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14
CHAPTER 2
LITERATURE REVIEW
2.1 INTRODUCTION
Early works on coaxial jets were motivated mainly by applications
in combustion and aircraft propulsion. Forstall and Shapiro (1950) were the
first to perform an experimental investigation on mass and momentum
transfer between the two streams of a co-flowing jet with very large
secondary flows in subsonic region, and showed that the velocity ratio of the
primary to secondary stream is the principal parameter determining the shape
of the mixing region, and proposed an empirical relation for the length of the
primary potential core.
Meanwhile, the feature of supersonic, dual, coaxial jet was first
investigated by Love et al (1959). They made theoretical studies on a jet
exiting into supersonic streams, and indicated that a supersonic outer stream
can permit an intersecting shock pattern at a much higher pressure ratio before
forming a Mach disk which is the case for a subsonic outer stream. Schadow
et al (1990) in their study of compressible spreading rates of supersonic
coaxial jets have identified that axisymmetric coaxial jet’s spreading rate
varies with the convective Mach number. Moreover the changes in spreading
rate depend on the axial distance.
A number of works have been made to investigate the subsonic and
supersonic dual, coaxial jet. These works have been mainly to investigate
1) jet noise suppression, 2) mixing enhancement, and 3) feature of shock
15
wav
e sy
stem
. H
owev
er,
ther
e ha
ve b
een
little
wor
k do
cum
ente
d on
the
supe
rson
ic je
t with
sec
onda
ry a
nnul
ar s
ubso
nic
stre
am, a
nd o
n th
e su
pers
onic
prim
ary
jet o
f non
-circ
ular
shap
es w
ith se
cond
ary
high
spee
d su
bson
ic st
ream
.
In th
is c
hapt
er, t
he p
revi
ous
wor
ks a
re re
view
ed to
exp
lain
the
dual
coax
ial j
ets
with
sec
onda
ry s
tream
. Im
porta
nt f
eatu
res
and
mai
n pa
ram
eter
s
dete
rmin
ing
them
are
dis
cuss
ed w
ith th
e pu
rpos
e of
giv
ing
an in
sigh
t int
o th
e
pres
ent s
tudy
.
2.2
CO
AX
IAL
JE
TS
2.2.
1 Je
t Str
uctu
re
One
of
dist
inct
fea
ture
s of
the
sup
erso
nic
jet
stru
ctur
e, w
hich
is
rela
ted
with
the
per
form
ance
of
indu
stria
l ap
plic
atio
ns,
is t
he s
hock
cel
l
syst
em, e
spec
ially
the
Mac
h di
sk.
D`A
ttore
and
Har
shba
rger
(19
65)
repo
rted
first
that
the
dist
ance
of
Mac
h di
sk fr
om th
e no
zzle
exi
t dec
reas
ed a
s the
out
er se
cond
ary
flow
vel
ocity
was
incr
ease
d, b
ut th
e de
taile
d ne
ar f
iled
stru
ctur
e of
the
supe
rson
ic c
oaxi
al
jet
for
a gi
ven
flow
con
ditio
n is
sho
wed
by
Dos
anjh
et
al (
1969
). B
uckl
ey
(197
5) h
as a
rgue
d th
at t
he l
ocat
ion
of t
he M
ach
disk
gen
erat
ed i
n a
supe
rson
ic, d
ual,
coax
ial j
et w
ill n
ot b
e al
tere
d si
nce
the
axia
l Mac
h nu
mbe
r
dist
ribut
ion
upst
ream
of t
he M
ach
disk
is in
depe
nden
t of t
he c
ondi
tions
of t
he
exte
rnal
stre
ams.
Mas
uda
et a
l (19
93, 1
994)
hav
e re
porte
d th
at th
e pr
esen
ce o
f
seco
ndar
y an
nula
r jet
has
a fa
vora
ble
effe
ct fo
r red
ucin
g th
e di
amet
er o
f Mac
h
disk
whi
ch is
form
ed in
the
prim
ary
inne
r jet
.
16
Mea
nwhi
le, N
aray
anan
and
Dam
odar
an (1
993)
hav
e ar
gued
that
the
Mac
h di
sk lo
catio
n an
d its
dia
met
er w
ill in
crea
se w
ith t
he p
ress
ure
ratio
of
seco
ndar
y an
nula
r jet
, and
sec
onda
ry a
nnul
ar s
tream
will
sign
ifica
ntly
cha
nge
the
Mac
h nu
mbe
r dis
tribu
tion
upst
ream
of t
he M
ach
disk
. Thi
s is
in a
ser
ious
conf
lictio
n to
the
repo
rt m
ade
by B
uckl
ey (1
975)
.
Rec
ently
, de
taile
d in
vest
igat
ion
on
the
near
fie
ld s
truct
ure
of
supe
rson
ic, d
ual,
coax
ial j
et w
as m
ade
expe
rimen
tally
by
Rao
et
al (
1996
).
They
inv
estig
ated
the
gas
dyn
amic
s pa
ram
eter
s in
fluen
cing
the
sup
erso
nic,
dual
, co
axia
l je
t, su
ch a
s th
e ge
omet
ric p
aram
eter
s of
the
inn
er n
ozzl
e, j
et
stat
ic p
ress
ure
ratio
(ra
tio o
f th
e ex
it pl
ane
stat
ic p
ress
ures
of
the
inne
r an
d
oute
r noz
zles
), an
d th
e ra
tio o
f out
er to
inne
r noz
zle
thro
at a
rea,
and
show
ed a
supe
rimpo
sed
oute
r an
d in
ner
jet
stru
ctur
e th
roug
h se
vera
l sc
hlie
ren
visu
aliz
atio
ns. T
hey
repo
rted
that
the
inne
r flo
w i
s co
mpr
esse
d by
the
oute
r
flow
, res
ultin
g in
the
for
mat
ion
of a
Mac
h di
sk, w
hich
is
also
sig
nific
antly
influ
ence
d by
the
jet s
tatic
pre
ssur
e ra
tio. T
hey
furth
er in
vest
igat
ed th
e no
zzle
dive
rgen
ce a
ngle
and
boa
ttail
angl
e ef
fect
s on
the
jet s
truct
ure.
Will
iam
s et
al (
1969
) in
vest
igat
ed th
e flo
w s
truct
ure
and
acou
stic
s
of s
ubso
nic,
com
pres
sibl
e co
axia
l je
ts, a
nd s
ugge
sted
em
piric
al r
elat
ions
for
thei
r pr
imar
y po
tent
ial c
ore
leng
th a
nd t
heir
nois
e em
issi
on. H
owev
er, t
hey
did
not
take
acc
ount
for
the
dens
ity r
atio
and
com
pres
sibi
lity
effe
cts,
whi
ch
17
flow. In general, mixing enhancement has a close relation with the
suppression of supersonic jet noise.
Westley and Lilley (1952) and Seiner and Gilinski (1997) have tried
to investigate the mixing enhancement of two streams using a lobed nozzle.
Ahuja and Brown (1989) and Samimy et al (1993) have made experimental
works of the mixing enhancement using a nozzle with a small tab at its exit.
Samimy et al (1998) have conducted an experimental investigation on the
effects of the configuration of the exit of nozzle. Strykowski et al (1993,
1996) have employed a counter flow method.
Although these methods could produce a significant increase in jet
spreading rate, reduction of the jet noise was not only insufficient but the jet,
sometimes, also was louder. Moreover, the thrust loss induced by mechanical
mixers exceeded about 10% (Seiner 1998; Samimy et al 1998), which was
unacceptable for effective and economic operation of the system.
Papamoschou (1996, 2000, 2003) has performed extensive works to
investigate the mixing characteristics in the dual, coaxial jet. He showed that
at a given primary flow condition, the spreading rate of the primary jet
increased with an increase in the secondary flow velocity in subsonic flow
speed range. He found that the spreading rate was significantly increased, as
the secondary flow reached a state very close to sonic conditions (correctly in
the approximate range of Mach number between 0.8 and 1.2). For further
increase in the secondary flow velocity over a supersonic range, the spreading
rate was decreased. This was due to the reduced velocity difference across the
shear layers, as the secondary flow velocity increased to the supersonic speed.
18
In general, the mixing enhancement reduces the length of the Mach-
wave-emitting region of the jet. This causes the near field Mach waves to be
more amplified, leading to appreciable thrust penalties. For example,
Nagamatsu et al (1972) argued that each dB of the noise reduction was
accompanied by about 1% thrust loss. Papamoschou (1996) has used a mixer
ejector system to overcome this problem. Louis Alpinieri (1984) studied the
turbulent mixing process between CO2 H2 central jets exiting into atmospheric
air and have identified that the product of local density and eddy kinematic
viscosity depends on axial coordinate.
Tillman et al.(1992) further studied the lobed mixer and ejector, and
made a significant progress for the mixing enhancement without
accompanying much thrust loss. They also found that the secondary flow
reduces the growth rate of the primary shear layer, and elongates the primary
potential core and the supersonic region of the jet. These results showed that
the entrainment rate of the coaxial jet was less than that of a single jet, and the
potential core length increased by as much as 68% when the secondary mass
flow rate was increased. They further investigated the effects of the primary
nozzle eccentricity on the mixing enhancement, and obtained that the
eccentric configuration led to substantial improvement in mixing.
2.2.3 Applications and Jet Noise
Applications of the supersonic, dual, coaxial jet to suppress jet noise
were systemically explored by Dosanjh et al (1969,1971). And investigated
various parameters such as nozzle geometry, pressure ratio, mass flow ratio
between two streams, etc., and found that the co-axial configuration reduces
jet noise emission. They also observed a significant noise reduction at certain
combinations of pressure ratios for the inner and outer streams at which the
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shock structure of the jet was significantly weakened. Wlezien (1989)
examined the coupled interaction of jets from two nominally identical
convergent/divergent nozzles as a function of nozzle spacing. The screech
modes of two coupled jets correspond to those observed for single plumes,
but the modal amplitudes are strongly dependent on nozzle spacing.
Dosanjh et al (1970) conducted an extensive experimental study of
supersonic coaxial jet. Using a small-scale nozzle, they investigated a
minimum noise condition for shock containing coaxial jet. As the outer nozzle
pressure ratio was fixed above a critical value, the inner nozzle pressure ratio
could be increased from a no flow condition to some condition at which the
measured overall sound level was a minimum less than the outer jet alone. For
higher inner pressure ratios, the noise increased. This minimum noise
condition always occurred for inner nozzle pressure ratios less than the outer
nozzle pressure ratios. Their optical shadowgraphs showed that the repeated
shock structures were destroyed at the minimum noise condition. Thus they
believed that the overall noise reduction was primarily due to a decrease in
shock-associated noise.
Recently, Tanna (1980) pointed out that the above results were very
sensitive to the nozzle geometry. He conducted the experimental work on
coaxial jet with the same conditions of Dosanjh et al (1979). Tanna and
Morris (1985) also defined a minimum noise condition based on overall jet
pressure level measurements. For a fixed outer nozzle pressure ratio, the
minimum noise condition was obtained when the inner nozzle pressure ratio
was about 1.9 (slightly above a critical value). Tam and Tanna (1985)
reported that the minimum noise condition is strongly dependent on the
nozzle configuration employed.
20
Bhutiani (1976) investigated the effect of the nozzle-lip thickness on
the supersonic coaxial flow, and showed that for the coaxial jet issuing from
two convergent nozzles with zero exit divergence angle, operated at unequal
pressure ratios for the inner circular and outer annular jets, the flow structures
are strongly influenced by the lip thickness. Thus, he concluded that a coaxial
convergent nozzle with a finite but thin lip results in substantial noise
reduction about 4-16 dB. Zamam (1999) identified that the characteristic
spreading of the tabbed jets is explained by the induced motion of the tab-
generated stream wise vortex pairs. The tabs, however, incur thrust loss; the
flow blockage and loss in thrust coefficient, vis-à-vis the spreading increase,
are evaluated for various configurations.
Farassat (1970) studied about the geometrical configuration of two
convergent coaxial nozzles and showed that an optimum area ratio for
minimum radiated noise was nearly unity and the lip thickness of each nozzle
plays an important role in the supersonic coaxial jet flow.
Further works on large scale supersonic coaxial jet were carried out
by Ahuja (1976), Bassiouni (1976) and Bhutiani (1976). They indicated that
the shocks were very weak or nonexistent for a correctly expanded flow
condition, at which appreciable overall noise reduction was obtained. Olson
and Friedman (1974) made experimental works over many kinds of coaxial
configurations, and concluded that the noise reductions at supersonic
conditions are qualitatively the same as that at subsonic conditions. The
results mentioned above revealed that the secondary flow reduces Mach wave
emission in supersonic jet, leading to the jet noise reduction. This has been
proven by Papamoschou and his co-workers (1996, 1997, 1998, 2000a,
2000b, 2001, 2002, 2003, 2004).
21
Such a Mach Wave Elimination(MWE) technique, where a
secondary flow prevents the formation of Mach waves from the primary jet
has been extensively applied by many researchers. Key elements of successful
implementation of MWE are the length of the Mach wave-emitting region of
the jet, and the ability of secondary flow to cover that region. However, a
major drawback of the coaxial arrangement is that the secondary flow reduces
the growth rate of the primary jet, hence lengthens the Mach wave-emitting
region of the jet. Consequently, a thick secondary flow is required to cover
the dominant noise-source region, and to achieve appreciable noise reduction.
In general, the effectiveness of the co-flow in reducing the Mach
waves can be dependent on four major factors as follows;
1) Convective Mach number of the jet eddies relative to the co-
flow. This number should be less than one. The lower it
becomes, the faster the attenuation of the signal within the co-
flow thickness.
2) Convective Mach number of the co-flow eddies relative to the
ambient gas. This number should also be less than one,
although one may tolerate some radiation from the co-flow if
the jet radiation is greatly suppressed.
3) Co-flow thickness. The greater it is, the more disturbances
have to decay sub-sonically before it reaches the ambient gas.
4) Coverage of the Mach wave-emitting region of the jet by the
co-flow. If the co-flow dissipates before the end of this region,
Mach waves will still be generated.
22
Erina Murakami and Dimitri Papamoschouy (2001) revealed that
the flow exiting in a convergent-divergent nozzle operated at off design
conditions exhibits an instability that causes mixing enhancement in the flow
itself and can destabilize an adjacent flow. The latter property enables mixing
enhancement of an arbitrary jet via parallel injection of a secondary gas flow.
In their study they extended the method of mixing enhancement using
secondary parallel injection (MESPI) to high-aspect ratio rectangular (2D)
jets and obtain data on flow structure and scalar mixing for both 2D and
axisymmetric jets. The turbulent structure is visualized using spark schlieren
photography and planar laser induced fluorescence (PLIF). They found that
the reduction in the peak molar concentration of a scalar injected in the
primary flow is 65% in round jets and around 40% in 2D jets.
Gladnick et al (1990) have identified that in turbulent coflowing jet
the inlet boundary profiles of velocity and concentration are found to
influence the decay of centerline, axial, mean velocity significantly.
Narayanan and Damodaran (1994) compared open mixing and confined
mixing cases of two coaxial high speed streams and have stated that confined
mixing needs a shorter distance for complete mixing.
Dhal and Morris (1997a, b, c) used the analytical and numerical
analyses to conduct a parametric study of supersonic coaxial jet. They
considered the instability waves as the dominant source of mixing noise
radiating into the downstream arc of a supersonic jet, when the waves have
the phase velocities that are supersonic relative to ambient conditions. They
calculated both normal velocity profile and inverted velocity profile coaxial
jets with the mixing length model. The stability calculations were used to
predict the noise radiation from coaxial jets with different operating
23
conditions. They showed that normal velocity profile jets can have noise
reductions, compared to the single equivalent jet, especially if the outer jet
stream is hotter than the inner jet stream. No noise reductions are found for
inverted velocity profile jets operated at the minimum noise condition
compared to the single equivalent jet. However, it is inferred that changes in
area ratio can provide noise reduction benefits for inverted velocity profile
jets.
2.3 AXISYMMETRIC JETS AND NON-CIRCULAR JETS
2.3.1 Circular Free Jets
George Papadopoulos et al (1999) presented the centerline velocity
data for a constant density axisymmetric jet having a non uniform initial
velocity distribution that was fully turbulent. The several Reynolds numbers
(Re) investigated showed distinctly the effect of Re on the development of the
jet, specifically the downstream shift of the virtual origin with increasing Re.
This shift of the centerline velocity decay curves was attributed to the initial
turbulence intensity distribution.
Antonia et al (2001) studied the two different circular jets. One
issues from a contraction with a laminar top-hat velocity profile. The other
exits from a pipe with a fully developed turbulent mean velocity profile. In
spite of the significantly different initial conditions, spectra of axial and radial
velocity fluctuations in the far field were analysed. In particular, the
characteristics of both large and small-scale motions remain the same in the
two flows.
24
Valentino Todde et al (2009) analysed the features of a low-
Reynolds number free submerged jet with special regard to statistical
quantities on the jet centerline. The results showed that, at low-Reynolds
numbers, the initial region of the jet is dominated by well-defined vortices in
the shear layer. This result is substantiated by both the statistical moments
and the spectral analysis.
2.3.2 Non-Circular Jets
Schadow et al (1988) have investigated the triangular jet for use as
a passive device to enhance fine-scale mixing and to reduce the coherence of
large-scale structures in the flow. The sharp corners in the jet injector
introduced high instability modes into the flow via the non-symmetric mean
velocity and pressure distribution around the nozzle. While highly coherent
structures could be generated at the flat side, the corner flow was dominated
by highly turbulent small scale eddies.
Dimitri Papamoschou and Marco Debiasi (1999) reported noise
measurements for perfectly expanded coaxial jets composed of a supersonic
primary stream at velocity of 920 m/s and a coflow stream at conditions
designed to prevent formation of Mach waves. Both the primary and
secondary streams consisted of helium–air mixtures to simulate
approximately the conditions of hot flows. The resulting sound field was
compared to that emitted by a single jet at the conditions of the primary
stream. It shows that Mach waves account for at least 85% of the sound field
most relevant to aircraft noise.
Shozo Koshigoe et al (1989) discussed the underlying mechanisms
for the deformation of coherent structures which occurs in the initial stage of
25
the axis switching of noncircular jets. The generalized shooting method is
applied to jets with elliptic core and equilateral triangular core regions of
constant flow. The qualitative behavior of the noncircular jets found through
the numerical analysis is compared with experimental results.
Quinn (1992) studied a turbulent free jet of air issuing from a sharp-
edged square slot. The quantities measured directly, using hot-wire
anemometry include the three components of the velocity vector, and three
Reynolds normal stresses. The higher numerical values of the Reynolds
normal and primary shear stresses in the square jet, compared to those found
in a round jet, indicate faster mixing of the square jet.
Katanoda et al (2000) studied the structures of the axisymmetric
free jets from supersonic nozzles with the exit Mach numbers of 1.5 and 2.0
with special attention to the decay of the Pitot pressures downstream of the
Mach disk. The Pitot pressure probe and schlieren method are used in the
experiments to diagnose the flow field. A TVD numerical method is also
applied to the Euler equations, and the computed jet structures are compared
with experiments. By comparing the numerical computation, it is concluded
that the turbulent momentum transfer to the central region from the region
outside the slip line where the stagnation pressure loss is small.
Sfeir (1979) studied the mean flow field and turbulent intensities of
air jets, issuing from rectangular slots having different geometries and aspect
ratio. In comparison to orifice jets, flows out of rectangular channels are
found to have a two dimensional region which extends further downstream.
Furthermore, turbulent structure of these jets approach closer to a state of self
- Preservation.
26
Lozanova et al (1998) presented an experimental investigation of
turbulent jets issuing from rectangular nozzle. Nozzles with aspect ratios
between 3 and 10 were used. Eight different initial conditions were studied.
The influence of the initial conditions on the similarity of the flow was
determined with respect to the mean axial velocity, turbulence intensity and
the Reynolds stresses.
Guillaume Balarac et al (2005) studied the mixing and coherent
vortices in turbulent coaxial jets. The mixing process is studied by seeding a
passive tracer first in the outer annular jet, then in the inner jet. They
demonstrates the important role played by coherent vortices in the mixing
mechanisms.
2.3.3 Non-Circular Coaxial Jets
There are several works on incompressible non-circular coaxial jets
and well documented. But only a few numbers of works on compressible non-
circular coaxial jets were documented.
Khodadadi et al (1989) studied the turbulent mixing of a primary jet
and its surrounding fluid in a pipe. Detailed profiles of the axial mean and
rms velocities have been measured with a laser Doppler anemometer.
Measured spectra of axial velocity fluctuations show the presence of coherent
structures.
Marco Debiasi and Dimitri Papamoschou (2001) carried out
experiments to characterize the acoustics of axisymmetric high-speed jets at a
variety of Mach numbers and velocities. Also at pressure-matched, over
expanded, and under expanded conditions. The effect of an annular secondary
27
flow on noise emission was also investigated. The secondary flow practically
eliminates the screech tones, but has little impact on broadband shock noise.
With exception of localized and weak screech tones, the fare field spectra in
the direction of peak noise emission (aft quadrant) are insensitive to nozzle
exit pressure and depend solely on the fully expanded Mach number and
velocity. Addition of the secondary flow produces substantial noise reduction
in the after quadrant, a consequence of Mach wave elimination, and modest
noise reduction in the lateral direction, an effect attributed to mean shear
reduction. Lowering the velocity and/or Mach number of the jet enhances the
benefit of the secondary flow by shortening the region of the principal noise
sources, thus improving the coverage of that region by the secondary flow.
Bitting (2001) studied the flows generated by equivalent coaxial
circular and square jets. Visualization results were obtained for three square,
coaxial configurations, and a reference circular coaxial nozzle, at different
velocity ratio of 0.15, 0.22, and 0.3.These indicated that the internal unmixed
region diminished with decreasing velocity ratio. Comparison between
circular and square jet indicated considerable mixing enhancement when
square nozzles were used.
Jimmy Sapede (2002) carried out the measurements in the field of a
coaxial rectangular jet with density variations induced by injection of carbon
dioxide. Numerical simulations were carried out using a commercial CFD
code with standard Reynolds turbulence models.
Papamoschou (2000) analyzed the flow exiting in a convergent-
divergent nozzle operated at off-design conditions. It exhibited a strong
instability that causes mixing enhancement in the flow itself and
destabilize an adjacent flow. The author presents an overview of experiments
28
at U.C. Irvine, and discusses the possible connection to supersonic nozzle
flow separation. A roadmap for better understanding and utilization of the
instability mechanism was proposed.
Sujith et al (2001) experimentally studied the five different
supersonic nozzles conical, elliptical, tabbed, radially lobed and two-
dimensional lobed- are compared their mixing performance under identical
operating conditions. The results of these investigations reveal the superiority
of mixing performance of the two-dimensional lobbed nozzle over
conventional circular and other non-conventional nozzles.
Nikitopoulus et al (2003) compared the mixing characteristics of
initially turbulent, Low-Velocity-Ratio circular and square coaxial jets. The
spectral characteristics of the circular and square nozzle combinations are
qualitatively similar. The dominant frequency at the end of the midfield
within the inner mixing region is lower in the case of the square nozzles
compared to that of the circular ones.
2.4 RECIRCULATION REGION AND VISUALIZATION
The recirculation region is practically important to stabilize a flame
in a combustion chamber technology since the re-circulated combustion
products create a reduced velocity region where flame speed and flow
velocity can be matched. Syred and Beer (1974), Lilley (1977), Leschziner
and Rodi (1984) and Naughton et al (1997) have made experimental works
to investigate the swirl jet, and found that the swirl jet leads to an increase in
the spreading and decay rates of jet, thereby increasing the entrainment rate of
ambient gas into the jet, when compared with the jets of no swirling. It is
known that the swirling intensity is a key parameter to determine the major
29
characteristics of jet. As the swirling intensity exceeds a certain critical value,
a region of reverse flow appears in the central region of the swirling jet since
the force due to axial adverse pressure gradient exceeds the kinetic forces of
the jet flow.
Much effort has been devoted to the recirculation region generation
mechanism in swirl jet. Some studies showed that the formation of free
stagnation point or recirculation region on the jet axis is due to the vortex
breakdown (Hall 1972; Martin 1975; Gore and Ranze 1964; Syred and Beer
1974; Lilley 1977, Vu and Gouldin 1982).
Erina Murakami and Dimitri Papamoschouy (2000) presented the
experimental results on mean flow development and mixing layer
characteristics of single and dual-stream compressible air jets. The results are
relevant to noise emission and mixing enhancement of high-speed turbulent
jets. Coaxial and eccentric nozzle configurations were investigated. In the
coaxial arrangements, the secondary flow reduces the growth rate of the
primary shear layer and elongates the primary potential core and the
supersonic region of the jet. The eccentric configuration shows substantial
improvement in mixing over the coaxial case and achieves an entrainment
rate comparable to that of the single jet when the thickness of the secondary
flow is relatively small. In the eccentric case, the maximum observed
elongation of the primary potential core was 20% relative to the single jet
case. An empirical model for predicting the primary and secondary potential
core lengths of a coaxial jet is proposed.
Champagne and Kromat (2000) studied the near flow filed of
coaxial swirl jet, using flow visualization and hot-wire anemometry, and
reported that the flow is sensitive to both the swirl number and the mass flow
30
ratio between the outer and inner jets. The necessary condition for the
formation of the recirculation zone is that the swirl number must exceed a
minimum value, which depends on the mass flow ratio.
Vu and Gouldin (1982) investigated a model swirl combustor under
non-combusting conditions, and reported that in the coaxial swirl jet
combustor, the outer swirl has a strong effect on the formation of the
recirculation zone, and on mixing characteristics in the jet shear layer. Ribeiro
and Whitelaw (1980), Merkle et al (2003), Guputa et al (2001), Durbin and
Ballal (1996), Gouldin et al (1985) investigated the effects of co- and
counter-swirling airflows on the flame characteristics. They found that
compared to the co-swirl configuration, the counter-swirl condition is better
in the flame stability, which is due to the feature of the recirculation zone.
Interaction of axi-symmentric supersonic twin jets mixing was
studied by enhancement study was carried out by Moustafa (1995) for
supersonic flow conditions. Pressure variation in inlet influences twin jet
propagation considerably. Ramesh Kumar and Job Kurian (1996) conducted
free jet studies using schlieren imaging and observed that secondary confined
flow generates stronger compression-expansion regions.
Durbin and Ballal (1996) measured a generic double-step swirl
combustor fuelled with methane and propane, using the co- and counter-
swirling configurations, and found that the counter-swirl conditions produced
an attached flame for moderate inner swirl intensity, leading to a better flame
stability. Cutler et al (1995) made use of swirl jet to enhance fuel/air mixing
in scramjets using the coflow nozzle, but they did not explain the effects of
the outer stream on the inner swirling jet characteristics.
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Erina Murakami and Dimitri Papamoschou (2000) studied the
morphology and evolution of large turbulent eddies in coaxial supersonic jets.
The study encompassed Mach 1.5, axisymmetric, perfectly expanded jets
composed of air or a mixture of helium and air. A double-exposure
planar laser-induced fluorescence (PLIF) system, with gaseous acetone as the
tracer molecule, enabled visualization of the turbulent structure and of its
evolution a short time later.
The convective velocity of the eddies was extracted from the PLIF
images by means of two-dimensional cross correlations. They found that all
turbulent motions in the coaxial helium-air jet are intrinsically subsonic,
leading to substantial reduction of Mach waves and reduction in noise. A
refined empirical model for eddy convection in compressible jets is proposed.
The results of this study are relevant to mixing, combustion, and jet noise.
The concept of swirl-enhanced mixing is not new. Swithenbank
and Chigier (1968) were early pioneers of the idea of mixing enhancement in
a supersonic flow by swirling the fuel jet. Swirl was generated by tangential
injection into the plenum, accelerated in a nozzle. The vortex breakdown
occurs in the jet, leading to increased entrainment of the ambient gas into the
jet. In their tests, the combustion intensity was increased, indicating a higher
mixing efficiency. Although their tests were limited mainly to subsonic flow
conditions, the reverse flow zone could be found in a transonic swirling jet,
which suggests that these effects could also be achieved in a supersonic jet.
Cutler et al (1999, 2001) further investigated the supersonic
swirling jets. The jet was created by tangential injections into a swirl
32
chamber, and accelerated through a convergent-divergent nozzle. They
observed (i) higher peak helix angles than those observed in the previous
subsonic studies, and (ii) the mixing layer growth rates increased considerably
with swirl.
Dimitri Papamoschu (2000) observed that flow exiting a CD nozzle
operated at off-design conditions exhibits a strong instability that causes
mixing enhancement in the flow itself and can destabilize an adjacent flow.
The later property enables mixing enhancement of an arbitrary jet via parallel
injection of a secondary gas flow.
Povinelli and Ehkers (1972) and Schetz and Swanson (1973)
investigated the swirling jet injected into a supersonic co-flow stream. They
believed that the addition of swirl did hardly enhance the mixing. In contrast
to these works, more recent investigations have documented that the swirl
does substantially enhance compressible turbulent mixing. To prove this,
Cutler et al (1993) performed an experimentation in a Mach 2.2 swirling jet,
using a planar laser scattering technique. Their results revealed that the shear
layer growth rate increased with the degree of swirl and was about three times
higher than that of the non-swirling jet.
Naughton et al (1997) carried out an experimental work for the
mixing enhancement in which various intensity swirls were added to an
axisymmetric jet, and showed that the addition of swirl to the jet increases
entrainment by up to 60%, compared to a corresponding non-swirling case,
and the swirl intensity required to achieve a significant mixing enhancement
is relatively small. They further concluded that the mixing layer growth rate
of the swirling jet is greater than that of non-swirling counterparts.
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Dimitri Papamoschou and Marco Debiasi (2001) demonstrated
the directional suppression of noise from a high-speed jet using an
asymmetric parallel secondary stream. The secondary stream attenuates Mach
wave radiation in the lower hemisphere of the acoustic far field, leaving
unaltered the upward-propagated Mach waves. An eccentric nozzle
arrangement with a Mach 1.5, 700-m/s inner stream and a Mach 1.0, 360-m/s
outer stream was used since it attributed to shorter potential core relative to a
concentric jet. The experiments also revealed the emission of strong crackle
from the untreated jet, a noise component arising from the nonlinearity of
Mach waves.
Gouldin et al (1985) studied the effect of the sensitivity of inlet and
boundary conditions on a swirl combustor by means of measuring the velocity
of the premixed coaxial jet, and reported that the recirculation zone can be
generated both in co-swirl and counter-swirl conditions for reacting flow, but
only in counter-swirl for non-reacting flow.
Dimitri Papamoschou (1997) presented the experimental results
that eliminates Mach waves from the exhaust of supersonic jets, hence
removes a string component of supersonic jet noise. Elimination is achieved
by surrounding the jet with an annular stream at prescribed velocity and
temperature so that all turbulent motions become intrinsically subsonice. No
mechanical suppressors are used. Implementation of the technique in a typical
turbofan engine is estimated to increase take-off thrust with minimal impact
overall fuel consumtion.
34
Cutler and White (2001) in an experimental and CFD study of a
supersonic coaxial jet have found that, if the center jet is of a light gas and the
coflow jet is of air, then the mixing layer between them is compressible. The
jet flow field ass characterized using Schlieren imaging, surveys with pitot,
total temperature and gas sampling probes. The results are compared to the
experiment for several variations of the k- turbulence model.
Lovaraju and Rathakrishnan (2006) analyzed the cross-wire
effectiveness for subsonic and sonic jet control. Mach 0.4, 0.6, 0.8, and 1.0
axisymmetric jets from a convergent nozzle with cross-wire along a diameter
at the exit were studied. The cross-wire was found to be effective in
promoting jet mixing right from the nozzle exit, at all Mach numbers. For the
Under expanded sonic jet at nozzle pressure ratio 3, 5, and 7, the cross-wire
influenced the core and the shock cells, causing significant reduction of core
length and weakening of the shocks at all levels of under expansion. The jets
from the nozzle with cross-wire spread faster in the direction normal to the
cross-wire.
Dimitri Papamoschou et al (2003) studied initially turbulent, low-
velocity-ratio circular and square coaxial jets. Visualizations and local
velocity measurements indicate modest mixing enhancement when square
nozzles are used compared to the axisymmetric ones. Dominant frequency at
the end of the midfield within the inner mixing region is lower in the case of
the square.
Abel vergas and Ahsan Choudhuri (2003) conducted studies on
elliptic coaxial jets. The near-field flow characteristics of a turbulent elliptical
coaxial jet with 0.55 and 1.45 velocity ratios (m = Uo/Ui) were numerically
computed and experimentally measured. Elliptical coflow analyses reveal the
35
jet approaching symmetry at x = 30.48 cm. The rms axial velocity is minimal,
less than 0.5 m/s at the center of the jet up to x = 25.4 cm for both velocity
ratios. Sarma et al (2003) in studying the spreading characteristics of jets
from several asymmetric nozzles compared a set of rectangular orifices
covering a jet Mach number range of 0.3–2.0. Curiously, the jet from a
‘lobed’ nozzle spreads much less at supersonic condition compared to all
other cases.
Pinnam Lovaraju and Rathakrishnan (2011) studied experimentally
the effect of an annular co-flow jet on the center jet at subsonic, correctly
expanded and under expanded sonic conditions. It is found that with co-flow
core length elongation of 40% and 80% were achieved for correctly expanded
and under expanded (NPR 7) sonic jets, respectively. Shadowgraph pictures
show that the co-flow is effective in preserving the shock-cell structures of the
inner jet, making the jet to propagate to a greater axial distance which
otherwise would have decayed faster.
Yu (2004) observed that the spreading rate at u = 0.5 was found to
be the highest among the nozzles investigated. Also found that the spreading
rate were higher for non circular jets than their circular counterparts under
similar flow conditions. Sharma et al (2008) found that the co-flow acts as
mixing inhibitor at all levels of overexpansion for Mach 2 circular nozzle. A
negligible core elongation was also observed.
Recent studies also have indicated that liquid core coaxial jets
operating at supercritical conditions (which are common in modern rocket
engines) scale in a similar-manner as single-phase coaxial jets (Davis et al
2006). Consequently, studying the mixing properties of gas/gas coaxial jets
can provide insight into the processes that are occurring in the supercritical
36
case and have the added advantage of being able to apply certain experimental
methods and having well defined fluid transport properties. Since coaxial
injectors already are used in legacy engines and posses a simple geometry yet
a complex flow field, they are ideal for validating the next generation of
modeling tools. Using gas/gas injection has the advantage that the mixing and
combustion models can be exercised without the added complexity of a
droplet breakup model.
In spite of a number of experimental works on the supersonic, dual,
coaxial jets, even the near field flow structures are not understood well.
Existing data are in conflict with other results and the effects of the outer
secondary flow on the structure of inner jet still remain ambiguous.
Especially, the data on supersonic, coaxial jet is few and very
limited to subsonic coaxial jets. Further study is needed to detail the
supersonic, coaxial free and non-circular jet for practical engineering
applications. These reasons and the desire to increase the basic understanding
of the complex interaction between turbulent shear flows, have motivated the
present systematic study of turbulent coaxial jet mixing.
The present study is initiated as a fundamental step of a long-term
project regarding the supersonic coaxial jet technologies.