Proceedings of FEDSM’98
1998 ASME Fluids Engineering Division Summer Meeting
June 21-25,1998, Washington, DC
FEDSM98-4994
RESEARCH
ON THE MIXING ENHANCEMENT
PERFORMANCE
NOZZLES BY USING PIV AND LIF
OF LOBED
Hu Hui, Toshio Kobayashi, Tatsuo Saga
Nobuyuki Taniguchi, Sigeki Segawa and Akira Ono
Institute of Industrial Science, University of Tokyo
7-22-l Roppongi, Tokyo 106, Japan
Email: huhui@cc.iis.u-tokyo.ac.jp
ABSTRACT
An experimental investigation of the vertical and
turbulent structures in the jet mixing flows of lobed nozzles
had been conducted. The techniques of Laser Induced
Fluoresce (LIF) and Particle Image Velocimetry (PIV) were
used to accomplish the flow visualization, instantaneous
quantitative concentration measurements and velocity field
measurements of the lobed jet mixing flows. The experimental
results showed that, besides the existence of the well known
streamwise vortices, compared with a circular jet flow, lobed
jet flows were found to have shorter laminar region, smaller
scale of the spanwise Kelvin-Helmholtz
vortices, earlier
appearance of small scale turbulent structures and bigger
turbulent intensity.
Based on the LIF and PIV results, two aspects of the
mechanism of mixing enhancement of a lobed nozzle are
suggested. One is that a lobed nozzle can accelerate the “cut
large scale spanwise Kelvinand connect” process of
Helmholtz vertical rings. Another is that the “stretch effect” of
streamwise vortices generated by the lobed nozzle on the
spanwise Kelvin-Helmhotz vertical rings also enhanced the
“energy cascade” process of turbulence. Both of them can
result in the creation of much small scale intense turbulence
and enhances the mixing of jet flow with ambient flow.
INTRODUCTION
A lobed nozzle which consists of a splitter plate with
convoluted trailing edge is extraordinary fluid mechanic
device for efficient mixing of two co-flow streams with
different velocity, temperature and/or spices. Such device had
been known since the earliest days of jet engines and received
considerable attention for reducing jet noise during the
1960’s. More recently, it has emerged as an attractive
approach for mixing core and bypass streams of turbofan
engine to improve propulsion efficiency, reduce the specific
fuel consumption (SFC) and suppress the infrared radiation
emission (Power et al.1994, Presz et al.1994 and Hu Hui et
a1.1996). Lobed nozzle/mixer has also been received attention
for using in supersonic ejectors for jet noise reduction at
aircraft take off and landing as well as in combustion
chamber for enhancing mixing between fuel and air (Tillman
et al.1993 and Smith et a1.1997).
About the mechanism of the mixing enhancement of a
lobed nozzle, many work had been conducted in the past, such
as Paterson (1982), Werle et a1.(1987), Elliott et a1.(1992) and
McCormick et a1.(1993). In the work of the Belovich et
al.( 1997), the results of these researches were summarized by
that the mixing process in a lobed nozzle/mixer is controlled
by three primary elements. The first is the streamwise vortices
generated due to the lobed shape. The second is the increase in
interfacial area between the two flows due to the special
geometry of the lobed structure, and the third is the BrownRoshiko type structures which occurring in any shear layer
due to the Kelvin-Helmholtz instabilities.
Although many important results had been got through
these investigations, much work still need to understand the
fluid dynamic mechanism of mixing enhancement by a lobed
nozzle more clearly, especially regarding to the vertical and
turbulent structures changes in the jet flow caused by a lobed
nozzle and the mechanism of how the streamwise vortices
caused by a lobed nozzle enhance jet flow mixing process. In
the meanwhile, most of the previous researches were
conducted
by using
Pitot
Laser
Doppler
probe,
Velocimetry(LDV) or Hot Film Anemeter (HFA), which are
very hard to reveal the vertical and turbulent structures in jet
mixing flows instantaneously and globally due to the limitation
of these experimental techniques. In the present study, both
Laser Induced Fluorescence (LIF) and Particle Image
Velocimetry (PIV) techniques were used to studied lobed jet
mixing flows instantaneously and globally.
1
Copyright 0 1998 by ASME
EXPERIMENTAL
SYSTEM AND TECHNIQUES
Lase
8/
Figure 1. Experimental Setup
Figure 1 shows the schematically experimental facility used
in the present research. The tested nozzles were stalled in the
middle of the water tank (550mm*550mm*600mm).
Fluorescent dye for LIF or PIV tracers (polystyrene particles of
d=lOOum) was premixed with the water in the jet supply tank,
and jet flow was supplied by a pump. The flowrate of the jet
flow, which was used to calculate the representative velocity
and Reynolds numbers of the jet flow, was measured by a flow
meter. A honeycomb structure was installed in the entrance of
the tested nozzle to insure the uniform flow entrance. A beam
of argon ion laser passed an optical system to form a plane
sheet. The investigated area was focused on a CCD camera and
then, recorded by Laser Videodisc Recorder. The images were
digitized by a s-bit gray-scale image processing system at a
spatial resolution of 640 by 480 pixels, which also can be
stored on a PC (host computer) and displayed on a PC monitor.
angc
outerpcnetratton
a. circular
nozzle A
c. lobed nozzle
-
C
b. lobed nozzle
B
d. three tested axial slices
Figure 2. Studied tested nozzles
Figure 2 shows the three tested nozzles: a baseline
circular nozzle A and two lobed nozzles with different lobe
numbers. The equivalent diameters of these nozzles at the exit
were the same, i.e. D=40mm. In the present study, the jet
velocities were O.O7m/s and 0.2Om/s, and the Reynolds
Number of the jet flows, based on the nozzle exit diameter,
were about 2,000 and 6,000.
In the present research, LIF technique was used to
conducted flow visualization and instantaneous concentration
measurement. Rhodamine B was used as fluorescent dye and
the fluorescent light was separated from the scattered laser
light with an optical filter. Rhodamine B solution in the jet
supply box has a low concentration (0.5mg/l) to insure the
strength of fluorescent light being linear with the
concentration of the fluorescent dye and the effect of laser light
attenuation being negligible as the laser light sheet propagated
through the flow.
The cross correlation method was used in the present
study to conduct PIV image processing. The post-processing
procedures
including subgrid interpolation (Hu Hui et
al. 1998), velocity outliner deletion (Zhou et al. 1996), and field
smoothing (Willert et a1.1991) were used to improve the
accuracy of the PIV measurements.
RESULTS AND DISCUSSIONS
Form the result of the flow visualization and instantaneous
concentration field measured for the circular jet flow (Fig,
3(a)), it can be seen that three different regions, which are
laminar, transition and turbulent regions, can be identified
clearly in the figure. At the end of laminar region (X/D=l.O),
spanwise Kelvin-Helmholtz vortices were found to roll up. The
pairing and combining of these spanwise vortices were
conducted in the transition region. In the downstream of
X/D=3.0, the small scale mixing (low concentration region)
began to occur and much small scale turbulence and vertical
structures began to appear in the flow field.
Figure 3(b) shows the result of the flow visualization and
instantaneous concentration field measured in the axial slice of
the lobe trough (Fig.2(d)) for the lobed nozzle B. Compared
with the circular jet flow, the jet flow for lobed nozzle B at this
axial slice had a shorter laminar region (X/D=O.2). The scale
of the spanwise Kelvin-Helmholtz vortices of the jet flow was
much smaller. The transition region in which the pairing and
combining process of the spanwise vortices were accomplished
also became shorter. In the downstream
of the
locationX/D=O.& much small scale mixing and small scale
turbulent structures (low concentration region) were found to
appear.
Figure 3(c) shows the result of the flow visualization and
instantaneous concentration field measurements in the axial
slice of the lobe peak (Fig.2(d)) for the lobed nozzle B. From
the figure, it can be seen that: the laminar region was not a
straight cylinder like that in the circular jet, and looked like a
expansive cut-off cone along the downstream of the lobed
structure instead. Compared with the flow structures shown in
Fig.3(b), the laminar region in the lobe peak axial slice was a
bit longer (X/D=0.4), but it was still shorter than that in the
circular jet flow. This is caused by the different thickness of
the boundary layer at the exit of the lobed nozzle (the work of
the Brink et al.(1993), had verified that the thickness of the
Copyright 0 1998 by ASME
boundary layer at the lobed trough is smaller than that in the
lobed peak), and the thicker boundary layer at the lobe peak
need a longer streamwise distance to roll-up the KelvinHelmholtz vortices (Hussain et al. 1989). In this axial slice, it
can also be seen that small scale mixing and small scale
turbulent structures (low concentration region) were found to
appear about in the down stream of location X/D=0.8.
Figure 3.(d) gives the result of flow visualization and
instantaneous concentration field measurement in the axial
slice of the lobe side (Fig.2(d)) for the jet flow of lobed nozzle
B. In this axial slice, streak flow structures can be seen in the
near field of the jet flow. These structures were the KelvinHelmholtz vertical tubes shed periodically from the lobe
training edge, which was observed and called “normal vortex”
by McCormick et a1.(1993). At the downstream of the location
X/D = 0.8, small scale mixing and small scale turbulent
structures (low concentration region) were found to appear in
the flow field.
Figure 4 is the flow visualization of the jet flow for the lobed
nozzle C in four cross planes. From the figures it can be seen
that, at X=lOmm (X/D=0.25, Fig.4(a)), the existence of the
streamwise vortices in the form of 12 petal “mushrooms” can
be seen clearly in the jet flow. As the streamwise distance
increased to X=20mm(X/D=OS),
the “mushrooms” grew up
(Fig.4(b)), which indicated the intersection and enhancement
of the streamwise vortices generated by lobed nozzle. As the
streamwise vortices intensified, some small scale structures
began to appear in the flow and the interaction between the
streamwise vortices and Kelvin-Helmholtz
vortices made
adjacent “mushrooms” merging with each other (X=30mm,
Fig.4(c), which indicated the process that the streamwise
vortices deform the Kelvin-Helmholtz vertical tube into pinchoff structure suggested by McCormick et a1.(1993). At X=
40mm (X/D=I.O, Fig.4(d)), the “mushroom” shape structures
almost disappeared and the flow was almost fully filled with
small turbulent structures.
2). PIV results
The PIV measurement results of the lobed jet flow and
circular jet flow were showed in the Fig. 5 to Fig. 8. The
instantaneous flow fields showed on these figures were
obtained at the frequency of 30 Hz and the mean flow fields
were got by the average of 100 frames of the instantaneous flow
fields. The instantaneous spanwise vorticity (Wzi,j,, ), mean
velocity (U i,Jand V i,j ) and mean turbulent intensity T, shown
on the figures were defined as:
r&zyG7
T = /=I
‘I
100
~j(.~,,,, -I/,.,)? +(v,,,,,-Y,,)’
=
100
In which ui,j,r and Vi,j,rare the instantaneous velocities in the
X and Y directions respectively, while u’i,j,t and v’i,j,t are the
instantaneous turbulent velocities.
From these figures, it can be seen that, compared with the
mean flow fields, the instantaneous velocity and vorticity fields
revealed the existence of many small scale turbulent in the
flow field, which cannot be observed in the mean flow fields.
This is also the advantage of the instantaneous full field
experimental
technique
like
PIV
over conventional
experimental techniques.
From the mean turbulent intensity fields, it can also be seen
that, the bigger turbulent intensity regions in the three axial
slices of the lobed jet were located almost in the near down
stream of the lobed nozzle, which means that the most of the
mixing process between the jet and ambient flows were
completed in near down stream region (X/D<3.0). While, for
the flow field of the circular jet, it is still in its core potential
region (Fig. 8). From the comparison the maximum values of
the turbulent intensity fields in the lobed jet (which are
0.095-n/s, 0.1 lOm/s and O.lOOm/s in the axial slice of lobe
trough slice, lobe peak slice and side of the lobe slice
respectively) and circular jet flow (which is O.O6Om/s),it also
can be seen that lobed jet flows were much more turbulent than
the circular jet, which also indicted the mixing enhancement
performance of the lobed nozzle.
Figure 9 shows the 100 frames PIV instantaneous velocity
fields in the axial slice passing the lobe trough of the lobe
nozzle B. In the figure, Z direction indicates the time step. The
iso-surfaces of the spanwise vorticity Wz=l.O(red) and Wz=l.O(blue) was also showed in the figure. From the figure it can
be seen that, at the entrance of the measurement region, for the
iso-surface of the spanwise vorticity Wz= I.O(red) and Wz=-1.0
(blue), some periodical structures were found along the time
direction (Z direction), which were corresponded to the
periodically rolled up process of the spanwise vortices due to
the Kelvin-Helmholtz instability in the shear layer. The size of
the iso-surface of these spanwise vortices decreased along the
flow direction (Y direction) due to the mixing process in the jet
flow. In the exit of the measurement region, the scale of these
iso-surface structures were much smaller than that in the
entrance of the measurement region.
3). Mechanism of the mixing enhancement of lobed nozzles
From above LIF and PIV results, it can be seen that,
compared with the circular jet flow, lobed nozzles can reduce
the scale of Kelvin-Helmholtz vortices, accelerate the process
of vortices pairing, produce streamwise vortices in the jet flow.
Small scale turbulence appeared earlier and bigger turbulent
intensity was found in the lobed jet. All these indicated the
mixing enhancement performance of lobed nozzles. However,
how the lobed nozzles enhance fluid mixing in the jet flow?
McCormick et a1.(1993) suggested that the interaction of
Kelvin-Helmholtz vortices with streamwise vortices generated
by lobed nozzles produces high levels of mixing which is the
mainly responsible for the enhanced mixing, but by what
means were these processes conducted? They did not explain
it.
Copyright 0 1998 by ASME
completed in the near field region (X/D<3.0) of the lobe jet
flow.
stretched
Doint
Kelvin- Helmhotlz
streamwise
stretchid
Figure 10. Idealization of the vertical evolution in a circular jet
flow conjectured by Hussain (1986)
It was well known that, for a circular jet, just like described
in the article of Hussain (1986, Fig.10) spanwise vortex rings
will be rolled up firstly due to the Kelvin-Helmholtz instability
(first instability) existed at any shear layer. As these spanwise
vortex rings move downsteam, they can not be two
dimensional vertical rings due to the self-interaction effect and
cross-interaction effect between them. They will be the
combinations of helical vertical tubes, i.e. toroidal vertical
rings through the effect of an additional instability (secondary
instability or azimuthal helical instability model). So, the two
dimensional spanwise vortex rings caused by the KelvinHelmholtz instability will be wrapped and developed into three
dimensional structures through secondary instability. With
undergoing interactions, the large scale toroidal vertical rings
will be broken down into many substructures through the “cut
and connect” process, which may be responsible for the
avalanche of three dimensional and smaller scale motions and
for the generation of high turbulence and Reynolds stress.
However, in a circular jet flow, the evolution of such process
will need a very long streamwise distance to complete.
For a lobed nozzle, because of its special geometry, it can
cause big perturbation along the azimuth of the jet flow, such
as the non-uniform momentum thickness of the boundary layer
at the exit of the lobed nozzle. The streamwise vortices
produced by the lobed nozzle enlarge the azimuthal
perturbation by the means of deforming the Kelvin-Helmholtz
vertical tubes into pinch-off structures (suggested by
McCormick et al.( 1993) and visualized in the Fig.4). All these
enhance the creation of the complex three dimensional vortices
and the helical instability of the jet flow. i. e., the “toroidal
effect” of the spanwise vertical structures is enlarged, and then
the “cut and connect” process of the vertical rings is
accelerated (which is the merging process of the adjacent
“mushroom” observed on the flow visualization in the cross
plane, Fig. 4), This means that the process of a large-scale
vertical structure breaking into smaller scale vertical structure
is conducted more rapidly, therefore, the mixing of the jet flow
with ambient flow is enhanced. All these processes can be
spanwise
vortices
point
Figure1 1. Stretch effect of streamwise vortices on the
spanwise Kelvin-Helmhotlz vertical tube
Besides this, the interaction between the large scale
streamwise vortices produced by the lobed nozzles and the
spanwise vortices caused by the Kelvin-Helmholtz instability
also results in that the spanwise vertical rings are
stretched(Fig. 11). According to the Helmholtz vorticity
conservation law, the scale of the vortices will be reduced when
the vortices are stretched, These also enhance the “energy
cascade” process of turbulence, and result in rapid reduction of
the scale of the spanwise instability (this is the reason why the
scale of spanwise Kelvin-Helmholtz vortices is smaller in the
lobed jet flows than that in the circular jet flow showed in the
above LIF results). These also result in the creation of much
small scale intense turbulence and the mixing enhancement of
the jet flow with ambient flow.
CONCLUSION
The LIF and PIV results of the investigation revealed the
great differences of the turbulent structure and vortex scale
between the lobed jet and circular jet mixing flow. Compared
with the circular jet flow, the lobed jet flow had shorter
laminar instability region, smaller scale of the spanwise
Kelvin-Helmholtz
vorices, earlier appearance of the small
scale turbulent structures and bigger turbulent intensity. All
these indicated the mixing enhancement performances of a
lobed nozzle over a circular nozzle.
Based on LIF and PIV measurement results, two aspects
of the mechanism of the mixing enhancement of a lobed
nozzle are suggested: One is that a lobed nozzle can cause
azimuthal perturbations in the jet flow,and the streamwise
vortices produced by the lobed nozzle enhanced these
azimuthal perturbations. These accelerate the “cut and
connect” process of the large scale spanwise Kelvin-Helmholtz
vortex rings to transfer the energy and vorticity from large
scale vortices to small scale vortices. Another is that the
interaction between the streamwise vortices and spanwise
Kelvin-Helmholtz vortices also enhanced the “energy cascade
process” of the turbulence, which also resulted in the creation
of much smaller scale intense turbulence and mixing
enhancement of the jet flow with ambient flow.
4
Copyright 0 1998 by ASME