(,,) overtone band at. mm. performed. Numerical calculations show that HCN is more abundantly present in rich flames. Therefore, HCN was measured in the premixed laminar flat flame with φ =. and u = 8.9 cm/s. The insert in Figure 9 shows part of the HCN P(,,) overtone band. The experimental data contains a lot of spectral interference from H O, and requires a careful consideration of the experimental spectrum. The lines which are assigned to HCN disappear at positions outside the flame region. Moreover, their intensity shows a completely different dependence on height above the burner than the H O lines. From the intensity of the J=7 line, a relative density distribution was constructed. The numerical total HCN density is included in the graph and shows a relatively good agreement. Conclusions Cavity ring-down spectroscopy was applied to a premixed flat flame of CH and air and has shown to be a valuable tool in quantitative experimental flame diagnostics. A CCD camera was found a necessity to recognise beam deflection and alignment problems. At atmospheric pressure, CH radical density distributions have been measured and compared to numerical values. Detailed analysis shows that the reaction mechanisms predicts the CH radical at too high a temperature and position. Complete HCO and CH density distributions could not be obtained at atmospheric pressure. However, measurements at x Pa show that radical profiles of HCO and CH can easily be measured. At low pressures, flame front curvature and stability have to be considered. In addition, the profile of H CO, which can hardly be detected at atmospheric pressure in the current configuration, has been recorded. Finally, some strong indications for HCN have been found which are currently extensively studied. Numeri cal density ( cm - ) Acknowledgements This work has been made possible by financial support of the Stichting voor Technische Wetenschappen (Technology Foundation) of The Netherlands. We gratefully acknowledge Dr. Ir. K.J. Bosschaart and and Ing. R.T.E. Hermanns from Eindhoven University of Technology for their assistance with the burner system and the numerical computations. References. Kohse-Höinghaus, K., Jeffries, J.B., ed. Applied combustion diagnostics. New York: Taylor and Francis (). Luque, J., Jeffries, J.B., Smith, G.P., Crosley, D.R., Scherer, J.J., Combust. Flame 6:7-7 ().. Mercier, X., Jamette, P., Pauwels, J.F., Desgroux, P., Chem. Phys. Lett. :- (999).. Lozovsky, V.A., Cheskis, S., Kachanov, A., Stoeckel, F., J. Chem. Phys. 6:88-89 (997).. Berden, G., Peeters, R., Meijer, G., Int. Rev. Phys. Chem. 9:6-67 (). 6. Evertsen, R. Pulsed cavity ring-down spectroscopy in combustion environments. Thesis, University of Nijmegen, Nijmegen, the Netherlands, ubn.kun.nl/ mono/e/evertsen_r () 7. Evertsen, R., Van Oijen, J.A., Hermanns, R.T.E., De Goey, L.P.H, Ter Meulen, J.J.., Combust. Flame :- (). 8. Van Maaren, A., Thung, D.S., De Goey, L.P.H., Combust. Sci. Technol. 96:7 (99). 9. Somers, L.M.T. The simulation of flat flames with detailed and reduced chemical models. Thesis, Eindhoven University of Technology, Eindhoven, the Netherlands, (99). De Goey, L.P.H., Van Maaren, A., Quax., R.M., Combust. Sci. Technol. 9:-7 (99).. Jongma, R.T., Boogaarts, M.G.H., Holleman, I., Meijer, G., Rev. Sci. Instrum. 66:8 (99).. Zalicki, P., Zare, R.N., J. Chem. Phys. :78-77 (99).. Hodges, J.T., Looney, J.P., Van Zee, R.D., Appl. Opt. :-6 (996).. Bosschaart, K.J., Versluis, M., Knikker, R., Van der Meer, Th. H., Schreel, K.R.A.M., De Goey, L.P.H., Van Steenhoven, A.A., Combust. Sci. Technol. 68:-9 ().. Evertsen, R., Stolk, R.L.,Ter Meulen, J.J., Combust. Sci. Technol. 9:9- (999). 6. Bowman, C.T., Hanson, R.K., Davidson, D.F., Gardiner, W.C., Lissianski, V., Smith, G.P., Golden, D.M., Frenklach, H. GRI-Mech. berkeley.edu/gri_mech/. (99) 7. Smith, G.P., Golden, D.M., Frenklach, M., Moriarty, N.W., Eiteneer, B., Goldenberg, M., Bowman, C.T., Hanson, R.K., Soonho Song, Gardiner Jr., W.C., Lissianski, V.V., Qin, Z. GRI-Mech. berkeley.edu/gri_mech/. () 8. Evertsen, R., Van Oijen, J.A., Hermanns, R.T.E., De Goey, L.P.H., Ter Meulen, J.J. Combust. Flame accepted (). 9. Derzy, I., Lokovsky, V.A., Cheskis, S., Chem. Phys. Lett. :-8 (999).. Lozovsky, V.A., Derzy, I., Cheskis, S., Proc. Combust. Inst. 7:- (998).

6 . Bosschaart, K.J., De Goey, L.P.H., to be published ().. Shin, D.I., Dreier, T., Wolfrum, J., Appl. Phys. B 7:7-6 ().