Confined transverse
jets
THE jet issuing into a crossflow
stream, or the transverse jet, has received extensive attention from
the fluid dynamics research community, both as a practical
configuration found in numerous engineering applications and as a
fundamental three-dimensional turbulent shear flow suitable for
validating turbulent flow models. The nonreacting transverse jet is
a configuration applicable for chimney stacks, V/STOL aircraft,
dilution of combustion gases in gas turbines, and film cooling.
Reacting transverse jets include the flame stabilization of a fuel
jet issuing into crossflow as a model of stack flares and secondary
combustion zones in gas turbine combustion chambers. Many previous
efforts focused on the detailed flowfield features, dimensionless
groupings, streamwise trajectory behavior, and scaling properties of
round jets issuing into unconfined crossflow. Vorticity dynamics
have played an important role in many studies, with emphasis on the
origin and dynamics of the counter-rotating vortex pair (CVP). Other
vortex systems present in the round transverse jet include the
horseshoe vortex formed upstream of the jet, the jet shear layer
structure, and the wake vortices that occur downstream of the jet
for sufficiently strong jet-to crossflow velocity ratios. For
experimental studies, flow visualization of this highly
three-dimensional flow has been an invaluable tool. Alternative jet
nozzle shapes have also received attention to explore a passive
means for mixing enhancement for transverse jets. Confinement has
also been considered, related primarily to gas turbine combustors.
The confined transverse slot jet in a rectangular duct has received
less attention, although a small number of studies have been
conducted. These efforts considered a slot jet that spans the entire
dimension of the confinement (i.e.,channel), and results show that
the generated flowfield is nominally two-dimensional with respect to
mean and turbulence statistics. It is not clear whether there is any
CVP-type structure found for these confined transverse slot jet
cases.
A rectangular test
section with a nominal aspect ratio of 4:1 and a short dimension of
12.5 mm was modified to allow spanwise uniform blowing at multiple
stations along one of the spanwise walls as shown in Fig. 2. The injection jet has a
width of 1.2 mm and a spanwise length of 48 mm. The injection jet is
spanwise centered in the channel and spans 96% of the total channel
depth. The area ratio of the injection jet to the channel cross
sectional area is larger than will be the subject of future studies
with combustion to minimize the required injection mass flow rate to
less than 5% of the main flow. Walls of the channel section consist
of Acrylic to allow optical access. Particle image velocimetry (PIV)
is the diagnostic employed to study the achieved flow control. The
Reynolds number of the sub-scale model is 19000 and was matched to
the Reynolds numbers that will be studied in the experimental setup
currently under construction. A mean velocity of approximately 26
m/s was needed to achieve the necessary Reynolds number under the
current nonreacting flow conditions. The channel flow was not
fully-developed and contains thin boundary layers and a laminar
steady potential core. The background turbulence intensity in the
core of the channel flow measured with PIV was found to be less than
1%.
Fig. 1 Schematic of the experiment
Two instantaneous
velocity-vector fields for a higher momentum ratio (MR) of 0.49 are shown in Fig. 2.
The mass flow ratio for this case is 0.22. The jet appears to
penetrate all the way across the channel, although entrainment of crossflow occurs along the trajectory. Acceleration and turning of
the crossflow is again observed above the deflected jet. Large
vortical structures are also observed, and large regions of
instantaneous reverse flow are present downstream of the injection
location.
Fig. 2 Instantaneous vector fields for a high blowing case MR = 0.49
The mean streamline
pattern is shown in Fig. 3 for the MR cases of 0.12 and 0.49. The
streamline patterns show that the transverse slot jet induces a
recirculation zone reminiscent of that produced due to a sudden
expansion. This is one of the important flow features required for
flame stabilization in a high-speed reacting flow. The streamline
pattern provides a means for defining the dimensions of the
recirculation zone, which are relevant when considering the dynamics
of the flame stabilization process. The height and length of the
recirculation are determined using the streamline that is attached
to the downstream edge of the injection slot; for two-dimensional
flow, this streamline establishes an attachment point at the
downstream end of the confined recirculation zone. Such a streamline
appears to exist for the low-momentum-ratio case, whereas for the
high-momentumratio case the streamline along the trailing edge of
the recirculation bubble has a bifurcating characteristic that
indicates the presence of mean flow three-dimensionality. The
length-to-height ratio of the recirculation bubble is in the range
of three to four, whereas for the 2:1 area ratio step flow, the
ratio is on the order of 7; hence, the transverse jet produces a
much more compact recirculation zone.
Fig. 3 Mean streamwise streamlines for (a) MR = 0.12, (b) MR = 0.49
The mean streamwise
velocity distributions for the two cases are shown in Fig. 4. The
velocity component is normalized by the maximum channel velocity
without injection, Uo, which was nominally 26 m/s. The
low-momentum-ratio case shows a slight acceleration of the main
channel flow due to the blockage created by the transverse jet. The
reverse velocity in the recirculation zone reaches approximately 20%
of the channel characteristic velocity. The acceleration and reverse
flow both increase with increasing MR. At MR = 0.49, the peak
velocity difference across the channel in the region of the
recirculation region is nominally 2.2 times the characteristic
velocity, providing a large shear magnitude for turbulence
production that will be beneficial for volumetrically efficient
turbulent combustion. The peak shear was nominally the same for the
2:1 rearward-facing step flow. The transverse jet acts as a virtual
nozzle for the channel flow. The turbulence of the slot jet must
also play a role, either as the initial turbulence that grows in the
mean shear, or act as a disturbance to the unstable separated
channel flow. One might expect that the high reverse velocities may
be etrimental to flame holding, because the residence time of a
typical reactant pocket may be too short for combustion to be
completed, although this may be alleviated if the turbulent flame
speed within the recirculation zone increases. The
high-momentum-ratio case provides a nominal 50% contraction of the
channel flow.
Fig. 4 Mean streamwise velocity for (a) MR = 0.12, (b) MR = 0.49
Figure 5 shows the
normalized turbulence levels for the two MR cases. For the low MR
case, the injection creates a highly turbulent flow that remains
near the injection wall, with nonturbulent crossflow flowing above
the turbulent region. For the higher MR case shown in Fig. 5b, the
turbulence levels are much higher and span the entire height of the
channel. In fact, the peak-turbulence contour is slightly skewed
toward the upper wall in the downstream half of the domain. The high
turbulence levels begin to decrease in the downstream half of the
domain. Reduced turbulent energy production caused by a drop in mean
shear as well as dissipation, convection, and diffusion of turbulent
energy is responsible for the reduction in peak turbulence levels. A
50% increase in turbulence level using a transverse slot jet
compared with the step flow. Additionally, the step-flow
configuration requires several channel heights of development before
peak turbulence levels are reached, whereas the transverse slot jet
quickly acquires high turbulence levels due to the increased shear
and potential effects of the jet turbulence.
Fig. 5 Normalized turbulence levels for (a) MR = 0.12, (b) MR = 0.49
It was shown from the
streamline pattern that the flow at the higher MR appeared to
contain mean flow three-dimensionality. Spanwise PIV was conducted
by rotating the experiment 90 deg about the streamwise axis, and the
measurement plane was adjusted to a number of different locations
relative to the plane of the injection wall for the transverse jet.
Figure 6 shows the mean streamline pattern in two spanwise planes
located at different distances from the injection wall. Note that
these are not technically streamlines, because the velocity
component out of the plane is generally nonzero. The apparent
three-dimensional nature of the flow is readily observed. For the
plane closest to the injection wall shown in Fig. 6a, located at y/H
= 0.16, some interesting features are captured. The crossflow
encounters the slot jet and creates a large stagnation point. The
crossflow turns toward the side walls on each side of the midspan
line and passes through the narrow gap between the slot jet and the
side wall. There is a slight asymmetry in the location of the slot
jet relative to the channel spanwise center. Two large-scale vortex
structures are observed near the outer edge of the slot jet. Fig. 6b
shows the spanwise mean streamlines for y/H = 0.32 and shows a
smaller recirculation region centered in the spanwise plane. A pair
of mean flow counter-rotating vortices is also present, although
they are located much closer to one another compared with the y/H =
0.16 location. The crossflow external to the recirculation region
passes around the recirculation bubble in a manner similar to the
flow around a smooth obstacle. Such three-dimensionality has not
been observed in confined slot jets in which the slot jet spans the
entire channel dimension.
Fig. 6 Mean spanwise streamlines for (a) MR = 0.12, (b) MR = 0.49
Figure 14 shows the
spanwise distribution of the mean streamwise velocity at the two
transverse locations. The y/H = 0.16 case shown in Fig. 7a shows
that the recirculation bubble a defined by the footprint of the
zero streamwise velocity contour has a triangular shape, a
qualitative feature in agreement with the streamline pattern shown
in Fig. 6a. The high mean reverse-velocity region, defined as U=Uo
<= 0.4, is limited to a small region in the center of the span. The
region near the side walls contains higher mean streamwise
velocities, because the crossflow accelerates as it passes between
the slot jet and side wall. The mean streamwise velocity
distribution shown in Fig. 7b for y/H = 0.32 shows a smaller
reverse-flow region. It is clear that the recirculation zone appears
to have a dome like shape that is centered in the span and has a
peak height in the midspan plane. This shape suggests that the slot
jet trajectory has maximum penetration near the spanwise center and
a lower trajectory for side-view planes that approach the side
walls. This trend is different from previous results for spanwise
transverse slot jets under confined conditions in which the slot jet
spans the entire channel depth, which has been shown to produce a
nominally spanwise uniform flow outside the side-wall boundary
layers. The vertically oriented high-speed band passing through near
x/H = 0 in Fig. 7b is the streamwise component of the transverse
slot jet that has begun to acquire a streamwise component due to
bending by the crossflow. The jet has a higher streamwise velocity
near the side walls, likely caused by the increased deflection of
the jet as the side walls are approached.
Fig. 7 Mean spanwise velocity for (a) MR = 0.12, (b) MR = 0.49
