Large Coherent Structures Visualization in a Swirl Burner ...ltces.dem.ist.utl.pt/lxlaser/lxlaser2008/papers/09.2_3.pdf · A DI 2200 FFT analyzer was used in order to recognize harmonics

14th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2008

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Large Coherent Structures Visualization in a Swirl Burner

Agustin Valera-Medina, Nicholas Syred and Anthony Griffiths

School of Engineering, Cardiff University, Wales, United Kingdom, [email protected]

Abstract The consumption of fossil fuels and their greenhouse emissions have increased research to develop new mechanisms for the generation of energy and a variety of industrial processes. However, many of the mechanisms under scrutiny have only been measured by indirect simulations. Swirling flows represent this category. Even thought they have been extensively analyzed, there are many uncertainties concerned to their behavior, especially those related to coherent structures and their relationship to the Central Recirculation Zone (CRZ), which is responsible for enhanced mixing and combustion stability. Although extensive programs on the coherent structures generated have been developed, structures such as the Precessing Vortex Core (PVC) and indeed the CRZ remain barely understood. Recent work using numerical simulations (DNS, LES, etc.) have predicted relationships between the PVC and CRZ. The problem lies in the lack of detailed experimental data to validate the results of the interaction, since the measurement has only been done using indirect techniques. Therefore, this paper adopts the approach of producing direct fundamental data on these structures in order to visualize the phenomenon under different conditions. Here the effects of combustion are ignored. Phase Locked Particle Image Velocimetry (PIV) provides results about the interaction of the PVC and CRZ, giving details of shape and dependence on non-dimensional parameters. Various cases were analyzed to find operational regions where strong perturbations occurred, which lead to the selection of cases that were inspected in detail. 3D holograms were produced in MatLab showing the real spatial interaction of these large structures. Shapes and interactions between different structures are compared and discussed in order to detail the relationship shared between different cases. The most stable and recognizable configuration was analyzed in detail under unconfined and confined conditions. The use of different triggering levels confirmed the accuracy of the technique, with implications of using different signals filters for future projects. No bifurcations or major perturbations were observed during the process, but the appearance of new structures made evident the high correlation between the geometry and type of flow in the burner. The results refuted the normally assumed helical shape of the PVC with a complex spiraling mechanism being revealed. The CRZ was shown in fact to consist of two normally separate, but intertwined CRZs both of which interacted with the PVC.

1. Introduction

Although swirling flows have been extensively studied and characterized in order to increase the

efficiency in burners and reduce emissions (Stambler 2006; Gupta et al. 1984; Syred et al. 1974;

Syred 2006), their highly complicated mechanisms and turbulent nature have made them difficult to

follow and virtually impossible to visualize, specially for those flows where the conditions are

highly transient (Gardner et al. 2005). This inconvenience has lead to simulations where the data

have only been corroborated by indirect techniques, avoiding a direct visual approach.

The use of advanced visualization techniques in fluid mechanics goes back to the 1980’s, where

the techniques became available for flow characterization. The use of techniques like Laser Doppler

Anemometry and Particle Image Velocimetry has been extensively applied for the acquisition of

data for swirling flows (Froud et al. 1995; Syred et al. 1994). The results have shown how these

flows increase combustion stability, decrease combustor size, avoid flashback and improve mixing

of reactants in order to reduce emissions and increase power density. Nevertheless, the visualization

of the former showed traces of structures previously recognized with indirect methods. One of these

structures is the Precessing Vortex Core (PVC) whose influence and relation with the Recirculation

Zone (RZ), where the mixing process takes place, has been analyzed but barely understood (Fick


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1998; Valera-Medina 2006; Froud 1992; Yazbadani 1995). Features about this zone and mixing

potential have been investigated superficially (Fick et al. 1997; Aleseenko et al. 1999), leaving

details as their effect in the mixing process, stability and coupling with other structure still uncertain.

Recent work with Large Eddy Simulation (LES) and Direct Numerical Simulation (DNS)

methods has indicated the occurrence of spiral structures in the flow confirming experimental data

from several sources (Freitag et al. 2006; Selle et al. 2006). Under such circumstances it appears

that the whole of the swirling (combusting) flow is wobbling, after the appearance of the PVC takes

place. It is not clear when the flow changes to a PVC, however, the simulation shows traces of a

helical structure, which as previously specified, has never been directly confirmed. Therefore, a

major problem is that the simulations and similarity modeling are being carried out on systems

where there is no detailed experimental data on the time dependent flow characteristics of the

predicted combustor, which is fundamental for validating these models.

The importance of these flows is their promising behavior to enhance combustion and mixing

when using both natural gas and high hydrogen content fuels. The use of fuel with calorific values

as low as 1.3 to 1.4 MJ/m3 have proved to be commercially possible (Syred et al. 1977), while the

burning of syngas and blends of hydrogen and methanol have attracted the attention of the research

community to reduce pollution and enable CO2 sequestration to efficiently occur (Ibas et al. 2006).

This paper thus adopts the approach of producing fundamental data on these large coherent

structures using a swirl burner that has been extensively characterized by Syred et al. (1974, 2006)

and Valera-Medina (2006). The use of Phase Locked Particle Image Velocimetry was the selected

technique in the visualization of the flow due to its reliability and high speed of acquisition. Since

the study would require vast amounts of frames per configuration, the use of the former not only

allowed the establishment of structures, but also information concerning the appearance of the latter

in a relatively fast manner.

2. Experimental Study

The experimental study was focused on a 100 kW Perspex version of a 2 MW swirl burner,

which has been extensively characterized. Two tangential entries together with variable inserts in

the entries allowed the measurement of different swirl numbers and examination of their effects on

the PVC. The system was fed by a centrifugal fan providing air flow via flexible hoses and a bank

of rotameters to measure the airflow rate as shown in figure 1.

Use of different inserts for the tangential inlets enabled a wide range of different swirl numbers S

to be characterized, ranging from 0.659 to 3.7. The variety of flow rates provided a range of Re

from 5,700 to 61,000. The opportunity was also taken to investigate the effects of uneven inlet flow

distribution by examining cases where the size of inlets was different in the 2 tangential inlets. The

inserts were designed to block 50% and 25% of the inlet area, allowing an inlet transversal area of

50% and 75% respectively. Another configuration at 0% blockage (no inlet) was used, providing 6

different configurations. Another 3 configurations were obtained turning off one of the rotameters

and keeping the other on with one of the mentioned inserts, giving a total of 9 configurations. The

rig was mounted on a stand with independent control of airflow to each inlet via valves and

rotameters.

Hot Wire Anemometry was employed to trigger the system. A DI 2200 FFT analyzer was used

in order to recognize harmonics of the flow. High Speed Photography was utilized to establish


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traces of the phenomenon, although no relevant results were found with this technique. The use of a

Dantec Phase Locked Particle Image Velocimetry (PIV) system was utilized to gather information

about the field. The system was triggered using the conditioned signal from a Hot Wire

Anemometer. The former was directed to trigger a BNC Model 500 Pulse Generator (PG), which

was set at different voltage levels to trigger the PIV system. A Nd:YAG Litron Laser of 532 nm, 15

Hz and a Hi Sense MkII Camera with a 60mm Nikon Lens were employed for resolution purposes.

The software used to analyze and interpret the results was FlowManager v 4.71 by Dantec. The

entire configuration can be seen in figure 2. A Vivid V-1 Water Nebulizer was used to seed the flow

for both experiments. The particle average size ranged between 2 to 5 µm, sufficient for particle

tracking.

After the analysis of unconfined cases was finished, the use of an enlarged ‘can’ allowed the

experimentation under confined conditions. The aim was to gather information about the behavior

of the flow using three different obstructions at the end of the can (burner); A) Open Burner (no

obstruction), B) Pyramidal Obstruction, length 1D and 1D2 square transversal area exit and C)

Sudden Obstruction, flat shape, 0.5D circular exit.

3. Results and Discussion

Once the flow had been characterized and interesting structures identified for further study based

on their behavior by Valera-Medina et al. (2008), the Phase Locked PIV system was used to obtain

a detailed visualization of the phenomena. The relations between the PVC and the reverse

flow/Recirculation Zone has been thought to be very close. Nevertheless, this relationship has never

been experimentally quantified. Moreover, the visualization of the phenomenon was conceived as a

Fig. 3 Confined cases with different obstructions

Fig. 2 PIV System Configuration

Fig. 1 Perspex Swirl Burner Model

A B C


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hologram formed by the superposition of various sections one over the other, simulating a

tomography of the flow. In the axial direction, images in the radial tangential plane over the region

were taken where the reverse flow zone and PVC were known to exist. Typically 40 images at 80

different vertical sections (separated by 3.25 mm) were obtained so as to be able to produce

averaged data at each section, although it was recognized that this was probably statistically

insufficient. This technique is based on the acquisition of data at certain triggering points, which

come from the TTL signal using the Pulse Generator. This generates the timing pulse using the

gradient of the strong PVC signal. The Pulse Generator was empirically set up at 90% of the highest

peak value recorded in the oscilloscope measurements over 5 minutes after the warming up of the

system. All the frames obtained were subject of a statistical procedure, using the available software

for the PIV system. The resulting sections were analyzed using a MatLab program which created a

volumetric matrix from the low velocity region of the area of interest. This was then smoothed and

covered by an isosurface, giving the desired holographic images. Figure 4 shows the procedure. The

total measured length was 3.175D, which covered the area where the phenomenon was thought to

take place. 40 frames were framed in pairs, with a delay of 20 µs between each pair, as required by

the PIV system to recognize displacement and velocity of the seeding particles.

After masking some planes to avoid extensive black zones, a correlation technique between

images was carried out at 32 x 32 pixels, with an overlap of 50% to reduce the noise caused by this

technique. In order to locate the position of low

velocity areas, a velocity filter was used over a

range of 3 m/s, leaving all those vectors up to this

value. Nevertheless, due to problems with the

algorithm per se, other filters were used to avoid

noise problems. This is recognized to be a problem

that can be solved by increasing the number of

frames per section.

From the analysis, a contour map had to be

created to distinguish the real shape of the flow

field in 3 dimensions. 30 colours were used to

Fig. 5 Hologram of PVC

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300pix

0

100

200

300

400

500

600

700

800

900

1000

pix

0.0 0.8 1.6 2.4 3.2 4.0 4.8 5.6 6.5 7.3 8.1 8.9 9.7 10.5 11.3

Signals of the System.

PIV Results

Low Velocity

Region

Overlaped Regions

Hologram

Fig. 4 Hologram Analysis


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describe velocities from 0 to 3 m/s. Each section is plotted in a 3D plane with the aim of acquiring a

higher correlation between each of them.

Each case was analyzed using more than 3,200 images. The results show the existence and a

strong correlation between the PVC and the CRZ, where the former is defined by the off centered

region of low velocity. Differences in cases were marked by their geometry and flow regime. After

the superposition of all the sections, a rendering process of the results was carried on to enable the

visualization of solid bodies. Figure 5 shows a typical hologram.

Afterwards, detailed phase locked data was obtained in the axial radial plane with sections

spaced 11.25°. Typically up to 40 images were recorded for each measurement bin as well. The

analysis was conducted comparing the value of V (or the axial velocity), in contrast to the

horizontal case, where the total length ([V2+U

2]1/2

) of each vector was taken into consideration. By

careful analysis of this data it was possible to define a boundary for the PVC and for the reverse

flow zone and determine their interrelationship. A velocity filter was used in the narrow range

between -0.200 to 0.200 m/s to recognize the former. Figure 6 shows the results at section 90°. The

analysis was repeated for every vertical section.

Inside of this boundary all the vectors were recognized to have a negative value, while those in

the outside were positive. This confirmed the existence of the reverse flow zone/recirculation zone.

Every case showed similar traces and the existence of the RZ.

The inner zone where the recirculation zone was located was analyzed using streamlines based

on the software results. The strongest areas showed traces of one major eddy, while those

positioned close to the former presented the existence of two or three eddies. This was linked to

Richardson’s cascade theory (Davidson 2004). The CRZ appears to be formed by a turbulent flow

which decays into smaller eddies. Smaller eddies appear in the weakest areas of the reverse flow

zone and obviously confirm the turbulent nature of the phenomenon. Figure 7 shows the bifurcation

and appearance of two smaller eddies in the core of the reverse flow zone.

3 dimensional representations of the PVC and the reverse flow zone have been brought together

to illustrate their interactions and dependencies. Figure 8 shows the result for the most stable case at

S = 0.985 and Re~26,700. It is observed that the PVC and CRZ co-exist, interacting between each

other. Contrary to LES results there is little twist to the single PVC visualized, which seems to sit

quite precisely around the reverse flow zone boundary. Initially the PVC diameter is quite small at

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300pix

0

100

200

300

400

500

600

700

800

900

1000

pix

Statistics vector map: Range, 83×63 vectors (5229)

Size: 1344×1024 (0,0)

Fig. 6 PIV vectors between -0.200 and 0.200 m/s in

axial plane

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300pix

0

100

200

300

400

500

600

700

800

900

1000

pix

Streamlines: Streamlines

Fig. 7 Streamlines of the flow. Visualization of the

RFZ core


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the burner exit, it then expands and appears to contract a little as it passes over the top of the reverse

flow zone. The PVC and the reverse flow zone appear to rotate together and be closely related,

possibly the PVC is distorting the reverse flow zone into the more elliptical shape. Figure 8 also

confirms several visualization studies of the PVC where gas has been persuaded to burn on its

boundary for visualization purposes and little PVC twist has been shown by Froud et al. (1995) and

Syred et al. (1975). The coherence of the PVC structure at the top of the reverse flow zone has not

been noticed before as the assumption has usually been made that it breaks up here. This is because

of the use of burning gas inside the PVC to improve visualization of the phenomenon (Gupta et al.

1984).

The appearance of bifurcations or projections in the flow was never seen. This could be

attributed to the statistical analysis which erases every trace of these projections if their appearance

was irregular and sporadic. There is also the problem of resolution that has decreased the final

quality of the results. Therefore, another set of experiments was carried out changing the triggering

level. The most stable [25-25] configuration, i.e. 25% blocked in each inlet, was again analyzed at

90%, 67%, and 34% positive and negative slope, of the highest peak recorded. The objective was to

find weak coherent structures. In order to minimize the resolution problem found using only 40

images, this time 150 images were obtained per section. This was the most stable case, confirming

findings by Syred (2006), Fick (1998) and Valera-Medina (2008), confirming the higher resolution.

The results of the hologram at 90% of the highest

peak can be seen in figure 9. It is evident in this

hologram, which behaves similar to those obtained

previously, that the PVC is spiraling for about 70-90

degrees downstream from the burner exit. The higher

quality of the results is evident when compared to figure

5. However, it stops this spiralling process and remains

steady after ~0.45-0.6D. Since no filtering was used, this

undoubtedly gives a better representation of the actual

flow field.

The same results were found at different triggering levels. The only noticeable difference

between experiments was the change in position of the PVC and the slight enlargement of the

vortex due to the variation of peaks. Nevertheless, no other structure, bifurcation or projection was

visualized in the process.

Upstream of section 0.366D a complex phenomenon suddenly appeared in all the measurements

at different triggering levels. Immediately after the strongest region of the CRZ, an inner vortex

inside the reverse flow zone started to form. Downstream, this vortex gained enough energy to

Fig. 8 Dimensional interaction of the PVC and CRZ. Top, left and right views respectively

Fig. 9 Detailed PVC, configuration at 90%

triggering level


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become similar in magnitude to the PVC which suddenly joined the former as soon as this

happened. From this point, the CRZ never regained its previous strength and the PVC stopped

spiraling around it. It is in this section where the PVC becomes steady, at ~0.6D. Figure 10 and 11

show the union of two vortices.

Something noticeable about the resulting sections was that the CRZ shape was smoother than in

the previous analysis due to the higher resolution of the results. Moreover, this CRZ was non

uniform, very narrow at the bottom, wider at the centre, whilst forming an off centred pyramid at

the top, figure 12. The widest dimension at the centre was 0.51D. The axial measured length was

2D.

The results from both axial and tangential

analysis were gathered to show the total flow and

the correlation between these structures. Analyses

were performed first on the resulting hologram of

the CRZ, figure 12. The lower and upper regions

of the flow present a non uniform shape, while the

middle is squeezed forming a canal in which the

PVC fits. The area is highlighted in.

The previous hologram and the position of the

canal were consistent with these finding and data

obtained from the tangential analysis. Therefore, it is believed that this is the region where the

helical region of the rotating PVC fits. The vortex is spiraling around and rotating with the CRZ,

whilst the interaction deforms the CRZ. Past the end of the canal in the CRZ, the PVC stops

spiraling and moves upwards, but still being attached to the CRZ, figure 13. The results indicate

very strong initial interaction between the PVC and CRZ with the PVC forcing the CRZ to rotate

with it. This is sufficient to retard the spiraling nature of the PVC so that by 0.5 to 0.6D the PVC is

straightened, but is still attached to and interacts with the CRZ over its remaining length (total~2D).

Comparison of figure 8 and figure 13 shows the advantages and smoother results obtained by

increasing image numbers to 150.

The next step was the use of the square confinement to the exit of the swirl burner. The use of an

open confinement, figure 3a, showed the existence of a new separate recirculation zone (CRZ2) in a

region close to the wall of the burner exit, unattached to the rig and opposite to the main CRZ

(CRZ1). The development of CRZ2 was followed using the same technique to obtain vertical

sections. Each of the latter showed traces of the separate recirculation zone, varying in size, position

Fig. 10 Appearance of a second vortex inside of the

CRZ, Section 0.447D

Fig. 11 Union between the PVC and the inner vortex

of the CRZ, Section 0.488D

Fig. 12 Recirculation Zone hologram. The

intermediate canal is highlighted


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and relation to the CRZ1. In some cases, it

was completely attached to the CRZ1, whilst

in other sections not so. In some sections

there were no traces of CRZ1. Figure 14

shows the development of CRZ1 and CRZ2

at different axial radial sections.

After gathering all vertical sections and

analyzing them with the MatLab software, it

was clear that the separate small recirculation

zone was a distinct CRZ(2), separate from

CRZ1. CRZ1 and CRZ2 are thus here

recognized as completely separate structures,

both of which interact with the PVC; the total

flow is seen from 3 different perspectives in figure 15. There are regions where CRZ1 and CRZ2

boundaries are ill defined and this region has been left opened, see first diagram in figure 15.

Possibly CRZ1 and CRZ2 are one very complex, 3 dimensional time dependent CRZ, this will need

even higher resolution to determine.

One structure frequently mentioned in the literature by Syred (2006), Valera-Medina (2006) and

Ibas et al. (2006) is an external eddy or CRZ (ECRZ) formed in the corner region where the burner

flow expands into the confinement. This is a structure that appears in this experimental study as

well. All sections have shown traces of the latter, and its toroidal shape around the exit of the swirl

burner is evident. Therefore, no holographic visualization was performed over this structure, as the

focus of this work was the CRZ and interactions with the PVC. Work done by Syred (2006) has

shown that the ERZ is not essential for flame stabilization and indeed is best eliminated by for

instance burner quarls to minimize combustion driven oscillations. Downstream of the CRZ near

the centre line the axial flow is of low velocity as the shear flow tends to stick in the wall region,

this is in accord with Syred (2006) and Gupta et al. (1998). The CRZs are clearly formed by the

radial pressure gradients caused by the swirling flow; their axial decay then causes the formation of

the CRZs. The highest rates of velocity decay of the swirling flow leaving the swirl burner occurs

with the freely expanding flow as external air is entrained; any form of exhaust confinement to the

swirl burner inevitably reduces this decay and makes the configuration of the CRZ much more

complicated as just described.

The analysis shows that the strength of the PVC seems to decrease in some sections downstream.

There is a point where the PVC completely vanishes into the low velocity reversed flow region

forming CRZ2. It is thought that the PVC is still present in the field, due to indentations in CRZ2,

which indicate a channel maybe formed by the PVC. There is also the visualization of an extremely

slow region in CRZ2 that is close to the point where the PVC was expected to be present. However,

Fig. 14 Different sections show CRZ2. A) CRZ1 and CRZ2; B) CRZ1 and CRZ2 merged; C) CRZ2 only

Fig. 13 The total flow. 90% triggering level

A B C


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no conclusive results on the real length of the PVC are available, and a more in depth study is

required. Nevertheless, it is confirmed PVC suppression compared to other cases is considerable.

The final flow is shown in figure 15, after the MatLab software was used to obtain the structures in

the field. CRZ1 and CRZ2 have a gap between them, see first diagram figure 15. This is an

indeterminate region which needs further higher resolution results.

The same technique was used with a pyramidal exit to the confinement, figure 3b. This

configuration made significant changes to the relationship between CRZ1, CRZ2 and the PVC,

figure 16 shows the total flow for the three structures. CRZ1 increases and CRZ2 decreases in size

somewhat. The PVC is much more pronounced, but again only spirals around the CRZ1 for ~0.4D,

then straightening and punching a hole through the middle of CRZ2. CRZ1 now extends back into

the swirl burner exit, part is clearly attached to the PVC at the rig exit; it also nearly extends as far

downstream as CRZ2. CRZ2 is still quite large but is lifted well away from the swirl burner exit.

Again there are regions of uncertainty where it is not clear whether or not the two structures are

joined together. After passing through CRZ2 the PVC continues upwards as a coherent form into

the pyramidal exhaust. Indeed depending on expansion conditions past this exit a second vortex

breakdown and PVC may form according to Syred (2006).

Finally, another study using a sudden exit of 0.5D to the confinement, figure 3c, was analyzed

using the same technique. A total flow hologram was assembled as before, figure 17.

Fig. 16 Real Flow seen from three different views. CRZ1 (purple), CRZ2 (yellow) and New Vortex (blue)

confirm the relation between structures

Fig. 15 Real Flow seen from three different perspectives. The CRZ1 (purple), CRZ2 (yellow) and PVC (blue)

confirm this complex structure

Fig. 17 Real Flow seen from three different views. CRZ 1(red) and PVC (blue) confirm the relation between

structures. Possible residual existence of CRZ2 is highlighted in green


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Here the reduced exit diameter has decreased the axial decay of radial pressure gradients and

hence the size and extent of the CRZs. Even so a large CRZ1 is shown, extending down into the

swirl burner exit. The PVC and CRZ1 are clearly attached for much of the initial length of CRZ1.

CRZ1 is considerably twisted and three dimensional; the region highlighted by the green circle

shows an appendage to CRZ1 which is probably the remnants of CRZ2, here it is clearly joined to

CRZ1. The PVC has some slight initial spiraling motion with CRZ1, then sharply straightens itself,

whilst separating from CRZ1, and passes as a coherent small vortex core into the final 0.5D exhaust

of the confinement.

4. Conclusion

A large number of experimental tests have been conducted in an isothermal flow regime to

analyze the effects of Swirl and Reynolds numbers on the PVC and corresponding RZ. Holographic

images have been developed from the experimental data that pictorially show the PVC and RZ

interaction. The interrelation between structures and their dependency on flow rate and

experimental geometry was confirmed. Under confined conditions, the existence of two central

recirculation zones, which interact with each other and the PVC has been shown. The extent of all

three structures has been shown to be dependent on the type of exhaust fitted to the confinement

added to the swirl burner. The use of phase locked PIV systems in this type of flow has shown new

information that can be derived and indicated possibilities of considerable work that needs to be

done in the future to characterize other flow features in these highly turbulent, three dimensional

time dependent flows.

5. Acknowledgments

Agustin Valera-Medina gratefully acknowledges the receipt of a scholarship from the Mexican

Government (CONACYT) and the assistance of Paul Malpas and Mario Alonso during the setup of

the experiments.

6. References

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Mechanics 382:195-243

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Fick W (1998), Characterization and Effects of the Precessing Vortex Core in Swirl Burners. PhD

thesis, Cardiff School of Engineering, October

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Documents

Large Coherent Structures Visualization in a Swirl Burner ...ltces.dem.ist.utl.pt/lxlaser/lxlaser2008/papers/09.2_3.pdf · A DI 2200 FFT analyzer was used in order to recognize harmonics