study of helical vortices in swirling jets and flames by...

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

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Study of Helical Vortices in Swirling Jets and Flames by Tomographic

PIV

V.M. Dulin1,2, L.M. Chikishev1,2, M.P. Tokarev 1,2, D.M. Markovich1,2*

1: Kutateladze Institute of Thermophysics, Siberian Branch of Russian Academy of Sciences, Novosibirsk, Russia

2: Novosibirsk State University, Novosibirsk, Russia * correspondent author: [email protected]

Abstract Helical vortices are formed in swirling jet flows and promote mixing in comparison to jets without swirl. However, influence of these vortices (including precessing vortex core) on stabilization of flames is not completely understood. Developing nowadays volumetric velocimetry techniques can provide deeper insight into this issue. The aim of the present study was to investigate 3D large-scale vortices in an open swirling turbulent jet and premixed methane-air flame by using tomographic PIV. The measurements reveal that for the high-swirl non-reacting flow, the vortex core co-existed with the pair of counter-rotating spiral vortices, which were located in the shear layers. Each vortex of the pair winded in the opposite direction to the sense of the flow swirl. The vortices in the shear layers were convected by the flow, and the double-helix structure rotated in the same sense as the jet swirling. This double-vortex structure was also detected in the swirling flame.

1. Introduction Helical vortices, formed in swirling jet flows, promote mixing in comparison to the jets without swirl. However, nature of these spiral vortices and their influence on stabilization of flames is not completely understood. In particular, role of precessing vortex core in flames dynamics is still a debated issue. A number of experiments has been carried out to obtain phase-averaged patterns of coherent helical vortices in swirling jets and flames. By synchronizing LDV measurements with recordings from the local pressure probe, Cala et al. (2006) introduced concept of a triple vortex structure for coherent velocity fluctuations in turbulent swirling jet flows with vortex breakdown: precessing vortex core and two secondary helical vortices. One secondary vortex is located in the outer shear layer, and the other one is between the recirculation zone and the annular swirling jet. By using phase-reconstruction from two the most energetic POD modes of 2D PIV data, several studies have examined applicability of this concept for other flow geometries (e.g., Legrand et al. 2010, Stöhr et al. 2011, Alekseenko et al. 2012). Some of them report only a double-helix structure in the phase-averaged velocity field, especially for combustion chambers, where the flow has a greater opening angle. Thus, another established concept of coherent vortices in swirling flows is a double-helix vortex structure that consists of the precessing vortex core (or helical vortex core) in the inner shear layer and the secondary helical vortex in the outer layer. Linear stability analysis for the mean velocity profiles of the strongly swirling jet by Oberleithner et al. (2011) revealed remarkable coincidence of the obtained global mode with coherent structure reconstructed from POD. Consequently, a scenario of vortex breakdown formation in high-swirl jets by Oberleithner et al. (2012) assumes appearance and growth of a permanent central recirculation zone with increase of the swirl intensity. The recirculation zone give rise to a global helical instability mode, corresponding precessing vortex core and the secondary helical vortex filament in the outer mixing layer. Developing nowadays volumetric velocimetry techniques, such as tomographic PIV (Scarano et al. 2013), can provide deeper insight into this issue. In the recent paper by Ceglia et al. (2014), 3D instantaneous velocity fields were measured by tomographic PIV in a non-reacting swirling flow in a model combustor of aeroengine. When the flow was not confined by the chamber, a third vortex structure around vertical axis could be detected in the POD reconstruction. Note that similar third vortex was detected in POD reconstruction from 2D PIV data by Stöhr et al. (2012) and was defined as exhaust tube vortex. From analysis of POD reconstruction and the instantaneous 3D velocity patterns in a strongly swirling jet, Alekseenko et al. (2013) concluded that a strong helix was present in the outer mixing layer along with two


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vortices inside the inner shear layer. The POD reconstruction resolved the outer helix, but smoothed the inner two helices into a single structure. Consequently, the latter work supports triple vortex structure of coherent fluctuations in swirling jet with vortex breakdown and explains why the POD reconstructions resolve only a double helix. The aim of the present study is to investigate 3D vortex structures in an open swirling turbulent jet and in premixed flame by using the tomographic PIV technique. The focus is placed on analysis of the large-scale vortices in the high-swirl flows. 2. Experimental setup and data processing The reacting and non-reacting jet flows were organized by a nozzle with the outlet diameter of 15 mm. The bulk velocity of the air flow was 5 m/s. A swirler was mounted inside the nozzle to produce high-swirl flows. The swirl rate is qualified as high, because the swirler produced jet flows with pronounced vortex breakdown and permanent central recirculation zone (in the non-reacting case). According to definition by Gupta et al. (1984) the swirl rate was equal to 1.0. To provide PIV measurements, the flow issuing from the nozzle was seeded by 4 µm Al2O3 particles. In case of the reacting swirling jet flow study, the equivalence ratio of the methane-air mixture was φ = 0.7. In the present study, a set of eight cameras was used to get reliable accuracy and spatial resolution of the tomographic PIV measurements in flames.

Fig. 1. Photograph of the experimental setup and scheme of laser beam passage A system of 8 cameras was mounted as shown in Figure 1. The bottom row of ImperX IGV-B2020 cameras was oriented horizontally (with the angles of −35º, −11º, 11º, and 35º, relatively to the horizontal normal of the mounting rail). The ImperX IGV-B4820 cameras in the top row observed the flow with same horizontal angles, and with nonzero vertical angles (−18º and −22º for the side and central cameras, respectively). The cameras were equipped with Sigma AF #50 lenses and band-pass optical filters (10 nm full-width at half maximum) by Edmund Optics with 60% transmittance at 532 nm. The seeding particles were illuminated by the second harmonic of a pulsed Nd:YAG double-head laser (Quantel EverGreen 200) with 200 mJ energy per each pulse. Two mirrors were used to organize multi-pass scheme for the beam (in a manner similar to Schröder et al. 2008, Ghaemi and Scarano 2010), as the sketch in Figure 2 demonstrates. The depth of the illuminated volume was 45 mm, but the tracer particles issued only from the nozzle. For each camera lens the aperture number was set to maximum (viz., #32) to provide large enough depth of field for the measurement volume of 37.5×37.5×57.4 mm without Scheimpflug correction. For each camera 4 Mpix

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images were captured.

Fig. 2. Sketch of volume illumination For calibration of the optical system, a plane calibration target (Edmund Optics, see Figure 3) was mounted above the nozzle. The nozzle and target were moved by a traverse system. The images were captured for the range of ±20 mm with the step of 4 mm. The cameras were calibrated by using a pinhole model from OpenCV library by Bradsky and Kaehler (2008). A self-calibration procedure (similar to that by Wieneke, 2008) was applied to align all camera models by using the experimental particle images and to get a perfect multiple ray correspondence through the measurement volume. The residual of the average disparity after three iterations of the self-calibration procedure was about 0.05 pixels with 0.15 pixels of the standard deviation.

Fig. 3. Calibration target images for the central plane.

The 3D images were reconstructed and processed by hybrid CPU-GPU realizations of MLOS-SMART (15 iterations) and MTE (Novara et al. 2010) algorithms. The server station with 2x16 AMD Opteron processors 6274, 2200 MHz (32 cores in total) with the graphics processor NVIDIA Tesla C2075 was used for the calculations. Figure 4 shows the 3D image intensity, averaged over x and z directions, for the non-reacting jet flow. The peak corresponds to the first passage of the beam and is located outside the jet flow. As can be seen the reconstruction contrast in the jet increases sufficiently when all eight cameras are used for the reconstructions. The normalized intensity variance (Novara and Scarano 2012) of the 3D images for these cases is equal to 18.5 and 12 for 8 and 4 cameras, respectively. An iterative cross-correlation routine with continuous volume shifting and deformation was applied to

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estimate 3D velocity fields. The final size of the interrogation volume was 403 voxels with 75% overlap factor.

Fig. 4. 3D intensity averaged over x and z coordinates .

3. Results To estimate images distortion introduced by the flames, the printed image with a random dot pattern was placed behind the nozzle (70 mm away from the central axis). To calculate the displacement of the background dots, a 2D cross-correlation algorithm was applied. For the laminar non-swirling flame, the result is shown in Figure 5. The maximal distortions, as it was expected, were caused by the flame front and by the region between the hot products of combustion and ambient air. The misalignment of the lines of sight is up to 1 pixel in these regions.

Fig. 5. (a) Direct image of the conical propane-air flame (φ = 0.9). (b) background dot image and (c) estimated dots displacement between the reacting and non-reacting flows.

In Figure 6b the reconstructed with 2 iterations of MLOS and15 iterations of SMART 3D image volume (768×768×1150 pixels in size) is shown. Density of the seeding particles decreases dramatically after the flame front, since the fluid density decreases. Nevertheless, the iterative cross-correlation routine, applied to estimate 3D velocity field, provides reasonable results (Figure 6c). Namely, the more-or-less uniform vertical velocity is resolved inside the cone and streamlines deflection due to the combustion takes place in the flame front. The data for the laminar conical flame was obtained by using only the bottom row of the cameras.

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Fig. 6. (a) Direct image, (b) reconstructed 3D volume flame (10% of voxels are shown) of tracers particles and (c) time- velocity field for a laminar premixed the conical propane-air flame (φ = 0.9)

The average over 35 snapshots and the instantaneous 3D velocity fields for the swirling non-reacting jet are shown in Figure 7 and 8, respectively. The data are obtained by using the set of eight cameras. Vertical cross-plane and the central recirculation zone (negative axial velocity Uz inside the light-blue surface) are shown in Figure 7a. Figure 7b plots horizontal cross-planes and two surfaces of constant positive values of Q criterion (greyscale) and squared axial vorticity component Ωz

2 (light-red). The derivative data were smoothed by 5×5×5 Gaussian filter (with full width of 40 voxels at half maximum). The former criterion (see definition in Jeong and Hussain, 1995) is useful to visualise 3D large-scale vortices in the instantaneous velocity fields, while the latter one is necessary to highlight the vortex core of the swirling jet. The horizontal planes demonstrate that through the whole thickness of the measurement volume the tomographic PIV resolves well the direction of the flow rotation, viz. annular swirling jet around the central recirculation zone.

Fig. 7. Cross-planes of the average 3D velocity distribution in a non-reacting high-swirl jet (Re = 5000, S = 1.0). (a)

Central recirculation zone is shown by Uz = 0. (b) Vortex core is visualized by positive Q criterion and Ωz2

From the analysis of the instantaneous velocity fields and distributions of Q criterion for the non-reacting flow, it was found that most of realisations included three long filaments of large-scale vortices. One helical vortex was in the outer mixing layer. Another helix was around the reverse flow region, i.e., in the inner shear layer. There was also third long vortex filament in the region with the negative axial velocity (see the

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red surface that corresponds to a modulus of the squared axial vorticity component). This vortex is determined as the core of the swirling jet.

Fig. 8. Cross-planes of the instantaneous 3D velocity distributions in a non-reacting high-swirl jet (Re = 5000, S = 1.0).

(a) Central recirculation zone is shown by uz = 0. (b) Vortices are visualized by positive Q criterion and ωz2

The direct image of the swirling methane/air flame is shown in Figure 9a. The optical distortions produced by the flame were evaluated in the same manner as it was done for the non-swirling conical flame. The strongest effect up to 1 pixel takes place around the flame front. For the region inside the flame, the effect corresponds to a uniform vertical shift of the image.

Fig. 9. (a) Direct image of the swirling methane/air flame (Re = 5000, φ = 0.7, S = 1.0). (b) Estimated background displacement between the reacting and non-reacting flows.

The instantaneous velocity snapshot for the reacting flow is presented in Figure 10. The size of the central recirculation zone decreases and it shifts downstream. This is probably caused by the sufficient smoothing effect for the annular swirling jet near the nozzle exit. In contrast, visualisation of the large-scale vortices by Q criterion shows large-scale structures only near the nozzle. This is explained by the fact that the density is not accounted for by the used criteria. Thus, a fixed threshold for them is a biased visualisation way. Nevertheless, double vortex structure, consisting of two helices (one is in the inner shear layer, and another

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one is outside the flame) could be detected.

Fig. 10. Cross-planes of the instantaneous 3D velocity distributions in a high-swirl methane/air flame (Re = 5000, φ = 0.7, S = 1.0). (a) Central recirculation zone is shown by uz = 0. (b) Vortices are visualized by positive Q criterion and

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4. Conclusions Non-reacting swirling air jet flow with vortex breakdown and flow of the fuel-lean methane/air flame were studied by means of a Tomographic PIV system, based on eight cameras. 35 snapshots of 3D velocity fields were processed for each flow case. The performance of the system to resolve the flow trough the thick measurement volume (about 40 mm) was also tested for a non-swirling laminar conical flame. The data support that the successful tomographic PIV measurements can be performed in premixed flames for such thick volumes. However, the deviation of the lines of sight can be up to 1 pixel. The measurements reveal that for the high-swirl non-reacting flow, the vortex core co-existed with the pair of counter-rotating spiral vortices, which were located in the shear layers. Each vortex of the pair, and the whole double-helix structure winded in the opposite direction to the sense of the flow swirl. The vortices in the shear layers were convected by the flow, and the double-helix structure rotated in the same sense as jet swirling. This double-vortex structure was also detected in the swirling flame. Acknowledgements Development of tomographic PIV was supported by the European Community 7th Framework programme (FP7/2007-2013) under the grant agreement No 265695. The experiments were funded by RFBR (grant No 12-08-33149). Dmitriy Sharaborin and Aleksey Lobasov are kindly acknowledged for their work during the experiments. References Alekseenko SV, Dulin VM, Kozorezov YS, Markovich D.M. (2012) Effect of high-amplitude forcing on turbulent combustion intensity and vortex core precession in a strongly swirling lifted propane/air flame. Combust. Sci. Technol. 184: 1862–1890 Alekseenko MV, Bilsky AV, Dulin VM, Markovich DM, Tokarev MP (2013) Tomographic PIV measurements in a swirling jet flow. Proc. 10th Int. Symp. on Particle Image Velocimetry, 1-3 July 2013, Delft, The Netherlands

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