12th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan (Online), July 19-22, 2022

INVESTIGATIONS OF VORTEX SHEDDING AND FLAME BASE
FLUCTUATIONS IN A DUAL SWIRL BURNER USING SPIV, OH PLIF AND
CHEMILUMINESCENCE

Masayasu Shimura
Department of Mechanical Engineering
Tokyo Institute of Technology
2-12-1 Ookayama, Meguro-ku,
Tokyo 152-8550 Japanshimura.m.aa@m.titech.ac.jp

Hiroki Hattori
Department of Mechanical Engineering
Tokyo Institute of Technology
2-12-1 Ookayama, Meguro-ku,
Tokyo 152-8550 Japanshimura.m.aa@m.titech.ac.jp

Mamoru Tanahashi
Department of Mechanical Engineering
Tokyo Institute of Technology
2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550 Japanshimura.m.aa@m.titech.ac.jp

ABSTRACT

ducted laser Raman scattering and reported that the unreacted
or partial-reacted gas exists in the same region because of local
flame extinction and ignition delay. Stöhr et al. (2012) haveconducted particle image velocimetry (PIV) and OH PLIF at 5
kHz and found that precessing vortex core (PVC) induces anunsteady lower stagnation point. Oscillation characteristics ofthe dual swirl burner was investigated using large eddy simulation (LES) Chen et al. (2019) and it is revealed that variationsof air mass split rate between dual swirl flows are significant
on fuel-air mixing characteristics in thermoacoustic oscillation
cycles. Recently, Litvinov et al. (2021) have investigated mixing and reaction characteristics in the dual swirl burner withseparately supplied air flows by using time-resolved stereoPIV, OH-PLIF and acetone-PLIF and revealed that PVC anddouble helical vortex are generated in inner and outer shear
layer and cause flame roll-up, mixing of burnt and unburnt
gases and subsequent ignition both in the inner and outer shear
layers. PVC and fluctuations of recirculation zones in swirl
burners have been studied. Oscillation mechanism due to PVC
were reviewed by Syred (2006).
In many previous studies, the ratio of two swirl flow rates
has not been set arbitrarily, and hence effect on the characteristics of turbulence and mixing of air and fuel in the shear
layer have not been well discussed. Therefore, in our research
project, a dual swirl burner in which the ratio of two swirl flow
rates is adjustable is constructed. The objectives of this study
are to clarify the effects of the air flow ratio on the characteristics of flame base fluctuation and a major factor causing flame
base fluctuation by using stereoscopic PIV, OH-PLIF, and OH
chemiluminescence measurement.

Simultaneous high-speed measurements of stereoscopic
particle image velocimetry (SPIV), OH planer laser induced
fluorescence (OH-PLIF), and OH chemiluminescence wereconducted in order to investigate relationship between fluctuation of flame base and flow structure in a dual-swirl burner,
in which peak frequency of fluctuations of flame base changes
according to equivalence ratio and the ratio of outer and inner
air swirl flow rates. Flame base is defined by height of upstream edge of OH chemiluminescence. Spectral analysis on
temporal data of flame base fluctuation and velocity revealed
one of major factors of flame base fluctuation. The flame baseis located in the shear layer between the inflow gas and theinner recirculation zone (IRZ). The velocity spectra show thatthe mainstream and radial velocity components have the same
peak frequency as fluctuations of the lift-off height in the same
locations. It is also found that the peak frequency is proportional to the magnitude of the mean velocity gradient at themean flame base position. Finally it is found that the Strouhal
number based on characteristics of shear layer between inner
swirl flow and inner recirculation zone was found to be constant regardless of the total flow rate and the flow rate ratio.

INTRODUCTION
For the gas turbine combustors utilized widely as propulsion engines and power generation systems, non-premixed andpartially premixed combustion are widely used because of norisk of flash back and easiness of control. Some of gas turbine
combustors have coaxial dual swirl flows, which is called dual
swirl burner hereafter, to stabilize flame and to enhance mixing of fuel and air. For the DLR dual swirl burner, Weigand
et al. (2006) conducted laser Doppler velocimetry (LDV) and
OH/CH-planar laser induced fluorescence (PLIF) and revealed
that intense velocity fluctuations occur in the shear layer between the inner recirculation zone (IRZ) and influent gas andreaction zone are strongly corrugated. Meier et al. (2006) con-

EXPERIMENTAL SETUP
Figure 1 shows a schematic of dual swirl burner used inthis study. The nozzles are co-axial triple cylinders for air,methane, and air in order from the inside. Both of the two
air nozzles are equipped with swirlers whose blade angle is 45

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12th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan (Online), July 19-22, 2022

Figure 2. Experimental setup of simultaneous stereoscopic
PIV, OH-PLIF, and OH chemiluminescence.

Figure 1.

Table 1. Experimental conditions of total mass flow rate, Qt[L/min], ratio of mass flow rates of inner and outer air flow,
Qo /Qi , and bulk velocities of inner and outer air flow (Vi [m/s]and Vo [m/s] ) and fuel flow (V f [m/s]).

Schematic of a dual swirl burner.

degrees to the center axis, and inner swirl flow is clockwise
and the outer swirl flow is counter clockwise viewed from thenegative region of the y axis. The diameters of the nozzles
are 24 mm, 26 mm and 39 mm, respectively and the outletof the inner air and fuel nozzle are located 5 mm upstream
from the bottom of the chamber. A combustion chamber has across-section of 110 mm × 110 mm and a height of 150 mm,
the sides are covered with quartz glass, which enables opticalmeasurements.
Figure 2 shows a schematic of simultaneous SPIV andOH chemiluminescence measurements system. In SPIV, 532
nm laser beam emitted from two Nd:YAG laser (Lee Laser
LDP-100MQG) is double-pulsed using a 1/2 wave plate andpolarizing beam splitter turned into a sheet and the guided toa cross-section including center axis of the burner. SiO2 tracer
particles (Suzukiyushi Industrial Corporation, E-6C) with anaverage diameter of 2 µm ware seeded in two air swirl flow respectively, and scattered light from the particles was capturedby two high-speed CMOS cameras (Photron, SA-Z) throughand macro lenses (Nikon, AF Micro-Nikkor 200 mm f/4D IFED) and optical filters (Edmund Optics, LONGPASS FILTER
BP center: 525 nm, band width: 50 nm). Three components
velocity was obtained at 10 kHz.
In OH-PLIF, Nd:YAG laser (Edgewave, IS120-2-L, 532
nm, 5 mJ/pulse) and dye laser (LIOPTEC, LiopStar-HQ, Rhodamine 6G) were used. In this study, Q1 (7) absorption linein the (1,0) vibrational band of A2 Σ − X 2 Π electronic transition at a wavelength of around 283 nm was used to excite
OH radical and fluorescence in the (1,1) and (0,0) vibrational
bands of A-X transition in range of 306-320 nm was detected.
The pulse energy was 45 µJ/pulse. Laser beam was alignedwith that of SPIV laser using dichroic mirror and guided to themeasurement area in the form of a sheet. The OH radical fluorescence was detected by a UV macro lens (Sodern, 100 mm
F/2.8 type CERCO 2178) equipped with a bandpass filter witha center wavelength of 320 nm and a half maximum full-widthof 44 nm (Semrock, FF01-320/40-50-D), amplified by an image intensifier (I.I.) (Hamamatsu Photonics, C10880-03F), andrecorded by a CMOS camera (nac, Memrecam ACS-3). The
gate width and gain of I.I. was set at 40 ns and 770, respectively.
In OH chemiluminescence measurement, the light emitted from the flame was captured by a UV-compatible lensequipped with a bandpass filter, amplified by an image intensifier, and recorded by CMOS camera (Photron, SA-X2). AQt

Qo /Qi

Vi

Vo

Vf

300

1.6

5.09

3.00

4.35

300

2.0

4.41

3.25

4.35

300

2.4

3.89

3.44

4.35

400

1.6

6.79

4.00

5.79

400

2.0

5.88

4.34

5.79

400

2.4

5.19

4.59

5.79

500

1.6

8.49

5.00

7.26

500

2.0

7.36

5.42

7.26

500

2.4

6.49

5.74

7.26

Figure 3. Mean distribution of OH chemiluminescence at total flow rate of 300 L/min and flow rate ratio of 2.0 and measurement area of SPIV and OH-PLIF (red line)

delay pulse generator (Photron, SA-X2) was used to synchronize the camera and I.I., and waveform generator (KeysightTechnology, 33512B) was use to trigger the start of the measurements.

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12th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan (Online), July 19-22, 2022

Figure 4. Mean distributions of velocity (a) and dv/dx (b) with contour lines of OH radical existence probability of 30%, 50% and
70% (green lines) at flow rate ratio of 1.6, 2.0 and 2.4 from the left for 300 L/min.
Table 2.

Experimental and Measurement Conditions
The equivalence ratio was fixed at 0.70. Total flow rate
was set at 300 L/min, 400 L/min, and 500 L/min. Experimental conditions of total mass flow rate, Qt [L/min], ratio of mass
flow rates of inner and outer air flow, Qo /Qi , and bulk velocities of inner and outer air flow (Vi [m/s] and Vo [m/s] ) and
fuel flow (V f [m/s]) are listed in Table 1. To investigate thecharacteristics of the flame base in relation to the flow ratio,
the ratio of the outer air flow to the inner air flow rate were set
at 1.6, 2.0 and 2.4. Table 1 shows the bulk flow velocity foreach condition.
Figure 3 shows the mean distribution of OH chemiluminescence at a flow rate ratio of 2.0 and the measurement areaof SPIV. The measurement area of SPIV and OH-PLIF was
19.7 mm × 19.9 mm (5.4 mm < x < 25.1 mm, 11.7 mm < y <31.6 mm) near the flame base. However, a region with laser intensity sufficient for OH excitation is limited in about 18 mm< y < 28 mm.
The camera resolutions are 79 µm × 79 µm for OH-PLIF,
111 µm × 111 µm for OH chemiluminescence and 21 µm ×
21 µm for PIV. Time separation of successive images of PIV
was 6.6 µs. The spatial resolution of PIV, which was definedas the size of interrogation region, is 672 µm × 672 µm, while
vector spacing is 336 µm × 336 µm. Laser sheet thickness for
OH-PLIF and PIV is 305 µm and 393 µm, respectively.
In order to evaluate the dynamic characteristics of theflame base fluctuation, it is necessary to define the lift-off
height. In this study, the lift-off height was defined as thelowest point of the OH region captured by the OH chemiluminescence, based on the method of Degeneve et al. (2021).
Otsu method was used for the discrimination of OH existence(Otsu, 1984). The lift-off height was calculated in each of thex > 0 and x < 0 regions. Mean and rms values of lift-off height(hm [mm] and h′rms [mm]) and peak frequency of flame basefluctuations ( f p [Hz]) (lift-off height fluctuations) are listed in

Characteristics of lift-off height fluctuations.

Qt

Qo /Qi

hm

h′rms

fp

300

1.6

15.4

3.14

224.6

300

2.0

14.6

2.80

195.3

300

2.4

14.3

2.80

166.0

400

1.6

15.5

3.12

312.5

400

2.0

14.7

2.73

273.4

400

2.4

14.9

2.70

239.3

500

1.6

16.6

2.51

439.5

500

2.0

14.6

2.48

341.8

500

2.4

13.9

2.36

278.3

Table 2.

RESULTS AND DISCUTTIONS
Figure 4 shows distributions of mean velocity and contours of 30%, 50%, and 70% existence probability of OH fluorescence for 300 L/min case. It can be seen that at the average flame base location, there is a large velocity gradient isgenerated by inflow gas and IRZ. The out-of-plane velocity
component shows a large gradient in a shear layer between the
inner and outer swirl flows. The shear layer between the inner
swirl flow and the IRZ also show the large out-of-plane velocity gradient in the radial direction, where the edge region of
OH radicals is located. As shown in Fig. 10, the reaction zone3

12th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan (Online), July 19-22, 2022

Figure 5. The velocity spectra at heights of y = 30 mm, 25 mm, 20 mm, and 15 mm at total flow rate of 300 L/min and the flow rate
ratio of 1.6, 2.0 and 2.4 from the left. Left, middle, and right columns are radial, mainstream and out-of-plane components of velocity,respectively.

tours of 30%, 50%, and 70% existence probability of OH fluorescence. Velocity gradients were calculated using a fourthorder central difference scheme for velocity data smoothed by3000 Hz cutoff filter. The absolute values of velocity gradientsof dv/dx are strongly correlated with the peak frequency of thelift-off height. The shear layer generated between IRZ and theinner air flow tends to have large velocity difference for lower
flow rate ratio and large velocity gradient at the location of average flame base, which results in higher frequency of vortex
shedding.
Figure 5 shows velocity spectra at heights of y = 15 mm,
20 mm, 25 mm, and 30 mm at the flow rate ratio of 1.6, 2.0 and
for Qt = 300 L/min. All the velocity components have a peakfrequency same as that of lift-off height specific to the flowrate ratio. The spectra of the mainstream and radial velocity
components have peaks near the average flame base position,
while the out-of-plane velocity component has peaks in theshear layer between the inner and outer air flows. It can beconcluded that the fluctuation of lift-off height is caused by
the intermittent motion of shear turbulence between the inflow
and the IRZ.
Figure 6 shows the Strouhal numbers with respect to the
Reynolds number Rei defined by the bulk velocity Vi , outer
diameter of inner swirl flow inlet Di and kinematic viscosityof air at 298 K. The Strouhal numbers are defined as:

Figure 6. Strouhal number defined by bulk velocity and inner nozzle diameter (top) and one defined by velocity gradientversus Reynolds number (bottom).

Sti = f p Di /Vi(1)Stθ = f p /(dv/dx)LMAX(2)

In Fig. 6, Sti tenda to increase with increase of Rei in highReynolds number region. This is caused by the change of swirl
numbe, and is consistent with the literatures by Syred (2006)
and Stohr et al. (2012). Note that the increase trend of Sti isvery weak. On the other hand, Stθ has nearly constant value in
Rei > 7000. The vortex shedding is caused by instability of inner shear layer, and hence the local maximum value of velocity
gradient at the mean lift-off height works well for alignment of
the frequency of lift-off height.

is engulfed by the vortex generated in the shear layer. At y =20 mm ∼ 25 mm, width between 30% and 50% lines of OH
radical existence is nearly same for the conditions tested. On
the other hand, width between 50% and 70% lines depends onthe flow rate ratio conditions, and that is large for the larger
flow rate ratio. However, this does not necessarily represent
less stability of inner recirculation zone, because higher flowrate ratio has larger IRZ and OH radical is possible to be moreconsumed in longer resident time. The consumption of OH
radical in the inner recirculation zone also causes a small region of OH existence higher than 70%.
Figure 4 also shows distributions of mean dv/dx and con-concludsions
Simultaneous high-speed measurements of stereoscopic
particle image velocimetry (SPIV), OH planer laser induced
fluorescence (OH-PLIF), and OH chemiluminescence wereconducted in order to investigate relationship between fluctuation of flame base and flow structure in a dual-swirl burner,4

12th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan (Online), July 19-22, 2022
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in which peak frequency of fluctuations of flame base changes
according to equivalence ratio and the ratio of outer and inner
air swirl flow rates.
The frequency of vortex shedding was discussed based on
velocity gradient at averaged location of lift-off height of theflames tested. The shear layer generated between inner recirculation zone (IRZ) and the inner air flow tends to have large
velocity difference for lower flow rate ratio and large velocity
gradient at the location of average flame base, which results inhigher frequency of vortex shedding because of the instabilityof shear layer. It has been clarified that the Strouhal number
based on the bulk velocity at the inner swirl and inner swirler
diameter has a trend to increase with increase of inner swirl
air flow rate, while the Strouhal number based on the velocity gradient tends to be a constant. Hence it can be concluded
that the characteristic frequency of flame lift-off height can bepredicted based on the velocity gradient at the averaged flamebase.

Acknowledgements
This work was supported by Fostering Joint International
Research (A) Grant Number 18KK0400.

REFERENCES
Chen, Z.X., Langella, I., Swaminathan, N., Stöhr, M.,
Meier, W., Kolla, H., 2019, “Large Eddy Simulation of a dual
swirl gas turbine combustor: Flame/flow structures and stabilisation under thermoacoustically stable and unstable condi-

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