Volumetric velocity measurements (V3V) on turbulent

Flow Measurement and Instrumentation 54 (2017) 46–55

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Flow Measurement and Instrumentation journal homepage: www.elsevier.com/locate/flowmeasinst

Volumetric velocity measurements (V3V) on turbulent swirling flows a,b,⁎

Toufik Boushaki a b c

c

a

, Amine Koched , Zakaria Mansouri , Florian Lespinasse

MARK

a

ICARE CNRS, 1C Avenue de la Recherche Scientifique, 45071 Orléans Cedex 2, France Université d′Orléans, IUT - GTE, 45067 Orléans Cedex 2, France TSI France, Hotel Technologique, Technopôle de Château-Gombert, 13382 Marseille, France

A R T I C L E I N F O

A BS T RAC T

Keywords: Swirling flow Turbulent flow Volumetric velocimetry V3V® 3D3C Stereo-PIV

This paper presents some results of Volumetric V3V 3D3C velocity measurements on a turbulent flow from a swirl burner. The flow out from the burner used is highly three-dimensional. The study aims at using a system of instantaneous 3D velocity measurements in order to characterize the turbulent swirling flow. The burner used consists of two coaxial tubes with a swirler placed in an annular part supplying the oxidant flow. The central pipe delivers the fuel radially (in the case of reacting flow) through eight holes symmetrically distributed on the periphery of the tube. The burner is placed at the bottom of a combustion chamber and the flow develops vertically along the confinement. The Volumetric 3-component Velocimetry V3V® technique commercialized by TSI is used for 3D velocity measurements in a non reacting flow. The measurement volume above the burner is located at 1.3Db (=49.4 mm) and is 50×50×22 mm3. The operating conditions considered in this study are 4.67 m/s of bulk velocity, Re=7531 of Reynolds number and Sn=1.4 of swirl number. There are very limited works on the application V3V technique on fluid flows. This paper presents the first results concerning an isothermal non-reacting gas flow. Instantaneous and mean velocity 3D3C volumes are measured and shown. Streamlines and velocity iso-surfaces are also analyzed together with different velocity profiles. The results are compared to previous SPIV (Stereoscopic Particle Image Velocimetry) measurements performed by the authors. A good agreement is found between the results of both techniques; the discrepancy did not exceed 10%. V3V results allowed a fine description of 3D aspects of the flow including the recirculation zone and the annular zone with swirling jet effects. The swirling part of the flow and the central recirculation zone are clearly identified by 3D fields of velocities and streamlines. Velocity volume indicates the presence of a central zone with a negative longitudinal velocity, which can reach −1 m/s at the burner center. Under the swirl effect, the tangential velocity is relatively high, particularly in the annular zone of the burner. Indeed, this velocity is varied between 3 and −3 m/s for a bulk velocity of 4.7 m/s.

1. Introduction

The recent papers have been interested in different aspects of swirling flows, as Huang et al. [11] on the mixing properties of the swirling double-concentric jets; Jourdaine et al. [12] on the stabilization of premixed swirling CO2 diluted methane flame; Benim et al. [13] on numerical investigation of turbulent swirling flames in a model gas turbine combustor; Elbaz and Roberts [14] on the effect of the quarl geometry on the flame structure of swirling non-premixed flame. Khalil and Gupta [15] studied the colorless distributed combustion for using swirling and non-swirling flowfield to provide ultra-high combustion intensity; Orbay et al. [16] were interested in structures of vortex breakdown on an industry swirl burner; Candel et al. [17] presented the progress on swirling flame dynamics particularly with acoustic waves; Lavante and Yao [18] investigated numerically turbulent swirling flows in axisymmetric internal flow configurations. Besides, fluid injection geometry plays a major role in flame

Swirl flows are extensively used to enhance mixing and improve flame stabilization in a variety of practical situations involving low calorific fuel, fuel spray, coal and biomass [1]. Strongly swirling reacting flow benefits are now well-known: flame stabilization enhancement due to the vortex breakdown phenomenon, responsible for the central recirculation zone (CRZ) occurrence [2–5], turbulent mixing improvements due to the Precessing Vortex Core (PVC) [3,6– 8] showed that the addition of swirl significantly changes the aerodynamic pattern and can be used to stabilize the flame. The helical fluid flow creates a recirculation zone, allowing dilution with combustion products, and the decrease of the flame temperature limiting the production of NOx [9,10]. Note that the swirled flows are extensively documented in the literature, in both cases not reactive and reactive.

⁎

Corresponding author at: ICARE, CNRS, 1C, Avenue de la Recherche Scientifique, 45071 Orléans, France. E-mail address: toufi[email protected] (T. Boushaki).

http://dx.doi.org/10.1016/j.flowmeasinst.2016.12.003 Received 18 July 2016; Received in revised form 24 November 2016; Accepted 8 December 2016 Available online 18 December 2016 0955-5986/ © 2016 Elsevier Ltd. All rights reserved.


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made to demonstrate the V3V measurement: a flow behind a cube, a flow around a vortex ring and the wake behind rough surface. Troolin and Longmire [32] used the V3V system for velocity measurements of vortex rings from inclined exits. Sharp et al. [33] were interested in the study of V3V measurements of the turbulent flow around a Rushton turbine. Both authors showed the vortex ring interaction with two trailing rings and highlighted the role of the V3V measurement for this kind of study. Flammanget al. [34] studied the three dimensional water flow around a fish with the V3V technique. The authors showed the vortex generation by the fish dorsal and anal fin and highlight the behavior of this vortex during the fish locomotion. Cambonie and Aider [35] warn against the influence of optical screening phenomenon for V3V measurements. This phenomenon can appear when the particles density is too high. To illustrate this screening phenomenon, they applied the V3V on transverse water jet in cross flow and determinate three regimes of measurements quality depending on particles density. They proposed a methodology to optimize volumetric measurements taking into account the system specifications, and defined a relation between particles density and optimal spatial resolution. Recently, the V3V technique has demonstrated its high ability to measure 3D velocities in more or less complex situations of flows. Kiebert et al. [36] used the technique in a cuvette with a fluid loaded SAW device (surface acoustic waves); Musta [37] studied the interaction of a vortex ring with a cutting plate; Wang et al. [38] performed flow fields inside a hydrocyclone; Ting and Reimnitz [39] measured using V3V instantaneous turbulent velocity fields induced by the breaking of plunging regular waves on a 1 in 40 plane slope. The originality of the present work is the application of the V3V on a gas flow, turbulent and swirled. In the flowing sections of the paper, the experimental setup with the burner description, operating conditions and the combustion chamber are given. Then, the V3V system used and processing technique of velocity measurements are presented in details. The fourth section is devoted to the results and discussions. 3D velocity fields of three components are shown. 2D mean and instantaneous velocity cartographies as well as velocity radial profiles are also illustrated and discussed. Through this work, we would like to demonstrate the feasibility of using the V3V technique to measure velocities in the case of turbulent swirling gas flow, in 3D and instantaneously. It would be necessary to complete the study in the future to have more details and analysis of swirling flows, in reactive and non reactive cases.

stabilization as studied by many authors [19,20]. Considering swirling annular jets and non-premixed flames, the fuel radial injection allows a better mixing than axial injection [21] at the close vicinity of the burner exit. Nevertheless, such turbulent swirling lean flames exhibit high sensitivity to combustion instabilities [22]. A solution was used by the authors in previous work concerned the use of oxygen enrichment to avoid these instabilities [23]. This later described an experimental study carried out on the same burner to assess the oxygen enrichment effects on flame stability and pollutant formation. In the present paper the authors focus on the dynamic study using the same swirl burner. The flow from the burner is highly three-dimensional and requires efficient 3D measurement techniques. The study aims at using a system of instantaneous 3D velocity measurementsin order to characterize the turbulent swirling flow. During these last years, the various techniques of particle imaging using a laser source, like PIV (Particle Image Velocimetry), have proven to be powerful measuring methods. Measuring instantaneous two components velocity fields in a plane (2D2C) is now very common. More and more complex optical diagnoses, involving large field of view measurements, simultaneous three velocity components measurements and high temporal and spatial resolution measurements, are now possible thanks to the improved technical specifications of the different parts composing a PIV setup (high repetition pulsed laser, dual frame cameras, powerfully computers, etc.). Three velocity components measured simultaneously in a 3D volume (i.e. 3D3C) is nowadays possible with a good spatial resolution and a high sampling frequency. During the last few years many innovative techniques have been developed to measure instantaneous 3D3C velocities as Holographic PIV (HPIV) [24], Tomographic PIV (TomoPIV) [25], or Defocusing Digital PIV (DDPIV) [26,27]. With increasing computing power, volumetric velocimetry should become popular in the near future. The Volumetric Three-Components Velocimetry (i.e. V3V) technique is based on the same principle as DDPIV which was first introduced for liquid flows [28–33]. More information of the first developments done on DDPIV can be found in Ref. [28]. In this article, Pereira and Gharib [28] applied DDPIV technique to measure the velocity of bubbles and particles in two-phase flows using particles tracking velocimetry algorithm which they described and discussed in terms of velocity accuracy. The performance of the DDPIV technique was established by the authors in terms of particle number densities and the void fraction. The DDPIV technique is used by Graff et al. [30] to measure flow field of bubbles generated by model ship propeller in a circulating water tunnel. The velocity field of this flow is resolved by the V3V technique, which is confirmed by the measurement of the vortex created behind the propeller. The V3V technique is based on the DDPIV but with a single-body for the camera that records tracer particle images from three different views simultaneously, as developed by Lai et al. [31]. The optical system, 3D imaging principle and the calibration method are clearly explained by the authors. Moreover, three experiments are

2. Experimental setup The experimental setup consists of a swirl burner, operating conditions and a confinement surrounded the burner. Fig. 1 shows a schematic diagram of the coaxial swirl burner apparatus. The burner used in this study consists of two concentric tubes with a swirler placed in an annular part supplying air as shown in Fig. 1. Eight guide vanes

Fig. 1. Diagram of the coaxial swirl burner, a) 3D view of the top part, b) vertical cross-section.

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3. V3V system and processing

are made with various vane angles to induce a swirl intensity variation. The central pipe delivers a fuel radially (in the case with a reacting flow) through eight holes symmetrically distributed on the periphery of the tube near the exit burner. Note that in this study of a non reacting flow, the fuel is replaced by air for safety reasons. The outer annular part diameter and the related radius are defined by Db and Rb respectively (38 mm and 19 mm). The degree of swirl for rotating flows is usually characterized by the nondimensional swirl number Sn which represents the ratio of the angular momentum flux Gx, and the axial momentum flux Gz times a characteristic distance of the radial dimension R (for example the outlet radius Rb). In the present work, it is defined as follows [8]:

Sn =

Gθ Gx R

The volumetric velocimetry technique V3V® is used to determine 3D velocity fields in the vicinity of the exit burner. This technique was first introduced by Willert and Gharib [26] and later used by Pereira et al. [29] in a configuration including three cameras which is the configuration considered in this study. The principle of measurement is similar to PIV: the flow is seeded with tracer particles and then illuminated by a dual pulsed Nd-YAG laser. Images of the flow are then recorded and processed to track the matched particles individually and then calculate their velocities. Particles of AL2O3 with 1 µm average diameter are considered to seed the flow. The particles were injected at the bottom of the burner in the annular region dedicated to air flow. The size of particles is in average 1 micro; however, there are some particles superior to 1 µm (to 10 µm) but its percentage is less to 10%. In all cases, the particles of 1–10 µm follow faithfully the flow because the criterion of Stokes [46] is respected (St < 1) in this study. Concerning the agglomeration, the particles were dried first, in order to limit agglomerates. Agglomeration of particles was possible out of the fluidized bed reactor used to generate the particles due to static charges of the solid particles among other things. However the proportion of aggregates compared to single particles was very small at the measurement volume due to the fact that the flow is being accelerated in the burner which leads to a deagglomeration of the aggregates. The laser is delivering a laser beam of 5 mm of diameter with a wavelength of 532 nm and an energy of 200 mJ/pulse at the laser head output. Regarding the small size of the particles involved for the seeding, a quite high energy of the laser is required to illuminate the particles so they can be detected by the cameras. The small size of the particles might limit also the size of the volume measurement with regards to the energy of the laser considered in this application (i.e. 200 mL/pulse). The laser head is equipped with two cylindrical lenses at the output to expand the laser beam in a volume of laser light used for the illumination of particles in flow. The cylindrical lenses are placed one in front of the other at the laser head with an adjustable angle of orientation relative to each other. The first lens with a focal length of f=−25 mm is used to expand the laser beam into an horizontal light sheet and the second one with a focal length of f=−15 mm is used to expand the horizontal light sheet vertically to obtain a cone of light to ensure the volume illumination. Black tape placed on the optical access is used to crop the volume of illumination to have the laser illuminating exactly at the position of the measurements. The angle between the two lenses is also adjusted to change the size of the volume of illumination and focus the maximum of the energy of the laser in the region of interest in the flow. Images of the flow are recorded with three CCD cameras mounted on a triangular specific probe, named TS frame, looking toward the flow. The cameras are equipped with 85 mm lenses and the aperture adjusted to the maximum value (i.e. f/22) to have the maximum depth of focus possible. The cameras recording is synchronized with the laser illumination using a timing control unit controlled by the software InsightV3V 4 G® from TSI: each image is illuminated with a single pulse from the laser. Pairs of images are recorded by each of the three cameras with a frequency of 15 Hz in dual frame mode and a time shift between the two successive images of the same pair (i.e. laser pulses) of 20 µs. The time shift is adjusted to have a maximum particle displacement of 5–6 pixels between Frame A and Frame B. This displacement allows a correct calculation of the velocity vectors of the particles using the tracking algorithm considered in this study. The experimental setup and the V3V arrangement are shown in Fig. 2. The V3V probe is fixed in front of the combustion chamber and looking toward the burner through a quartz window. The focal length f is fixed at 450 mm in this configuration: all three apertures on the cameras were adjusted to focus on the reference plane. According the Pereira and Gharib [28], the measurement volume should be located between 0.92×f and 0.87×f which means that the maximum depth of the measurement volume will be 22 mm. A mirror was used to reflect

(1)

The geometrical swirl number Sn related to this configuration is defined as [1]:

Sn =

1 ⎛ 1 ⎞ 1 − (Rh / R )4 ⋅⎜ ⎟⋅ tan α0 1 − ψ ⎝ 2 ⎠ 1 − (Rh / R )2

(2)

where ψ is the blockage factor and α0 is the vane angle. R and Rh are nozzle and vane pack hub radii respectively. The swirl number based on Eq. (2) is fixed to 1.4 for this work. The swirler position above the burner exit plane is indicated by h and fixed at 60 mm. The Reynolds number in the annular part, based on Db=38 mm and the fixed bulk velocity 4.67 m/s, is Re=7531. The experiments are conducted within a square cross-section chamber of 48×48 cm2 and 1 m high operating at atmospheric pressure as shown in Fig. 2. Six windows are placed in each face of the chamber allowing optical access to all the potential flow zones. Theburneris placedat the bottom of the chamber andthe flowdevelopsvertically along the confinement. The velocity measurements are carried out inside this chamber using the V3V (3D3C) technique developed by TSI.

1

2 3 4

5

6 7

Fig. 2. V3V system on the combustion plant; 1) flue gas evacuation, 2) combustion chamber, 3) visualization windows, 4) Nd Yag laser, 5) V3V probe with 3 CCD cameras, 6) burner, 7) seeding system.

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study is H×W×D=50×50×22 mm3. Fig. 4b shows a reconstruction of the measurement volume obtained from the calibration process. The size of the volume of measurement was mainly limited by the quality of the signal emitted by the particles in the measurement volume: this is mainly related to the small size of the particles considered (i.e. 1 µm) and the energy of the laser (i.e. 200 mL/pulse). For the purpose of this study the final size of the volume was OK to allow a comparison with previous SPIV measurements performed in the same region of the flow. The measurement volume is located above the burner from z=1.3 Db (Db=38 mm) as shown in Fig. 4c. Once the calibration is done, the laser is turned on and the cameras start capturing images of the particles in the flow. The images recorded using V3V are then processed using specific algorithms. First, a preprocessing is applied on the raw images to improve the contrast between the background and the particles: removing the background image (average intensity image) is one of the pre-processing that is considered to remove the laser reflections on the wall, bubbles etc. In addition, a Gaussian filter is applied with a filter size of 10 pixels to retrieve particles which will make their detection easier later. Processing of images to obtain velocity data involves four steps:

back the laser light on the measurement volume to minimize the intensity attenuation along the laser beam axis and improve the effective laser energy. The resulting illumination was uniform and to increase the SNR a preprocessing of the raw image was considered to remove the local average intensity calculated from the neighbor pixels of each pixel in the images. Each particle in the measurement volume far from the reference plane will be seen by the three cameras as a triplet (Fig. 4a). The triplet size is used to evaluate the position of the particles along the optical axis. In the focal plane (i.e. reference plane) of the cameras the triplets collapse as single dots. To find the axial position of a particle in the volume of measurement using the triplet size, a calibration of the measurement volume is necessary. The calibration is performed using a backlighted calibration target with dimensions of 100×100 mm2 with a regular grid of circular dots with a distance of 2 mm between dots in vertical and horizontal directions. A fiducial mark at the center of the target is considered for the determination of the grid origin and the axis orientation. A traverse system is used to move the calibration target in different position along the optical axis in the measurement volume. The maximum distance between the planes is usually equal to the distance between dots of the calibration target. In this case a distance of 2 mm between the planes is considered and a total number of 11 planes were recorded. The calibration process corrects for the mechanical misalignment error of image sensors detected by plotting the signature graph of the fiducial mark on the target at the different positions of the calibration planes. The dewarping error is also determined from the calibration: the standard deviation of grid dot position error after dewarping is plotted and was found ≤0.25 pixels (i.e. ~0.1 pixels). The magnification of the cameras was also compared to the pinhole camera at each calibration plane. The deviation found is corrected by adjusting the pixel size in each of the three views during the triplet search. A deviation less than 5% was obtained during the calibration process performed in the current study. Fig. 3 shows the results of the calibration process. The final size of the volume of measurement is determined by the intersection of three volumes: the volume determined by the calibration, the illumination volume defined by the position of the laser light, the window masks and the width of the channel and the volume defined by the particle processing masks in case those are added to the recorded images. The volume of measurement size obtained in this

– Identification of individual particles in the 2D images recorded by the cameras: intensity peaks are detected in the regions of interest of three pairs of images. This peak detection uses an intensity threshold. A particle overlap is also considered to retrieve overlapped particles. Using these parameters, each particle can be fitted by a Gaussian intensity profile through an iterative scheme on neighboring pixels (Levenberg–Marquardt algorithm). Each 2D particle is then defined by its center and its fitted diameter as shown in Fig. 5. – The reconstruction of the 3D particle repartition in the measurement volume: Particles identified on one camera are matched with corresponding particles seen by the other two cameras using the calibration results performed earlier. Fine and coarse search tolerances respectively 0.5 and 1 pixel are used in this matching process to retrieve a triplet of coordinates for each particle. During the triplet reconstruction processing “ghost particles” [28] can be created. This occurs when noise background is detected as “false particles” and reconstructed as real particles. The number of false particles obviously increases with increasing 2D concentrations of particles in the image. The ratio of the number of valid particles in

Fig. 3. Results of the calibration process in V3V®.

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Fig. 4. a) Triplet reconstruction, b) reconstruction of the measurement volume obtained from the calibration process, c) measurement volume above the burner 50×50×22 mm3.

A minimum number of one vector inside a voxel of 4×4×4 mm3 of size and with a voxel overlap of 75% and a smoothing factor of 20.5 are the parameters considered for the processing of the velocity fields in this study. The spatial resolution of the velocity vectors is then equal to 1 mm in each direction. The V3V results are compared to previous Stereoscopic Particle Image Velocimetry (SPIV) measurements performed by the authors [45]. A comparison of results is done in the next section. The main elements of the SPIV are reported here briefly. A double-pulsed Nd:YAG laser (Minilite 25 mJ/pulse) with a wavelength of 532 nm operating at 10 Hz is used as the light source. The two cameras mounted on Scheimpflug adapters (TSI Powerview Plus® 4MP, 12-bit output and 2048×2048 pixels2) are oriented on the same side of the laser sheet. Nikon AF Micro-Nikkor 105 mm F/2.8 lenses and bandpass filters centered at 532 nm with a bandwidth of 10 nm and a transmission of 50% are used to collect Mie particle scattering. TSI Insight 3G® v10 software is used to analyze S-PIV images. The processing of the images was done using an iterative FFT crosscorrelation processor with interrogation areas of 32×32 pixels, with an overlap of 50% between adjacent cells and a magnification ratio of 0.11. The spatial resolution achieved for the velocity vector grid is 1.1 mm in both directions.

3D (3D particles count: PC3D) over the number of detected particles in 2D is calculated and considered as the indicator of an efficient 3D particle reconstruction; the higher the yield, the more efficient the reconstruction. However, an efficiency value higher than 100% is indicative of presence of ghost particles. In this study the yield values ranged between 50% to ~85% in most of the cases. – The 3D particle tracking algorithm used in this study for the velocity calculation is the relaxation method based upon the works of Ohmi and Li [41] and Pereira et al. [27]. The velocity of each individual particles identified in the volume is calculated. The relaxation method was also described by Baek and Lee [42], Bernard and Thompson [43] and Graff and Gharib [40]. Robust point matching or nearest neighbor algorithms can also be considered [44]. The vectors determined by the 3D particle tracking step were spaced randomly according to the particle locations. – The last step is optional and consists in the velocity interpolation: the random distribution of velocity vectors in the volume is interpolated on a regular volumetric grid. This last step is achieved by a Gaussian- weighted interpolation algorithm. In fact an interpolation grid is created, composed of voxels with a specific size an overlapping could also be considered between adjacent voxels and also a smothing factor to take into account the influence of adjacent voxels. If there are not enough raw velocity vectors in the voxel, the interpolation is not successful [28].

4. Results and discussions Fig. 7 shows the V3V mean velocity field with the longitudinal velocity (Vz) in color scale. The mean velocity 3D field is obtained

Fig. 6 shows an example of the processing mentioned before:

Fig. 5. Particle identification, a) raw image, b) 2D particle identification.

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Fig. 6. Velocity calculation, a) random velocity field (in blue: the velocity vectors; in yellow the particles), b) interpolated instantaneous velocity field (contours of vorticity, velocity vectors in black).

Fig. 7. V3V mean velocity field (longitudinal velocity Vz in color scale) with 4 slices, vertical one at the burner center and 3 transversal ones along the flow. The blue isosurface encloses the recirculation zone of a negative vertical velocity (−0.5 m/s value). Streamlines show the swirl effect.

Fig. 8. V3V mean velocity field (tangential velocity Vθ in color scale) with 4 slices, vertical one at the center of the burner and 3 transversal ones along the flow. Streamlines show the swirl effect.

based on 250 instantaneous acquisitions. The duration of acquisition was fixed based on the previous tests performed using SPIV where the statistical convergence of the mean velocity field was obtained for a minimum of 15 s of acquisition. Then based on the frequency of the camera and laser considered in this study, a set of 250 instantaneous acquisitions were considered. Four slices with velocity vectors are plotted, a vertical one at the center of the burner y=0 and three horizontal ones at z=1.4 Db, 1.9 Db and 2.4 Db. Streamlines in grey color are added in 3D along the flow. The blue iso-surface encloses the recirculation zone of a negative vertical velocity with −0.5 m/s as value in this figure. Figs. 8 and 9 illustrate V3V mean velocity fields with tangential component (Vθ) and radial component (Vr) in color scale, respectively. These V3V results are very interesting because they show the 3D aspect of the flow simultaneously and in the same volume. The swirling jet and the

central recirculation zone (CRZ) are clearly identified. The swirl effect is illustrated through streamlines plotted along the flow and inclined vectors in the traversal slices as shown in Fig. 7. A recirculation zone has appeared in the center of flow where the longitudinal velocity is negative. This CRZ is due to the swirl effect and the presence of the central tube in the burner. It is noted that the swirl through the central recirculation zone allows a better stabilization of flames as studied by Nazim et al. [45]. The velocity vectors right vertical slices of Figs. 7–9 are not visible because the flow is directed to the opposite direction of the plane. Fig. 8 shows that the tangential velocity is relatively high since its value varies between 3 and −3 m/s for a bulk velocity of 4.7 m/s. This is expected because of the helical flow induced by the swirler. Red and blue colors are observed on the right and on the left in the vertical slice of the Vθ field. These are positive and negative velocity values because the flow 51


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To quantify velocity values, radial profiles of Vz and Vθ are plotted in Fig. 13 at six height positions from the burner. The longitudinal velocity reaches 3.5 m/s at z=1.5 Db and decays to 2 m/s at z=2.5 Db in the right part of Vz profiles. To the left of Vz profiles the values varies from 2.5 to 1.5 m/s between z=1.5 Db and z=2.5 Db. This indicates that the velocities are not symmetrical, because the location of swirler vanes which are different for a given vertical plane in the burner. In the central recirculation zone, Vz reaches −1 m/s at z=1.5 Db under the swirl effect. The tangential velocity is about 2.6 m/s in the annular part and zero at the burner center for the height z=1.5 Db. The decay of tangential velocity is significant from 1.5 Db and 2.5 Db despite the small distance between the two positions. This reveals that the swirl effect is high near the burner but decreases quickly along the flow. A comparison with the stereo-PIV data is done in order to validate the consistency of V3V measurements. Figs. 14 and 15 show the comparison of SPIV and V3V of longitudinal (Vz) and tangential (Vθ) mean velocity fields. It is based on data from the longitudinal plane at the center of the burner which y/Rb=0. Note that height (z) and radius (x) are normalized by the diameter (Db) and radius (Rb) of the burner, respectively. A good agreement between the two measurement techniques for longitudinal and radial velocities has been found. The discrepancies did not exceed 10% on average in the region of interest of the flow. Note that the field of view of V3V is smaller than the SPIV one because the size of the studied domain is limited. This is observable, particularly on the right of the cartographies.

Fig. 9. V3V mean velocity field (radial velocity Vr in color scale) with 4 slices, vertical one at the center of the burner and 3 transversal ones along the flow. Streamlines show the swirl effect.

goes in the (x, z) plane on one side and goes out the (x, z) plane of the other side. The tangential velocity decays with height as the jet slows down and the swirl effect decreases along the flow. The radial velocity is lower compared to the longitudinal and tangential velocities as shown in Fig. 9. Note, however, some high values of Vr near the limit of annular part as illustrated in transversal slices by a blue color (eg. −2.6 m/s at Z=1.4 Db). Figs. 10 and 11 show V3V instantaneous velocity fields with the longitudinal velocity (Vz) in color scale in Fig. 10 and tangential velocity Vθ in color scale in Fig. 11. Two slices are plotted from the vertical plane y=0 Db and y=−0.6 Db and one slice from transversal plane z=1.4 Db (Db=38 mm). The high variation of velocity values and discontinuous streamlines demonstrate the highly three-dimensional aspect of flow. Indeed, the velocity is comprised between +5 and −2 m/ s for the longitudinal component and between +5 and −5 m/s for the tangential component. Fig. 12 illustrates 2D mean velocity fields from V3V data at the central plane of the burner y=0. The longitudinal component (Vz) is plotted on the left and the tangential component (Vθ) is plotted on the right in terms in color scale. The maximum values of Vz are found in the annular part and negatives ones are located in the central zone. The tangential velocity is also high in the annular part of the burner, whereas it is almost zero in the central zone.

5. Conclusion The present study concerns the use of the volumetric velocity system (V3V 3D3C from TSI) to investigate a turbulent swirling flow. The 3D aspect of this flow requires three dimensional techniques as V3V for velocity measurements. This work provides the first results of the application of V3V technique to a turbulent gas swirling flow. The burner used is a coaxial type with a swirler placed in the annular part. The main results can be summarized as follows: – The V3V results obtained are very interesting since the 3D evolution of swirling flow is correctly represented. The swirling part of the flow and the central recirculation zone are clearly identified by 3D fields of velocity and streamlines. – Velocity volume indicates the presence of a central zone where the longitudinal velocity is negative. This velocity can reach – 1 m/s at 1.4 Db of height for a bulk velocity of 4.7 m/s. – This central recirculation zone is due to the presence of the swirler

Fig. 10. V3V instantaneous velocity field (longitudinal velocity Vz in color scale) with 2 slices, vertical one (y=0 in left, y=−0.6 Db in right) and horizontal one at 1.4 Db (Db=38 mm).

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Fig. 11. V3V instantaneous velocity field (tangential velocity Vz in color scale) with 2 slices, vertical one (z=0 in left, z=0.6 Db in right) and transversal one at 1.4 Db (Db=38 mm).

Fig. 12. 2D mean velocity fields from V3V, longitudinal (Vz) and tangential (Vθ) components in color scale in left and right, respectively.

Fig. 13. Radial profiles of the longitudinal (Vz) and the tangential (Vθ) mean velocities at the center of the burner for different positions from the burner.

– Instantaneous 3D velocity fields and 2D profiles reveal the absence of symmetry and the highly three-dimensional aspect of the flow. – The V3V results are compared to previous Stereoscopic Particle Image Velocimetry (SPIV) measurements performed by the authors. A good agreement has been found between the two techniques results.

and central tube in the burner. – Results show that the tangential velocity is relatively high (from 3 and −3 m/s) because of the helical flow induced by the swirler. However, along the flow the tangential velocity decays rapidly because of the decrease of the swirl effect. The radial velocity is lower compared to the longitudinal and tangential ones.

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a) SPIV

b) V3V

2,5

(m/s) 4 3

2

2 1 0

1,5 0

x/Rb

0

1

x/Rb

1

-1

Fig. 14. Comparison SPIV/V3V of longitudinal mean velocity fields (Vz) at the center of the burner y/Rb=0. Height (z) and radius (x) are normalized by the diameter (Db) and radius (Rb) of the burner.

a) SPIV

b) V3V

2,5

(m/s) 3 1,8

2

0,6 -0,6 -1,8

1,5 0

0

1

1

-3

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