study of large-scale structures in isothermal and reacting swirling...

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

- 1 -

Study of Large-Scale Structures in Isothermal and Reacting Swirling Jets

Sergey V. Alekseenko1,2

, Vladimir M. Dulin1, Yuriy S. Kozorezov

1,2

Dmitriy M. Markovich1,2*

, Sergey I. Shtork1

1: Institute of Thermophysics of SB RAS, Novosibirsk, Russia

2: Novosibirsk State University, Novosibirsk, Russia

*: [email protected]

Abstract The current work reports on detailed experimental characterization of the coherent structures emerging in non-reacting and reacting swirling jet flows. Spatial distributions of instantaneous velocity and turbulent kinetic energy (TKE) components were measured with stereo PIV. Additional attention was paid on the ability of periodical forcing to control structure of the swirling jets. It was found that for the jet with high swirl rate, with pronounced vortex breakdown, the application of forcing at some range of frequencies with relatively high amplitude results appearance of large-scale regions rotating in opposite direction to the mean flow and increase of turbulent mixing. The paper also contains the results of stereo PIV study of a number of combustion jet regimes with variation of Re numbers, fuel-air ratio and swirl rates.

1. Introduction Swirling jets are widely used in a variety of industrial applications such as mixing devices, heat

exchangers, combustors, etc. Particularly, in combustion installations high swirling rate is often applied

for generation of a central reverse flow, which provides the flame anchoring near the burner outlet.

The bases of swirling flows are deeply described in books by Gupta et al. (1984) and Alekseenko et al.

(2007). Depending on the swirl rate and on the manner in which the swirl is applied, substantially

different flow regimes can be observed for the isothermal swirling flow. Different types of swirling jet

flows were studied in a number of experimental, numerical and theoretical works, considering different

inflow geometries (e.g. Billant et al. 1998, Alekseenko et al. 1999, Loiseleux and Chomaz 2003,

Gallaire et al. 2004, Liang and Maxworthy 2005, Cala et al. 2006, Duwig and Fuchs 2007). Most of

these works are devoted to the analysis of azimuthal instabilities and dynamics of the vortex breakdown

in the swirling jets at comparatively small Reynolds numbers (up to 1,000). It is widely acknowledged

by many authors that Kelvin–Helmholtz instability in the axial shear layer leading to vortex rings

formation, dominates non-swirling and weakly swirling jets. For a high enough swirl rate (but before the

vortex breakdown) strong helical waves were usually observed in the mixing layer of the jet. Further

increase in a swirl rate leads to vortex breakdown appearance, which is known to have different states

(Alekseenko et al. 2007): spiral, bubble, or conical, where the last two can be either symmetric or

asymmetric (e.g. Billant et al. 1998). Besides, strong influence of buoyancy effects on the vortex

breakdown state was shown in the recent paper of Mourtazin and Cohen (2007).

For the non-swirling jet, it is well known that process of the vortices formation and downstream

evolution can be controlled by flow external excitation (Crow and Champagne 1971, Zaman and

Hussain 1981). It was found that the forcing at the prevailing ('natural') frequency (typically ranging

from St = 0.3 to 0.6) leads to the strongest intensification of the ring-like vortices formation, and thus to

the greatest turbulent mixing in the initial region of the jet. Similarly to the non-swirling jets, external

low-amplitude forcing of inlet velocity can be a key to control development of large-scale vortices and

thus the turbulent mixing in swirling jets and flames. For flow conditions preceding the vortex

breakdown, Panda and McLaughlin (1994) and Gallaire et al. (2004) showed that axisymmetric or

azimuthal forcing intensifies development of corresponding instability modes and results domination of

axisymmetric or helical vortices, respectively. However, swirling jets at high swirl rates were so far

considered to be insensitive to the external excitation (Gallaire et al. 2004), at least for low forcing

amplitudes.


- 2 -

The structure of the jet flows with combustion is significantly more complex in comparison to

isothermal flows. Presently, starting from the works of Harris et al. (1949), Wohl et al. (1949), Chen and

Churchill (1972) the structure and stability of a non-swirling (Bunsen) hydrocarbon jet flame, is widely

acknowledged for a wide range of inlet velocities and fuel-air ratios. Generally, the flame can be

stabilized between two limiting values of the flow rate. When the flow rate is decreased below of a

certain minimum value, the flame can’t longer anchor itself to the lip of the burner rim and it flashes

back inside the burner. This threshold value is called as a flashback limit. In contrast, when the flow rate

is increased above a certain value, the flame abruptly becomes detached from the burner lip, rises to a

certain distance from the burner and continues to burn as a 'lifted' flame. When the flow rate is further

increased and it exceeds a blow-off limit, the flow can’t be longer stabilized in the region of interest and

it is carried away by the upstream flow. In practice, modern combustion devices utilize lean premixed

flames to achieve a low level of NOx emissions (see Tacina 1990). However, lean premixed flames are

prone to instabilities induced by various sources, including flame unsteadiness and blow-off (Lieuwen et

al. 2001, Meier et al. 2007). Thus, application of a relatively strong swirl and appearance of the vortex

breakdown are usually used for the lean flame stabilization. The vortex precession itself as well as

large-scale vortices, appearing from vortex breakdown (recirculation zone) and peculiarities of the

swirling shear layers, increase the flame stabilization via high turbulent mixing rate of the fresh and

burnt gases in the back-flow region. This allows to maintain the combustion at rather lean conditions.

Observations of different authors show that for the turbulent swirling flames a great variety of

combustion regimes can exist resulting from a number of effects. Thermal expansion affects flow

structure in a high degree since the shape of recirculation zone is sensitive to the buoyancy effects

(Mourtazin and Cohen 2007) and, additionally, it increases velocity of the backflow. For example,

Hartung et al. (2008) have shown that the presence of combustion strongly affects shape and size of the

recirculation zone in the non-swirling bluff-body stabilized wake flow. Schneider et al. (2005) reported

that the combustion suppresses vortex precession in the swirling flame in comparison to the isothermal

flow. For the turbulent flow, it is well known that thermal expansion effect suppresses the turbulence in

the gas passing thru the flame layer (e.g. Treurniet et al. 2006). From the other hand, turbulent

fluctuations tends to increase propagation speed of the flame front in premixed combustion (Peters

1999, Treurniet et al. 2006). Besides, large-scale vortices interacting with the flame front result its

intensive fluctuations and, consequently, fluctuations of the heat-release that leads to increase of

acoustic noise level (Schuller et al. 2002). Thus, the mutual flame/vortex interactions can result

intensification of large-scale vortices via pressure fed-back effects which can significant increase of

combustion rate in jet flames (see Yoshida et al. 2001). At the same time, such interaction can lead to

harmful resonance effects in enclosed combustion facilities (Meier et al. 2007). Thus, the study of large-

scale vortices role in the swirling isothermal and combusting flows is the relevant task and Particle

Image Velocimetry appears to be most suitable for measurements.

The present work is devoted to experimental study of the large-scale vortices emerging in non-reacting

and reacting swirling jet flows and their role in turbulent combustion and flame structure. Additional

attention was paid on the ability of external periodical forcing to control structure of the swirling jets.

The paper reports spatial distributions of instantaneous velocity as well as statistical velocity moments,

particularly, turbulent kinetic energy (TKE) components measured with stereo PIV. The paper also

contains results of visualization and PIV measurements in a number of combustion jet regimes, with

variation of Re numbers, fuel-air ratio and swirl rates.

2. Experimental setup and apparatus The measurements were performed both in a hydrodynamical loop and in a combustion facility

providing similarity of boundary conditions. The hydrodynamic loop was equipped with a pump, a

flowmeter and a temperature stabilizing device. The jet flow was organized by a contraction nozzle

located at the bottom of a rectangular working section. The section was made of plexiglas in order to

provide PIV measurements (see Fig. 1a). Water seeded by 20 µm polyamide particles was used as a

working fluid. External periodical axisymmetric perturbations of inlet velocity were imposed on the

flow by means of an exciter connected to a diaphragm mounted at the bottom of a mixing chamber. The


- 3 -

mixing chamber was connected to the nozzle plenum chamber by a pipe of one meter length.

The combustion rig (Fig. 1b) consisted of a burner, air fan, plenum chamber, flow seeding device,

premixing chamber, and a section for the air and fuel (propane) flowrate control. The burner represented

the profiled contraction nozzle with the same geometry as was in the water jet experiment. For the flow

external forcing, a loud speaker connected to an amplifier and a function generator was used. The flow

was seeded by oxide aluminium particles (1-3 µm diameters). The experiments were performed at

normal pressure and temperature.

The contraction nozzle was designed to provide a 'top-hat' velocity distribution at the nozzle exit for the

non-swirling flow (see Fig. 1c). The nozzle exit diameter d was of 15 mm and the area contraction rate

was 18.8. For organization of the flows with swirl, smoothing grids in a plenum chamber of the nozzle

were replaced by a swirl generator (see Fig. 1d). The Reynolds number Re was defined on the basis of

the mean flow rate velocity U0, nozzle diameter d, and kinematic viscosity v. The forcing was performed

at fe frequency characterized by the Strouhal number St. The definition of the swirl rate was based on

the swirler geometry according to Gupta et al. (1984):

0ReU d

v= ;

0

St ef d

U= ;

3

1 2

2

1 2

1 ( / )2tan( )

3 1 ( / )

d dS

d dϕ

−=

− (1)

Here, d1 = 7 mm is the diameter of a centerbody supporting the blades, d2 = 27 mm is the external

diameter of the swirler, and φ is the blade inclination angle. By using swirlers with various blades

inclination angle, the swirl rate S was varied from 0 to 1.0. The definition of the swirl rate based on the

swirler geometry was used as the reference to the experimental setup case only. In Alekseenko et al.

(2008) more details can be found about geometry of the nozzle and values of the swirl rate calculated

from the initial velocity distributions.

a) b) c) d)

Figure 1. Schemes of facility and measurement system arrangement: (a) hydrodynamic loop; (b) combustion rig. (c) sketch of contraction nozzle and (b) scheme of swirler arrangement.

Particle Image Velocimetry allowing comprehensive study the vortex structure of turbulent flows was

used in the experiment. To date, there is a number of papers devoted to application of PIV to

experimental study of the flows with combustion (e.g. Li et al. 2008) presented in literature. Many of

them (e.g. Meier et al. 2007) present simultaneous application of PIV and PLIF for measurement of

instantaneous spatial distributions of gas velocity and flame radicals (OH, CH, etc.). The aspects of PIV

application to turbulent flames, were deeply described in Stella et al. (2001). In particular, it was shown

that for laboratory-size flames, inaccuracy resulting from refractive index variation is quite small in

terms of laser sheet deflection and image deformation. Also it was shown that Al2O3 particles, surviving

the flame, are able to follow the flow properly, except for flame front regions and small-scale velocity

fluctuations because of relaxation time. However, PIV resolution is mainly affected by size of the

interrogation window. In the present work, a "PIV-IT" Stereo PIV system consisting of a double cavity

Nd:YAG pulsed laser, couple of CCD cameras and a synchronizing processor was used for the

measurements. A couple of narrow-bandwidth optical filters were used for registration of the laser

emission and for suppression of flame radiation. During the experiments, the system was operated by a

Laser

Laser

CCD Cameras CCD Cameras

Seeder

Flowmeters

Exciter

Propane

Air

0U

d d

1d

2d


- 4 -

computer with "ActualFlow" software. Measured images were processed by an iterative cross-

correlation algorithm with an image deformation and with a final interrogation area size of 32×32 pixels

and 50% overlap. Calculated instantaneous velocity fields were validated by using a signal-to-noise

criterion for cross-correlation maxima and by an adaptive median filter proposed in Westerweel and

Scarano (2005). Identified "outliers" were removed. Before the stereo reconstruction, the "holes" were

interpolated by using a 3rd-order 3×3 linear interpolation filter. Stereo calibration was performed by

using a plane calibration target and a 3rd-order polynomial transform. Additionally, an iterative

correction procedure of the laser and target planes misalignment was performed. All the measurements

were performed in a central plane of the jet/flame.

Additional processing procedures were elaborated for the experiment with combustion. A special

modification of the cross-correlation algorithm was made to account to the non-uniform seeding of the

flow by the tracers (in particular, in regions of the ambient air). Thus, before calculation of the cross-

correlation in a certain domain of the image, the presence of at least five seeding particles inside of the

interrogation window was tested. Otherwise the vector was not calculated. The particles detection was

performed by means of a particle mask correlation approach (Etoh and Takehara 1998). Besides, in

some experimental conditions, for high-luminosity flames, certain regions in the images contained

residual intensity from the flame radiation that was not enough suppressed by the optical filters. These

regions reduced signal-to-noise ratio of the cross-correlation function. To avoid this effect, digital non-

linear filtering of the images was performed by extraction of 9´9 median value from original intensity

value.

3. Experimental results. Water flows In the water jet experiments, the Reynolds number was equal to 8,900. The water temperature was

maintained constant at 26°C with an accuracy of ±0.2 °C. On the basis of 1,000 measured instantaneous

3-component velocity fields, spatial distributions of the mean velocity and components of turbulent

kinetic energy (TKE) were calculated.

At first, the flow field measurements were performed for non-swirling jet flow. Scenario of vortex

formation in the initial region of non-swirling jets is well acknowledged. Growth of Kelvin-Helmholtz

instabilities in the shear layer of the axial velocity leads to formation of large-scale ring-like vortices,

and thus results increase of the TKE downstream in the mixing layer. In the present experiments for the

non-swirling jet flow, clearly identifiable vortices were observed in the mixing layer at z/d > 0.8 that is

reflected by locally high values of ⟨υ2⟩ in this region (see Fig. 2b). The forcing at St = 0.5 led to a more

periodical formation of the large-scale vortices appearing in the vicinity of the nozzle exit (see Fig. 3)

and to significant increase of in-plane TKE components (namely, ⟨u2⟩ and ⟨υ2⟩). Obviously, increase of

TKE provides a greater turbulent mixing rate in the initial region of the jet due to greater velocity

fluctuations. The highest observed value for TKE components during the forcing at relatively high

amplitude (a = 15%) was ⟨υ2⟩ = 0.13U02 in the mixing layer of the jet at z/d = 0.6.

-1 -0.5 0 0.5 10

0.5

1

1.5

2

-24

-20

-16

-12

-8

-4

0

4

8

12

16

20

24

a) 0 0.5 10

0.5

1

1.5

2

b) 0 0.5 10

0.5

1

1.5

2

c) 0

0.003

0.006

0.009

0.012

0.015

0.018

0.021

0.024

0.027

0.03

Figure 2. (a) Instantaneous velocity and vorticity fields, and spatial distributions of the (b) radial and (c) axial

components of TKE for isothermal jet at S = 0, Re = 8,900. Unforced case.

z

d

z

d

z

d

r d r d r d

2

0U×

*

0

d

U

θω


- 5 -

-1 -0.5 0 0.5 10

0.5

1

1.5

2

-24

-20

-16

-12

-8

-4

0

4

8

12

16

20

24

a) 0 0.5 10

0.5

1

1.5

2

b) 0 0.5 10

0.5

1

1.5

2

c) 0

0.012

0.024

0.036

0.048

0.06

0.072

0.084

0.096

0.108

0.12


components of TKE for forced isothermal jet at S = 0, Re = 8,900. Forcing at St = 0.5, a = 15%.

Structure of instabilities and large-scale vortices in swirling jets is known to be significantly more

complex: helical waves of different modes become dominant with increase of swirl intensity. Further

increase of the swirl rate leads to a vortex breakdown (VB). Usually, for the flow with VB, strong

helical waves are observed in the outer mixing layer of the jet with single and/or double helixes in the

vortex breakdown region.

In the present work, for S = 0.41 case, the axial mean velocity was found to have positive value in the

whole jet core, that corresponded to the absence of the recirculation zone and no clear VB could be

determined from the average velocity field. At the same time, as it is seen from Fig. 4a, the local weak

reserve flows exist near the jet axis. Strong shear of the axial velocity near the jet axis produced intense

vortices which were observed to have helical structure. The vortices resulted the local maximum of the

axial component of TKE ⟨u2⟩ = 0.22U02 at z/d = 0.5 (Fig. 4). Less pronounced helical vortices were also

observed in the jet outer mixing layer where the greatest value of ⟨u2⟩ was of 0.04U02. Thus, one can

conclude that the greatest turbulent mixing rate for the present flow was near the jet axis.

-1 -0.5 0 0.5 10

0.5

1

1.5

2

-24

-20

-16

-12

-8

-4

0

4

8

12

16

20

24

a) 0 0.5 10

0.5

1

1.5

2

b) 0 0.5 10

0.5

1

1.5

2

c) 0.006

0.018

0.03

0.042

0.054

0.066

0.078

0.09

0.102

0.114

0.126

0.138

0.15


components of TKE for swirling isothermal jet at S = 0.41, Re = 8,900. Unforced case.

As it was expected, the effect of the forcing on structure of the swirling jet at S = 0.41 was quite similar

to the S = 0 case. Nearly symmetric (ring-like) large-scale vortices (which were clearly identified up to

z/d = 1.2) replaced helical vortices and became dominant in the outer mixing layer of the forced swirling

jet (see Fig. 5a). This led to significant increase of ⟨u2⟩ and ⟨υ2⟩ in the outer mixing layer. The forcing

effect was most pronounced for St = 0.5; however the swirling jet was less sensitive to the forcing in

comparison to S = 0 case.

z

d

z

d

z

d

r d r d r d

2

0U×

*

0

d

U

θω

z

d

z

d

z

d

r d r d r d

2

0U×

*

0

d

U

θω


- 6 -

-1 -0.5 0 0.5 10

0.5

1

1.5

2

-24

-20

-16

-12

-8

-4

0

4

8

12

16

20

24

a) 0 0.5 10

0.5

1

1.5

2

b) 0 0.5 10

0.5

1

1.5

2

c) 0.006

0.018

0.03

0.042

0.054

0.066

0.078

0.09

0.102

0.114

0.126

0.138

0.15


components of TKE for forced swirling isothermal jet at S = 0.41, Re = 8,900. Forcing at St = 0.5, a = 15%.

For the swirling jet at S = 1.0 the pronounced VB took place and a recirculation zone was observed in

the initial region of the jet up to z/d = 1.8. Analyzing structure of large-scale vortices plotted in Fig. 6,

one can conclude that intensity of the vortices emerging inside and outside of the recirculation zone is

significantly greater than for the jet with low swirl (cf. TKE components in Fig. 4 and 6). Structure of

the vortices was found to be quite similar to the observations made in the work of Liang and Maxworthy

(2005) for the jet at S = 0.9 and Re = 1,000. One can clearly identify two linked together strong vortices

with opposite sign, where one of the them is located inside the recirculation zone and the other one in

the outer mixing layer. If one consider the left and right sides of the jet cross-section, the vortex pairs

are observed to propagate asymmetrically, indicating odd mode of the inner and outer helix (similar

structure of the turbulent flow at high swirl rate was observed by Cala et al. 2006).

-1 -0.5 0 0.5 10

0.5

1

1.5

2

-24

-20

-16

-12

-8

-4

0

4

8

12

16

20

24

a) 0 0.5 10

0.5

1

1.5

2

b) 0 0.5 10

0.5

1

1.5

2

c) 0.02

0.06

0.1

0.14

0.18

0.22

0.26

0.3

0.34

0.38

0.42

0.46

0.5


components of TKE for swirling isothermal jet at S = 1.0, Re = 8,900. Unforced case.

Generally, for the unforced flow at S = 1.0, ⟨u2⟩ had the maximum of 0.23U02 in the mixing layer inside

the recirculation zone, ⟨υ2⟩ reached values of 0.21U02 and 0.17U0

2 at the jet axis and in the outer mixing

layer, respectively. The forcing at St = 0.5 with various amplitude almost did not change the flow

structure. However, when the forcing was applied at St = 1.2 (corresponding to a maximum of

preliminary calculated one-dimensional spatial spectra), its effect on the TKE distributions was found to

be substantial. For forcing amplitudes below 5%, magnitude of ⟨w2⟩ grew in some degree, while ⟨u2⟩ and

⟨υ2⟩ remained almost unchanged. When the amplitude value was more than 5%, the flow structure

changed abruptly: ⟨u2⟩ and ⟨υ2⟩ decreased in amplitude (their magnitude became 2 times lower), while

⟨w2⟩ became great (up to 0.7 of U02, see Fig. 7d), as well as the total TKE (TKE increased in 1.4 times).

z

d

z

d

z

d

r d r d r d

2

0U×

*

0

d

U

θω

z

d

z

d

z

d

r d r d r d

2

0U×

*

0

d

U

θω


- 7 -

-1 -0.5 0 0.5 10

0.5

1

1.5

2

a) -1 -0.5 0 0.5 10

0.5

1

1.5

2

b) -2

-1.5

-1

-0.5

0

0.5

1

1.5

2

0 0.5 10

0.5

1

1.5

2

c) 0 0.5 10

0.5

1

1.5

2

d) 0.02

0.06

0.1

0.14

0.18

0.22

0.26

0.3

0.34

0.38

0.42

0.46

0.5

Figure 7. (a), (b) Instantaneous azimuthal velocity and (c), (d) spatial distributions of the azimuthal component of TKE

for swirling isothermal jet at S = 1.0, Re = 8,900. (a) and (c) unforced case; (b) and (d) forcing at St = 1.2, a = 5.1%

It was found that for unforced and forced at St = 1.2 with amplitude above 5% jets, the flow structure

was quite similar and slightly lower intensity of vortices was observed in the forced case. More

significant difference was observed in distributions of the instantaneous azimuthal velocity w* plotted in

Fig. 7a and b. While for the unforced jet the azimuthal velocity was mainly positive to the left of the jet

axis and negative to the right of the axis (in the coordinates used), respectively, the large regions of w*

having opposite values were observed for the forced case at St = 1.2, a > 5%. These regions were found

at the most of the instantaneous velocity fields, witnessing the large-scale helical structures presence in

the flow. These large-scale energy-containing structures must be the reason of large values of ⟨w2⟩, and

thus are expected to be a key tool to increase turbulent mixing in highly swirling jets.

Generally, for the unforced flows, the highest values of TKE components were observed for the flows

with swirl, and especially for the flow with the pronounced vortex breakdown for which large areas of

high velocity fluctuations were observed near the nozzle exit. In all cases the forcing did not produce

significant effect on the mean velocity distributions, but caused significant change in structure of large-

scale vortices and distributions of TKE components. It was found for the jet with high swirl and clear

vortex breakdown, forcing at relatively great amplitude can result significant increase of azimuthal

velocity fluctuations. In particular, this effect can be used for stabilization of swirling flames via

enhanced turbulent mixing.

4. Experimental results. Air-propane reacting jets During the experiments on the combustion rig, a relatively wide range of Re numbers was covered:

from 500 up to 8,000. The equivalence ratio Φ of air-propane mixture was varied from 0.5 to 10.

Similarly to the water flow experiments, the swirl rate S was varied from 0 to 1.0. On the first stage,

images of the combustion regimes were taken, and on the second stage 2,500 instantaneous 3-

component velocity fields were measured by stereo PIV technique for the most interesting cases. In

order to provide the generality of results, the influence of the seeding particles on the flame structure

was tested. No significant difference was found between seeded and not seeded flows for a whole range

of Re and Φ. Figure 8a shows combustion regimes at different Re-Φ conditions together with the blow-

off curve for the non-swirling flow (S = 0). The blow-off curve corresponds to the upper value of Re for

the fixed Φ for which flame still exists. If the Re was increased above the critical value, the flame was

blown away by the up-stream flow. For low Reynolds numbers the flame flash-back was observed

(approximately shown by unshaded area at the lower part of regime map).

Depending on Re and Φ values, various combustion regimes exist between an attached classical Bunsen

flame and a remote from the nozzle 'lifted' flame. For the stoichiometric premixed flame (Φ = 0.95) at

Re = 1,200 a distinct flame cone with a thin flame layer was observed. According to the blow-off curve

the maximum Re number for Φ = 1.0 combustion regime is about 1,400 for the present nozzle

configuration. With Re and Φ increase along the blow-off curve, the flame became detached from the

nozzle and the 'lifted' flame regimes realized. The distance from the flame to the nozzle increased while

moving along the curve.

z

d

z

d

z

d

r d r d r d

2

0U×

*

0

w

U

r d


- 8 -

Figure 8b shows Re-Φ diagram with various combustion regimes for the flow with high swirl rate

(S = 1.0). Similarly to the non-swirling case, the 'lifted' flame regimes were observed for relatively large

values of Re and Φ, however, the range of Re, where stable 'lifted' or attached flames realized, was

much more wide than for non-swirled flames. The typical examples of combustion regimes of the are

shown in Fig. 8b: 'lifted' regime (Φ = 3.4 and Re = 4,000), and attached regime (Φ = 3.5, Re = 1,000)

with outer and inner flame layer, where strong helical waves dominated in the outer layer. The majority

of regimes observed at Re-Φ diagram besides laminar and 'lifted' flames represented the flames,

penetrating inside the nozzle and forming a quasitubular flame structure (see, for example, Re = 2,000

and Φ = 2.5). For the case of S = 0.41, the most of the turbulent combustion regimes and shape of the

blow-off curve were found to be similar to the S = 1.0 case. Thus, for space saving, the case of S = 0.41

is not presented. Generally, introduction of a swirl significantly increased blow-off limit for the flow in

comparison to the non-swirling flow.

In the present work a low amplitude acoustical forcing at various amplitudes and frequencies was

applied for the studied jet flames. There was no major effect observed for the swirling flames, that

seems to be due low amplitude of forcing. The most pronounced effect for the non-swirling flames was

observed for the 'lifted' flame for St ≈ 1. The forcing led to significant decrease of the flame-to-nozzle

distance that is considered to be due to the faster increase turbulent mixing rate with the distance from

the nozzle in case of the forced flow.

0 1 2 3 4 5

0

1000

2000

3000

4000

5000

a) 0 1 2 3 4

0

2000

4000

6000

8000

b) c)

Figure 8. Re-Φ diagram with blow-off curve and examples of typical flame regimes for (a) S = 0, and (b) S = 1.0 cases.

(c) Effect of acoustical forcing (St = 1.0) on non-swirling 'lifted' flame at Re = 3,450; Ф = 2.55. (Upper) unforced

flame; (bottom) forced flame.

-1 -0.5 0 0.5 10

0.5

1

1.5

2

a) -1 -0.5 0 0.5 10

0.5

1

1.5

2

b) 00.10.2

0.30.40.50.60.70.80.91

1.11.21.31.4

Figure 9. Distributions of the mean velocity for non-swirling flame at (a) Re = 1,600 Φ = 1.1; (b) Re = 2,700 Φ = 2.7.

Re

Φ

Blow-off

region

Re

Φ

z

d

z

d

r d r d

0U×


- 9 -

Figure 9 shows velocity distributions of two typical laminar combustion regimes. The flame front is

schematically shown by dashed line. For the cone regime (Fig. 9a), the vector field shows the tendency

of the flame to deflect the flow due to its acceleration while passing through the flame layer. For the

second case (Fig 9b), in spite of relatively high Re number, the large-scale vortex structures, emerging

in non-reacting flow, were suppressed in the initial region of the jet and combustion was dominated by

molecular diffusion. The greater role of the large-scale vortices in combustion was observed in the

'lifted' flame regimes. Figure 10a shows example of instantaneous velocity field for the flame at

Re = 3,200 and Φ = 2.2. The flame front (schematically shown by the dashed line) was found to

oscillate around z/d = 1.3 position. It is considered, that the flame front represented bottom of a torus

(Su et al. 2000). A locally great spreading rate was observed after the flame front due to thermal

expansion effect. Figure 11 shows instantaneous velocity field and spatial distributions of TKE

components for the isothermal flow with the same parameters at for the 'lifted' flame. The distributions

for isothermal gas and water flows were found to be quite similar. Comparing the gas-air flows with and

without combustion, one can conclude that the vortices located above the flame boundary are slightly

suppressed, while the vortices before the front are more pronounced in comparison to the flow without

combustion. This can be explained by the following effects. From the one hand, turbulent fluctuations

become suppressed after the gas passes thru the flame that is reflected by the spatial distributions of

TKE. From the other hand, slightly promoted vortices before the flame front and locally greater values

of TKE can be explained by the forcing effect produced by the flame noise. When the flame front

interacts with the large-scale vortices, pressure waves are generated due to fluctuations of the flame

surface, and consequently, of the heat release rate (see Schuller et al. 2002). The pressure waves can be

responsible for the forcing of the jet shear layer and faster growth of the vortices in the initial region of

the jet in comparison to the isothermal flow. Thus, one can state that the 'lifted' premixed flame partially

stabilizes itself in a certain region via the nonlinear feed-back mechanism. This is partially supported by

the forcing effect on the 'lifted' flame.

-1 -0.5 0 0.5 10

0.5

1

1.5

2

-24

-20

-16

-12

-8

-4

0

4

8

12

16

20

24

a) 0 0.5 10

0.5

1

1.5

2

b) 0 0.5 10

0.5

1

1.5

2

c) 0

0.003

0.006

0.009

0.012

0.015

0.018

0.021

0.024

0.027

0.03


components of TKE. 'Lifted' flame at S = 0; Re = 3,200; Φ = 2.2.

-1 -0.5 0 0.5 10

0.5

1

1.5

2

-24

-20

-16

-12

-8

-4

0

4

8

12

16

20

24

a) 0 0.5 10

0.5

1

1.5

2

b) 0 0.5 10

0.5

1

1.5

2

c) 0

0.003

0.006

0.009

0.012

0.015

0.018

0.021

0.024

0.027

0.03


components of TKE. Air jet flow at S = 0; Re = 3,200.

z

d

z

d

z

d

r d r d r d

2

0U×

*

0

d

U

θω

z

d

z

d

z

d

r d r d r d

2

0U×

*

0

d

U

θω


- 10 -

Figure 12 shows spatial distributions of instantaneous velocity and components of TKE for the swirling

'lifted' flame at S = 1.0, Re = 4,000 and Φ = 3.4. Intense helical vortices were observed in the inner and

outer mixing layers of the jet, thus producing region of great values of TKE components (see Fig. 12b

and c), which were found to be similar (but slightly greater near the nozzle) to the case of isothermal

water jet at Re = 8,900. Similarly to the non-swirling lifted flame, rapid suppression of fluctuation

intensities was observed after the gas passed thru the flame layer.

-1 -0.5 0 0.5 10

0.5

1

1.5

2

-24

-20

-16

-12

-8

-4

0

4

8

12

16

20

24

a) 0 0.5 10

0.5

1

1.5

2

b) 0 0.5 10

0.5

1

1.5

2

c) 0

0.03

0.06

0.09

0.12

0.15

0.18

0.21

0.24

0.27

0.3


components of TKE. 'Lifted' flame at S = 1.0; Re = 4,000; Φ = 3.4.

-1 -0.5 0 0.5 10

0.5

1

1.5

2

-24

-20

-16

-12

-8

-4

0

4

8

12

16

20

24

a) 0 0.5 10

0.5

1

1.5

2

b) 0 0.5 10

0.5

1

1.5

2

c) 0

0.03

0.06

0.09

0.12

0.15

0.18

0.21

0.24

0.27

0.3


components of TKE. 'Quasi-tubular ' flame at S = 1.0; Re = 6,800; Φ = 1.0.

-1 -0.5 0 0.5 10

0.5

1

1.5

2

a) -1 -0.5 0 0.5 10

0.5

1

1.5

2

b) -24

-20

-16

-12

-8

-4

0

4

8

12

16

20

24

0 0.5 10

0.5

1

1.5

2

0

0.03

0.06

0.09

0.12

0.15

0.18

0.21

0.24

0.27

0.3

c)

Figure 14. (a), (b) Instantaneous velocity and vorticity fields, and (c) spatial distribution of the axial components of

TKE for flame at S = 1.0; Re = 1,600; Φ = 1.85.

Spatial distributions of instantaneous velocity and components of TKE for the stoichiometric flame

(Φ = 1.0) at S = 1.0, Re = 6,800 are shown in Fig. 13. For this regime, the flame front is considered to

have quasi-tubular structure in the initial region of the flame (z/d < 1). The flow structure significantly

differs from the previous case. The backflow region is less pronounced and there is rather low level of

z

d

z

d

z

d

r d r d r d

2

0U×

*

0

d

U

θω

z

d

z

d

z

d

r d r d r d

2

0U×

*

0

d

U

θω

z

d

z

d

r d r d r d

2

2

0

u

U

*

0

d

U

θωz

d


- 11 -

velocity fluctuations inside the recirculation zone. Close to the inner and outer sides of the flame, dense

chain of intense vortices, resulted from the strong shear of the mean velocity, were observed, producing

locally great values of TKE components. Another considerably different combustion regime is shown in

Fig. 14. In this case the combustion was dominated by molecular diffusion in the outer mixing layer of

the flow. Distributions of instantaneous velocity show that mainly the large-scale vortices appear in the

flow inducing flame front distortion in the outer mixing later (cg. Fig. 14a and b). It was observed that

the vortices represented even mode in certain periods and odd mode in another.

5. Conclusions The impact of large-scale vortices, emerging in non-reacting and reacting swirling jet flows, on

turbulent mixing and combustion was studied experimentally by using stereo PIV technique. Spatial

distributions of instantaneous velocity and turbulent kinetic energy (TKE) components were measured.

Generally, for the isothermal jets, the highest values of TKE components were observed for the flows

with swirl, and especially for the flow with clearly determinated intensive vortex breakdown, for which

large areas of high velocity fluctuations were observed near the nozzle exit. Additional attention was

paid on the ability of imposed periodical forcing to control the structure of the swirling jets. In all cases

the forcing did not produce significant effect on the mean velocity distributions, but caused significant

change in structure of large-scale vortices and distributions of TKE components. It was found for the jet

with high swirl rate and pronounced vortex breakdown, forcing at relatively great amplitude can result

significant increase of azimuthal velocity fluctuations and, as a consequence, increase of turbulent

mixing rate.

For the turbulent swirling flames a great variety of combustion regimes was observed depending on

Reynolds number, fuel-air ratio and swirl rates. For the non-swirling 'lifted' flame it was found that the

flame front suppresses velocity fluctuations in the gas passing though it but, at the same time, the flame

is stabilized in the regions of high velocity fluctuations. Besides, comparison of the turbulence statistics

for the flows with and without combustion, and the results of external forcing application showed that

the flame/vortex interaction partially support the flame stabilization via feed-back effect.

Acknowledgements The current work was partially supported by RFBR (grants 07-08-00213, 07-08-00710, 07-08-00296)

and Integration Research Projects of RAS and SB RAS. Authors would like to thank Tokarev Mikhail

for assistance in data processing for combustion experiment.

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