B Lecordier

S Grout

J M Foucaut

M Stanislas

K H Tran

S Coudert

P Desgroux

A Cessou

Combined measurements of

published or not. The documents may come

Introduction

Soot emission is an important issue in many combustion devices such as aircraft gas turbines, automotive engines and furnaces. Soot emission is unwanted and negatively impacts environment and health. Soot is a limiting factor in operability because it enhances thermal radiation and consequently reduces combustion efficiency [1][2][3]. Its formation depends on numerous chemical and physical parameters such as fuel type [4], oxygen rate, residence time and so on the mixing characteristics within a flame [5]. Some models, able to predict the formation of soot, are available today for simple test cases. Soot formation has been widely investigated in laminar flames because they allow stable and well-defined conditions. In practical combustors, it is strongly influenced by its interaction with the turbulent flow environment. The soot formation in a turbulent flame is still unclear owing to the complex interactions among the turbulence, thermochemistry, diffusion, nucleation and agglomeration processes. Exhaustive data base is available for laminar flames covering different fuels, large parameter fields as well as exact defined systems. However, suitable turbulent systems with sufficient soot and a comprehensive data base are quite few in literature [5][6][7].

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Specific objective

This present aims to provide a new data base for modelling soot formation in turbulent flame. The method is based on the Stereoscopic Particle Image Velocimetry technique (SPIV) coupled with Laser Induced Incandescence (LII). The SPIV has the advantage to measure the instantaneous 3 components velocity fields in a plane [8], whereas LII provides instantaneous 2D soot distribution. LII relies on the detection of increased thermal radiation from soot particles heated to very high temperature (~ 4000 K) by a short laser pulse. Under selected conditions, LII signal intensity is proportional to the soot volume fraction noted f v [9]. This work is part of the European project AFDAR (Advanced Flow Diagnostics for Aeronautical Research 2010-2013) which involves 10 academic partners and 3 private companies. The combination capability of SPIV and LII techniques is validated on a model burner. The PIV diagnostic associates two highresolution sCMOS cameras (Lavision) to a double pulse Nd-Yag laser at 532 nm. The LII is performed at 1064 nm from a single pulse Nd-Yag laser and the LII signal is collected by an intensified camera (Roper Scientific -PIMAX 3). Influence of aerodynamic on soot formation processes will be then analysed and experimental measurements will supply an advanced database for the modelling of the soot formation and the distribution in turbulent jet flames. The experimental combined SPIV and LII set up is developed in sight of future industry applications.

Experimental set up

The burner consists of a circular nozzle of 3 mm inner diameter from which the fuel emerges. The tube has an external diameter of 4 mm. Co-annular dry air flows through a contoured nozzle which has an external diameter of 400 mm over a vertical distance of 370 mm and provides a homogeneous co-flow oxidizer. Glass balls and two circular hives were fixed into the co-flow to obtain a laminar co-flow. Fuel mixture of pure ethylene and pure methane were metered into the central tube at a rate of 10L/min via two electromechanical flow control units (Bronkhorst multiflo 7300 -SC17 and SC 18). This corresponds to a bulk flow-velocity of 23 m/s. To change the measurement position, the burner was placed on an elevator. The reference height (0 mm) is fixed when the nozzle is at laser axis level. The burner was placed around 140 cm under the laboratory smoke extractor. Four fuel mixtures of ethylene and methane have been used. Flame compositions are given in Table 1: Ethylene/methane fuel compositions Fig. 1 shows photographs of sooting flames for different fuel compositions. The visible flame heights were found between 550 and 650 mm. The higher the ethylene mole fraction is the brightest the flame is. This is consistent with the increase of the soot volume fraction with ethylene proportion. The SPIV experiments were performed using a double pulse Nd:YAG laser (SplitLight Compact 300 Innolas 10 Hz optimal, 150 mJ/pulse max energy) operating at 532 nm with a pulse duration of 5-7 ns.

100% E-0 % M 75% E - 25% M 50% E - 50% M 30% E - 70% M
The 3 beams are combined after reflection (532 nm) and transmission (1064nm) through a dichroic mirror and transformed into three coincident planar laser sheets. The shape and the dimension of the laser sheets are determined using a combination of spherical (f = +1000 mm) and cylindrical (f=-100 mm) lenses. The vertical sheet is approximately 0.5 mm thin and 60 mm high on the flame axis. A telescope, placed at the output of the PIV laser allows to refine the 532 nm-laser beams focusing accordingly to the 1064 nm one.

Figure.3: SPIV/LII Experimental optical set up

The LII signal from 1064 nm excitation was detected perpendicularly to the incident laser sheet by an intensified CCD camera (PIMAX 3 Roper scientific, 1024x1024) at 10 Hz and equipped with a camera lens ( f = 100 mm, F2.8). The two SCMOS cameras (i.e. LaVision-PCO SCMOS) used for SPIV system have a maximum recording rate in full frame of 50 images/s with a 2560x2160 pixel scientific CMOS chip (6.5x6.5 um pixel size) and a 16 bit dynamic range. Each SCMOS camera is equipped with a Scheimpflug mount and a lens (Nikkor 105 mm, F8, d=1.5 m, FOV=6x8 cm). The two cameras are imaging the illuminated flow particles from different angles. Scheimpflug arrangements keep the planes in focus. The combination of both camera projections allows the reconstruction of the "real" particle displacement inside the measurement area and the evaluation of all three velocity components. Interferences filters centered at 532 nm are placed in front of the camera lenses. As detailed in section 2.3.3 their bandpass has been adjusted to reduce spectral interference from flame emission, laser-induced fluorescence (LIF) of polycyclic aromatic hydrocarbons (PAH) and laser-induced incandescence from soot particles. A head-on PMT photomultiplier tube (Hamamatsu R2257) placed at about 45° from the incident laser sheet, served to determine the temporal response of the LII signal. The LII decay-time is a complex function of the soot size distribution in the collection volume and of the rate of the energy transfer during and after the laser heating. Even though these transfers are not examined in detail in this task, duration of the LII signal along the flames has to be known to avoid interference between PIV and LII temporal signals. Signals were collected using a combination of two plano-convex lenses (50 mm diameter, f= 200 mm and f= 400 mm). The collection volume is selected using a narrow 0.7 mm horizontal slit parallel with the laser sheet axis and placed in front of the PMT. A bandpass filter centered at 400 nm with a bandwidth of 10 ns was set in order to improve the signal to noise ratio. Signal from PMT were acquired and digitised by an oscilloscope. The fuel stream was seeded with zirconium oxide particles (ZrO 2 , purety > 99%, Alfa Aesar GmbH & co) via a solid particle seeder placed on the fuel circuit. The co-flow was seeded with olive oil particles by a liquid particle seeder. Oil droplets are generated using a laskin nozzle. The synchronisation process is completely controlled via Davis 8 software (Lavision) by the PTU (Programmable Timing Unit, Lavision) in internal mode at 10 Hz frequency. The PTU pilots simultaneous excitation and acquisition systems.

Results and Discussion

Once the laser energy was fixed, 2D-LII image acquisition was performed with the DIF (Dual Image Feature) of the PIMAX 3 camera. This mode can capture a pair of images within a short and adjustable delay. A first gate of 60 ns duration before the laser shot was used to acquire the background flame luminosity (ref. The two sCMOS cameras were used in their double frame mode (i.e. short first exposure of 10 μs and second one depending on recording rate) and acquisition rate was 10 Hz PIV (i.e. 20 images/s on each camera), so second exposures were reduced to 16 ms. The LII system was recorded at 5 Hz (i.e. 5 images/s). Hence only one over two SPIV acquisitions is correlated to a LII measurement, limiting the rate of combined LII/SPIV measurements to 5 Hz. SPIV images have around 5 pixels particle image diameter and a concentration of 0.01 particle per pixel. SPIV technique was calibrated as detailed below.

A multiple planes calibration was used: it consists of a planar target with black crosses over a transparent sheet that is moved by an accurate motorized translation stage in the direction perpendicular to target plate. The scanned volume was 10 mm by step of 1 mm centered on the light sheet position (i.e. -5 mm to +5 mm with 1 μm position accuracy; setup lays in Fig. 3). Cameras are setup on each side of the transparent target, so 2 white screens are placed in their fields of views behind the target to get a proper contrast between black crosses and white background. Moreover, the LII camera is recording each target position during the SPIV calibration, so all images of the 2 systems could be mapped on the same physical plane. Standard multi-pass PIV analysis with polynomial reconstruction method [10] and misalignment correction [11] was used. The first pass is 64x64 pixels and the last one is 32x32. For misalignment between calibration and laser sheet planes correction, it is recommended to record images with co-flow only (i.e. no flame), so particle images lay all over the frames.

In this section, fields of velocity and soot volume fractions obtained using simultaneous SPIV and LII techniques are shown. Experiments were performed with different fuel compositions (% ethylene, % methane) noted in table 1 at different HAB (20, 50, 100 mm).

The soot volume fraction is proportional to the LII signal acquired. An average over entire image was performed on fifty LII images corrected for flame background luminosity with different fuels and different HAB. In the flame portion area, the soot volume fraction increases with HAB. It is enhanced by ethylene addition. These data are consistent with the emission observations reported in section 2.1. Fig. 6 shows simultaneous and instantaneous PIV images from SCMOS camera 1 and 2. This experiment was performed at 50 mm HAB and a fuel composition of 50% ethylene and 50% methane. Fuel and air flows were seeded with olive oil and zirconium oxide respectively. A satisfactory seeding is observed in the fuel flow and the co-flow. Nevertheless, the co-flow oil particles are vaporized in the vicinity of the flame fronts leading to absence of particles in a zone of around 50 pixels in width.

The PIV vectors are superimposed on the PIV image. PIV vectors were obtained by cross correlating the raw image using Davis 8 sotfware. The Davis multi-pass algorithm with an interrogation window decreasing size from 64 x 64 to 32 x 32 pixels and 50% overlap was used to process PIV, no post-processing was performed after calculation. The main displacement is vertical and the color scale represents the displacement in pixel. The main velocity was calculated at 13 m/s at HAB of 100 mm while the ejection velocity at the burner nozzle was calculated at 23 m/s. In the boundary region between flame front and air of the co flow where there is no particle, a lot of wrong vectors appear. Experimental set up need to be improved to seed this area. Fig. 7 presents the resulting SPIV image with the three vector components. SPIV vectors were obtained by stereo reconstruction using Davis 8 sotfware. The Davis multi-pass algorithm with an interrogation window decreasing size from 64 x 64 to 32 x 32 pixels and 50% overlap was used to process SPIV, post-processing was performed using a vector median filter which removes vectors if the difference to average is less than 3 r.m.s of neighbours. Vectors were also deleted if the correlation values are less than 0.2. An interpolation was used to fill up empty spaces and smoothing mode (smooth 3 × 3) was used.

Figure 1 :Figure 2 :

 12 Figure 2: photographs of different fuel compositions lift off height The experimental set up for simultaneous 2D-LII and SPIV measurement is shown in Fig.3 The 10 Hz pulsed Nd:YAG laser (Quantel Brilliant B) operating at 1064 with a pulse duration of 7 ns was used for LII excitation. The SPIV experiments were performed using a double pulse Nd:YAG laser (SplitLight Compact 300 Innolas 10 Hz optimal, 150 mJ/pulse max energy) operating at 532 nm with a pulse duration of 5-7 ns.

Fig. 4 (

 4 a)) and a second gate with the same duration started simultaneously with the laser pulse to acquire the LII signal (ref. Fig.4 (b)). Both gates were separated by 5.5 μs. This time is short enough to neglect flow convection. In the DIF mode, the acquisition rate is 5 Hz. One thousand images were acquired to perform a statistical study. The intensifier gain is set to 10 (maximal gain =100). The first post-processing consists of the subtraction of raw LII images by the background flame luminosity as shown in Fig.4 (c). It must be noticed that background LIF from PAH is not excited at1064 nm facilitating the images interpretation.

Figure 4 :

 4 Post-processing of LII image (100% ethylene, 100 mm HAB, 90 mJ, 1064 nm excitation wavelength). (a) The background flame luminosity (max intensity about 1100 u.a), (b) The LII signal + the background flame luminosity (max intensity about 11000 u.a), (c) resulting image after post-processing (max intensity about 10000 u.a)

Fig. 5 Table 3 :

 53 illustrates LII signal images for 20, 50, 100 mm HAB with 100% ethylene as fuel. HAB = 20 mm HAB = 50 mm HAB = 100 mm Figure 5: Average LII image over fifty images for different HAB with 100% ethyleneEvolution of soot volume fractions (a.u.) was determined from these data and represented in table3. Qualitative evolution of soot volume fraction from LII signal average (a.u.) for 4 fuel compositions at 3 HAB.

Figure 6 :

 6 Figure 6: instantaneous PIV image with corresponding velocity field of SCMOS camera 1(upper) and camera 2 (bottom) for 50 mm HAB in the 50E-50M flame

Figure 7 :Fig. 8

 78 Figure 7: Resulting instantaneous SPIV image at 50 mm HAB in the 50E-50M flame. (a) Vector length in y direction, (b) Amplitude of the planar velocity (x,z) 2 2 z x   Fig.8represents the instantaneous LII image recorded simultaneously with the previous SPIV ones, LII intensity varies from 400 to 7000 counts, while the background flame luminosity intensity ranges from 900 to 1200 counts (the mean intensity background is around 700 counts). Globally an excellent signal to noise ratio was found both in the LII images and the processed SPIV images thanks to the selection of optimized experimental parameters.

Figure 8 :

 8 Figure 8: Instantaneous background flame luminosity intensity (a) and LII images (b) for at HAB of 50 mm and a fuel composition of 50% ethylene and 50% methane Conclusion To our knowledge SPIV technique is demonstrated for the first time in sooting flames. To this aim, different strategies were applied to reduce the natural or laserinduced emission due to soot particles and PAH present in the flames. SPIV technique has been combined with instantaneous LII measurements in several turbulent sooting flames. Statistical investigation is currently under investigation to determine the correlation between soot formation areas and the turbulent flow in the different flames. Knowledge of the three velocity

Acknowledgements

The work is supported by the AFDAR project (Advanced Flow Diagnostics for Aeronautical Research) funded by the European Community's Seventh Framework programme (FP7/2007-2013) under grant agreement no 265695.