H Ajrouche

A Lo

P Vervisch

A Cessou 

D single-shot spontaneous Raman scattering for temperature and multi-species measurements in flame. Comparison of back-illuminated CCD with BI-EMCCD and ICCD cameras

    1D single-shot spontaneous Raman scattering for temperature and multi-species measurements in flame. Comparison of back-illuminated CCD with BI-EMCCD and ICCD cameras H. Ajrouche, A. Lo, P. Vervisch, Armelle Cessou

Introduction

Validation of theoretical and numerical combustion models has motivated the development of Spontaneous Raman Scattering (SRS) as a multispecies diagnostic [1,2]. Due to its low efficiency, the gas analysis by SRS has been limited for many years to large control volumes and long exposure times. These constraints make the SRS unsuited for the analysis of turbulent flames requiring single-shot measurements with high spatial and temporal resolutions. Therefore, such measurements require on one hand high laser energies, greater than 1J/pulse [3], associated with long pulse duration to avoid optical breakdown [4], and on the other hand very sensitive detectors. Moreover, the weak SRS is usually embedded in the flame emission, thus requiring a detection system not only with high efficiency (high sensitivity, low noise, high dynamic range) but also with a fast temporal gating. Therefore, single-shot SRS measurement in flames are performed either with back-illuminated CCDs (BI-CCD) [5,6] associated with a home-made shutters [7,8] or with ICCD cameras [9][10][11][12]. The advantage of each solution is the high sensitivity for BICDD and fast gating for ICCD. Their main drawback is the requirement of developing a shutter for BI-CCD and the high shotnoise for ICCD [13,[START_REF] Wehr | Proc. Eur. Combust. Meet[END_REF] . The recent development in the field of signal detection is the electron multiplying CCD camera BI-EMCCD, which combines extremely high quantum efficiency (QE), when back-illuminated (BI-EMCCD), with the ability to eliminate the readout noise detection limit [START_REF] Gregor | [END_REF][16][17] but keeps the need for a fast shutter.

We have previously proposed a new experimental set-up for 1D single-shot measurements of temperature and concentration of major species by SRS [START_REF] Ajrouche | 17th Int. Symp. Appl. Laser Tech. to Fluid Mech[END_REF][START_REF] Ajrouche | 14éme Congrès Francoph[END_REF], where the ability of a Pockels cell shutter (PCS) to enhance the signal-to-noise ratio (SNR) has been demonstrated [START_REF] Lo | Proc. Eur. Combust. Meet[END_REF]. The applicability of PCS as optical gating for BI-CCD and BI-EMCCD makes these cameras suitable for Raman measurements in flame [17].

Specific objectives

In the present work, we studied the potential applicability and the limitation of the three detector types for instantaneous 1D Raman measurements of temperature and species concentrations in flame. The ability to measure single-shot scalar values accurately in flames is assessed by comparing the BI-CCD, BI-EMCCD and ICCD detection systems. First, a comparative analysis of the performances of these devices has been performed, principally to illustrate the effect of each detector on fundamental SNR considerations. Second, thermometry by SRS, which offers the advantage of not requiring reference temperature, has been proposed in previous work [START_REF] Ajrouche | 17th Int. Symp. Appl. Laser Tech. to Fluid Mech[END_REF][START_REF] Ajrouche | 14éme Congrès Francoph[END_REF]. The accuracy and uncertainty of temperature in near-adiabatic CH 4 /air flames according to the detector used are analyzed and compared to adiabatic 1D freely propagating laminar flames modeling. Third, 1D singleshot density measurements of the 3 detectors are compared for N 2 concentration quantified when crossing the flame front and for density corresponding to signal close to the detectability limit by probing CO in a nearstoichiometric rich premixed flame.

Experimental setup

The experimental set-up has been described in detail elsewhere [START_REF] Ajrouche | 17th Int. Symp. Appl. Laser Tech. to Fluid Mech[END_REF][START_REF] Lo | Proc. Eur. Combust. Meet[END_REF] and only a short summary will be given here. Fig. 1 shows the layout of the SRS set-up. The laser source consists in a Nd:YAG laser (Agilite Continuum) operating at 10 Hz providing about 1.2 J with top-hat pulse with a long pulse duration adjustable from 200 ns till 1 µs. A long pulse duration provides a large energy deposit suitable for single-shot SRS [3], with good spatial resolution and without optical Fig. 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) breakdown occurring when instantaneous and local irradiance is greater than a threshold value of 34 GW/cm 2 [4]. The pulse duration used in this study is 310 ns. The laser beam is focused using a convergent lens with a 1 m focal-length providing a probe volume of 170 µm-thick (1/e 2 ). The SRS light is collected at right angle to the laser beam with a large solid angle (f/2) using two telescopes composed of achromatic lenses. 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.

In this study, three types of camera are compared with different quantum efficiency (QE) and noise factor (NF), which originates from the amplification process, defined as: the ratio of the output noise of the amplifier to the product of the multiplication gain by the input noise. An ideal amplifier will therefore have a noise factor of 1. The consequence of the stochastic nature of the gain in both BI-EMCCDs and ICCDs is a fundamental parameter affecting the SNR.

The ICCD detector used is a 16 bit CCD camera equipped with a GenIII intensifier (PI-MAX UNIGEN, Princeton Instruments, 512 x 512pixels, pixel size 23µm, readout rate of 1 MHz). The maximal QE provided by this device is 38% between 400 and 700 nm and its NF ranges between 1.6 and 3.5 [START_REF] Robbins | [END_REF]22]. The image intensifier was operated in gated mode with a gate width of 500 ns, suitable to suppress non-laserinduced emissions, such as flame luminosity.

The back-illuminated CCD camera is a full-frame CCD (Pixis 400B, Princeton Instruments, 1340x400 pixels, pixel size 20 µm, NF=1). This camera offers approximately 94 % of QE with 16-bits of dynamic range and readout noise (<13e-). Different analog-to-digital converters (ADC) are available as 1-2 MHz or 100 KHz. Because the readout noise of CCD arrays increases with the readout rate, the 100 KHz ADC is chosen to enhance the SNR for Raman active species.

The last camera is an back-illuminated electronmultiplying CCD camera (BI-EMCCD) (ProEM, Princeton Instruments, 1600x200pixels, pixel size 16 µm, 94% of QE). The selected EM gain setting was approximately (x200) and was sufficient to make the readout noise negligible. The readout rate used in this study is 1MHz. A fast electro-optical shutter is used for SRS measurements in flame with the 2 non-intensified detectors (BI-CCD or BI-EMCCD). The PCS consists of a large aperture Pockels cell (LAP-50, KD*P, 50 mm aperture, Quantum Technology) between 2 crossed polarizers (19WG-50, Quantum Technology), The two crossed wire-grid polarizers have high transmission (85% of the polarized incident light for each polarizer) leading to PCS transmission of about 72% of the Raman signal [START_REF] Ajrouche | 17th Int. Symp. Appl. Laser Tech. to Fluid Mech[END_REF][START_REF] Ajrouche | 14éme Congrès Francoph[END_REF][START_REF] Lo | Proc. Eur. Combust. Meet[END_REF]. 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. After the PCS, an achromatic half-wave plate (AHWP10M-600, THORLABS) is placed in front of the spectrograph in order to fit collected SRS signal to the polarization of the grating.

Results and Discussion

Single-shot SRS temperature and multispecies measurements from BI-CCD, BI-EMCCD and ICCD are compared by measurements in a premixed laminar flame, stabilized downstream a Bunsen burner fed with a methane-air mixture with equivalence ratios of Ф=1 and 1.4. The measurements are compared to modeling calculation of 1D freely propagating laminar flame by the COSILAB [23] software, using the GRI-Mech 3.0 chemical mechanism [24]. Since the laser beam does not cross the flame front perpendicularly, the experimental profiles are corrected from the angle effect assuming that the tangential temperature gradients are negligible at the height probed, far from the flame tip and burner lip. In first approximation the stretch effect on the temperature and species profiles are neglected. This assumption will be discussed afterwards. Most of the measurements are performed at Ф=1, for which the adiabatic temperature determined by modeling is 2136 K. The Ф=1.4 flame is used to assess the cameras for measuring low level of concentrations, especially in probing CO which is a species difficult to probe by SRS in flame [25]. 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). The SRS radial profiles performed at height of 8 mm above the burner exit, labeled FG, offer measurements in various conditions: fresh mixture near the centerline, homogeneous burnt gas at the periphery, and sharp temperature and composition gradient for the intermediate radii, when crossing the preheat and reaction zone. At this height the flame front is stable. The second height, labeled (BG), is located 24 mm above the burner exit in the burnt gas areas of the flame, in a region of homogeneous temperature and composition, close to equilibrium conditions.

The possibility of measuring single-shot temperature from SRS has been demonstrated in previous work using BI-CCD camera with a PCS shutter [START_REF] Ajrouche | 17th Int. Symp. Appl. Laser Tech. to Fluid Mech[END_REF][START_REF] Ajrouche | 14éme Congrès Francoph[END_REF][START_REF] Lo | Proc. Eur. Combust. Meet[END_REF]. Here the single-shot temperature measurements from BI-CCD, BI-EMCCD and ICCD are compared and analyzed in terms of uncertainty and accuracy. The temperature is determined by simulating the vibrationrotation spectra of N 2 by theoretical spectra. These spectra are calculated and convoluted with the in-situ instrumental functions [START_REF] Ajrouche | 14éme Congrès Francoph[END_REF]26]. The SNR values obtained from single-shot spectra are defined as the ratio of peak Raman intensity of fitted spectra, considered as the "true" signal value, divided by the root-mean-square (rms) fluctuations of the difference between experimental and fitted spectra calculated in the wavelength range of the SRS signal.

Fig. 2 shows an example of single-shot Raman spectra with a spatial resolution of 160 µm, obtained on the axis of the collection system at BG for the three detectors, illustrating their different nature of noise. The two spectra acquired with the BI-EMCCD and BI-CCD show the SRS signal of the rovibrational bands of CO 2 , N 2 and H 2 O (Fig. 2 a,b). Fig. 2 (c) shows that the single-shot spectra obtained with BI-CCD and BI-EMCCD are weakly noisy, SNR of 9.92 and 8.89 respectively, and very reproducible from one shot to another. While instantaneous spectra acquired with ICCD is altered by shot noise, and varies from one shot to another. Here, SNR of 5.84 much lower than for the 2 other camera is noticed. In the following, the COSILAB modeling will be considered to provide the reference data (temperature and major species) of the flame. The comparison of temperature accuracy between the BI-EMCCD and BI-CCD detectors highlights their ability to provide accurate temperature measurements with an error smaller than 1% and shows the reliability and the reproducibility of experimental procedure proposed. Temperature fluctuations are almost equal for these two detectors with slight higher uncertainties obtained with BI-EMCCD (160K) than those with BI-CCD (120K). We can point out that when BI-EMCCD gain is set to 1 similar fluctuation levels (124K) to those with the standard BI-CCD are obtained. For ICCD, the temperature measurements are clearly affected by the higher NF of this device degrading the SNR from 9.92 with BI-CCD to 5.84 with ICCD and resulting in an average temperature of 1926K clearly smaller than the calculated temperature (2136K) and very high uncertainties (340K). This is due to the decrease in the spectral resolution of the vibrational bands because of the higher pixel size and the higher shot noise of this detector.

Fig. 4: 400-shot averaged radial profiles of temperature from stoichiometric methane-air flame, measured by the three detectors at FG compared to temperature values calculated by COSILAB.

Fig. 4 compares averaged single-shot temperature to the calculated profile for the three detectors. The profiles obtained from both BI-CCD and the BI-EMCCD at 160 µm spatial resolution are in agreement with the COSILAB calculations. The profiles fit well with the modeling with a maximum shift at the inflexion point of 50 K for BI-EMCCD, which is reduced to 24 K for BI-CCD. This agreement shows that the stretch rate does not affect the temperature profile in the present work.

For ICCD, the temperature profile is significantly affected by the higher shot noise, which degrades the fitted spectra and leads to an underestimation of the temperature on the plateau due to the shot-to-shot signal variation. However, the averaged experimental temperature values obtained from mean spectra acquired with ICCD are more accurate. Temperature of 2100 K is observed on the temperature plateau at BG, which implies temperature measurement in our configuration can be performed with ICCD only for average spectra where the noise effect is reduced.

The 3 detectors are now compared by measuring species concentrations firstly with high concentration levels by probing N 2 in the stoichiometric flame and then with low concentration levels by probing CO in the Ф=1.4 flame. Density is determined using the area of theoretical spectra and the fitted temperature. The accuracy of N 2 density measurement is first assessed at BG. The measured densities are 2.483x10 18 ± 1.8x10 17 cm -3 , 2.623x10 18 ±3.812x10 17 cm -3 and 3.95x10 18 ± 1.04x10 18 cm -3 for BI-CCD, BI-EMCCD and ICCD respectively. Density values obtained with BI-CCD are very close to the density calculated by adiabatic flame modeling COSILAB (2.48x10 18 cm -3 ). Density obtained with BI-EMCCD is slightly underestimated due to the higher uncertainty of temperature with this detector. If the modeled temperature from COSILAB is used instead of the instantaneous temperature measured, the density value is 2.36 x10 18  2.66x10 17 cm -3 , showing that for this camera the uncertainty is due to the temperature uncertainty measured from SRS.

For ICCD, the underestimation and uncertainties of density measurements are high. These results suggest that temperature is very important parameter which affects density measurements. When the modeled temperature is used, the results show more accurate density values 2.56x10 18 cm -3 instead of 3.95x10 18 cm -3 and lower uncertainty values which are significantly reduced from (1.04x10 18 cm -3 ) with temperature measured by ICCD to (4.48x10 17 cm -3 ) with temperature provided by simulation. In this case, if ICCD detector is used for density measurements, temperature should be measured by an independent simultaneous measurement, like Rayleigh scattering or SRS with BI-CCD associated with a fast shutter. Scatter plots of instantaneous temperature measurements versus density of N 2 with 160 µm spatial resolution measured at FG when crossing the front flame are plotted for different cameras in Fig. 5 superimposed to the modeled profile. The BI-CCD measurements reproduced well the hyperbolic behavior. The dispersion of 9.5% from either side of the modeled curve is acceptable especially if we consider the small probe volume. This dispersion is reduced to 6.7 % for a probe volume of 300µm. The density measurements performed with BI-EMCCD present approximately similar results to those obtained with BI-CCD with higher dispersion of the values due to the higher uncertainties of temperature measurements performed with BI-EMCDD (Fig. 5 (b)).

The scatter plot dispersion of N 2 density measurements performed with ICCD is very broad due to the cumulated uncertainty of density and temperature measurements (Fig. 5 (c)). The question arises whether the temperature is the cause of the loss of the precision on the density measurements characterized by the increase in density uncertainties. The results discussed in the previous paragraph show that temperature is the key parameter which affects density measurements and that for ICCD camera the temperature measurements must be performed by another method.

Fig. 6 presents an example of single shot SRS spectra of N 2 and CO acquired in burnt gases (160 µm spatial resolution) recorded with each camera, with their respective calculated fit. For illustration, the spectra were chosen among the 400 single-shot spectra for the temperature corresponding to the COSILAB temperature in burnt gases at Ф=1.4 (T=1940 K). In Fig. 6 (a) acquired from BI-CCD, the CO peak of few counts (~5) is difficult to distinguish with a SNR value of 0.92 (Table 1). The small peak value of CO obtained with BI-CCD can be embedded for some single shot spectra in the background signal and therefore information about CO will be lost. As for Fig. 2 (a) above, the improvement in BI-EMCCD signal quality is immediately noticeable in Fig. 6 (b). Therefore, the use of BI-EMCCD increases the detectability of CO peak to 350 counts with a SNR of approximately 1.5. BI-EMCCD can detect very low CO signal. For ICCD, the high shot noise decreases drastically the SNR of the CO peak to 0.84, and makes the detectability of the peak very low. We have to remember here that CO density measurement by SRS is not usual due to the low level of signal and that this issue is enhanced here by the small probe volume (160 µm). The COSILAB value of CO density is 2.91x10 17 cm -3 for Ф=1.4 methane-air premixed flame. Despite the acceptable accuracy of density measurements of CO performed with BI-CCD for 160 µm spatial resolution (3.02x10 17 cm -3 ), the uncertainties are high (50%) compared to the values obtained with BI-EMCCD (35%). These high values are due to the low level of detectability of these detectors for low signal in the hot gases. Again ICCD presents a shift in the average value from COSILAB value and high uncertainties (66%). The use of an alternative for temperature measurement, like Rayleigh scattering, is essential to perform low concentration species with ICCD detector.

Detector

For BI-EMCCD, density values of CO are very close to the density calculated by adiabatic flame modeling, showing the benefit provided by the electron multiplication gain. To compensate the loss in detectability for BI-CCD, SNR improvement has to be achieved either by enlarging the probe volume, increasing the laser energy or by increasing the efficiency of the collection system. When the length of the probed volume is increased to 300 µm leading to an improvement of about 44% in SNR. An improvement in accuracy is obtained from both BI-CCD and the BI-EMCCD and a reduction in uncertainties by a factor of about 1.38 appears (Table 1).

Conclusions

In flames, where the SRS signal is embedded in continuous background radiation, different types of cameras can be used for signal detection: BI-CCD, BI-EMCCD and ICCD. BI-CCD and BI-EMCCD offer advantages for SRS measurements, due to its high quantum efficiency and limited shot-noise. However, measurements in flame with these types of cameras require the use of a fast shutter device. Here a Pockels cell shutter PCS is used as optical gating for BI-CCD and BI-EMCCD, in order to assess accuracy and uncertainties of SRS measurements in flames and to compare their performances to those of ICCD cameras. Results obtained with BI-CCD and BI-EMCCD for temperature, temperature gradient, and high density are in good agreement with laminar flame calculations. Fluctuations in the measured temperature with BI-CCD and BI-EMCCD for high spatial resolution (160 µm) are below 7% in burnt gases. Temperature measurements performed with ICCD camera are not so accurate and present high uncertainties due to the high shot noise. The measurements with ICCD are limited to larger probe volume and density measurements must be associated to another temperature measurement than SRS proposed here, as Rayleigh scattering for instance.

PCS offers time gates comparable to ICCD, and it makes on one hand the BI-CCD, the most efficient detection systems for single-shot temperature measurements but for single-shot density measurements with low detectability the measurements are readout noise limited. On the other hand, the BI-EMCCD is the powerful tool for best detectability of low concentration species such as CO. The powerful improvement for BI-EMCDD is obtained because this detector removes the readout noise detection limit by applying a low-noise gain process, to enhance the signal above the noise background. This study opens prospects for the analysis of turbulent reacting flows by simultaneous 1D measurements of temperature and concentrations of major species.

Fig. 2 :

 2 Fig. 2: Samples of single shot Raman spectra acquired in stoichiometric methane-air flame with 160 µm spatial resolution (2130 K) using (a) BI-EMCCD (b) BI-CCD (c) ICCD.

Fig. 3 :

 3 Fig. 3: PDF of Raman Temperature measurements with the 3 detectors at BG with 160µm. spatial resolution.

Fig. 3

 3 Fig.3shows temperature PDF from stoichiometric methane-air flame, measured with the 3 detectors at BG. The average temperatures measured are 2140K, 2131K for BI-EMCCD and BI-CCD respectively. They are very close to the temperature calculated by adiabatic flame modeling COSILAB (2136K)[23] and demonstrate that the combustion can be considered almost adiabatic in this area.

Fig. 5 :

 5 Fig. 5: Scatter plots of instantaneous measurements of temperature versus density of N 2 at front flame for different detectors: BI-CCD (a) BI-EMCCD (b) and ICCD (c) compared to laminar flame calculation (solid red curve)

Fig. 6 :

 6 Fig. 6: Example of single shot N 2 and CO Raman intensities in premixed methane-air flame Ф=1.4 (blue points ). The solid red curve is the theoretical best-fit obtained for BI-CCD (a) BI-EMCCD (b) and ICCD (c)

Table 1 :

 1 Table of representative SNR of detectability,

		BI-CCD BI-EMCCD ICCD
	SNR (160µm)	0.92	1.49	0.84
	Density x10 17 cm -3 (160µm) 3.02	2.89	3.35
	Uncertainties (x10 17 cm -3 ) 1.54	1.03	2.22
	Accuracy (%)	3.7	0.6	15
	Density x10 17 cm -3 (300µm) 2.92		3.01
	Uncertainties (x10 17 cm -3 ) 1.11		1.69
	Accuracy (%)	0.3		3.4
	measured density of CO, accuracy and uncertainties
	obtained in methane-air premixed flame (Ф=1.4) at a
	homogenous temperature zone (z=24 mm) for different
	detector.

Acknowledgements

This work was supported by the Haute-Normandie Region for the PhD fellowship of H. Ajrouche.


H Ajrouche

A Lo

P Vervisch

A Cessou

Detector Assessment for 1D Single-Shot Spontaneous Raman Scattering for Temperature and Multi-Species Measurements in Flames

Keywords: Spontaneous Raman Scattering, Temperature, species number density, Combustion

Detector Assessment for 1D Single-Shot Spontaneous Raman Scattering for Temperature and Multi-Species

Measurements in Flames H. Ajrouche, A. Lo, P. Vervisch, A.

Introduction

Validation of theoretical and numerical combustion models has motivated the development of Spontaneous Raman Scattering (SRS) as a multispecies diagnostic [START_REF] Barlow | Laser diagnostics and their interplay with computations to understand turbulent combustion[END_REF][START_REF] Smith | Demonstration of high-speed 1D Raman scattering line imaging[END_REF].

Due to its low efficiency, turbulent flame investigation by SRS is most of the times limited to 1D measurements even if few works of imaging are proposed [START_REF] Kelman | Quantitative technique for imaging mixture fraction, temperature, and the hydroxyl radical in turbulent diffusion flames[END_REF], and it requires on one hand high laser energies, greater than 1J/pulse [START_REF] Meier | Laser Raman Scattering in Fuel-Rich Flames : Background Levels at Different Excitation Wavelengths[END_REF], associated with long pulse duration to avoid optical breakdown [START_REF] Cleon | Long-cavity Nd : YAG laser used in single-shot spontaneous Raman scattering measurements[END_REF], and on the other hand very sensitive detectors. Moreover, the weak SRS is usually embedded in the flame emission, thus an efficient detection system is required, not only with high efficiency (high sensitivity, low noise, high dynamic range) but also with a fast temporal gating. Thus the critical aspect of 1D single-shot Spontaneous Raman Scattering (SRS) experiments in flames is the requirement of high efficiency of the detection system associated with a fast temporal gating.

In the present work, the potential applicability and the limitation of three detector types are analyzed for instantaneous 1D Raman measurements of temperature and species concentrations in flames: back-illuminated CCD (BI-CCD), electron multiplying CCD (BI-EMCCD) and intensified CCD (ICCD). Single-shot SRS measurements in flames are usually performed either with BI-CCD [START_REF] Fernandez | Quantitative diagnostics of a methane/air mini-flame by Raman spectroscopy[END_REF]) associated with a home-made shutters [START_REF] Kojima | Observation of Turbulent Mixing in Lean-Direct-Injection Combustion at Elevated Pressure[END_REF]Nguyen 2008, Miles 2000) or with ICCD cameras [START_REF] Geyer | Turbulent opposed-jet flames: A critical benchmark experiment for combustion LES[END_REF]. The advantage of each solution is the high sensitivity for BICDD and fast gating for ICCD. Their main drawback is the requirement of developing a shutter for BI-CCD and the high shot-noise for ICCD. Back-illuminated cameras require a fast shutter, due to their full-frame architecture. Without fast shutters, the pixels may be exposed during the readout time, causing smearing, especially when the camera is placed in front of a luminous medium like flames. The electron multiplying CCD camera (EMCCD), which have extremely high quantum efficiency (QE) when back-illuminated (BI-EMCCD), offers the ability to eliminate the readout noise detection limit [START_REF] Gregor | A quasi-adiabatic laminar flat flame burner for high temperature calibration[END_REF] but keeps the need for a fast shutter. We have previously proposed a new fast shutter for 1D single-shot measurements of temperature and concentration of major species by SRS [START_REF] Ajrouche | Assessment of a fast electro-optical shutter for 1D spontaneous Raman scattering in flames[END_REF], where the ability of a Pockels cell shutter (PCS) to enhance the signal-to-noise ratio (SNR) has been demonstrated. This fast shutter is used to assess the ability to measure single-shot scalar values accurately in flames by comparing the BI-CCD, BI-EMCCD and ICCD detection systems. The assessment of the detectors is performed by comparing measurements performed in near-adiabatic CH4/air flames.

First, the SNR of these detectors is compared. Second, the accuracy and uncertainty of temperature measurements by SRS, with a procedure presented in previous work [START_REF] Lo | Spontaneous Raman scattering: a useful tool for investigating the afterglow of nanosecond scale discharges in air[END_REF], are analyzed for each detector used and compared to adiabatic 1D freely propagating laminar flames modeling.

Third, 1D single-shot density measurements of the 3 detectors are compared for N 2 number density quantified when crossing the flame front and for density corresponding to signals close to the detectability limit by probing CO in a near-stoichiometric rich premixed flame.

Experimental setup

Fig. 1 shows the layout of the SRS set-up. The laser source consists in a Nd:YAG laser (Agilite Continuum) operating at 10 Hz providing about 1.2 J with top-hat pulse with a long pulse duration adjustable from 200 ns till 1 µs. A long pulse duration provides a large energy deposit suitable for single-shot SRS [START_REF] Meier | Laser Raman Scattering in Fuel-Rich Flames : Background Levels at Different Excitation Wavelengths[END_REF], with good spatial resolution and without optical breakdown due to instantaneous and local irradiance is smaller than threshold value of 34 GW/cm 2 [START_REF] Cleon | Long-cavity Nd : YAG laser used in single-shot spontaneous Raman scattering measurements[END_REF]. The pulse duration used in this study is 310 ns. The laser beam is focused using a convergent lens with a 1 m focal-length providing a probe volume of 170 µm-thick (1/e 2 ). The SRS light is collected at right angle to the laser beam with a large solid angle (f/2) using two telescopes composed of achromatic lenses. 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. In this study, three types of camera are compared with different quantum efficiency (QE) and noise factor (NF), which originates from the amplification process, defined as the ratio of the output noise of the amplifier to the product of the multiplication gain by the input noise. An ideal amplifier has therefore a noise factor of 1. The consequence of the stochastic nature of the gain in both BI-EMCCDs and ICCDs is a fundamental parameter affecting the SNR.

The ICCD detector used is a 16 bit CCD camera equipped with a GenIII intensifier (PI-MAX UNIGEN, Princeton Instruments, 512 x 512pixels, pixel size 23µm, readout rate of 1 MHz). The maximal QE provided by this device is 38% between 400 and 700 nm. The image intensifier was operated in gated mode with a gate width of 500 ns, suitable to suppress non-laser-induced emissions, such as flame luminosity.

The back-illuminated CCD camera is a full-frame CCD (Pixis 400B, Princeton Instruments, 1340x400 pixels, pixel size 20 µm, NF=1). This camera offers approximately 94 % of QE with 16bits of dynamic range and readout noise (<13e-). Different analog-to-digital converters (ADC) are 18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 -7, 2016 available as 1-2 MHz or 100 KHz. Because the readout noise of CCD arrays increases with the readout rate, the 100 KHz ADC is chosen to enhance the SNR.

The last camera is a back-illuminated electron-multiplying CCD camera (BI-EMCCD) (ProEM, Princeton Instruments, 1600x200pixels, pixel size 16 µm, 94% of QE). The selected EM gain setting was approximately (x200) and was sufficient to make the readout noise negligible. The readout rate used in this study is 1MHz. A fast electro-optical shutter is used for SRS measurements in flame with the two non-intensified detectors (BI-CCD or BI-EMCCD). The PCS consists of a large aperture Pockels cell (LAP-50, KD*P, 50 mm aperture, Quantum Technology) between 2 crossed polarizers (19WG-50, Quantum Technology). The two crossed wire-grid polarizers have high transmission (85% of the polarized incident light for each polarizer) leading to PCS transmission of about 72% of the Raman signal [START_REF] Ajrouche | Assessment of a fast electro-optical shutter for 1D spontaneous Raman scattering in flames[END_REF]. 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. After the PCS, an achromatic half-wave plate (AHWP10M-600, THORLABS) is placed in front of the spectrograph and oriented in order to obtain the best efficiency of the spectrograph.

Results and discussion

Single-shot SRS temperature and multispecies measurements from BI-CCD, BI-EMCCD and ICCD are compared by measurements in a premixed laminar flame, stabilized downstream a Bunsen burner fed with a methane-air mixture with equivalence ratios of Ф=1 and 1.4. The measurements are performed at two locations (Figure 2): in burnt gas downstream the flame tip (GB) and along a profile from fresh gas to burnt gas crossing the flame front (GF). GB is located in a region of homogeneous temperature and composition, close to equilibrium conditions. Measurements in GF offer measurements in various conditions: fresh mixture near the centerline, homogeneous burnt gas at the periphery, and sharp temperature and composition gradient for the intermediate radii, when crossing the preheat and reaction zone. Since the laser beam does not cross the flame front perpendicularly, the experimental profiles are corrected from the angle effect assuming that the tangential temperature gradients are negligible at the height probed, far from the flame tip and burner lip. The measurements are compared to modeling calculation of 1D freely propagating laminar flame by the COSILAB (COSILAB 2007) software, using the GRI-Mech 3.0 chemical mechanism. In first approximation the stretch effect on the temperature and species profiles are neglected. This assumption will be discussed afterwards. In the following, the COSILAB modeling will be considered to provide the reference data (temperature and major species) of the flame. Most of the measurements are performed at Ф=1, for which the adiabatic temperature determined by modeling is 2136 K. The Ф=1.4 flame is used to assess the cameras for measuring low level of concentrations, especially in probing CO which is a species difficult to probe by SRS in flame (Barlow et al, 2009). The experimental spectra are fitted by theoretical spectra by the procedure described by Lo et al [START_REF] Lo | Spontaneous Raman scattering: a useful tool for investigating the afterglow of nanosecond scale discharges in air[END_REF]. From this procedure instantaneous temperature can be measured [START_REF] Ajrouche | 1D single-shot thermometry in flames by Spontaneous Raman Scattering through a fast electro-optical shutter 17th[END_REF]. Figure 3 shows the temperature measurements across the flame front obtained from the average spectra collected by the three detectors. Despite the difference of the apparatus functions illustrating by the N 2 SRS peak (Fig. 3, right), the temperature profiles illustrate how the procedure developed provides accurate and reproducible temperature measurements. measurements from BI-CCD, BI-EMCCD and ICCD are compared and analyzed in terms of uncertainty and accuracy. The temperature is determined by simulating the vibration-rotation spectra of N 2 by theoretical spectra. These spectra are calculated and convoluted with the in-situ instrumental functions [START_REF] Lo | Spontaneous Raman scattering: a useful tool for investigating the afterglow of nanosecond scale discharges in air[END_REF] [START_REF] Ajrouche | 1D single-shot thermometry in flames by Spontaneous Raman Scattering through a fast electro-optical shutter 17th[END_REF]. The SNR values obtained from singleshot spectra are defined as the ratio of peak Raman intensity of fitted spectra, considered as the "true" signal value, divided by the root-mean-square (rms) fluctuations of the difference between experimental and fitted spectra calculated in the wavelength range of the SRS signal. For ICCD, the temperature measurements are clearly affected by the higher shot-noise and the higher NF of this device degrading the SNR from 9.92 with BI-CCD to 5.84 with ICCD and resulting in an average temperature of 1926K clearly smaller than the calculated temperature (2136K) and very high uncertainties (340K). Scatterplots of instantaneous temperature measurements versus density of N 2 when crossing the flame front (Fig. 10) show that the BI-CCD measurements reproduced well the hyperbolic behavior. The dispersion of 9.5% from either side of the modeled curve is acceptable especially if we consider the small probe volume (160 µm). This small spatial resolution, which leads to low signal levels with our experimental setup, has been chosen for the assessment of the detectors with low signal-to-noise ratio conditions. Results with a probe volume of 300 µm (Fig 11) show the decrease in dispersion of the data (6.7%). The number density measurements performed with BI-EMCCD present approximately similar results to those obtained with BI-CCD with higher dispersion of the values due to the higher uncertainties of temperature measurements performed with BI-EMCDD (Fig. 10b). concentration measurements in GB, where temperature is known, are performed by using the temperature value given by COSILAB (Table 2). The accuracy of the average values is clearly enhanced but it remains low for the ICCD camera, showing that the high level of noise of this detector has an impact even on the area measurement of the SRS peak. To assess the detector for very low signal levels, CO number density are measured in GB. In fig 9a

acquired from BI-CCD, the CO peak of few counts (~5) is difficult to distinguish with a SNR value of 0.92 (Table 3). The small peak value of CO obtained with BI-CCD can be embedded for some single shot spectra in the background signal and therefore information about CO will be lost. The improvement in BI-EMCCD signal quality is immediately noticeable in Fig 9b . BI-EMCCD can detect very low CO signal. For ICCD, the high shot noise decreases drastically the SNR of the CO peak to 0.84, and makes the detectability of the peak very low. We have to remember here that CO density measurement by SRS is not usual due to the low level of signal and that this issue is enhanced here by the small probe volume (160 µm).

Conclusion

In flames, where the SRS signal is embedded in continuous background radiation, different types of cameras can be used for signal detection: BI-CCD, BI-EMCCD and ICCD. BI-CCD and BI-EMCCD offer advantages for SRS measurements, due to its high quantum efficiency and limited shot-noise. However, measurements in flame with these types of cameras require the use of a fast shutter device. Here a Pockels cell shutter PCS is used as optical gating for BI-CCD and BI-EMCCD, in order to assess accuracy and uncertainties of SRS measurements in flames and to

Fig. 1

 1 Fig. 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, half-wave plate

Fig. 2

 2 Fig. 2 photo of the CH4-air flame and locations of the SRS measurements (GB) and (GF)

Fig 3

 3 Fig 3 temperature profiles obtained from averaged SRS spectra for the three detectors

Fig 4 :

 4 Fig 4: Samples of single shot Raman spectra acquired in stoichiometric methane-air flame with 160 µm spatial resolution (2130 K) using (a) BI-CCD (b) BI-EMCCD (c) ICCD

Fig 5

 5 Fig 5 Samples of single shot Raman spectra acquired in stoichiometric methane-air flame with 160 µm spatial resolution (2130 K) using (a) BI-CCD (b) ICCD

Fig 6 Fig 7

 67 Fig 6 Temperature measurements with the 3 detectors at BG with 160µm. spatial resolution in a stoichiometric methane-air flame

Fig 8 400 -

 400 Fig 8 400-shot averaged radial profiles of temperature and temperature fluctuations from stoichiometric methane-air flame, measured by the three detectors at FG compared to temperature values calculated by COSILAB.

Fig 10

 10 Fig 10 Scatterplots of instantaneous measurements of temperature versus density of N 2 at front flame with a probe volume of 160µm for different detectors: BI-CCD (a) BI-EMCCD (b) and ICCD (c) compared to laminar flame calculation (solid red curve)

Fig 11

 11 Fig 11 Scatterplots of instantaneous measurements of temperature versus density of N 2 at front flame for different detectors: BI-CCD with a probe volume of 300 µm

Table 2

 2 average number densities measured in GB with the temperature calculated by COSILAB

	Détecteur	BI-CCD	BI-EMCCD	ICCD
	Densité x10 18 molécules.cm -3	2.481	2.46	2.56
	Incertitudes (%)	8.5	11.2	17.5
	Précision (%)	99.9	99.2	96.8

Table 3

 3 representative SNR of detectability, measured density of CO, accuracy and uncertainties obtained in methane-air premixed flame (Ф=1.4) in GB for different detector.

	Detector	BI-CCD BI-EMCCD	ICCD
	SNR (160μm)	0.92	1.49	0.84
	Density

x10 17 cm -3 (160μm)

	3.02	2.89	3.35
	Uncertainties

(x10 17 cm -3 )

		1.54	1.03	2.22
	Accuracy (%)	3.7	0.6	15
	Density

x10 17 cm -3 (300μm)

	2.92	3.01
	Uncertainties

(x10 17 cm -3 )

		1.11	1.69
	Accuracy (%)	0.3	3.4

Acknowledgements

This work was supported by the Haute-Normandie Region for the PhD fellowship of H. Ajrouche.

The concentration measurements are also compared first for N 2 , a species providing the highest signal levels, and secondly for CO in the Ф=1.4 flame, providing very low signal levels. Figure 9 presents an example of single shot SRS spectra of N 2 and CO acquired in burnt gases (160 µm spatial resolution) recorded with each camera, with their respective SRS model fit.

Density is determined using the area of theoretical spectra and the fitted temperature:

Table 1 provides the concentration measurements in GB: average, fluctuations, and accuracy obtained by comparison with the adiabatic number density calculated by COSILAB, and Fig. 10 shows the results obtained when crossing the flame front in GF. Density values obtained with BI-CCD are very close to the density calculated by adiabatic flame modeling COSILAB (2.48x1018 cm-3). Density obtained with BI-EMCCD is slightly underestimated due to the higher uncertainty of temperature with this detector. For ICCD, the underestimation and uncertainties of density measurements are high.


Lecordier

C Lacour

A Lo

J Marrero

F Lefebvre

P Vervisch

A Cessou

B Lecordier

Characterisation of electric discharge in laminar flow with optical diagnostics

Keywords: electric discharge, laminar flow, PIV, Raman scattering, spark imaging

Characterisation of electric discharge in laminar flow

with optical diagnostics

Introduction

Significant reduction of carbon dioxide emissions due to spark ignition engines can be achieved thanks to a downsizing approach associated with diluted combustion. Stratified combustion and exhaust gas recirculation (EGR) are combined strategies to maximize the cycle efficiency.

However, a major limitation to these combustion modes is the ignition stability, especially in lean mixtures. The potential of combustion ignition by means of electric discharges remains the most affordable answer to these ignition constraints. The optimisation of such systems requires a better understanding of the coupling between the plasma discharge in a turbulent gas flow with variable composition and ignition mechanisms. The effect of the size and duration of the spark as well as the time evolution of the deposited energy needs to be handled in such flow situations for the identification of key parameters to be optimised for the future ignition systems enhancement.

Coupling between physico-chemical characterisation of the discharge and the topology of the flow and the discharge in turbulent flows by means of optical diagnostics would provide new insights in such a context. Such results will also aim at providing a large database for the simulation. First results obtained under turbulent conditions have led to investigate the discharge behaviour in a 18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・ LISBON | PORTUGAL ・JULY 4 -7, 2016 laminar flow to build a reference case and to feed the simulation database. In the present paper, only results under laminar flow conditions will be presented.

To achieve these objectives, an electric discharge generated by a commercial Audi pencil coil is characterised by means of optical diagnostics. The electrodes of the spark plug have been replaced by a pin-to-pin configuration to make the use of laser diagnostics easier and in a sake of simplicity for simulation and modelling. The investigation is firstly performed in quiescent air under atmospheric pressure condition. In complementary to these velocity measurements, a set of selected temperature fields obtained by Spontaneous Raman Scattering are presented for a better analysis of the energy transfer processes in the gas surrounding the spark. In a second step, the discharge is investigated in a laminar air flow of different velocities by means of high-resolution phase-locked Particle Image Velocimetry and high-speed intensified imaging techniques. The impacts of the discharge on the surrounding gas flow but also the dynamic behaviour of the arc discharge distortions with the flow (growth, shape, and duration) are analysed. It should be noticed that during the test campaign, for all the operating conditions, the time evolution of the current and voltage are also monitored for evaluating the maximum available electric energy at the electrodes, which included both the energy transfer to the gas and the heat loses. These electric discharge properties are essential inputs for the modelling and the simulation of ignition processes.

Experimental set-up and optical diagnostics

The schematic representation of the experimental setup is sketched in Fig. 1. The electrodes in a pin-to-pin arrangement are placed at the exit of a laminar air jet of 20 mm in diameter. The body of the blower is equipped with a glass balls bed and honeycombs. It has been designed with a double smooth convergent profiles with a section ratio 1:100. The exit flow is stable and laminar with a very low residual turbulence and has a top-hat velocity profile. The tip curvature radius of the electrodes is 150 µm with a parabolic profile. The inter electrode gap distance is 3 mm, a value intentionally fixed larger than in a real car engine ignition systems to reach a high spatial resolution of optical measurements, which is a critical point for the optical diagnostics implemented for the present study. Moreover, in future this « academic » reference case will be compared to a 0.8 mm pin-to-pin electrodes gap and a commercial spark plug.

The high voltage pulse applied to the electrodes is provided by a pencil coil used in Audi cars. It is powered by a 12V DC Power Supply. The duration of the coil actuating signal, corresponding to the coil charging time (dwell time) is fixed to 4ms, similarly to the dwell time imposed in car engine.

The potential difference applied between the electrodes is negative to be representative of internal combustion engine configuration. The aerodynamics interactions of the arc with surrounding air flow are investigated by means of Particle Image Velocimetry with specific adaptation. The acquisition system consists of a MicroMax Roper detector (1280x1024@12 bits) equipped with a 105 mm lens with a macro feature (Nikon 105mm f/2.8 AF-D). The recorded area was 8 mm x 6 mm with a spatial resolution 165 px.mm -1 .

The excitation source is a Nd:YAG double head 532 nm Twins Ultra from Quantel (25 mJ). The laser sheet formed by means of spherical convergent (f = 600 mm) and cylindrical divergent lenses (f =-100 mm) is focused on the central plane of the blower exit. The thickness of the laser sheet has been reduced about 500µm through adjustable knife edges. Specific geometrical arrangement of the set-up is made to let the laser sheet enter as close as possible to the inter-electrode gap and avoid intense laser diffusion on the electrodes. Electrodes had been slightly tilted with an angle of 20° regarding the laser line-of-sight and have been positioned between the laser and the camera (Fig. 1). It has been ensured that this titled positioning of the electrodes does not affect nor the electric characteristics of the spark (voltage and current time evolution) neither the spark topology, as that has been confirmed from the arc visualization in top and front directions using a short-gated imaging technique (see section 2).

Fig. 1: Schematic representation of the experimental set-up

A time delay generator is used for the external control of the camera and laser to perform phaselocked measurement relative to the breakdown phase. Apart from the difficulties related to the small size of the investigation domain, the main difficulty for optical diagnostic in our set-up lies in an accurate synchronisation of the acquisition timing to the breakdown phase. Indeed, with a coil, the breakdown phase occurs with a time uncertainty of a few microseconds relative to its actuating signal. To overcome this difficulty, and then obtain an accurate acquisition timing, a photomultiplier (PM) coupled to an optical fibre has been used to record the light emitted by the spark during the breakdown phase. The PM output voltage is turned into TTL signal to drive the external trigger channel of the delay generator controlling the whole acquisition timing. For this optical synchronisation, all the electronic components and devices have been carefully selected to maintain the total response time below 200 ns and permit accurate phase-locked measurement as close as possible to the breakdown phase.

The gas temperature T is measured in the gas surrounding the spark by Spontaneous Raman Scattering (SRS). The experimental Raman set-up is only briefly described here and details can be found in [START_REF] Ajrouche | 1D single-shot thermometry in flames by Spontaneous Raman Scattering through a fast electro-optical shutter[END_REF][START_REF] Ajrouche | Assessment of a fast electro-optical shutter for 1D spontaneous Raman scattering in flames[END_REF]. The scattered light is induced by a long pulse Nd YAG laser (Continuum) with pulse duration of 310 ns and energy of 1.1 J operating at 10Hz [START_REF] Ajrouche | Assessment of a fast electro-optical shutter for 1D spontaneous Raman scattering in flames[END_REF]. The laser beam is focused to form a probe volume (150µm diameter, 3mm length) with a 1m focal-length lens. The spontaneous Raman scattered light is collected at right angle to the laser beam in an imaging spectrograph (Acton SpectraPro 300i). A fast electro-optical shutter (PCS) provides high-speed gating up to 500 ns. 1D Raman scattering spectra are acquired using a full-frame back-illuminated CCD camera. The gas temperature T is obtained by least squares fitting of the experimental Raman spectra to the theoretical ones. More details on the Raman spectra modelling and on the fitting procedure can be found in [START_REF] Lo | Spontaneous Raman scattering: a usefull tool for investigating the afterglow of nanosecond scale discharge in air[END_REF][START_REF] Ajrouche | 1D single-shot thermometry in flames by Spontaneous Raman Scattering through a fast electro-optical shutter[END_REF].

The deformation and elongation of the spark with time is investigated by means of high speed and short gated intensified imaging. A phantom V10 high speed camera is equipped with a double stage Hamamatsu intensifier and a UV Cerco 100 mm lens. The spatial resolution is 20µm/px. The acquisition rate is fixed at 10Hz and a gate of 3 µs was chosen as the best compromise between signal level and time integration. The high speed camera and intensifier were synchronised with light emission of the breakdown phase through the PM signal, as detailed for the PIV synchronisation. The spectrum of emission of the plasma generated at the electrodes ranges from the blue to the ultraviolet wavelength, with a strong emission in UV range. A band-pass UG5 filter was used to reduce the spectrum around 300 nm and thus the emission intensity. However, the very first moment of the spark (breakdown phase) saturated the image whereas at later time, it is elongated downstream of the electrode and its emission decreases rapidly. To overcome this difficulty, a thin glass sheet of a few millimetre thickness was placed between the camera lens and the electrodes. The height of the glass sheet has been adjusted to cover only the spark gap and thus to attenuate the intense emission occurring at early stages of the discharge.

Electrical characterisation of the spark.

Maly and co-authors were the first to describe in details the spark generated by a coil as a succession of four steps [START_REF] Maly | Initiation and propagation of flame fronts in lean CH4-air mixtures by three modes of ignition spark[END_REF][START_REF] Maly | Initiation model for spark discharges and the early phase of flame front growth[END_REF][START_REF] Albrecht | New Aspect on spark ignition, SAE technical paper[END_REF][START_REF] Ziegler | Ignition of lean methane air mixtures by high pressure glow and ARC discharge[END_REF].

The first phase is named pre-breakdown or streamer propagation and consists of an intense voltage rise enabling the establishment of ionised gas channel between the electrodes. Then, during the breakdown phase the current settles in the channel. This second step occurs for a few nanosecond and consists in very high voltage (around 10kV) and intense current peaks (up to 100A). The gas between the electrodes is assimilated to a plasma with temperature reaching thousands of Kelvin. Next, the third phase, called arc phase, is characterised by a rapid drop of voltage to a few hundred volts across the electrodes gap and produces a very emissive plasma channel. The last step, called glow phase is the longer one and can last more than a few millisecond. The voltage maintains to a hundred of volts and the current drops progressively to zero. Maly evaluated the total energy delivered at the spark plug to 50 to 100 mJ for conventional systems, distributed for about 1 mJ to the breakdown phase and 30 to 100 mJ for the following steps, mainly attributed to the glow phase. It is also relevant to note that the author assesses the thermal energy lost at the electrodes around 6%for the breakdown phase and 70% during the glow phase. The energy transfer to the gas is thus evaluated to 94% for the breakdown phase but only to 30% during the glow phase [START_REF] Mantel | Contribution à la modélisation de la combustion dans les moteurs à allumage commandé avec prise en compte de la phase d'allumage[END_REF][START_REF] Ternel | Contribution au développement de l'allumage par laser pour les moteurs à combustion interne[END_REF].

For the electrical characterisation of the discharge in the present set-up, the high voltage U at the The energy delivered by the coil to the electrodes during the glow phase was obtained by a time integration of the U.I product, after a time rescaling. A sample of this energy measurement is presented in Fig. 2 (green curve) showing that the energy deposited to the electrode in air at rest and atmospheric pressure is around 80 mJ. While the time integration of U by I product, is successful during the glow phase, it fails to provide an accurate energy measure during the breakdown phase due to variable time shift between U and I measurements at the nanosecond scale and the difficulty from the voltage probe to resolve in time the extremely rapid voltage rise.

Another methodology has then been applied for the breakdown phase. As the maximum of the voltage peak is accurately measured, the energy amount delivered during the breakdown phase can be deduced by eqn. 1:

𝐸 = 1 2 𝐶𝑈 𝑝𝑒𝑎𝑘 2 (1)
The capacitance of the near-gap circuit has been estimated based on voltage and current measured during the rising period of the applied high voltage to the electrode (pre-breakdown phase). The time evolution of current is integrated through the pre-breakdown phase and the capacity is estimated after dividing by the breakdown voltage peak. This methodology has been evaluated and validated by comparing the results obtained from various ignition systems (real NGK spark-plug, resistive and non-resistive, pin-to-pin electrodes) and with different approaches, such a direct derivation of the pre-breakdown or breakdown voltage. Finally, a capacity about 9 pF is deduced for our 3 mm gap pin-to-pin electrode configuration charged by a commercial Audi coil.

With a voltage peak reaching 20 kV (see Fig. 2), the energy amount deposited to the electrodes during the breakdown phase is 1.3 mJ. The energy ratio between the breakdown (1.3 mJ) and the glow (80mJ) phases measured in our study is coherent with the literature [START_REF] Maly | Initiation and propagation of flame fronts in lean CH4-air mixtures by three modes of ignition spark[END_REF][START_REF] Mantel | Contribution à la modélisation de la combustion dans les moteurs à allumage commandé avec prise en compte de la phase d'allumage[END_REF][START_REF] Ternel | Contribution au développement de l'allumage par laser pour les moteurs à combustion interne[END_REF].

This well characterised electric discharge will be generated in a laminar flow in order to characterize the interaction of the spark with the flow regarding both heat release and aerodynamics. As a first step, the aerodynamics interactions are investigated in an air flow initially at rest. The shock wave location is witnessed by a strong velocity step which drops to zero and results

show that it exits the measurement window after 11 µs. These results show the flow induced by the shock wave, but the unsteady shock wave itself and the strong velocity gradient are not accessible from PIV due to time response of the seeding particle. Untill 41 µs, the flow topology is symmetric regarding a horizontal line crossing the electrodes and at 41 µs large flow velocity magnitudes are observed along the electrodes, giving evidence of strong re-entry of fresh air (7 m/s) in the spark central area induced by the pressure drop following the gas expansion after the moving of the shock wave. Such recirculation of fresh gases into the spark gap have already been observed [ (Borghese, Sforzo et al., 2015[START_REF] Bane | Investigation of the effect of electrode geometry on spark ignition[END_REF]. After 41 µs, velocity magnitude decreases and the instabilities of flow structures induced by the steep gradients of density and pressure develop. This transition toward a chaotic development of the kernel of hot-gases after spark was observed by several authors [START_REF] Borghese | Development of hot nitrogen kernel, produced by a very fast spark discharge Symposium[END_REF][START_REF] Schneider | Turbulent decay of after-spark channels[END_REF]. The hot gas zone then expands due to the conductive heating of the gas.

Transverse velocity profiles (d) present a symmetry for the first instant (till 41µs), in accordance with the gas expansion noted on Fig. 4 Vertical velocity (h) presents "M" shape profiles linked to the expanding hot gas which has a cylindrical shape in the gap but ended by a more spherical shape. After 41 µs, the velocity decreases rapidly.

Heating of the gas due to the spark in air at rest

In the same experimental condition (air, atmospheric pressure, at rest), the heat release of the spark in surrounding air is evaluated from the local measurement of the air temperature using the Spontaneous Raman Scattering (SRS) with the set-up presented in section 2. Fig. 5 presents the mean gas temperature evolution with time for vertical profiles at x=0 mm. The temperature time evolution extracted at x=0 mm and y=0.75 mm is marked by a red dot on Fig. 4 (f). At this position, a first temperature peak reaching 1000K occurs about 100 µs after the breakdown. This temperature peak is due to the convective heating of the gas induced by the rapid expansion of extremely hot gas channel leaving the inter-electrode gap following the shock wave. At 500 µs, the gases cool down to nearly 700K, due the entrance of cold gas induced by the rarefaction wave [START_REF] Maly | Spark Ignition: Its Physics and Effect on the Internal Combustion Process[END_REF]. A second temperature rise is observed after 1000 µs and reaches more than 2500K at 2 ms corresponding to the conductive heating of the gas by the remaining glow discharge.

After that time, the temperature decreases but still reaches 1000K at 4ms, although the arc is totally extinguished at this time.

The temperature profiles extracted for y=1 mm to 1.75 mm indicate that before 50 µs, the gas is not yet heated at the probed positions as suggested by the presence of oil particles in the PIV images (Fig. 4 The spark is then generated in these three flow conditions. The topology and deformation of the spark in this aerodynamic is then investigated by means of high speed and short gated intensified imaging. The optical set-up is detailed in section 2. In Fig. 8, column (d) presents the recorded emission of the spark with time in a laminar flow (case 10 m/s).

The intensity has been normalised. For delays shorter than 6 µs after the breakdown, the spark is not deformed and its shape is similar to the case in air at rest. Then it presents a slight curvature, and its elongation rises with time. At 691 µs, the spark presents a wavy shape giving evidence of the wake formed downstream of the electrodes as seen on the velocity fields. The energy and duration of the glow phase, obtained from integration of voltage and current measurements are presented in Fig. 12. These results are obtained by averaging more than 50 single-shot acquisitions. The energy deposited to the electrodes rises from 75 mJ to 105 mJ with the flow velocity, whereas the spark duration decreases from 2.8 ms at rest to less than 1.5 ms at 15 m/s. These results show that with an increased flow velocity through the electrodes, a larger amount of energy is deposited to the electrodes in a shorter time. To explain this behaviour, a constant available amount of energy is to be considered in the secondary stage of electric circuit, corresponding to about 55% of the energy delivered by the primary electrical circuit of the coil evaluated at 300 mJ. The total amount of available energy is partly consumed by the internal coil resistance but also in the glow phase of the spark. As the spark resistance increases with flow velocity, the portion of consumed energy in the glow phase of the spark increases. The energy consumed in the coil resistance consequently decreases to conserve the total energy in the secondary circuit and current lies down to zero in a shorter time (Fig. 11(a)).

Finally, with larger flow velocity, the spark elongation and thus its resistance rises in a shorter time. The limited amount of available energy is thus consumed more rapidly, and the duration of the spark is reduced with time. The elongation of the spark in the transverse flow induces an increase of its resistivity and thus a rapid consumption of the available energy.

Velocity fields obtained by PIV put forward a strong modification of gas flow surrounding the spark. Shortly after the breakdown (41 µs), strong air re-entries of fresh air occur towards the spark central area, induced by the pressure drop following the gas expansion after the moving of the shock wave. Such gas aspiration is evidenced with significant velocity magnitude in the case of a flow initially at rest but is perturbed by the presence of a transverse laminar flow.

Velocity fields measured at later timings after the breakdown show that velocity magnitude decreases rapidly and the gas flow surrounding the spark is perturbed by hydrodynamic instabilities generated in the after-spark flow.

The time evolution of temperature measured by Spontaneous Raman scattering put forward a first convective heating of the gas surrounding the spark induced by the rapid expansion of extremely hot gas channel leaving the inter-electrode gap following the shock wave. A second temperature rise is observed later on corresponding to the conductive heating of the gas by the remaining glow discharge. After that time, the temperature decreases but still reaches 1000K at 4 ms, although the arc is totally extinguished at this time. Results obtained by this complementary set of diagnostics build a database which could provide data for ignition models validations and can further help the investigation of the plasma-aerodynamics interactions.

18th

  International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 -7, 2016

  on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 -7, 2016

  electrodes and total current I have been respectively measured with a high voltage probe (Tektronix P6015A, resistance of 100 MΩ and a bandpass at 75 MHz) and with 0.05 Ω resistive probe (T&M Research SNBC-5-5 with bandpass at 800 MHz). A sample of voltage and current measurements in air at atmospheric pressure is presented in Fig.2. Two distinct measurements with dedicated time resolution are required to get the results during the breakdown and glow phases since their timescale differ of two orders of magnitude.

Fig. 2 :

 2 Fig. 2: Voltage and Current measured at the electrodes during the breakdown (left) and the glow phase (right) for a discharge in air at rest under atmospheric pressure.

Fig. 2 .

 2 Fig. 2. (a) shows that the voltage peak is around 20 kV and the current peak reaches 30 A during the breakdown phase in quiescent air under atmospheric pressure. Voltage and current peaks are negative as in car engine operating conditions. The breakdown phase duration is no longer than

4.

  Fig.4shows mean velocity field of the gas surrounding the spark for increasing delay after the breakdown discharge ranging from 3 µs to 1191 µs. Mean velocity fields are obtained from 50 instantaneous acquisitions synchronised in phase with the breakdown (see section 2). Velocity vectors are coloured by their magnitude and the particle images are superimposed. It should be noted that the presence of oil droplets in the flow does not affect the electric characteristics of the arc. This point has been ensured by similar measurement of voltage and current with and without seeding.

Fig. 3 :

 3 Fig.3: Mean velocity fields (a) at the exit of the laminar blower (without spark). Velocity vectors are coloured by their magnitude. Associated mean and fluctuating velocity profiles at y=1mm (b).

  on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 -7, 2016

  (a) and (b). After 41 µs, the profiles evidence a converging central flow. For later delays (191, 1191µs) the negative and positive peaks are inversed compared to the observations made at first instants. The flow surrounding the spark is now converging. The dissymmetry of the profiles corresponds to the flow recirculation on the anode noted on Fig.4 (g).

Fig. 4 :

 4 Fig.4: Gas velocity fields surrounding the spark after breakdown in quiescent air. Velocity vectors are coloured by their magnitude. Associated velocity profiles at y=1mm (d), (h).

  (a), (b) and (c)). The hot gas in expansion inducing convective heating, reaches the position y=0.75 mm at 50 µs. The hot gas zone then expands to upper positions while cooling down as shown by temperature profiles measured at 1mm. For downstream positions, the convective temperature peak is merged with the second temperature peak occurring at later time.

Fig. 5 :

 5 Fig.5: Temperature evolution with time at x=0 mm along a vertical profile (y=0.75, y=1.75mm)

  Fig.8presents mean velocity fields of the surrounding air flow obtained in the presence of the spark. The first column presents results obtained with an exit velocity of 2 m/s, the second column corresponds to 5 m/s and the last concerns 10 m/s results. The symmetry of the topology of the flow surrounding the spark is still observable for the first instants of the spark, at 3 µs after the breakdown and during the rapid expansion of shock wave. It should also be noted that the position of the spark (observable by the light emission between the two electrodes) is not perturbed by the flow since it still forms a straight horizontal line. For this early stage of the spark the surrounding flow has no impact on the start of the shock wave neither on the initial topology of the spark. The velocity ranges and flow time scale involved are drastically different from the breakdown phenomenon to the laminar jet flow.Shortly after the start of the shock wave, at 6 µs, the horizontal symmetry of the flow is lost due to the incoming laminar flow. Particularly at 41 µs, higher velocities are measured downstream of the spark. The flow velocity downstream of the spark is accelerated nearly by a factor of two, certainly due to convection of hot gases. Upstream of the spark a stagnation point is observed in the flow, this is attributed to the competition between the expansion of the gas and the flow coming from the blower. The deformation of the spark is now visible on the particle image superimposed to the velocity field. It presents a slight curvature in accordance to the direction of the flow. The deformation of the spark is increased by the magnitude of the flow. Similarly to the case at rest (Fig.4(c)), gas re-entries are still noticeable along the electrodes but this flow is perturbed by the upcoming vertical laminar jet. For later timings, the hot gas zone is transported downstream of the electrodes and is perturbed by the hydrodynamic instabilities generated by the after-spark flow.Transverse velocity profiles Fig.9(a) present a symmetry for the first instant (till 41 µs), with negative and positive peaks, characterizing the outward expansion of the jet. After 41 µs, the profiles evidence recirculation along the electrodes. For later delays (191, 1191µs) transverse velocity profiles shows a converging flow toward a central vertical line. Vertical velocity profiles (b) are characteristic of the expansion of the hot gases for the first instants, then, after 41 µs, the velocity rapidly decreases.

  Fig. 9 presents the length of the spark obtained by a thresholding of the spark light emission region and a contour 18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 -7, 2016 extraction. As expected, the spark is maintained between the electrodes with a length estimated around 3 mm for the flow at rest. The more the flow velocity increases, the more the spark Gas velocity field surrounding the spark for mean transverse laminar low at 2m/s (a), 5 m/s (b) and 10m/s (c). Mean emissionn images of the spark (d). Normalised grey level Mean transverse (a) and vertical (b) velocity profiles at y=1mm above the electrodes for flow 5m/s.

Fig. 10 :

 10 Fig.10: Spark length evolution with time for different flow exit velocities.

Fig. 11 :

 11 Fig.11 : Single-shot voltage and current measured at the electrodes during and the spark phase for a discharge in air at V=0m./s, 5m./s and 10m./s under atmospheric pressure (a). Deduced spark equivalent resistance during the glow phase (b).

Fig. 12 :

 12 Fig.12: deposited energy and spark duration of the arc phase in air versus air jet velocity.

18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 -7, 2016

Acknowledgements

This work was funded by the National Research Agency (ANR) under grant no. ANR 12 VPTT-0002, FAMAC project.


Irfan A Mulla

Jérôme Yon

Guillaume Lefevre

Bertrand Lecordier

David Honoré

Armelle Cessou
email: armelle.cessou@coria.fr

Determination of soot volume fraction and particle size distribution in turbulent nonpremixed butane and ethylene jet flames through LII, PPS and SMPS measurements

The present work reports soot volume fraction determined in turbulent sooting non-premixed jet flames using a Pegasor particle sensor (PPS) and also using the non-intrusive laser-induced incandescence (LII) technique. Additionally, soot particle size distribution is determined using Scanning Mobility Particle Sizer (SMPS). Turbulent flames with different fuels, namely: butane (Re = 26,000), 70% butane + 30% ethylene by volumes (Re = 20,000), and ethylene (Re = 10,000) are investigated. The axial distribution of PPS derived soot volume fraction is qualitatively in excellent agreement with that obtained using LII. The possible sources of quantitative discrepancy between soot concentration obtained through LII and that of the PPS are discussed. Axial evolution of the soot size along with the volume fraction is presented. These results form part of the proposed comprehensive database to validate advanced soot models.

Introduction and Objectives

The soot particles produced during combustion processes poses serious environmental and health risks. Soot deposited on snow surfaces reduces albedo, accelerating glacier melting. Similarly, at higher altitude soot absorbs sunlight which may contribute to global warming. Soot nanoparticles pose risks to respiratory organs. Carcinogenic compounds may get deposited on the soot, which in turn could aggravate health risks. These environmental and health hazards are prompting stringent regulatory particulate emission norms.

To reduce soot emissions, it is necessary to gain further insights into soot formation process to enhance prediction capabilities. Most of the experimental studies in turbulent sooting flames have been performed in fuel-jet/co-flow air configuration. Lee et al. [START_REF] Lee | Measurements of soot, OH, and PAH concentrations in turbulent ethylene/air jet flames[END_REF] investigated correlation between soot formation processes and flame front (OH). Qamar et al. [START_REF] Qamar | Soot volume fraction in a piloted turbulent jet non-premixed flame of natural gas[END_REF] reports soot volume fraction in a turbulent non-premixed jet flame with a natural gas fuel. Shaddix et al. [START_REF] Shaddix | Understanding and predicting soot generation in turbulent non-premixed jet flames[END_REF] reports modeling and experimental data in turbulent non-premixed jet flames. The data included: soot volume fraction (LII), OH-PLIF and PAH-PLIF, and large eddy simulations in ethylene and a kerosene-based JP-8 fuel. Köhler et al. [START_REF] Köhler | Sooting turbulent jet flame: characterization and quantitative soot measurements[END_REF] reported wide range of parameters in a turbulent nonpremixed ethylene jet flame. The database was used to assess numerical simulations [START_REF] Köhler | Experimental characterization and numerical simulation of a sooting lifted turbulent jet diffusion flame[END_REF].

The present work is a sub-task of a consortium project "Advanced Soot Model for Aeronautics and Piston Engines" (ASMAPE) aimed at enhancing soot prediction capabilities. In this context modeling community require experimental database for fuels apart from ethylene for validation of novel models. The proposed database shall quantify soot volume fraction, flame temperature, velocity field, soot particle size, fuel concentration field, flame front, and soot-precursor. The present work is a part of this database, and focuses on the determination of soot size and volume fractions for ethylene, butane and butane/ethylene mix turbulent flames.

Various techniques have been used in the past to measure soot volume fraction. Among them, Laser-Induced Incandescence (LII) is generally preferred due to its non-intrusive nature, which provides planar and instantaneous description of the soot volume fraction in the flame. In this technique, soot particles are heated well-beyond the flame temperature with an incident laser pulse. The resulted incandescence signal which is proportional to soot volume fraction is imaged. The primary soot particle size can also be deduced based on LII decay rate with an appropriate modelling, however, the particle aggregate size cannot be determined. The methodology and assumptions in LII modelling and calibration are reported in a recent review [START_REF] Michelsen | Laser-induced incandescence: particulate diagnostics for combustion, atmospheric, and industrial applications[END_REF]. In the present work, the soot volume fraction is determined using Pegasor Particle Sensor (PPS), an ex-situ technique mainly used for characterizing particulate emission in Diesel engine. The particle size distributions are determined by Scanning Mobility Particle Sizer (SMPS), which is also an ex-situ technique.

The objectives of the present work is to report the soot size and volume fraction in the target flames of interest to modelers, and to compare the volume fractions determined by aerosol sampling based PPS technique with well-established in-situ LII technique.

Experimental methodology and data reduction 2.1 Burner

A jet flame configuration is chosen, as it provides relatively simple axisymmetric configuration. The fuel is injected through a tube of 2 mm inner diameter and 0.5 mm wall thickness throughout the tube length. The fuel tube is surrounded by coflowing air, which is issued through a square duct of 400 mm width. The co-flow duct also acts as a settling chamber that accommodate flow conditioning devices, which includes honeycomb, glass beads, and screens which reduces flow non-uniformity. The flame is confined up to 750 mm height above burner (HAB). Quartz windows provide optical access for the laser diagnostics. The confined configuration avoids flame perturbations caused by ambient currents.

A square exhaust hood of 750 mm size extracts the combustion gases. The larger exhaust size ensures uniform suction. The whole burner can be translated vertically to allow imaging at different HAB. The flame perturbation due to entrainment of ambient air from the co-flow exit plane (at HAB = 750 mm) is mitigated by a square annular shield curtain of air flow. These precautions allowed stabilization of target flames with high reproducibility, despite of varying distance between exhaust hood and burner. The air and fuel flowrates are controlled with thermal mass flow controllers (Bronkhorst). These thermal flow controllers are independently calibrated using a Coriolis calibration bench, ensuring high accuracy.

Flame conditions

In this work, n-butane is used as a base fuel since it has a balanced combination of cetane number and sooting tendency as of a fuel of practical interest. The flame conditions are listed under Table 1, where the Reynolds number (Re) is defined based on the tube diameter and bulk jet velocity. Each flame is identified with the mass flow rate and fuel type (e.g. mb = 0.30 is a butane flame with flow rate of 0.30 g/s). Apart from butane, an ethylene flame is also investigated, for which the Re (10 000) is chosen to mimic an earlier reported flame [START_REF] Köhler | Sooting turbulent jet flame: characterization and quantitative soot measurements[END_REF]. Additionally, a mixture of butane/ethylene (mb/e, 70/30 by volume) is used to assess effect of soot-enhancing agent. For the modeling convenience, the velocity of mb/e flame is kept same as of mb = 0.3 flame. The mass flow rate of co-flow air is kept high enough (5 g/s, in all cases) to allow over-ventilated flame condition.

Photographs of the flames listed under Table 1 are shown in Fig. 1. The camera exposure of 1/20 s is set to mimic human eye vision. A low-sooting blue region at the flame base suggests partial air/fuel premixing.

LII and extinction-based calibration

The fundamental wavelength (1064 nm) of Nd:YAG laser (Quantel, Brilliant) is used to heat soot particles. Infrared wavelength is used to avoid excitation of PAHs florescence. Laser delivers 6 mm beam with 7 ns pulse width, which is converted into light-sheet using a set of cylindrical and spherical lenses. Only a portion of the light-sheet within fairly uniform intensity (within 70% of the peak) is used. Thus, the effective light-sheet was 90 mm high and 0.2 mm thick. Thickness of the sheet is measured using a beam profiler (DataRay Inc., WinCam). This beam profiler is also used to deduce light-sheet profile. The intensity at the edge (along the height) of light-sheet dropped by 30% relative to peak, which is acceptable, provided that LII is performed in a plateau regime. LII needs to be performed at the optimum fluence (plateau), just before the significant soot sublimation. The fluence was controlled by varying laser energy using a custom made attenuator. Thus, the corresponding fluence along the sheet height was within 0.4 to 0.6 J/cm 2 range. The LII signal is collected normal to the lightsheet using an ICCD camera (Princeton Instruments, PI-Max RB, resolution: 512 pixel × 512 pixel, 16 bit). The camera was equipped with imaging lens (UV-Nikkor 105 mm), resulting in a spatial resolution of 5.75 pixel/mm. The signal is collected in blue spectrum through a 400 nm filter (FWHM 25 nm). The camera is gated to an effective prompt gate width of 50 ns, which minimizes flame radiation.

LII signal is calibrated using laser-extinction method in butane turbulent flame at HAB = 313 mm. Laser-extinction measurements were also performed in ethylene flame at the same HAB. The calibration constants in butane and ethylene flame were found to be nearly identical. To avoid background flame emissions and low signals, we used a pulsed laser. A small portion of the LII light-sheet (~1.3 mm tall) is used. The pulse intensity is measured using two photodiodes (upstream and downstream of flame). The upstream photodiode is used to account for incident laser energy drift from run-to-run. The intensity with flame (I) and without flame (I0) is measured using the downstream photodiode. Each of these measurements was averaged over 1000 shots, providing mean transmittance ratio I/I0. The corresponding soot volume fraction fv is calculated following Beer-Lambert-Bouguer's law, as: I/I0 = exp(-Ke.fv.L/𝜆𝜆), where Ke is a dimensionless extinction coefficient (= 5.01 at 1064 nm, following [START_REF] Köhler | Sooting turbulent jet flame: characterization and quantitative soot measurements[END_REF]), L is optical path length, 𝜆𝜆 is incident wavelength. Since the soot concentration along the path length is not constant in jet flames, only the path integrated soot volume fraction, Fv-int = ∫fv..dL (ppm.au) can be obtained. Nevertheless, LII calibration constant can be deduced by integrating signal along the same optical path length, as: ∫SLII.dL (au.mm). Subsequently, the calibration coefficient C (ppm/au) is found as: Fv-int = C.∫SLII dL.

PPS and SMPS

Pegasor particle sensor (PPS) is used to measure soot concentration, whereas scanning mobility particle sizer (SMPS) for soot particle size. Both SMPS and PPS measurements require extraction of an aerosol sample from the flame. Fig. 2 shows the schematic of experimental layout used.

The sample is drawn through a 1 mm orifice drilled in a tube with 4 mm inner diameter and 1 mm wall thickness. A mild vacuum is generated by a pump and controlled by a bypass valve, which draws the sample. The sample is diluted by rapid mixing of nitrogen (N2) that flows through the tube at 10 lpm rate. Apart from dilution, N2 assists in quenching the reactions and limiting the soot aggregation. In the absence of pump, flame perturbations caused by injection of N2 were observed. On the other hand, a stronger suction (without a bypass valve) caused extraction of larger sample, interpreted from mild shortening of flame and excess heating of the probe. Additional, dilution (with air) is achieved using two diluters units (PALAS, VKL 10) connected in tandem, providing an additional dilution factor of 100 which assists in minimizing saturation of SMPS and PPS. The sampling probe generally disturbs the flame through alteration of temperature and upstream flow. Yet, wealth of information can be extracted with fewer assumptions and lesser complexity than that of non-intrusive techniques. In laminar flame such perturbation effects were observed to be within 3 mm ahead of probe [START_REF] Abid | On evolution of particle size distribution functions of incipient soot in premixed ethylene-oxygen-argon flames[END_REF]. Furthermore, in turbulent flow the flame perturbation effects are expected to be even lower, due to larger flow momentum.

PPS (PPS-M 2000HC PEMS) has been primarily used in the past to measure particulate emission in Diesel engines. PPS provides temporal resolution high enough (100 ms, in this work) to track soot evolution in the flame. In PPS technique particles interact with ionized air produced by a corona discharge (diffusion charging, abbreviated as DC). Subsequently, ions that are not attached to particles are trapped in an ion voltage trap. The exiting charged particles provide estimates of escaping current, which is proportional to soot concentration. Detailed working principle and recent developments in PPS technology can be found in Ref. [START_REF] Amanatidis | Measuring number, mass, and size of exhaust particles with diffusion chargers: The dual Pegasor Particle Sensor[END_REF]. Once escaping current is measured, soot mass concentration can be deduced as Cs = Kpps.Ipps/Qv, where: Ipps is escaping current in ampere, Qv is sample flow rate in m 3 /s (5 lpm), Kpps is a constant (1.05, based on Diesel soot applications). Soot volume fraction can be deduced, as fv-PPS = Cs/ρsoot, where: ρsoot is assumed to be 1700 kg/m 3 . SMPS is used to determine soot size distribution (TSI, Classifier 3080 + CPC 3010). SMPS counts the particle concentration by sequential selection of classes of electrical mobility diameters over certain range. Such scan is long (3 min for 7-290 nm), and thus this technique is generally used in laminar flames [START_REF] Abid | On evolution of particle size distribution functions of incipient soot in premixed ethylene-oxygen-argon flames[END_REF][START_REF] Gu | Particle size distribution of nascent soot in lightly and heavily sooting premixed ethylene flames[END_REF]. Nevertheless, for the size range of interest (7-30 nm), the scan duration was short (23 s). The duration for each class scan needs to be long enough to provide statistically stationary description of fluctuating flame, so that the measurement provides temporally averaged quantity. The obtained size distributions are lognormal. In this paper, only geometric mode diameters are reported.

The probing system and dilution level were qualified using well-characterized miniCAST soot (Jing, miniCAST 5200). Soot generated by miniCAST propane (60 mlpm) / air (1.5 lpm) flame is stable and reproducible. The combustion products (including soot) are quenched by nitrogen gas (7 lpm) and diluted with air (20 lpm). SMPS measurements were performed at various N2 flowrates through the probe, and 10 lpm was found to be enough to avoid soot aggregation in the sampling line. Additionally, PPS measurement showed a remarkable stability in the soot concentration with time, indicating no blockage of orifice for the miniCAST configuration. The dilution ratio (N2/sample) is determined (24:1) through PPS measurements with the sampling probe and directly without the probe. Recall, the additional sample dilution (×100) of air is used. Thus, the effective dilution ratio is 2400:1 by volume.

Results and discussion

Mean LII

Mean soot volume fractions fv (ppm) determined by LII technique are reported in Fig. 3 for the three targeted flames. These mean fields are determined by averaging 2000 instantaneous LII images for HAB > 225 mm and 1500 images below this HAB. As the flame is confined, certain regions are optically inaccessible. In ethylene flame (me = 0.166), measured axial peak fv is 0.51 ppm, which is in close agreement with Köhler et al. [START_REF] Köhler | Sooting turbulent jet flame: characterization and quantitative soot measurements[END_REF]. Pure butane flame indicates much lower peak fv (0.06 ppm) than ethylene as expected, since C/H ratio of butane (C4H10) is lower than that of ethylene (C2H4). Also note that, fv depends on both: fuel type and turbulence level (residence time, strain, etc.). Addition of 30% ethylene (by volume) increases fv by nearly twice (0.14 ppm). Recall, that the bulk velocities in butane and butane/ethylene mix are kept constant to isolate flow dependencies (residence time, strain) from fuel type, and also for a modeling convenience.

Probe characterization in a jet flame

A preliminary test is performed with a butane jet flame (mb = 0.10 g/s). The sample is extracted at r = 0, HAB = 250 mm. Figure 4 shows an evolution of measured soot concentration.

The profile shows initial spike as flame ignites. The peak concentration is associated with the transitional soot volume fraction. Following the peak, measurement shows continuous drop in soot concentration, which originated due to blockage of probe orifice. There exists a distinct post-peak transition marked by circle, which is assumed to be the soot concentration in the flame before any obstruction of the sampling orifice. This procedure is adapted for all target flames. The blockage of probe orifice is lesser in turbulent butane, and butane/ethylene mix flames due to lower soot concentration.

Value used

Soot volume fraction LII vs. PPS

Soot volume fraction measured using LII is referred here as, fv-LII and that of the PPS as fv-PPS. Axial fv profile is extracted from mean LII image shown in Fig. 3, along with the corresponding fv-PPS, which is measured at 8 heights. Excellent qualitative agreement is observed between LII and PPS (Fig. 5) for the axial distribution of soot volume fraction, whereas the absolute values differ. However, a minute offset is present in ethylene case which may be due to a mild radial misalignment between sampling orifice and LII plane. The PPS trends provide valuable complementary information at the location where LII data is unavailable; however, the values of fv-PPS are not in agreement with fv-LII.

In the analysis, from the LII and PPS profiles, we deduce a ratio: maximum fv-PPS / maximum fv-LII, irrespective of axial location. These ratios for butane, butane/ethylene mix, and ethylene flames are 4.3, 3.2, and 1.8 respectively. This varying discrepancy warrants a detailed discussion. There are multiple sources of uncertainty: 1) Unlike LII, fv-PPS is measured only at discrete HABs, thus there is a possibility of missing an actual peak fv. 2) Dilution ratio has been determined at lower temperature in miniCAST soot, whereas in the jet flame soot samples were extracted at higher and varying temperatures (with HAB), leading to modification of dilution ratio through gas expansion in the probe. 3) Extinction coefficient Ke used in LII calibration is a function of fuel and soot maturity, whereas Ke is assumed to be constant across fuels and throughout the flame. This assumption is reasonable, since nearly identical fluence curves and calibration constants were found (not presented here) in ethylene and butane flames 4) PPS calibration constant Kpps and soot density depend on various parameters. PPS response is sensitive to particle size [START_REF] Giechaskiel | Review of motor vehicle particulate emissions sampling and measurement: from smoke and filter mass to particle number[END_REF] and soot morphology [START_REF] Shin | The effect of particle morphology on unipolar diffusion charging of nanoparticle agglomerates in the transition regime[END_REF]. Also, PPS has been factorycalibrated to a fixed size distribution (50 nm mode diameter for Diesel soot), which leads to error with deviation from factory reference [START_REF] Amanatidis | Measuring number, mass, and size of exhaust particles with diffusion chargers: The dual Pegasor Particle Sensor[END_REF]. Nevertheless, the particle size is very small (Sec. 3.4), indicating a negligible aggregation, and thus errors attributed to soot morphology are less relevant.

Moreover, varying particle size does not affect the PPS interpretation. For instance, in mb = 0.3 flame, soot size (Fig. 6.a) varies from 12 to 22 nm over HAB of 250 to 600 mm, yet fv-PPS (Fig. 5.a) is qualitatively in excellent agreement with fv-LII over entire HAB. In order to have a quantitative agreement between PPS and LII results, Kpps need to be 0.24 (instead of 1.05 as used here) which seems to be outside the uncertainty range (based on our unpublished data). Although above discussion identifies potential sources of uncertainty in fv-LII and fv-PPS, definitive conclusion requires targeted investigations.

Soot volume fraction and particle size

The SMPS derived, geometric mode soot diameter (Dm) is plotted in Fig. 6 along with fv-LII. We retain LII derived fv as a reference since encouraging agreement is found between present fv-LII in ethylene flame with an independently reported value [START_REF] Köhler | Sooting turbulent jet flame: characterization and quantitative soot measurements[END_REF].

Figure 6 shows that the Dm ranges from 10-25 nm (with geometric standard deviation typically from 1.4 to 1.9 nm), suggesting negligible soot aggregation (either in flame or in sampling line), unlike measurements in miniCAST flame (Dm = 105 nm) with identical sampling configuration. The location of the peak fv is generally associated to the transition where soot oxidation process begins to dominate the surface growth. As shown in Fig. 6, this location is not systematically correlated with a peak soot size. For instance in butane flame (Fig. 6.a), soot size increases with concentration, and appears to increase beyond peak soot concentration location; however, a definitive conclusion could not be drawn due to limited data points past the peak fv location. Nevertheless, such conclusion could be drawn from butane/ethylene flame (Fig. 6.b), since the flame is relatively compact, allowing measurements well past the peak fv location. Soot size appears to peak slightly downstream of the peak fv location.

Ethylene flame (Fig. well before peak fv location. A second local peak is observed around y = 450 mm. Such trend has also been reported through simulations in a similar flame [START_REF] Köhler | Experimental characterization and numerical simulation of a sooting lifted turbulent jet diffusion flame[END_REF]. Although, simulated trends are in good agreement with the present data, the predicted [START_REF] Köhler | Experimental characterization and numerical simulation of a sooting lifted turbulent jet diffusion flame[END_REF] peak (axial) mean particle size is nearly twice (64 nm) as of the present peak mean diameter (28 nm). Also, note that in simulations [START_REF] Köhler | Experimental characterization and numerical simulation of a sooting lifted turbulent jet diffusion flame[END_REF], soot particles were assumed to be spherical to save the computational time, whereas in this experimental work we measure mobility diameter of soot which may have been coagulated, although the smaller size suggests otherwise. The assumption of neglecting soot coagulation may affect the numerical predictions. The discrepancy warrants further work on both experiment and simulation fronts. Independent simulations by ASMAPE consortium will provide soot size, and thus will contribute to convergence of database accuracy.

The correlation between soot size and volume fraction is not trivial. Such relationship depends on the particle mobility, aggregation and fragmentation rates, which in turn depend on local temperature, particle number concentration, and concentration of oxidant species.

Conclusions

The present work reports soot volume fraction and soot size in the flames of interest to modeling community. fv is deduced using two different techniques, namely LII, and PPS. Axial peak fv-LII is measured as 0.06 ppm, 0.14 ppm, and 0.51 ppm in butane, butane/ethylene, and ethylene flames respectively. Trends in fv-LII and fv-PPS are in excellent agreement. The quantitative discrepancy between fv-LII and fv-PPS is attributed to various sources: uncertainty in LII extinction coefficient, uncertainty in sample dilution ratio, limited data points of PPS along the flame height, and PPS calibration uncertainties originated from different calibration reference size, soot morphology and composition.

Soot particle mode diameter along the flame axis lies within 10-25 nm range in different flames. Soot size increases with concentration in butane and butane/ethylene mix flames, and peaks slightly downstream of the peak fv location. In ethylene flame soot size peaks well upstream of fv location, with another local peak appearing in the downstream of peak fv location.

The data in butane and butane/ethylene flames is a new addition to the existing literature, and will become a part of the ambitious comprehensive database. Further measurements will be reported (OH-LIF, PAH-LIF, velocity, and mixing field) to complete the proposed database.

Fig. 1 .

 1 Flame photographs, (a) mb = 0.3, (b) mb/e = 70/30, (c) me = 0.166. Exposure = 1/20 s. Images are 645 mm in height.

Fig. 2 .

 2 Fig. 2. Schematic of sample extraction layout for SMPS and PPS measurements.

Fig. 3 .

 3 Mean fv (ppm), (a) mb = 0.3, (b) mb/e = 70/30, (c) me = 0.166.

Fig. 4 .

 4 Fig. 4. Soot volume fraction evolution using PPS

Fig. 5 .

 5 Fig. 5. Soot volume fraction measured using LII (line) and PPS (circle), (a) mb = 0.3, (b) mb/e = 70/30, (c) me = 0.166.

Fig. 6 .

 6 Fig. 6. Soot volume fraction measured using LII (line) and soot mode diameter (circles), (a) mb = 0.3, (b) mb/e = 70/30, (c) me = 0.166.

  6.c), shows markedly different behavior than other flames. Soot size peaks

Table 1 .

 1 Target flame conditions.

	No.	mf-butn (g/s)	mf-ethy (g/s)	V (m/s)	Re (-)
	1	0.30	-	39.0	26 000
	2	0.21	0.044	39.0	20 000
	3	-	0.166	45.0	10 000

Acknowledgements

This project (ASMAPE) was funded by the National Research Agency (ANR) under contract no. ANR-13-TDMO-0002. The authors thank Dr. Gilles Godard for assistance in laser installation.