Advances in Materials Physics and Chemistry, 2012, 2, 134-137
doi:10.4236/ampc.2012.24B035 Published Online December 2012 (
Mixing Enhancement in a Coaxial Jet Mixer
Valery Zhdanov, Egon Hassel
Rostock University, Department of Technical Thermodynamics, Rostock, Germany
Received 2012
Experimental investigations of mixing in a coaxial jet mixer have been carried out applying Particle Image Velocimetry (PIV) and
Planar Laser Induced Fluorescence (PLIF) methods simultaneously. A developed turbulent jet of an aqueous solution of Rhodamine
6G issued from the nozzle was mixed with co-flow water. Velocity and scalar fields were studied quite far downstream flow to con-
trol the formation of a quasi homogeneous mixture. The intensity of mixing was varied by mouthpieces with rectangular and triangu-
lar vortex generators of different sizes installed in the nozzle. The formation length of the quasi homogeneous mixture was reduced
about 10 jet diameters by the tabs. The triangular tabs were more effective than the rectangular ones.
Keywords: Mixing; Turbulent Flow; Vortex Generators; PIV; PLIF
1. Introduction
Mixing enhancement of gas and fluid flows is interesting for
different technical applications from turbo-jet engines to jet
mixers in chemical industry. In jet co-flows passive and active
methods are applied already long time to increase mixing. If
active methods (acoustic radiation, jet oscillation, nozzle vibra-
tion and so on) require an additional energy to influence the
flow, passive ones solve the same problem using the flows
energy by changing initial conditions of the mixing layer for-
mation. For this purpose nozzles of different forms (square,
triangle and lobed nozzles [1-6]) and nozzles with vortex gen-
erators [1,7-11] are usually used. These tools enhance mixing in
comparison with round nozzles of correspondent sizes in sub-
sonic and supersonic flows [9].
Nozzles with vortex generators (tabs) intensify mixing
stronger than any no round nozzles or the active methods (pe-
riodic forcing). As have been noted in [8]… “ in terms of the jet
centerline velocity decay, none of these active methods pro-
duces as much effect as observed with the tabs”. The effective-
ness of tabs was demonstrated in studies of jet flows issued
from nozzles with the formation of laminar [1,8-10] or turbu-
lent boundary layers [9,12]. On the base of these investigations
a physical model of tabs effect was proposed: a pair of
counter-rotating streamwise stationary vortices was generated
behind the tab and these vortices changed the vorticity distribu-
tions in the flow downstream [8-10]. As a sequence of this ac-
tion, the cross section of the original round jet was hard dis-
torted. The jet diameter was reduced in one direction and be-
came wider in the other direction (jet bifurcation). The extent
and the form deformation depended on the number of tabs. Two
sources for the generation of streamwise vorticity behind the
tab have been identified [11]. The dominant source (denoted as
1) comes from the pressure hill formed upstream of the tab. The
second source (denoted as 2), again owing to the pressure gra-
dients on the tab’s surface, is the vortex shed from the sides of
the tab. The application of the tabs in nozzles of different con-
figurations has shown the higher effectiveness of the triangular
vortex generators [1,9].
The effect of tabs has been investigated mostly by Pito tube,
Laser Doppler Anemometry (LDA) or Thermo Anemometry
systems in studies of the jet flows. The mixing enhancement
was estimated by the intensity of the mean velocity decay or
calculating the entrainment (the “mass flux”) [1,9]. This inte-
gral quantity was used as the primary measure for the compara-
tive study and due to the fact details of mixing were hided. As
has been shown in the reference [13], the scalar field develops
faster than the velocity one and to get more comprehensive
information on mixing the direct measurements of scalar varia-
tions are required.
In the present study the development of velocity and scalar
fields in the coaxial jet mixer is studied applying PIV and PLIF
methods simultaneously. The measurements interval extended
long enough downstream the nozzle to control the formation of
a quasi-homogeneous mixture. Apart from the known investi-
gations, where the issued jets were characterised by some art of
boundary layers, the present study deals with mixing of devel-
oped turbulent jet flow. As was noted in the reference [8], the
tab effect was observed only if the tab height was large relative
to the boundary-layer thickness of the issued jet. A tab with
height substantially smaller than the boundary-layer thickness
was not effective.
2. Experimental Set Up
2.1. Equipment
The measurements have been carried out in the closure water
channel (Figure 1).
Mixing of fluids took place into a coaxial jet mixer. The clear
water from tank 1 was pumped to buffer tank 2 and through
tube 3 (L=5 m) entered mixer 4. Dye solution of Rh 6G from
tank 5 pumped through heat exchanger placed in tank 2, came
to vessel 9, where the air was removed from the fluid and fi-
nally was ejected through nozzle 6 into the coflow. The mixed
fluids were collected in tank 7 to eliminate the dye before being
Copyright © 2012 SciRes. AMPC
drained into the environment. The accuracy of the flow rate in
both supply lines was not worse of 1%.
The test section of the channel consists of the mixer and
equipment. The mixer was formed by two co-axial tubes: glass
tube 1 with an inner diameter of D = 0.05 m and steel tube 2
with an inner diameter of d = 0.01 m (nozzle), which was
co-axially positioned with an accuracy of ±0.1 mm (Figure 2).
The horizontal length of the nozzle was equal 60d, so the
developed flow was formed at the nozzle exit. To reduce the
image distortion of the object in flow due to the curvature of the
mixer surface it was placed into the glass rectangular box filled
with the water.
Identical CCD cameras (14 - bit PCO 1600, with a resolution
of 1600x1200 pixels and frame rate of 30 fps at full resolution)
were used to measure the velocity and scalar fields. The cam-
eras were placed on a common assembly plate and they ob-
served the same flow image using the beam splitter plate (BP)
(Figure 2). The assembly plate was fixed to the profile fastened
on a linear stage. This stage also carried lenses and mirror
which were needed to produce a laser sheet of 0.7103 m thick-
ness, and a laser Nd:YAG. The mutual displacement of the
cameras and the laser sheet, the lenses and the laser did not
change when the measurements at different distances from the
nozzle were carried out. Both cameras were equipped with
Nikkor 50 mm lens and separating rings PK-11A. The image
magnification was 0.173 for both cameras. The image acquisi-
tions run in double frame mode for both cameras with a fre-
quency of 15 Hz. The host computer synchronized the cameras
and the laser. The pulse laser Nd:YAG (Nano 50-50) had a
pulse width of 5 ns, a variable pulse repetitive rate of 4–50 Hz,
and the energy stability of 50 mJ ± 2% at 532 nm.
Figure 1. Scheme of the water channel.
Rh 6G
Figure 2. Scheme of the test section.
2.2. Running Conditions of Experiments
The investigations have been carried out for the coflow-to-jet
flow rate ratio equal to 5 at which the exit jet velocity was tur-
bulent at the Reynolds number Red =104.
The intensity of mixing was controlled by mouthpieces that
were installed in the nozzle (Figure 3).
Four mouthpieces with the same inner diameter (d=0.01 m)
were used: the reference one without tabs (D0), the mouthpiece
with rectangular tabs h = 1.5 10-3 m (D1), the mouthpieces with
triangular tabs h=1.3 and 1.8 10-3 m (D2) and (D3) correspon-
dently. So, the influence of sizes and configurations of the tabs
on mixing can be estimated. The exit cross sections of the
mouthpieces were reduced to 12 % for D1 and to 8 and 16% for
D2 and D3 in comparison with the cross section of the refer-
ence one (D0). This reduction of cross sections resulted in the
higher exit velocities because the value of the jet flow rate was
the same in these investigations.
The laser sheet crossed the mixer in the vertical plane along
its centre line (z = 0). This light was reflected from the particles
in the flow and exited the dye molecules, which started to radi-
ate the light at the longer wavelength. The reflected light off the
particles was collected by camera with the laser-line band pass
interference filter 532nm (Edmund Optics). The radiated light
of the dye molecules passed through the broad pass filter
BP600 nm with 50nm FWHM (Edmund Optics) and was col-
lected by another camera.
Preliminary studies of different but uniform dye concentra-
tions of Rh 6G were executed. A short glass cylindrical volume
identical to the mixer was filled with dye solutions and placed
into the same glass box with the water. Series of 200 images
were recorded and then averaged at each pixel. Besides, series
dark images were recoded to determine the grey value offset for
each pixel. The difference of these images yields the light in-
tensity distribution that corresponds to the determined dye con-
centration. Due to the each pixel calibration, the Gaussian na-
ture of the laser beam, which results in the variations of the dye
intensity over the laser sheet, were taken into account.
To calculate statistical characteristics of the velocity and
scalar fields of the mixed flows 2000 images were captured by
each camera at seven positions along the mixer length.
3. Results
The jet bifurcation was developed just behind the nozzle with
the mouthpieces D1-D3. Therefore the measurements were
done for two positions of these mouthpieces in the planes dif-
fering by 45°. At first the mouthpiece was installed in the noz-
zle so that the vertical laser sheet coincided with the two oppo-
site tabs (the plane of 0°). In this case the pairs of the
counter-rotating vortices generated by the tabs produced the
vertical fluxes to the jet axis, i.e. the positive cross velocities
appeared at the lower jet part and the negative one at the upper
jet part (Figure 4(a)).
Figure 3. The mouthpieces applied in experiments, from left to
right D0, D1, D2 - D3.
Copyright © 2012 SciRes. AMPC
The longitudinal velocity and scalar profiles in this plane
ases of the
y field started to be uniform at x/D=9 where the
came narrower (left part of Figure 4(b),(c)). The second row
measurements were done when the mouthpieces were rotated
by 45°. At this position the fluxes generated by the vortices
from different pairs were ejected outside the jet axis and the
velocities and concentrations profiles became wider (right part
of Figure 4(b),(c)). The distributions of the velocity and con-
centrations in the measured cross sections were normalized on
their values at the mixer centre line (U0 and C0).
The jet bifurcation decays downstream and in the c
outhpieces of D1 and D2 the longitudinal velocity and fluc-
tuations profiles at the distance x/D=3 did not differ in both
measured plans while the concentrations profiles showed some
differences even downstream in the case of D2 (Figure 5). The
jet expanded stronger than the jet issued from the reference
The velocit
uctuations did not practically distinguish for all considered
cases (Figure 6(a)). The cross velocity and its fluctuations had
the same order and varied a few across the mixe r (Figure 6(b)).
Figure 4. The distributions of the cross velocity (a) longitudinal
velocities (b) and concentrations (c) just behind the nozzle at dif-
ferent positions of the mouthpieces.
Figure 5. The velocity and concentration distributions behind the
mouthpieces D0 and D2 at the distance x/D=3.0.
Figure 6. The velocities and scalar distributions at the distance x/D
he uniform scalar field was formed already at x/D=7 when
= 7, 9.
mouthpieces D1-D3 were used (Figure 6(c)). The difference in
the concentration distributions in two planes was already insig-
nificant. The application of the mouthpiece D3 provided the
Copyright © 2012 SciRes. AMPC
Copyright © 2012 SciRes. AMPC
inal mean velocities, con-
oncentration (U, C) and the
e en-
s in mixing, as already has
us mixture, where the
faster formation of the quasi-homogeneous mixture in com-
parison with the other mouthpieces.
Decays of the normalized longitud
ntrations and their fluctuations along the mixer axis down-
stream the nozzle demonstrate the dynamics of the tabs influ-
ence on mixing (Figure 7(a),(b)).
Exit values of the velocity and ci i
lues of these parameters at the mixer axis (U0 and C0) at the
measured cross sections were used for the normalization of the
fluctuations downstream the flow. The first normalisation shows
the evolution of velocity and scalar fields along the mixer length
and the second one gives the dynamics of the fluctuation–to-
local velocity ratio. This parameter presents the development of
the turbulence level in the flow. The correlation of the present
measurements with the known ones [13] was quite well.
The tabs forced the co-flow entrainment into the jet. Th
ainment started earlier and was accompanied by the decrease
of the mean velocity and the concentration, and by the growth
of the fluctuations. Maximum of the fluctuations was moved to
the nozzle directions (at x/D=0.3 for D3). Because the mean
values of the velocity and the concentration decreased stronger
than the fluctuations ones, the local ratios u’/U0, c’/C0 at the
mixer axis increased downstream. The scalar parameters de-
cayed faster than the velocity one.
The advantage of the triangle tab
en noted in the references [1,9], can be seen within the inter-
val of 0 < x/D < 3 for the mouthpiece D2 against D1. With
smaller blockage effect the mouthpiece D2 more intensively
involved the co-flow fluid to the jet and resulted in the same
mixture quality to the distance x/D=5.
The formation of the quasi-homogeneo
locity and scalar gradients fast degenerated and the fluctua-
tions distributions across the mixer were nearly uniform, was
completed in the case of the mouthpiece D3 minimum about
10d earlier in comparison with the case when the jet was issued
from the mouthpiece D0.
Figure 7. Decay of the velocity and concentration fields inhe
l jet mixer was investigated applying si-
fluxes generated by tabs resulted in the earlier en-
y the German Research Founda-
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4. Conclusions
Mixing in the coaxia
multaneously methods PIV and PLIF. The development of the
velocity and concentration field was controlled by the mouth-
pieces with different tabs installing in the nozzle. The mixing
enhancement has been observed at all kinds of investigated tabs,
i.e., the tabs of relative small sizes (0.13 h/d 0.18) are quite
effective also in the developed turbulent jet. The advantage of
the triangular tabs against the rectangular ones also has been
The cross
ainment of the co-flow fluid to the jet and forced the jet to
expand faster.
5. Acknowledgements
The study has been supported b
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