Chapter 2

LITERATURE REVIEW

Rocket Based Combined Cycle (RBCC) engines have high specific impulse

to operate over a wide range of Mach numhers 4 . Such vehicles would open up low

cost access to space. Since there is no single propulsion system capable of' operating

in the entire range. h ybrid vehicles have been evolved. Such system comprises

difThrent propulsion devices operating in different regimes of flight and has both air

breathing and non-air breathing stages

In the air breathing stage of the hypersonic vehicles Scram* engines are

used. Various con figurations have been suggested for the Scramjet engine concept.

An important one is to combine various propulsion systems to achieve better

performance and wide range of application. A dual combustor rarnjet engine (DCR)

incorporates the features of a ramjet and a Scramjet. In the E)CR cycle, a portion

of the inlet air is diffused to subsonic speed and admitted to a subsonic combustor

hich operates in a fuel rich condition. The hot gases issuing from this combustor

carry unhurnt fuel. which undergoes combustion inside the supersonic combustor to

hich a major portion of the inlet air is admitted. Another composite engine which

makes use of supersonic combustion is the air augmented rocket (AAR) 9 II) which

combines the features of a rocket and a Scramjet.

2.1 REVIEW ON MIXING OF SUPERSONIC FLOWS

A major feature of the Scramjet development is the quality of mixing and

combustion inside the combustor. The mixing of coaxial jets is governed by the

growth of the shear layer between them. Mixing in supersonic shear layer is

dependent on the compressibility efThct in addition to the velocity and density across

the shear layer. The compressibility level is described by a parameter called the

convective Mach number (Mc) by Papanioschu and It is defined as the

relative convective speed of the major streams of the shear layer to one of the

streams, normalized by the speed of sound of that stream. Elliot and Samimy'2

documented the slow growth rate of compressible shear layers as compared to

incompressible ones under identical conditions. l]ie effect of compressibility on

molecular mixing in free shear layers is shown by hall et al''. Heat release due to

chemical reaction is used to characterise the extent of molecular mixing between the

fuel and oxidizer. The results revealed that the compressibility significantly reduced

the entrainment iii comparison to the theoretical predictions for incompressible

flows.

lhe studies on the shear la yer reveal that in high-speed jets the shear layer

groth is extremel y slow due to the effects of compressibility and hence the shear

induced mixing is retarded. A suitable mixing technique is necessary to achieve rapid

and effective mixing of the high speed flows for the Scramjet combustor.

l)ifl'erent injection have been proposed with par.ticular concern

lr rapid held mixing. These injection strategies. both flush mounted injectors and

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intrusive in;eCtOrs. typically rel y on the generation of strong stream wise counter

rotating vortices. As a result. mixing is enhanced both in macro scale by entrainment

of large quantities of air into the fuel and in micro scale due to stretching of the fuel

and air interface. Stretching increases the interface area. simultaneously increasing

the local concentration gradients. thereby enhancing the diffusive micro scale

mixing. Micro scale mixing is required for combustion because chemical reactions

occur at the molecular level. however, efficient mixing of fuel and air does not

directl y initiate the combustion process.

The important factors 2 ' for the design of an injection s y stem is ignition and

flame holding. Once ignition is established, the efficiency of combustion depends

directly on the efficiency of mixing. The ignition is considered to he accomplished

cvcii though no appreciable heat has been released when sufficient free radicals are

formed to initiate the reaction system.. The primary objective of a flame holder in

supersonic combustion is to reduce the ignition delay and to provide a continuous

source of radicals for the chemical reaction to he established within a short distance.

Flame holding is achieved by organization of a recirculation area here the

fuel and air can he mixed partially at low velocities, interaction of a shock wave with

partially or full mixed fuel-oxidizer formation of coherent streams containing

unmixed fuel and air. Here a diffusion flame occurs as the gases are convected down

stream.

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One of the simplest approaches for flame holding is the transverse injection

of fuel from a wall orifice. As a fidel jet intersects across the Supersonic flow, a how

shock wave is produced. As a result. the upstream wall boundar y layer separates.

providing a reuion where the boundary layer and the jet fluid mix sub-sonically

upstream of the jet exit. This region is important in transverse injection flow fields

because of its flame holding capability after combustion 24 . I lowever. the transverse

injection configuration has stagnation pressure loss due to strong three dimensional

hov shock fornied by the normal jet penetration, particularly at high flight velocities.

Another way of achieving flame stabilization-' 26 is by means of a step followed by

transvcrse injection. The step creates a large recirculation area where hot gases serve

as a continuous source for ignition. This approach provides sustained combustion

however it has the disadvantage of stagnation pressure losses and increase in drag

due to the lo flow pressure base behind the step.

Mans researchers reported to have examined the flow field of modified

nozzles and their effectiveness in mixing enhancement. Quinn 27 studied the flow

field of a turbulent jet from a rectangular nozzle. 1 he results show enhanced mixing

of rectangular jet as compared to a turbulent circular jet. Schadow et al 2 compared

subsonic and supersonic elliptic jets and found increased growth rate in the case of -

supersonic flow. Husain and Hussain 29 reported the use of elliptical nozzles to

enhance mixing and jet noise control. Gutmark et a 13 studied mixing performance

of elliptical and rectangular jets and compared it with conical jet in subsonic and

supersonic flow regimes. It is observed that the elliptical jet gives better mixing in

sonic and supersonic streams.

Ihe mixing enhancement found in non-conventional nozzles as compared to

axismmetric nozzles of the same area led to the development of forced mixers. In

the flow ticld of forced mixers. large-scale stream wise vortices influence the mixing

process'. Werle et al discussed the flow field structures in an array of axial

vortices generated h forced mixed lobes. Tilman et al 37 demonstrated the use of

lobbed nozzle to enhance mixing between a supersonic jet and a co-flowing subsonic

jet. The flow field of the jet is studied by mapping the temperature and velocity

distributions. The uni forrnitv of distribution indicates the extent of mixing of the

Acts.

lxperimental studies using petal nozzle for mixing enhancement carried out

by Narayanan and l)arnodaran showed effectiveness of mixing of a supersonic

jet mth a co-flowing sonic jet in an axisy rnmetric mixing tube. Manohar ci al-1

observed better mixing performance of radially lobed nozzles in supersonic mixer

ejectors. Ramesh Kumar and Kurian 42reported that with increased lobe height

and number of lobes enhance mixing albeit with large total pressure losses.

Srikrishnan ci a1 44 demonstrated the thermal mixing performance of radially lobed

petal nozzles. Ken ci al introduced ramps for mixing enhancement of supersonic

flow and observed increased shear layer growth rate in the presence of ramps.

Rradhurv and Khadem 46 examined the efficacy of inserting tabs in to supersonic

stream to achieve better mixing. A number of researchers also have reported the use

of vortex generators47 in the flow field for mixing enhancement. 1lari 8 ' compared

effectiveness of ramp nozzle and a nozzle with secondary tabs with that of a conical

nozzle for mixing ola supersonic jet ith a co-axial sonic jet.

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Ihe revie 38-53 of different methods for mixing of supersonic flows reveal the

inevitabilit y of large stagnation pressure losses incurred, though some brought out

very good results as regards to mixing. Hence further exploration of other better

techniques to enhance high speed mixing is warranted. The present study focuses on

the novel method of mixing of' co-axial supersonic flows viz., the use of cavity

induced acoustic oscillations.

2.2 REVIEW OF FLOW 1AST CAVITIES

In this context. \all mounted cavities impose acoustic oscillations which

influence the compressible shear layers in high-speed jets and hence enhances

mixing. Considerable amount of work 4 ' has been carried out in the past on cavities

andand their effect in the flow field. The motivation of most of these studies is to

suppress the high amplitude oscillations incurred due to the presence of depressions

like heel wells and bomb bas that caused vibrations and structural failure of the

aerod y naniic bod\ oh air-crafts. Ihe possibility of the use of ca ities for supersonic

mixing has been proposed er y rccentlN as cavity oscillations can induce instabilities

in the shear layer. I arlier in I 95. Krishnamurth found that acoustic oscillations

are generated due to instabilities in the shear layer at the lip of the cavity when how

occurs over rectangular cutout.

Charwat et al' studied several types of separated regions such as blunt base

aLes and cavities formed in cutouts in the boundaries and ahead of or behind tvvo

dimensional steps in supersonic and subsonic flow. Describin g the ca itieS as open

or closed, they reported that there is a maximum length of the separated free shear

la er to the depth of the depression in the boundar y beyond which the cavity

collapses leaving behind independent and separated regions at each of' the

protrusions. The y revealed the extent of- heat transfer from the cavity depending on

the thickness of the oncoming houndar layer.

tliang and Fdwards 7 experimentally studied supersonic flow past tandem

cavities. l'hev ftund that the flow characteristic of cavity is affected by the presence

of a preceding cavit y and the effect varied depending upon the mode of' oscillations

of the first cavit y . When the two ca l. ities are of transverse mode oscillatory type. the

first cavity tends to damp the high frequency modes and the second cavity enhances

the dominant mode. Oil hand. when the cavities are Of longitudinal mode

oscillatory t y pe, the disturbances generated by the first cavit y are carried downstream

and amplified b y the second ca Ity but the drag in this case is f'ound to be more.

Sarno and Franke proposed two methods for suppressing the flow induced

pressure oscillations in shallow ca ities for a range of subsonic to supersonic flow

Conditions. A static fence at the leading edge of the cavity with thickness equal to or

greater than the houndar layer thickness is found to he most effective in suppressing

the oscillations. A stead y and pulsating flow injection at 450 is also fOund suitable

fOr suppression. but parallel injection in both the cases is not effective.

I lardin and Pope studied the sound generated by lo Mach number and low

Re'nolds number flow over cavities. lhcir computational studies demonstrated the

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dependence of ca itv oscillations oil boundarx layer height. Fxperiments of

I baker et a1 6° for characterisation of acoustic field induced in supersonic confined

flow over cavities showed that the pressure oscillations from the cavity are not

merel y acoustic in nature but are a1S0 shock related.

2.3 REVIEW OF ACOUSTICS ON MIXING PROCESS

1 he cl'fect of' acoustic field on jet spread and augmentation of' mixing have

been anaNsed by a few researchers. (Ilass studied the effect of acoustic feed hack

Oil the spreading of' supersonic free jets by reflectors. The acoustic waves are just

pressure disturbances emitted by unstable shear layer that rolls up as vortices at the

jet boundary . I hese acoustic waves get reflected from the reflectors kept near the jet

boundary and interact \\ ith other parts of the shear layer in the jet boundary and alter

the jet spread rate. It is concluded that there is a substantial increase in spreading of

jet clue to the acoustic feed back from the reflectors.

Pimshtcin carried out experiments oil supersonic jet flow

\ ith externall y imposed acoustic waves at a frequency of 10 kl-lz and up to an

intensit y of' nearl y 170 dB by changing the direction with respect to free stream and

the distance From the nozzle exit. The shadowgraph images of the flow revealed the

quantitative information about the shear la yer thickness. The result also indicated the

high jet spread rate fr an oblique incidence of the acoustic wave to the jet boundary

and the interaction of sound with the jet in the near field of the nozzle exit.

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Raman and Rice studied the effect of two harmonicall y related tones, one at

fundamental mode and the other at its sub harmonic mode. on the jet spread. They

aricd the initial phase difference and the amplitude of' the two modes. The

remarkable outcome is that the two-frequency excitation is more eflctive in Jet

spread than the single f'i'equenc excitation and with higher sub harmonic levels

higher jet spread is possible.

The outcome of' the above studies provided the direction for further

investigations to explore the use of cavities as a potential means to enhance mixing

of' supersonic flows. [he ca liv flow oscillations can actuall y he used to enhance

mixing in supersonic flow because the instabilities in the shear layer develop long

avelength fdr Kelvin-I lelmholtz instabilit y ayes. Studies by Kumar ci al 4 proved

that use of' an oblique high frequency oscillating shock wave increases K-11

instability.

2.4 REVIEW ON CAVITY ASSISTED MIXING

[he studies on ca ity assisted mixing of' supersonic streams are scant and

only a reports are mailable for reference. Yu and Schado have demonstrated

the enhancement of mixing of' supersonic reacting and non-reacting jets by a cavity

attached at the exit of' a supersonic nozzle. The cavities are two dimensional as well

as semi annular and longitudinal ith rectangular cross section. When the cavities

are tuned for certain frequencies. large-scale highly coherent structures are produced

in the shear layer substantially increasing the growth rate. The spreading rate of' the

initial shear layer increased b y a factor of' three for a jet ith cons ecti\ e Mach

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number M0.5 and for a let with M 1.4. a 50% increase in jet spread is

observed. For reacting jets. the cavity actuated forcing technique reduces the

allerhurning flame length b y 20-30% with modified intensity.

Sato et aI 1,6 studied the effect ot' acoustic waves emitting from the cavity and

the impingement on the initial mixing layer. Their experiments revealed that the

mixing is enhanced by the acoustic disturbances and the rate of increase is controlled

by cavity shape while the total pressure loss is negligibly small.

2.5 REVIEW ON CAVITIES AS FLAME HOLDERS

In the past few ears researchers have proposed the use of cavities as flame

holders in supersonic combustors. The idea is to create recirculation zone inside the

cavity with a hot pool of radicals. in order to reduce the induction time so that auto

i gnition of the fuel and air mixture is achieved. This in turn pros ides a region

conduci e fur stable combustion. In recent ears. cavit y flame holders. an integrated

fuel in j ection\llame-holding approach. have been proposed as a new concept for

flame holding and stabilization in supersonic combustors. Cavity flame holders.

designed by the Central Institution of Aviation Motors (ClAM) in Moscow. are

used for a joint Russia/l-'rench dual mode Scramjet flight test. using hydrogen as fuel.

V inogrado' et a l < experimented with cavities as flame holders on kerosene operated

supersonic combustors. A ro\ of hydrogen fuel injectors placed in front 01'a cavity

is used to achic c sustained combustion.

Gruber et al extensively studied experimentally and computationall) the

efficacy of' cavities as flame holders in supersonic flow In their studies. they used

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long cavities of difThrent oliset ratios with various all ramp angles. Their results

shoved that the all ramp angle characterises the nature of shear layer that spans the

length of the cavit\ Decrease iii all ramp angle below 90"gives more stable two-

dimensional fields and the separation wave changes 1mm compressive to expansive

as the alt ramp angle is reduced. The cavit y fure wall pressure steadil y decreases

ith decreasing ramp an g le. Reduction in ramp angle results in higher drag

coellicients and shorter residence time within the cavity.

Yu ci al studied several wall mounted cavities in a non-reacting Mach2

llo I he influence of cavity geonletry on the combustion of a fuel jet injected

upstream of the cavit y is studied. Their results suggested that small aspect ratio

cavities provide better flame holding than longer ca ities ith inclined all ramp

angles.

l-4en-Yakar and I hanson have investigated a hiuh speed. high total

temperature flo\ over cavities to understand the fluid dynamics and the flame

holding characteristics. 1 lie shock wave structure around the cavity changes from

compressive to expansive in nature as the ratio of length to depth of the cavity

increases from, I .'I) 3 to 7. For large values of L./I), the leading edge shock pattern

gets diminished. Schlieren flow images reveal the shock wave fluctuations aroLind

the jet caLised by the pressure oscillations inside the cavity.

Mathur et al' conducted experiments oil cavity based flame holder with

low angle flush wall fuel injection upstream of the cavity using gaseous ethylene in

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t o lIicilitv nozzles to simulate supersonic combustor. with the inlet flow properties

appropriate to flight Mach numbers between 4 and 6 at a d y namic pressure of

47.9kPa. The results reveal that the cavity flame holder concept proved the

efIctiveness over a wide range of operating conditions and the combustor fuel and

air equivalence ratios.

Hsu et al 7 ' injected foci and air into an axisvmmetric cavity to achieve low

speed flame stabilisation. Stable flames resulted for cavity length between 0.45 and

0.65 of all ail diameters. Longer cavities produced unstable flames, while shorter

cavities lacked enouh volume of flame holding. With direct injection of fuel and

air. stable combustion is obtained at a cavit\ aspect ratio that corresponds to

minimum drag and entrainment. Similar results are obtained by Katta et a1 74 'The

computational fluid d y namics calculations for a limited I ID range is suffice for

ac hi cvi n a stable flame tone.

Davis and Bov ersox 7 ' used a combined computational fluid

d\ namics/periectly stirred reactor methodology as a design guide for sizing of the

cavity . l'hey recommended that initial cavity size may be estimated based on the

minimum residence time required to obtain ignition h\ assuming a perlctl stirred

reactor ca itv flow. l3aurle et a1 7 studied the cold flow characteristics of cavity flow

fields for Scramjet flame holding applications b y computational means. Their results

reveal that the ca\: ity length has the largest influence on the mass entrainment rate

hile the ca ity depth primaril y determines the residence time.

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Segal et al 77 carried out experiments on a wall mounted cavity formed by a

rearward facing step which is directly connected to a to dimensional wind tunnel at

Mach number oil .8. they studied the stability of' the flame in a recirculation region

is formed within the ca it. Pilot flame is provided by the ignition of hydrogen

injected through the base of' the cavity. Kerosene is injected into the boundary layer

Upstream of the step for obtaining a combustible mixture capable of' maintaining

cavit y flame once h\ drogen injection is stopped. three diIl'erent step configurations

al-e studied. The results revealed that the cavities are regions of' rich mixture in

hich the flame stabilit y is strongly affected by the stagnation temperature.

Stagnation temperature of' 1000K results in rich mixture in the cavity and addition of'

pre-injected kerosene cause flame extinction except in case of' large hydrogen flow

rates. This is due to the richer boundar y layer at high temperatures which entrains

onl\ less fresh oxidizer at this region. The recirculation reiogn increased with

increased hydrogen llo rates. At stagnation temperature of 300 K the flame is held

even at larue kerosene llo rates.

Morgenstern et al on h y personic flow past a cavity observed large vortex

structures at the rear of the cavit y in a comparative study by experimental and

computational nieans. Self' sustained oscillating lTh)tiOfl occurs v ithin the cavity over

a range of Re y nolds numbers and cavity dimensions. The frequency spectra of the

oscillations show good agreement with the computed ones.

Yu et al conducted experiments on combustion stabilization in liquid

kerosene-fueled model Scramjct combustor Utilizin g the concepts of' effervescent

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atomization with either air or hydrogen being a harhotage gas. and cavity as a flame

holder. The entry Mach number is fixed at 2.5 and the total pressure and temperature

of ' vitiated air varied around 1.0 to 1 .3 MPa and 1700K to 1900K respectively.

l3arhotaged atomization using h ydrogen promotes the overall burning and increases

the combustion efficiency by 15 to 20 % compared to the pure liquid atomization

case. The combustion performance is found to increase with increasing cavit y depth.

hich essentially characterises the cavity residence time. Further increasing the

depth of the cavity flame holder from 12 mm did not provide much variation in

conihistion pertornance.

Yu et al have investigated the stable and unstable characteristics of a cavity

flow with an emphasis oil phenomenon of flow induced cavit y resonance. It is

fund that the stable and unstable cavities could he used for flame holding and

mixing enhancement. respectively. As such, combining open and closed cavities in

tandem would be a promising approach to provide both flame holding and mixing

enhancement.

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