Flame front detection using formaldehyde laser ...

Flame front detection using formaldehyde laser induced fluorescence in turbulent lean premixed flames Internal report, June 2004

Niclas Tylli, Sabine Schenker and Rolf Bombach Paul Scherrer Institut CH-5232 Villigen PSI, Switzerland Abstract The present work aims at suggesting the excitation / detection scheme most suited for laser induced fluorescence measurements of formaldehyde in turbulent lean premixed flames. In the literature, three different excitation schemes have been suggested, with excitation around 339 nm, 352 nm, and 368 nm, respectively. These excitation schemes were systematically tested and advantages and disadvantages for each scheme is discussed. In addition, a number of filter combinations were tested to ensure maximal signal strength for 2-D LIF applications.

1

Introduction

The motivation for the present work was to define a suitable flame front marker for performing laser-induced fluorescence (LIF) measurements in lean turbulent flames. Here the requirement for applications of visualizing the turbulent flame front implicitly means that the signal intensity should be sufficient for single shot measurements. The CH radical has been successfully used in a number of studies, see e.g. Allen et al. (1986), Chen & Mansour (1997), Lemaire et al. (2003), and Giezendanner et al. (2003). However, the strong adverse dependence of CH mole fraction on air/fuel equivalence ratio (λ) makes CH LIF impractical for λ ≥ 1.3. Sutton & Driscoll (2003) mention λ = 1.18 as the lean limit for CH LIF in both methane and propane flames, whereas Chen & Mansour (1997) quote λ ≈ 1.25 as the lean limit. For richer flames, Sutton & Driscoll report an increased CH signal until a maximum is reached for λ = 0.8 and 0.74 for methane and propane, respectively. Sutton & Driscoll also suggest hydrogen addition to hydrocarbon fuels as a means to increase the CH LIF Signal to Noise Ratio (SNR). The increased SNR is in part due to a higher CH mole fraction but, above all, due to reduced background from soot precursors. In order to estimate the CH and CH2 O volume mole fractions which can be expected in the test rig, numerical simulations of a 1-D freely propagating flame, including detailed chemistry, were performed for different equivalence ratios. For these simulations, the PREMIX code (Kee et al., 1985) with the GRI-Mech 3.0 (Smith et al., 1999) reaction scheme was used. The conditions for which the simulations were run, representative for those of the experiment, were atmospheric pressure and a preheating temperature of 673 K. The CH mole fraction dependence on λ can be seen in Figure 1(a). For λ = 1, a CH mole fraction of 10 ppm is predicted whereas less than 0.1 ppm is predicted for λ = 2. As we need a flame front marker suitable for single shot measurements at lean conditions, λ ∼ 2, CH cannot be used. This has also been confirmed by extensive experiments by our group. As can be seen from Figure 1(b), a significantly lower mole fraction dependence on flame equivalence ratio is predicted for formaldehyde. As CH2 O is present in relative abundance even at lean flame conditions, our efforts were directed toward identifying the excitation / detection scheme best suited to perform LIF measurements of this species. As a comparison of the CH2 O mole fractions predicted by the present simulations and experiments, it can be mentioned that Shin et al. (2001) report 994 ± 298 ppm for λ = 1.11, in 2

(a) CH

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Figure 1: CH and CH2 O volume mole fraction for different air/fuel equivalence ratios.

good agreement with the 1160 ppm obtained in the present simulations. LIF of naturally occurring formaldehyde in flames was first reported by Harrington & Smyth (1993). In this study both the Nd:YAG third harmonic at 355 nm as well as a tuneable dye laser in the wavelength range 350-355 nm ˜ 1 A2 41 vibronic manifold. Even was used for excitation into the A˜1 A2 ←− X 0 though significantly less laser power was available with dye laser than with Nd:YAG third harmonic excitation — 0.2-0.9 mJ compared to 15-60 mJ for excitation at 355 nm — the authors report a better discrimination against background fluorescence of polycyclic aromatic hydrocarbons (PAH) when using tuneable excitation. This is due to the possibility to choose a stronger transition using dye laser excitation. However, for a given excitation wavelength, increasing laser power increases the signal to noise ratio due to the fact that the PAH fluorescence signal saturates at relatively low laser fluencies whereas the formaldehyde transitions remain in the linear excitation regime. B¨ockle et al. (2000) used a XeF laser with a maximum of 25 mJ for excitation at 353.2 nm. This laser power was sufficient for single shot measurements with a 20 mm wide laser light sheet. For excitation around 339 nm of the 210 410 band, Giezendanner et al. (2003) employed a 30 mm wide light sheet with 2 mJ energy per laser pulse. However, the authors do not report single shot formaldehyde LIF images for this configuration. These literature data concerning the required excitation power are in good agreement with the experience from the present work. For excitation around 352 nm, it was found that 10 mJ per laser pulse is sufficient for single shot measurements 3

with a 30 mm wide light sheet. To obtain absolute, or even relative, CH2 O concentration profiles via LIF, correction for the variation in collisional quenching rate and partition function of the ground states of the excited transition as a function of spatial location in the flame is required. Harrington √& Smyth (1993) used measured temperature profiles to apply a global 1/ T correction to account for quenching as well as a calculated formaldehyde partition function and Boltzmann factors to obtain relative concentration profiles. More recently, Shin et al. (2000) used measured effective lifetimes to apply relative quenching corrections and calculations from the HITRAN database to arrive at relative concentration profiles. Absolute formaldehyde concentration measurements are reported by Shin et al. (2001), who calibrated the recorded LIF signal with that obtained in a low pressure reference cell with known CH2 O vapor pressure. Klein-Douwel et al. (2000) suggest excitation of the formaldehyde hot band 401 around 370 nm in order to reduce the temperature dependence of the LIF signal. The tradeoff is a reduced overall signal level, rendering single shot measurements difficult. As Klein-Douwel et al. report a possible LIF signal variation by a factor of 5, for a given formaldehyde mole fraction over the temperature range 700-1800 K, significant corrections of the recorded signal are also needed when using the hot band excitation, to obtain relative concentration profiles. An advantage with the CH2 O hot band excitation scheme is the possibility to excite CH in the same wavelength range, thus allowing for detection of these two species with the same laser dye, see also section 3.4. Bombach & K¨appeli (1999) used an innovative arrangement of the dye frequency conversion crystals to simultaneously obtain laser light in the 355 nm as well as the 427 nm region. This way it was possible to record CH2 O and CH or CN and CH LIF simultaneously. Using the conventional mixing after doubling arrangement of the frequency conversion crystals, the authors could additionally reach the 258 nm wavelength region without changing the dye. By tuning the laser to 258.6 nm, a frequency exciting both OH and CHO was found. Simultaneous LIF measurements of these radicals were therefore also possible. Bombach & K¨appeli demonstrated the excitation schemes described above on a rich methane-air flame of a Wolfhard-Parker burner, clearly showing the flame regions in which the different radicals are present. The authors also show that predominantly hot or cold formaldehyde may be excited by carefully choosing the excitation wavelength. In internal combustion engine research, B¨auerle et al. (1994) used 2-D 4

formaldehyde LIF as an indicator of hot spots. For knocking engine cycles, the authors found that formaldehyde is formed and consumed in localized spots in the end gas before the regular flame front has reached these regions. More recently, Schiessl & Maas (2003) applied LIF measurements of formaldehyde to deduce temperature variations in the endgas of an internal combustion engine. To this end, correlations between CH2 O mole fraction and temperature were calculated with measured cylinder pressure traces as boundary condition. Assuming a homogeneous endgas, and using the relative formaldehyde fraction determined from the LIF measurements, the calculated CH2 O / temperature correlation was used to obtain the temperature fluctuations. Using this technique, Schiessl & Maas arrived at temperature fluctuations of more than 20 K along a one-dimensional measurement path. Najm et al. (1998) used both experiments and computations to identify the best measure of heat release rate. The authors suggest the formyl radical (HCO) as the most suited in this respect. However, as CH, HCO mole fraction also decreases rapidly with increasing air/fuel equivalence ratio. The PREMIX simulations referred to above gave 83 ppm and 8 ppm for λ = 1 and λ = 2, respectively. As an alternative, suitable for single shot imaging of the flame front and as an indicator of heat release, Paul & Najm (1998) propose a pixel by pixel product of simultaneously recorded OH and CH2 O LIF. However, even if this approach would work in principle, it was not attempted within the present work due to the obvious need for two laser systems.

2

Experimental techniques and apparatus

In the present work, a Bunsen burner was used as model and reference flame. The Bunsen burner offered the great advantage of being a free flame (i.e. excitation and detection does not have to be done through windows), therefore having negligible stray light levels. After having optimized the laser excitation on the Bunsen burner, the experiments with the turbulent premixed flame were performed in an axisymmetric sudden expansion burner with an expansion ratio of 1:3. The desired flow velocity, u, and preheating temperature, T , was achieved by a throttling valve and by controlling an electrical heater, respectively. The preheating temperature was measured by a thermocouple located in the burner head. The gas flow rate was regulated by a flow controller. Typical operating ranges of the sudden expansion burner within 5

the present work were T = 673 − 783 K, u = 20 − 40 m/s, and λ = 1.4 − 2.1. The test section downstream of the sudden expansion consisted of a quartz tube, giving unobstructed optical access. However, it was soon noticed that the quartz tube fluoresced over a wavelength range broader than 100 nm, spectrally coinciding with the fluorescence of formaldehyde (see also Figures 12 and 15). A spectral separation of the two processes was therefore impossible. As a way to circumvent this, vertical slits were machined into the quartz tube, allowing the laser beam to pass without hitting the glass. However, the open slits modified the flow field and it was impossible to stabilize the flame over a broad range of operating parameters. As a compromise between stray light level and flame stabilization, slit covering arms were constructed, the ends of which were covered with Suprasil quality quartz windows, see Figure 2. By using quartz windows with a low fluorescence

Figure 2: Axisymmetric sudden expansion burner with slit covering arms. level, and placing the windows away from the test section, the fluorescence problem could be alleviated. A Nd:YAG pumped dye laser (Quantel YG781C/TDL50) was used to obtain the desired excitation frequencies around 339 nm, 352 nm and 368 nm. These wavelengths were obtained as follows:

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• 339 nm: Mixture of DCM and Pyridine 1 dye and doubling of the dye laser frequency. • 352 nm: Pyridine 1 dye and doubling of the dye laser frequency. • 368 nm: Mixture of Rhodamine 110 and Rhodamine 6G dye and mixing of the dye laser beam with the Nd:YAG laser fundamental at 1064 nm. For a part of the experiments, even some Rhodamine B dye was added. Using beam shaping optics, an approximately 3 cm wide light sheet was obtained. In the present work it was found that 10 mJ is enough to obtain good quality single shot CH2 O LIF data with a light sheet of these dimensions, for excitation around 352 nm. Tests with an 7.5 cm wide light sheet were also performed, but the available laser power was too low to allow for single shot measurements with an adequate signal to noise ratio. To tune the dye laser to the desired wavelength, excitation spectra of a Bunsen flame were recorded with a photomultiplier (Hamamatsu Photosensor Module H5783-01) connected to a boxcar integrator (SRS Gated Integrator SR250 and SRS Computer Interface SR 245). The boxcar integrator was needed to gate the recorded signal for a short time interval (typically ∼30 ns) around the laser pulse. In addition, Schott filters KV389 and BG39 (3 mm thickness) were placed in front of the photomultiplier in order to reject stray light of the excitation beam and long wavelength light. Knowing the excitation spectrum, the dye laser could be tuned to the desired peak. For the dye laser used in the present work, it was found that the dye laser frequency could shift by ∼2 pm over a 15 min period. This required regular checks of the excitation frequency for cases when extended measurements were performed. Apart from optimizing the excitation scheme, it is evidently also important to ensure that as much of the fluorescence signal as possible reaches the detector, be that a photomultiplier or camera. To this end, filters with an as high transmission as possible in the wavelength range in which the excited species fluoresces should be used. For the case of formaldehyde LIF, this wavelength range is between 400 nm and 550 nm, see also Figure 12. Tests with different filter combinations showed that a combination of the Schott filters KV389 and BG39 provide good transmission properties in this wavelength range. As can be seen in Figure 3, a transmission of almost 60% or more is reached in the relevant wavelength range.

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In the present work, dispersed fluorescence measurements were also carried out. In these measurements, the Bunsen burner flame was employed and the CH2 O LIF signal was focused on the entrance slit of an 25 cm imaging spectrograph (Chromex 250i). In addition to the spectrally resolved measurements of formaldehyde LIF, such measurements were also carried out for the test rig components, such as the quartz tube. For the 2-D LIF measurements, an intensified CCD camera was employed (LaVision FlameStar 2F).

3 3.1

Results Excitation around 339 nm, 210 410 band

In Figure 4, a formaldehyde excitation spectrum for the wavelength range 337.5-340.5 nm is shown. As can be seen, a number of possible excitation frequencies exist. To choose the most suitable frequency, dispersed fluorescence spectra of the strongest peaks were recorded. The peaks chosen are indicated in Figure 4 and the corresponding dispersed fluorescence spectra are shown in Figure 5. For Figures 5(a) and 5(b) different spectrometer gratings were used and the intensities can therefore not be directly compared. However, the fluorescence spectrum with excitation at 338.065 nm (dye laser frequency 676.130 nm) is recorded with both gratings, allowing for a comparison relative to the signal strength recorded for this wavelength. Performing this comparison, it 8

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Figure 4: Excitation spectrum from 337.5 nm to 340.5 nm. Peaks for which dispersed fluorescence measurements were performed are indicated in the plots.

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Figure 5: Dispersed fluorescence measurements for the excitation wavelengths indicated in Figure 4. The color coding of the lines in the above plots correspond to that of the wavelength markers in Figure 4.

is found that the fluorescence signal strength is strongest for excitation at 338.927 nm (dye laser frequency 677.854 nm) and 339.144 nm (dye laser frequency 678.288 nm). As the peak 339.144 nm provides closer access to an off-resonance frequency, this wavelength was chosen for subsequent measurements.

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Excitation around 352 nm, 410 band

3.2

An excitation scan between 351.75 nm and 353.25 nm is shown in Figure 6. In the figure, the peak at 352.37 nm, which was used within the present study, is indicated. This peak provided the advantage of a high signal strength com703.5

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Figure 6: Excitation spectrum from 351.75 nm to 353.25 nm. The peak at 352.37 nm, used in the present work, is indicated with an arrow.

bined with a clearly separated valley region at slightly shorter wavelengths, suitable for off-resonance measurements. Figure 7 shows a formaldehyde LIF image recorded at the sudden expansion burner. The inlet flow velocity was 30 m/s, the air preheating temperature 400◦ C, and λ = 1.7. Here, the off-resonance signal has been subtracted. In the figure, the v-shaped flame can be clearly distinguished. The flame

Figure 7: Laser light sheet width 7.5 cm, excitation at 352.385 nm, 10 mJ per pulse. Camera settings: gain 80, 2x2 binning, integration over 100 shots. Inlet flow velocity 30 m/s, air preheating temperature 400◦ C, λ = 1.7.

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Figure 8: Single shot CH2 O LIF. Laser light sheet width 7.5 cm, 10 mJ per pulse. Camera settings: gain 99 (max), 2x2 binning. Inlet flow velocity 30 m/s, air preheating temperature 400◦ C, λ = 1.7.

front appears to be quite thick, but this is only due to integrating over 100 single shot images. As the flame front position fluctuates significantly for these turbulent lean operating conditions, this results in a smeared out integrated image. The flame front fluctuations are illustrated in Figure 8, in which representative single shot images are shown for the same conditions as for Figure 7. For these images, the signal to noise ratio is rather poor and is due to ∼10 mJ being insufficient laser power for a light sheet of approximately 7.5 cm width. This can be clearly seen when comparing to Figure 9. Here, the light sheet is 2.5 times smaller and the resulting LIF signal in-

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Figure 9: Single shot CH2 O LIF. Laser light sheet width 3 cm, 10 mJ per pulse. Camera settings: gain 99 (max), 2x2 binning. Inlet flow velocity 30 m/s, air preheating temperature 400◦ C, λ = 1.6.

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tensity is significantly higher. Note that the image scaling is 0-500 counts in Figure 8 and 0-1000 counts in Figure 9. With the camera used in the present experiments, and the image intensifier at max value (gain 99), 500 and 1000 counts correspond to ∼14.5 and ∼29 photons, respectively. The small difference in equivalence ratio — λ = 1.7 versus 1.6 — still allow for a comparison between the two figures. In order to test whether the LIF signal is saturated when using the smaller light sheet, an experiment with focused and unfocused light sheet was performed. As can be seen in Figure 10, the signal level is similar for both cases,

(a) Focused light sheet.

(b) Unfocused light sheet.

Figure 10: Saturation test: Laser light sheet width 3 cm, 10 mJ per pulse. Camera settings: gain 80, integration over 100 laser shots, 2x2 binning. Inlet flow velocity 20 m/s, air preheating temperature 400◦ C, λ = 1.7.

indicating that the excitation was still in the linear regime for the 3 cm wide light sheet and 10 mJ laser power per pulse.

3.3

Comparison of excitation at 339 nm and 352 nm

To compare the signal strength from excitation at 339 nm and 352 nm, a few measurements were recorded under similar conditions. Due to the fact that the laser dye was not changed, the laser power was different by a factor of 2; 7-8 mJ versus 3.5-4 mJ for 339 nm and 352 nm, respectively. In Figure 11, the on-resonance, off-resonance and the difference image is shown for both excitation wavelengths. Here, the signal level is higher for excitation at 339 nm, Figure 11(a), than for excitation a 352 nm, Figure 11(b). This 12

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Figure 11: Comparison of on-resonance to off-resonance signal ratio for excitation at 339 nm and 352 nm. The region for which the image intensity was considered is indicated with a rectangle. Camera settings: gain 80, 2x2 binning, integration over 100 shots. Inlet flow velocity 20 m/s, air preheating temperature 500◦ C, λ = 1.9.

is partly due to the difference in laser power. However, due to the fact that fluorescence of the quartz tube walls is much more prominent in Figure 11(a), one can assume that the laser light sheet position was not equally well optimized in both cases. Despite this, a comparison in relative signal strength, on-resonance to off-resonance, can be made. For this purpose, the average image intensity was calculated for the region within the rectangle indicated in the figure. It was found that the on-resonance to off-resonance signal ratio was 1.9 and 2.4 for excitation at 339 nm and 352 nm, respectively. 13

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As a further test of the suitability of these two excitation wavelengths, dispersed fluorescence spectra were recorded. As can be seen in Figure 12, the signal to noise ratio is better for excitation at 352 nm. Here the curves

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Dispersed fluorescence spectra formaldehyde with excitation at 339.14 nm and 352.09 nm. Integrated signal contribution versus λ traces for both cases are also shown.

have been adjusted to account for the difference in laser power. From these experiments it can be concluded that excitation at 352 nm is a better choice than 339 nm, providing that approximately the same laser power is available for both wavelengths.

3.4

Excitation around 368 nm, 401 band

Excitation around 368 nm provides the advantage of a less temperature dependent signal level compared to the other excitation schemes. This is due to the energy levels excited at 368 nm being populated mainly for hot formaldehyde. The band around 368 nm is therefore referred to as the hot-band. As pointed out by Klein-Douwel et al. (2000), CH LIF can also be performed for wavelengths around 368 nm, by exciting the B-X (1,0) band. In the excitation scan shown in Figure 13, the CH peaks are in fact the strongest. Here, the four CH peaks are indicated with arrows. Several weak formaldehyde peaks are present in the wavelength range between 368.18 nm to 371.59 nm, the strongest being at 368.27 nm. Figures 14(a) and 14(b) show the CH2 O and CH LIF images of a Bunsen burner flame recorded with excitation at 368.27 nm and 367.04 nm, respec14

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Figure 13: Excitation spectrum from 368.18 nm to 371.59 nm. CH peaks present in this wavelength range are indicated with arrows.

tively. The figures correspond to an average of 20 images, each of them integrated over 200 laser pulses. For the formaldehyde LIF image, the offresonance signal recorded at 367.97 nm has been subtracted. As can be seen in Figures 14(a) and 14(b), formaldehyde and CH are present in different flame regions. Formaldehyde is present before the reaction zone and is therefore found closer to the flame center whereas CH indicates the actual reaction zone and is therefore found further away. To illustrate the possibility of exciting CH2 O and CH in the same wavelength region, a 1-D scan was recorded. For this, the laser wavelength was continuously scanned and images integrated over 40 laser pulses were recorded. The region indicated by white lines in Figures 14(a) and 14(b) was binned to one single line, yielding one pixel column in the scan image in Figure 14(c). In Figure 14(c), the image has been rotated 90 degrees counter clockwise. Close to 368.86 nm, three distinct peaks can be seen. The positions of these peaks in the 1-D scan, corresponding to different lateral positions in the flame, indicate that the signals correspond to two different species; formaldehyde closer the flame center and CH further away.

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(a) CH2 O LIF with excitation at (b) CH LIF with excitation at 368.27 nm, average of 20 images, 200 367.04 nm, average of 20 images, 200 shots each. The off-resonance image shots each. recorded at 367.97 nm has been subtracted. dye wavelength 565

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Figure 14: Formaldehyde hot-band measurements of a Bunsen burner flame. The 2-D LIF images of CH2 O and CH in (a) and (b) show in which flame regions these radicals are present. The region of interest (ROI), which is binned to one single line, in the 1-D scan, figure (c), is indicated with white lines.

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3.5

Fluorescence of different quartz qualities

During the course of the work it was found that broad band fluorescence of test rig parts may be a significant limitation for the applicability of formaldehyde LIF. In the case of the present sudden expansion burner, fluorescence of the quartz tube was so strong that slits had to be machined in the tube. Because this destabilized the flame, slit covering arms were constructed, see Figure 2. These slit covering arms consisted of a stainless steel construction allowing for placing windows of Suprasil quality quartz farther away from the test section. By disjoining the windows through which the excitation beam passes from the test section, spreading of fluorescence light in the test section tube was avoided. In this way the fluorescence problem could be alleviated and the flame stability maintained. In general it can be said that windows pose a major obstacle for performing formaldehyde LIF, unless the excitation beam windows can be disjoined from the detection windows and placed geometrically away from the measurement area. This is clearly illustrated by the dispersed fluorescence spectra of three different quartz qualities in Figure 15(a). For the measurements presented in this figure, the standard Quarzrohr mit Isolationsmaterial Quarzrohr Corning Suprasil

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Figure 15: Dispersed fluorescence measurements of different test rig components. The excitation beam was at 365 nm.

quartz tube used for the sudden expansion burner as well as Corning and Suprasil 2 quality quartz windows were placed in the laser beam and their fluorescence was recorded with an imaging spectrograph. The laser wave17

length was at 365 nm and the peak close to 555 nm in the figure is due to a small amount of the dye laser fundamental beam hitting the quartz tube. Comparing the wavelengths at which quartz glass fluoresces, Figure 15(a), with those at which the formaldehyde LIF signal is present, Figure 12, it is clear that a spectral separation of the two processes is impossible. As both processes also coincide temporally, no means other than geometrical separation of the test section and the glass windows remain. One of the test rigs of the Combustion Laboratory at the PSI comprise a quartz tube surrounded by insulation material. In order to estimate the influence of the insulation on the prospects of successful CH2 O LIF measurements, dispersed fluorescence measurements were also carried out for the combination quartz glass tube with insulation, see Figure 15(b). As can be seen from the figure, the fluorescence stays spectrally unshifted whereas its maximum is increased by a factor of 2 compared to the case without insulation.

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Giezendanner, R., Keck, O., Weigand, P., Meier, W., Meier, U., Stricker, W. & Aigner, A. 2003 Periodic combustion instabilities in a swirl burner studied by phase-locked planar laser-induced fluorescence. Combust. Sci. Technol. 175, 721–741. Harrington, J. E. & Smyth, K. C. 1993 Laser-induced fluorescence measurements of formaldehyde in a methane air diffusion flame. Chem. Phys. Lett. 202, 196–202. Kee, R. J., Grcar, J. F., Smooke, M. D. & Miller, J. A. 1985 A fortran program for modelling steady one-dimensional premixed flames. Tech. Rep. SAND85-8420. Sandia National Laboratories. Klein-Douwel, R. J. H., Luque, J., Jeffries, J. B., Smith, G. P. & Crosley, D. R. 2000 Laser-induced fluorescence of formaldehyde hot bands in flames. Appl. Optics 39 (21), 3712–3715. ¨ hringer, K. Z., Rolon, J. C. & Lemaire, A., Meyer, T. R., Za Gord, J. R. 2003 Vortex-induced flame extinction in two-phase counterflow diffusion flames with CH planar laser-induced fluorescence and particle-image velocimetry. Applied Optics 42 (12), 2063–2071. Najm, H. N., Paul, P. H., Mueller, C. J. & Wyckoff, P. S. 1998 On the adequacy of certain experimental observables as measurements of flame burning rate. Combust. Flame 113 (3), 312–332. Paul, P. H. & Najm, H. N. 1998 Planar laser-induced fluorescence imaging of flame heat release. Proc. Combust. Inst. 27, 43–50. Schiessl, R. & Maas, U. 2003 Analysis of endgas temperature fluctuations in an SI engine by laser-induced fluorescence. Combust. Flame 133, 19–27. Shin, D. I., Dreier, T. & Wolfrum, J. 2001 Spatially resolved absolute concentration and fluorescence-lifetime determination of H2 CO in atmospheric-pressure CH4 /air flames. Appl. Phys. B-Lasers Opt. 72, 257– 261. Shin, D. I., Peiter, G., Dreier, T., Volpp, H. R. & Wolfrum, J. 2000 Spatially resolved measurements of CN, CH, NH, and H2 CO concentration profiles in a domestic gas boiler. Proc. Combust. Inst. 28, 319–325. 19

Smith, G. P., Golden, D. M., Frenklach, M., Moriarty, N. W., Eiteneer, B., Goldenberg, M., Bowman, T., Hanson, R. K., Song, S., Gardiner, W. C., Lissianski, V. V. & Qin, Z. 1999 Grimech 3.0. http://www.me.berkeley.edu/gri mech/. Sutton, J. A. & Driscoll, J. F. 2003 Optimization of CH fluorescence diagnostics in flames: range of applicability and improvements with hydrogen addition. Appl. Optics 42 (15), 2819–2828.

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