1d single-shot thermometry in flames by spontaneous...

17th International Symposium on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2014

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1D single-shot thermometry in flames by Spontaneous Raman Scattering

through a fast electro-optical shutter

Hassan Ajrouche1, Amath Lo1, Pierre Vervisch1, Armelle Cessou1,*

1: CORIA UMR 6614 CNRS – Normandie Université, University & INSA of Rouen, Saint Etienne du Rouvray, France

* Corresponding author: [email protected] Abstract High temperatures generated by combustion processes have a direct impact on a variety of factors including process efficiency, pollutant levels and product quality. It is therefore of great importance to determine temperature with high accuracy. A new experimental set-up for 1D single-shot measurement of temperature and concentration of major species with Spontaneous Raman Scattering (SRS) in hydrocarbon flames is proposed. For the majority of single-shot 1D SRS experiments in flames, the critical aspect is the temporal gating scheme to remove flame emission such as chemiluminescence of different excited species. Therefore, the need of fast shutter to take full advantage of the high quantum efficiency, the larger dynamic range, the good spatial resolution and the low readout noise offered by full-frame back-illuminated Charged Coupled Device (CCD) cameras is essential. The use of fast electro-optical shutter composed of a large aperture Pockels cell placed within two crossed polarizers (PCS) is proposed for quantitative results. The PCS is assessed by studying its capacity to reduce the background radiation from methane-air flame and to enhance the signal-to-noise ratio (SNR) for Raman-active species. The temperature is determined by comparing experimental and theoretical vibrational-rotational Stokes spectra of N2. For assessing the abilities of the system to measure simultaneously and instantaneously temperature and multi-species concentration, measurements with two spectral resolutions are compared. And their SNR, uncertainties are discussed for assessing the abilities of the system to perform simultaneously and instantaneously temperature and multi-species concentration measurements. Measured temperatures are compared to flame modeling. The spatial resolution being key point of 1D SRS measurement, its influence on uncertainty and spatial averaging is analyzed the effect of spatial averaging within the probe volume is thus analyzed. 1. Introduction More detailed understanding and optimization of combustion processes to improve energy efficiency is one of the major challenges to reduce fuel consumption, pollutant emissions, cost and to increase reliability. Combustion models play an increasingly important role in design and optimization procedures and still require validation data [2]. Thus, the need for reliable data has motivated the increasing development of non-intrusive laser diagnostics. Particularly, Spontaneous Raman Scattering (SRS) presents high potential for flame probing [2-3-4]. Using SRS, as a multispecies diagnostic for combustion flames [2], provides practical benefits: 1) it offers absolute temperature without any calibration; 2) it is independent of collisional quenching effects; 3) it offers a probing of major species with a single laser wavelength. However, its low efficiency has long limited the gas analysis by SRS to large control volumes and long exposure times. The analysis of turbulent flames is tricky due to the need of single-shot measurements with high spatial and temporal resolutions. Therefore, such measurements require on one hand high laser energies, greater than 1J in each pulse [5], associated to long pulse duration to avoid optical breakdown [6] and non-linear effects. On the other hand, they need very sensitive detectors. During the last decades, powerful lasers, high-speed detecting and processing devices have prompted the development of SRS as a combustion diagnostic [2-5-7-8]. From an experimental point of view, flame emission and soot radiation can produce a strong background emission that makes Raman detection very difficult, even impossible in certain spectral locations [9-10]. This flame luminosity in combustion environments makes difficult the accurate determination of temperature and species concentration. Thus, the implementation of a


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temporal gating scheme to remove this interference and to limit its effect on Raman measurement is the way to get a quantitative analysis [1]. Therefore, for signal detection in single-shot Raman measurements, two choices of CCD camera can be made to study the flame. The first consist of performing measurements with Intensified CCD cameras ( ICCD) [4-5] [12] [13] taking profit of their short electronic gating (<10 ns) despite the less quantum efficiency and the high shot-noise levels, which lead to additional uncertainties on single-shot Raman measurements. Regarding single-shot Raman measurements, detectors of high quantum efficiency and high dynamic range are essential for accurately capturing the full range of weak Raman signals. The second choice proposed is the use of full-frame back-illuminated CCD cameras, preferred for their high quantum efficiencies of more than 90%, their larger dynamic range, their good spatial resolution and their low readout noise (<13e-) [14][15][16]. However, due to the full-frame architecture, these cameras require extremely fast shutter shutters for measurement in flames. Various devices have been proposed to solve the gating issue of full-frame CCD cameras. A ferroelectric liquid-crystal shutter has been used to gate a non-intensified CCD camera [17] for one-dimensional multispecies Raman flame imaging [18]. A transmission efficiency of about 40 % is reported on the detection of Raman-active species in H2–air flames with gate width in the range of 40-80 µs. For turbulent flames, this transmission factor is too low and the gate width is too large. A shutter without optical transmission loss has been suggested by Barlow et al. [19]. This mechanical shutter based on rotary chopper wheels has been used for line-imaging Raman measurements in flames allowing gating as short as 9 µs with 800 µm slit [19]. Nguyen et al. [20] has also proposed a high-speed mechanical shutter using a rotary optical choppers providing a gate lower than 10 µs at 30 Hz. More recently, Kojima et al. [21] has described a time-gated detection method for SRS using frame-transfer CCD sensor operating in a subframe burst-gating mode. Then, gate width less than 5 µs were achieved for point-wise measurements. These gate widths are attractive relative to time scales of turbulence and help to reduce much of flame emission. However they are much longer than the duration of the laser pulses used and could be further reduced to study the intense luminous flames. Based on the conservation of polarization by SRS, electro-optical shutter using the fast electronic gating of a Pockels cell can be used as an active polarization-rotating element, decreasing the on-off time to 100 ns. The use of such electro-optical shutter has been reported for application in infrared videography [22] and for probing materials at very high temperatures by Raman scattering spectroscopy [23]. Due to the limited field of view of the small Pockels cell’s aperture used 10 years ago, the long optical path through the cell and the high-voltage pulse tailoring for such long cell, limitation in using Pockels cell’s as a shutter for videography was noticed in the first study[22]. While the second study [23] shows the importance of this type of shutter for point-wise measurements, no use of such system for 1D single-shot measurements in turbulent flames is found in literature. The purpose of this paper is to asses such electro-optical shutter for 1D measurement of SRS from multiple species in turbulent flames. In the present work, we describe first the development and application of a Pockels cell-based electro-optical shutter (PCS) for 1D single-shot Raman multispecies measurements in flames. The PCS is assessed by measuring visible spectrum of the Raman scattering in CH4-air flame with and without shutter. The greatest advantage of using this type of shutter to enhance the signal-to-noise ratio (SNR) for Raman active species is shown. After the assessment of the PCS, the capability of the setup proposed for accurate temperature measurements along a line in flames is analyzed. The temperature is determined by comparing experimental and theoretical vibrational-rotational Stokes spectra of nitrogen (N2) in the flame. The possibility of performing simultaneously single-shot multispecies concentration measurements is investigated by testing different gratings. Averaged temperature measurement in burnt gases are verified and then compared to corresponding temperature laminar flame calculations by COSILAB [24]. Once, the average temperature is confirmed, the possibility of performing 1D single-shot temperature measurements with high and low spatially resolved gratings at flame temperature for high spatial resolutions are presented. Their SNR and uncertainties are also discussed. The compromise between uncertainty and spatial resolution is analyzed. 2. Raman Diagnostics and Experiment


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The layout of the SRS set-up is shown in Figure 1. A long-pulse frequency-doubled solid state Nd:YAG laser (Agilite, Continuum) operating at 10 Hz providing about 1.1 J was employed as the excitation laser source. The laser provides a top-hat pulse with a pulse duration adjusted to 310 ns (FWHM). The laser beam is focused using a convergent lens with a 1 m focal-length providing a probe volume of 150 µm- thick (1/e2).

Figure 1: Overview of the experimental set-up: S, slit; LS, Lens (AR coated @400-700nm) PC, Pockels Cell; P,

Polarizer; BD, beam dump; PM, Power meter; NF, Notch filter (532 nm, FWHM 20 nm); WP, Waveplate

(λ/2, AR coated @400-700nm).

The spontaneous Raman scattered light is collected at right angle to the laser beam and focused onto a spectrometer after passing through the Pockels cell shutter (PCS). Achromats, with large aperture are used to provide good collection efficiency and to match the f-number of the spectrograph. Thus, the light is firstly imaged with a magnification ratio of 2 using a first telescope composed of 2 achromats of 150 mm (f/2) and 300 mm (f/4) focal-lengths. After the first telescope at the focal point of the second lens, a 400 µm-wide adjustable slit aperture acts as a spatial filter. This adjustable slit prevents excessive background illumination (i.e. chemiluminescence of different exited species such as CH*, C2*, CO2*) from reaching the detector. The PCS requires the collected light to provide a high extinction ratio, thus a second pair of achromats with 200 mm (f/4) and 300 mm (f/4) focal-length is used to collimate light coming from the first telescope and to reimage the probe volume to the 200 µm-wide entrance slit of an imaging spectrograph f/4 (Acton Research SpectraPro-300i). The scattered light at the laser wavelength is rejected by a notch filter (HNPF-18702, Kaiser Optical System, OD=6, FWHM 20 nm, transmission efficiency in passbands >80%) placed in the second collimated part of the optical collection system. Then, a periscope is used to rotate the image of the laser beam parallel to the entrance slit of the spectrograph, which is adjusted to 200 µm. Two gratings blazed at 500 nm are used, one with 1200 grooves per millimeter (g/mm) and the second with 600 g/mm. An achromatic half-wave plate (AHWP10M-600, THORLABS) is placed after the PCS in order to fit collected SRS signal to the polarization of the grating. The main idea in the use of the PCS for SRS is to take advantage of the polarization feature of SRS and the fact that the SRS is induced during the laser pulse duration (310 ns), whereas flame emission is continuous. The PCS relies on the electro-optical effect induced by the application of a high voltage (HV) to alter the delay of a birefringent crystal leading to fast switching of polarization [25]. The PCS consists of a leading polarizer (19WG-50, Quantum Technology), to establish initial polarization, a large aperture Pockels cell (LAP-50, KD*P, 50 mm aperture, Quantum Technology) and a final polarizer oriented orthogonally to the leading one. The two crossed wire-grid polarizers have high transmission (85% of the polarized incident light for each polarizer) leading to PCS transmission of about 68% of the Raman signal. A high voltage (HV) generator (HVP-5LP, Quantum Technology) drives the Pockels cell crystal. The duration of the HV


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pulse is externally adjustable (from 200 ns to 1 ms). In this study, the applied high voltage is around 4.85 kV and its duration is adjusted to 500 ns. Due to the optical alignment dependence of the Pockels cell performance, the crystal is inserted into an optical mount allowing pitch, yaw and roll settings. The light coming from the probe volume, which contains lights from flame emissions and Raman scattering is polarized by the first polarizer before entering the Pockels cell crystal (Figure 1). With no HV impressed across the Pockels cell, the light is blocked by the second orthogonally oriented polarizer and the system acts as a closed shutter. When HV is applied, the cell rotates the polarization of the collimated lights by 90° allowing the light to pass through the second polarizer. In this case, the PCS acts as an opened shutter. The single-shot spectra are acquired on a full-frame back-illuminated camera (Pixis-400, B, Princeton Instruments, 1340 x 400 pixels of 20 x 20 µm2). This type of camera offers approximately 90 % of quantum efficiency with 16-bits of dynamic range. In this work, the 100 KHz ADC converter is used to enhance the SNR for Raman active species in burnt gases. Wavelength calibration using a neon lamp and intensity calibration carried out with calibrated tungsten lamp are performed. With the magnification of 3 chosen (3.13 measured), the field of view along the laser beam is almost 2.4 mm long and 150 µm wide, limited by the width of the entrance slit. The length probed is divided by hardware binning to provide different spatial resolutions ranging from 600 µm to 100 µm. Thus, the number of simultaneous spectra acquired ranges from 4 to 25. For each location, 400 single-shot spectra are acquired. 3. Results and discussion The measurements are performed in a laminar flame, stabilized on a Bunsen burner fed with a methane-air mixture with equivalence ratio (Φ) between Φ=0.75 to Φ=1.4. The burner is mounted on x-y-z translation stages with an accuracy of 100 µm. The SRS measurements are performed at two heights above the burner (8 and 24 mm) as shown superimposed to the CH4-Air flame photograph in Figure 2. The inner cone (main luminous front) was 19 mm height and 7 mm wide at half-maximum for stoichiometric methane-air flame. The first measurement height labeled (FG) is located at 8 mm from the burner, probes the fresh gases and crosses the front flame where sharp temperature gradients are present. In addition, this location ensures to have a stable flame. The second one labeled (BG) is located at 24 mm from the burner in the burnt gas areas of the flame, in region of homogeneous temperature close to adiabatic equilibrium temperature. We have to point out that these two zones are chosen to analyze high, at FG, and low, at BG, levels of Raman scattering with maximum levels of flame emission, since in the two cases the line-of-sight flame emission is collected.

Figure 2: Probe volume location superimposed to a typical flame photograph of a laminar premixed CH4-Air

flame at Φ=1.


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3.1 Influence of the PCS on the accuracy of the measurement Due to the full-frame architecture of the back-illuminated camera used, the pixels may be exposed during the readout time causing smearing, especially when the camera is in front of an intense luminous media like flames. When the PCS is switched off, the unpolarized flame emission is integrated over long exposure time denoted by the readout time of the camera (24 ms). Since the full-frame back-illuminated CCD camera is not protected during this period, the flame emission signal becomes very important compared to Raman signal present during 310ns. For this reason, the collected light without PCS is mainly dominated by flame emission. With the PCS switched on, the flame emission is integrated on a small time interval (500 ns), and its contribution on the spectra becomes negligible. This result has been shown in a previous work [26]. The spatial performance of the PCS active-response, which can affect the collected signal of both point-wise and 1D Raman measurements was verified in details in a previous study [26]. The main results reported on the spatial homogeneity PCS active-response are contrast ratios higher than 1:100 in almost 80 % of the aperture of PCS [26]. The highest value 1:800 is measured at the axis and the lowest 1:20 at the periphery. This effect can be attributed to residual stress-induced birefringence on the Pockels cell crystal and slight optical misalignments leading to beam depolarization without any applied electric field [27]. In the present work, the radial efficiency is almost flat throughout the measurement volume. A slight decrease of 20 % in the radial efficiency is noticed on the periphery due to the very short focal length, large aperture collection system and to the lower contrast ratios obtained at the periphery. In the present configuration, CH4-air flame emissions are not much luminous this implies less significant parasite signal than those obtained in previous configurations [26]. Despite the small background radiation presented in the actual experiment, the ability of the PCS to reduce flame emission is illustrated in Figure 3. Figure 3 (top) shows 16 single-shot averaged spectra, acquired with the PCS switched off and parallel polarizers, from a laminar premixed CH4-Air flame (Φ=1). In this situation, the SRS signal from O2, N2, and CH4 acquired at FG with the 600 g/mm grating (Figure 3 (a)) are superimposed to flame background of CO2* emission and the edge of C2* emission at 560 nm, which as intense as the SRS signal in fresh . . At BG (Figure 3 (b)), the acquired Raman signal from N2 and H2O does not clearly emerge from the background. The level intensity of the background near N2 and O2 Raman signals, of about 300 counts without the PCS, is reduced to less than 2 counts with the PCS switched on (Figure 3 (c)). At BG, a clear rotational and ro-vibrational Raman spectra of major species N2, H2O, and CO2 are well observed with the PCS switched on. A significant reduction of flame emission is observed and a noticeable enhancement in the detection of Raman-active species is achieved.

FG (a) BG (b)


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Figure 3 : 16 single-shot averaged spectra acquired from a laminar premixed CH4-Air flame (Φ= 1) along the

2.4 mm-wide probe volume, with the PCS switched off (top) and with the PCS switched on (bottom) The SNR values obtained from single-shot spectra are defined as the ratio of peak intensity to the root-mean-square (rms) fluctuations of noise. After the treatment of 400 single-shot spectra acquired on each radial position, radial profiles of mean SNR values for N2 obtained with the PCS switched on is compared to the corresponding values acquired without any shutter as seen in Figure 4. At FG, an enhancement of the SNR value as high as 5 times is achieved with the PCS switched on. A slight decrease in SNR values denoted at FG is noticed on the periphery for |r|> 0.9 mm. This result is related to the lower contrast ratios obtained at the periphery, which induces a little decreasing in transmission of the signal at this position, and to the reduction of Raman signal according to the increasing temperature. At BG, the relative SNR value obtained with the PCS become 3 times higher than that obtained without the shutter. According to the high temperature (2000 K) in these burnt gas areas of the flame, this low enhancement on the detection of Raman active-species at BG compared to that obtained at FG is explained by the low acquired Raman signal which is slightly higher than the readout noise of the CCD camera. In these regions the SNR is limited by the readout noise.

Figure 4: Radial profiles of SNR value for N2 measured in stoichiometric CH4-air flame with the PCS

switched on (open circle) and without the shutter (cross) at FG (straight red line) and BG (dashed blue line).

Summarizing from above results, the fast shutter system with hundreds of nanosecond gate duration reduces the effects of the background light leading to a significant increase in the SNR of the active species SRS compared to a conventional electro-mechanical shutter. The set of these results permits to improve the quality of the signal and then to consider 1D single-shot temperature measurements by SRS.

FG (c) BG (d)


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3. 3 Instrumental function shape Model The temperature is determined by inversion of vibration-rotation spectra of N2 offering the advantage of not requiring reference temperature. Theoretical spectra are calculated and convoluted with the instrumental function. The determination of an accurate instrumental function shape model is the key to obtain reliable temperature measurements. Considering the moderate dispersion of the spectrograph and the atmospheric pressure condition, the collisional and Doppler widths are assumed negligible compared to the instrumental function. Due to the very short focal length of the collection lens and to the acquisition of spectra at the boundary of the focal plane of the spectrograph, the determination of the instrumental function is very sensitive to the focus of the optical set-up. It is then necessary to determine the instrumental function for each of the simultaneous spectra acquired in situ [28]. The N2 spectra in cold air are adjusted at each region of interest (ROI) defined along the probe volume. For each location, the instrumental function is determined by minimizing the difference between synthetic spectra calculated at room temperature with the experimental spectra. Various mathematical models can be chosen for the instrumental function as Voigt, Lorentzian, Gaussian profiles. Figure 5 gives an illustration of the least-square fitting obtained for different shape models profile used to convolute the theoretical spectra of N2 in cold air for probe volume located at the left, the middle and the right of the length probed. In the present work, the best result is obtained with a Gaussian function for the 1200 g/mm grating and with the combination of 3 Gaussian functions for the 600 g/mm grating: 2 for the wings and one for the peak to take into account the astigmatism. The shapes of the instrumental function at different locations, as expected, is much wider on the periphery than at the center of the focal plane (the full width at half maximum (FWHM) is equal to 17 cm-1 at the center and 23 cm-1 at periphery). The FWHM values show that the assumption of neglecting the collisional (0.1 cm-1 at 300 K and 0.03 cm-1 at 2000 K) and Doppler widths (0.03cm-1 at 2000 K) [29] is validated.

Figure 5: Curve fits to N2 spectra, acquired in cold air with the 600 g/mm grating, using some general

models of instrumental function b) residuals in the fits. 3.4 Average temperature measurement in burnt gas The accuracy of the experimental procedure is assessed by measuring the temperature at the BG height in CH4-air premixed flame of different equivalence ratio from averaged single-shot

(a)

(b)


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spectra. At this height, the temperature is almost homogenous along the length probed (2.4 mm). Examples of average spectra at Φ=1 are shown in Figure 6 for the two gratings with 160 µm spatial resolution.

Figure 6 : Example of N2 Raman intensities in stoichiometric methane-air flame (blue points). The solid red

curve is the theoretical best-fit a) for grating 1200 g/mm at 2139 K b) for grating 600 g/mm at 2129 K. The

green curve represents the residuals values. The value of temperature measured for the 1200 g/mm grating (2139 K) is close to the temperature calculated by COSILAB (2136 K). To consider performing multi-species measurements, average temperature measurements are performed with 600 g/mm grating, less resolved than the first one but providing a wider spectral range. For this grating, the residual is slightly higher and leads to an average temperature (2129 K) that deviates by as much as 10 K from the result obtained with the 1200 g/mm grating and 7 K from the COSILAB value. Further information on the accuracy of the measurement setup for the two gratings are derived from the comparison of the averaged experimental temperature value to the temperature calculated by COSILAB for CH4-Air flame of equivalence ratio ranging from 0.75 to 1.4 with spatial resolution of 160 µm (Figure 7).

Figure 7: Averaged experimental measured temperatures over 400 single-shot spectra versus equivalence

ratio for laminar premixed CH4-air flames for the two gratings compared to results from laminar flame

calculations by COSILAB. Averaged results were obtained in the 1880 K-2190 K. For the 1200 g/mm grating, the agreement between all the measurements and the COSILAB values is good for all the CH4-air flames. And if we assume that the COSILAB value is the theoretical one, the accuracy is lower than 25 K or 1.5 %. The measured temperature by the 600 g/mm grating may differ by as much as 25 K

(a) (b)


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from the calculated temperature. Contrary to the 1200 g/mm, the measured temperature by the 600 g/mm grating is slightly above the calculated temperature. This result is due to the shape of the vibrational bands which is less resolved and to an underestimation of the width of the instrumental function linked to a greater astigmatism of this grating. Despite this effect, the agreement keeps very good at the high temperature measurements. Thus, the accuracy of 600 g/mm grating is always less than 1.5 % in the temperature ranges of the flame.

Grating

Case (160µm) Φ=0.8 Φ=1 Φ=1.2

Modeling Temperature (K) 1972 2136 2093

Region of interest (ROI) 1 6 14 1 6 14 1 6 14

1200

g/mm

Measured Temperature (K) 1952 1958 1956 2142 2138 2146 2088 2101 2098

Accuracy (%) 1 0.7 0.8 0.2 0.1 0.5 0.2 0.4 0.2

600

g/mm

Measured Temperature (K) 1958 1996 1966 2148 2132 2151 2113 2125 2110

Accuracy (%) 0.7 1.2 0.3 0.5 0.2 0.7 1 1.5 0.8

Table 1: Measured temperature in methane-air premixed flame at a homogenous temperature zone (z=24

mm) for different ROI. The comparison of the accuracy between the two gratings highlights the importance of resolved grating for accurate temperature measurements and the possibility to realize temperature measurements for 600 g/mm grating with accuracy less than 1.5 %. The temperature measured with the two spectral resolutions are very close to the temperature calculated by COSILAB and demonstrate that the combustion is almost adiabatic in this area. Table 1 shows the average temperature values measured at the edges of the length (ROI 1 and 14) and in the middle (ROI 6) with a spatial resolution of 160 µm. Due to the in-situ determination of the instrumental function, the dispersion of the values is small. 3.5 Single-shot temperature measurements To illustrate the feasibility of instantaneous temperature measurements with our experimental setup, we have studied the temperature measurement changes in methane-air premixed flames of equivalence ratios ranging from 0.75 to 1.4 at all radial positions along the 2.4 mm-length probe volume for FG and BG. Instantaneous temperatures and their uncertainties will be presented. 3.5.1 Influence of spatial resolution

First, the accuracy of our SRS system for 1D single shot temperature measurements is

estimated at z=24 mm with different gratings and different resolutions for stoichiometric methane-air flame. At each location, 400 single shots spectra are acquired. Figure 8 shows an example of single-shot SRS spectra of N2 acquired in burnt gases with a spatial resolution of 160 µm, this example corresponds to the worse SNR case due to the small probe volume.

The average values of the single-shot temperatures measured in homogeneous regions of the burnt gas are 2134 K ± 138 K and 2126K ± 153 K for 1200 g/mm and 600 g/mm respectively. These temperature values obtained from the single-shot spectra are within the accuracy range of the temperature from the mean spectra (Figure 7).


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Figure 8: Example of single shot N2 Raman intensities in stoichiometric methane-air flame (blue points). The

solid red curve is the theoretical best-fit for grating 1200 g/mm (a) and for 600 g/mm grating (b). From the temperature results of different spatial resolutions presented in Table 2, temperature measurements were obtained in the 2124 K-2154 K for a stoichiometric methane-air flame. There is no difference between the two gratings in terms of accuracy and the accuracy is better than 1%. According to the uncertainties measurements listed in Table 2, the values of uncertainties increase significantly when the size of the probe volume is reduced.

Grating

Resolution (µm) 600 300 250 200 160 100

SNR 21 12 11 10 9 5

1200 g/mm

Temperature (K) 2154 2153 2152 2151 2150 2149

Uncertainties (%) 2.8 4.2 5 5.8 6.5 9.2

Accuracy (%) 0.85 0.8 0.75 0.7 0.65 0.6

600 g/mm

SNR 31 19 17 15 13.5 8

Temperature (K) 2146 2134 2131 2130 2128 2125

Uncertainties (%) 3.4 4.8 5.6 6.1 7.2 9.8

Accuracy (%) 0.5 0.1 0.2 0.3 0.37 0.4

Table 2: Measured temperatures in stoichiometric methane-air flame at a homogenous temperature zone

(z=24 mm) for different spatial resolutions. For the measurement acquired with the 1200 g/mm grating, uncertainties range from 2.8% for

a spatial resolution of 600 µm to reach 9.2 % for a spatial resolution of 100 µm. For this grating, a SNR of 11 or higher is necessary for an uncertainty lower than 5%. Despite the higher SNR values obtained with the 600 g/mm, the uncertainties of temperatures are slightly higher than those obtained with the more resolved 1200 g/mm grating due to the worse spectral resolution of the vibrational bands. Uncertainties for temperature measurement performed with the 600 g/mm grating range from 3.4% for a spatial resolution of 600 µm to reach 9.8% for a spatial resolution of 100 µm. In addition, when the length of the probed volume is reduced to half leading to a decrease of about 43% in SNR, an increase in uncertainties by a factor 1.5 appears. It indicates that the SNR has to be greater than a threshold value to provide small uncertainty of the single-shot

(b) (a)


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temperature measurements. Thus, the uncertainty in the value of the measured single-shot temperature can be attributed to

three main sources. It arises from small oscillation of the flame tip, from low values of the SNR and from accuracy of the model used for the calculation of the instrumental function.

3.5.2 Influence of increasing the SNR The SNR can be enhanced either by increasing the efficiency of the collection system, here new polarizers with higher transmission could be envisaged, or by increasing the energy deposited through the probe volume. The second solution is adopted and the laser energy is multiplied by 1.8 to reach 2 J, by back-propagating the laser through the probe volume using a prism and a 1m-length lens. Samples of single-shot Raman spectra with spatial resolution of 160 µm acquired respectively by 600 g/mm grating and 1200 g/mm grating with and without the double-pass system are plotted in Figure 9. The increase in SNR with the double-pass device makes the detection of certain species like CO2 and H2O in the hot gases possible. At BG, the peak intensities of CO2, N2 and H2O for example increase from 12, 35 and 11 counts without the double pass system to 20, 62 and 19 respectively, leading to increase in SNR from 4, 13.5 and 7.5 to 7, 21 and 11 respectively. Thus, an increase of 65 % in SNR is noticed.

Figure 9: Comparison between 1-pass and 2-pass single-shot spectra for methane-air premixed flame (160

µm) in fresh gases (top) measured by 600 g/mm grating (a) and 1200 g/mm grating (b) and same

experiment in burnt gases (bottom)

Grating Case (160µm) 1-pass Φ=0.75

2-pass Φ=0.75

1-pass Φ=1

2-pass Φ=1

1-pass Φ=1.3

2-pass Φ=1.3

1200 g/mm

SNR 7 12 9 14.5 8 13

Temperature (K) 1887 1897 2150 2170 2023 2042

Uncertainties (%) 6.9 4.3 6.5 4.4 8.1 4.9

(a) (b)

(c) (d)


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Accuracy (%) 0.8 0.4 0.65 1.5 0.2 1.1

600 g/mm

SNR 10.5 18 13.5 21.5 12 19

Temperature (K) 1916 1930 2128 2164 2050 2054

Uncertainties (%) 7.5 4.9 7.2 5.3 8.3 5.4

Accuracy (%) 0.6 1 0.37 1 1.5 1.6

Table 3: Table of representative SNRs, temperature, accuracies and uncertainties for 1 and 2 passes of the

laser beam in the CH4-Air flame (z=24 mm, r=0mm) with spatial resolution of 160 µm. Table 3 shows the influence of SNR increasing on the accuracy of single-shot temperature measurements in burnt gases for rich (Φ= 1.3), lean (Φ= 0.75) and stoichiometric methane-air premixed flames with a resolution of 160 µm. According to the 1-pass and 2-pass temperature measurement performed for different flames, the values of accuracy are not changing and keep a value lower than 1.6 %. A very small but systematic increase in the average temperature is observed when changing from 1-pass to 2-pass configuration. This slight increase shows the loss of detectability affects first the hot vibrational bands. However the temperature difference is very small and remains within the uncertainty range. The results indicate that 65 % improvement in the SNR induced 40 % reduction of the uncertainty without changing the accuracy in temperature measurements. The set of these results points out the need to improve the SNR to reduce uncertainties. 3.6 Spatial averaging effect The temperature measurements in the hot burnt gases show that the temperature of 2100 K can be measured with accuracy better than 1.5 % and an uncertainty around 5 % when SNR is high enough. Due to the decrease in SNR when the size of the probe volume decreases, the uncertainty of single-shot measurements increases when a better spatial resolution is required. Thus, when probing media with temperature or composition gradient, the minimum spatial resolution is the result of a compromise with a correct SNR. When the spatial resolution is too large compared to the gradients, the SRS spectra collected result from molecules with different temperature, and thus the temperature cannot be determined from their inversion. Thus, spatial averaging is a key point of the quality of SRS measurements in turbulent combustion. The spatial averaging effects are analyzed with measurement through the flame front at FG height (z=8 mm). At this location, the laser beam crosses the front flame and the preheat zone where high temperature gradients occur. The temperature profile is compared to the COSILAB modeling in neglecting in first approximation the stretch effect on the temperature profile for our Bunsen burner flame. This assumption will be discussed afterwards. Since the laser beam does not cross the flame front perpendicularly, the experimental profiles are corrected of the angle effect assuming the tangential temperature gradients are negligible at the height probed, far from the flame tip and burner lip. The radial positions are multiplied by cos(20°) then translated by 3.75 mm horizontally. Figure 10 compares averaged single-shot temperatures to the calculated profile for several spatial resolutions. The experimental profiles obtained with the two gratings at high spatial resolution (length of measurement volumes lower than 300 µm), are all in agreement with the COSILAB measurements. The width of the profile and the shape of the profiles fit well with the modeling with a maximum shift at the inflexion point of 60 K for the resolution of 300 µm, which is reduced to 14 K for the resolution of 160 µm. This agreement shows that the stretch rate does not affect the temperature profile in our configuration. A small disagreement in the temperature values, within the accuracy range, between the 1200 g/mm and the 600 g/mm gratings can be noticed in the high temperature plateau due to the repeatability of the measurements from one day to another and the effect of the astigmatism of the 600 g/mm grating previously discussed.


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Figure 10: Radial profiles of the temperature from stoichiometric methane-air flame for different spatial

resolutions, measured by SRS at front flame compared to temperature values calculated by COSILAB For low spatial resolution of 600 µm, the measurement of the temperature profile is clearly affected by the averaging effect: the width is thickened of a factor 2 and the temperatures are underestimated in the temperature range leading to a shift of 190 µm of the inflection point. The accuracy of the temperature measurement is also linked to the ability of resolving the vibrational bands. Thus, it is interesting to analyze the temperature measurements when temperature decreases. Figure 11 shows the temperature profiles acquired with a spatial resolution of 200 µm along the edge regions of the plume of burnt gases. This region corresponds to a region of mixing of the burnt gases with ambient air where the boundary of the hot region is affected by the buoyancy and oscillates. In such conditions no theoretical data are available for comparison with the experimental measurements.

Figure 11: a) Radial profiles of the temperature and their uncertainties from stoichiometric methane-air

flame for different spatial resolutions, measured by SRS at burnt gases. b) Pdfs of Raman measured

temperatures at r=0 mm (left) and r= 14 mm (right) Again, the small difference between the measurements obtained with the two gratings shows the reliability and the reproducibility of experimental procedure proposed. Above 1400 K the difference is less than 1%. For lower temperature (< 1400 K), the measurements with the 600 g/mm are affected by the disappearance of the hotter bands and the astigmatism of this grating. The standard deviation increases below 1400 K due to the flapping of the hot plume boundary as shown by the temperature pdf in Figure 11 .At this location, the oscillation of the hot plume leads to measure either hot gases around 1270K or colder gases around 550K. The increase in the standard deviation is thus due


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buoyancy and is not representative of the uncertainty of the measurements. The temperature fluctuations are almost equal for the two gratings for all the temperature range showing that the temperature good quality of the measurement. Further investigation will be planned to analyze the accuracy of the temperature determination at moderate temperature. The temperature being determined from ro-vibrational spectra, the accuracy of the measurement must be dependent on the population level of the hot vibrational levels. 4. Conclusions A new experimental setup for 1D SRS in flames has been proposed. In such environments where the SRS signal is embedded in continuous background radiation, the use of CCD back-illuminated camera is essential to improve the SRS measurements. Measurements in flame with this type of camera can be realized only if the detector is used in combination with a fast shutter device. The applicability of PCS for 1D single-shot high-performance SRS diagnostics system in flames is demonstrated. Despite its efficiency still limited by the transmission of the polarizers, the PCS is able to achieve fast gating while preserving enough signal to realize single-shot measurement with high spatial resolution (around 200 µm). The capability of SRS to measure temperature with the advantage of not requiring reference temperature is demonstrated and proves that single-shot instantaneous temperatures in flames can be measured with accuracy high enough for the quantitative diagnostics of combustion processes. To consider multi-species measurements, single-shot temperature measurements with 600 g/mm grating were performed leading to the same accuracy as with the 1200 g/mm grating and to slightly higher uncertainties. Impact of SNR on reducing systematic uncertainties shows that 65 % improvement in SNR induced 40% reduction of uncertainty. The limits of the single-shot measurement in flame are determined through radial temperature profiles in flames temperature ranges. The effect of spatial averaging on SRS temperature measurements is analyzed and show first that the determination of the preheat zone thickness requires a spatial resolution smaller than 300 µm. With a slight higher energy deposited in the probed volume, instantaneous single-shot temperature measurement with high spatial resolution as 100 µm can be considered if necessary. This study opens prospects for the analysis of turbulent reacting flows by simultaneous 1D measurements of temperature and concentrations of all the major species. References [1] R. S. Barlow, “Laser diagnostics and their interplay with computations to understand

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