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

19

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.