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CHAPTER 2

REVIEW OF LITERATURE

2.1 INTRODUCTION

Exhaust diffusers are critical components of a gas turbine in both the

propulsion and power system applications. The flow through these diffusers is

receiving a considerable attention in the present because of its significant

impact on overall efficiency and range of applications. In the area of diffuser

flow analysis, emphasis has been placed on the radial, planar, and conical

configurations with noticeably fewer models on annular diffusers. A

considerable amount of investigations have been made on annular diffusers.

These investigations can be classified into two categories namely (a)

Experimental investigations and (b) Numerical investigations. In this chapter,

a survey of the existing literature in the field of diffusers is presented. The

scope of the present investigation is also presented.

2.2 EXPERIMENTAL INVESTIGATIONS

In this section the survey of literature on experimental investigations

on annular, conical and other forms of diffusers are presented.

2.2.1 Annular Diffuser

Adenubi (1976) investigated the effects of transient inlet flow

parameters on flow regime, the performance and the mechanism of the

annular diffusers in the downstream of the turbomachinery. Three diffuser

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characteristics were used, all having non- diverging cylindrical inner walls

and conical outer walls with divergence angles of 5o, 10o and 15o resulting in

area ratios of 1.5, 2.0 and 2.6. He studied the parameters such as pressure

recovery, total pressure loss, mean velocity profile and turbulence structure

along the length of diffusers. The author reported that the static pressure

recovery of the 5o and 10o diffuser agreed well with Sovran and Klomp (1967)

configuration which implies that the standard design techniques are valid even

for operating downstream of turbomachines. He concluded that the

improvement of diffuser static pressure recovery with inlet Reynolds number

may be due to an increase in turbulence intensity.

Japkise and Pampreen (1979) tested two automotive gas turbine

diffusers to a variety of inlet swirl and inlet Mach number parameters in

various exit conditions. They analysed the performance of two automotive gas

turbine diffusers. They concluded that pressure recovery in interstage

diffusers is comparable to values that would be achieved in equivalent straight

walled diffusers; the recovery achieved in the exhaust diffusers falls below

the equivalent straight wall recovery levels.

Lohmann et al (1979) experimentally investigated the performance

of a series of diffusers of various lengths, area ratios and cant angles with

swirl. They reported that increase in the inlet swirl angle and the cant angle,

lead to increased distortion of the meridional velocity profiles at the diffuser

exit. They inferred that high diffuser cant angles lead to high losses and the

presence of swirling through flow alters the structure of turbulence in the

diffuser causing premature separation from the inner wall.

Kumar and Kumar (1980) have investigated the effect of swirl on

flow through annular diffusers having diverging hub and casing boundaries.

The static pressure distributions and the axial and radial velocities were

measured using a three-hole cobra probe. It was observed that the inlet swirl

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tends to increase the overall pressure recovery of the diffusers; the increase is

more significant for stalled diffusers than for well behaved diffusers.

Pfeil and Going (1987) presented the boundary layer measurements

at the outer wall of an annular diffuser behind a one stage axial compressor.

The measurements were performed to show the character of turbulent

boundary layers in diffusers behind a turbomachine. The characters of the

boundary layers at the two lines of traverse positions were different, caused

by secondary effects and wake flows of stator blades. Their results showed

that the boundary layers between two stator blades are two dimensional and

the law-of-the-wall region showed a three dimensional character.

Hobson and Jedwab (1990) investigated the effect of eccentricity on

the unsteady fluid forces on the centre body of an annular diffuser. They used

a vibration test rig in an annular diffuser, where the fluid force on a centre

body fixed or forced to vibrate over a range of frequencies and amplitude

were observed directly. They found that, with a fixed centre body, the flow

can exist in two states characterised by the presence or absence of vortex

shedding. They concluded that the self induced components of the fluid forces

on the centre body are consistent with negative fluid damping when no vortex

state in contrast to the positive fluid damping found with the vortex state.

Fric et al (1996) have studied various strut designs in an annular

exhaust diffuser. They conducted the tests in three phases – the first two with

scaled down models and the third with a full – scale gas turbine. They used

five strut designs namely baseline, vortex generator, tapered chord, long

chord and vented struts. From the first test, they observed and measured the

vortex shedding from the baseline strut. The new strut designs were tested in

order to achieve vortex shedding amplitude reduction and frequency shift.

They reported that the vortex shedding modification was achieved with

passive control techniques such as a tapered chord, vortex generators, and

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vented struts. From the scale-model tests, it is seen that the dual tapered

design resulted in an amplitude reduction and shift in wake frequencies to

higher levels. From the full-scale tests at full-speed, no-load (FSNL)

conditions, the authors have reported that the tapered struts were totally

effective in eliminating vortex shedding amplitude. It also reduced the broad-

band noise from the exhaust frame from 1 to 5 dB, depending on the inlet

guide vane setting.

Djebedjian and Renaudeaux (1998) have done numerical and

experimental investigations in an axial steam turbine engine exhaust diffuser

with and without swirl at inlet. The measurements were carried out with five

hole probes and hot film probes. They used 22 cylindrical struts (20 inclined

and 2 vertical struts) in the exhaust diffuser passage for the rigidity of the

outer casing. The numerical code used was based on the resolution of the

time-averaged equations of conservation of mass and momentum. They used

standard k - model of Launder Spalding (1974) and Reynolds stress model

for modelling the turbulence. The code is based on Finite – volume approach

using a non-staggered grid arrangement. Computations were performed for

turbulent flow in a sector of 1/8 of exhaust diffuser (2 struts/45° of the total

geometry). Hence, only 16 struts were considered for computations while

experiments were conducted for 22 struts. From the experiments, they

concluded that the pressure recovery is more or less influenced by the

interaction between the two struts’ wakes. Both turbulence models over

predicted the pressure recovery coefficient compared to that of the

experimental values. However, the RSM model predications are better when

compared to that of standard k - model. The authors concluded that over

prediction by both models is due to the discretisation scheme, wall function

approach and swirl treatment in the model.

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Ubertini and Desideri (2000 a) have analysed the effect of struts on

the performance of an annular diffuser. They used a scale down model of an

exhaust diffuser of the PGT 10 industrial gas turbine. They reported that

higher pressure recovery gradient was observed behind the struts. It was

concluded the overall diffuser loss is increased by the presence of struts. They

suggested that different shapes of the struts could be studied in order to

develop an optimum strut design. They (2000 b) also studied the mean flow

characteristics and turbulent length scales in the exhaust diffuser of the gas

turbine. They used 35% scale down model of a gas turbine diffuser. A Dantec

make split film were used for the measurement. They reported that the flow in

the diffuser was distorted by the presence of the struts. There was interaction

between the struts and inlet guide vane wakes in the separation process

around the hub and the shell. They calculated the turbulent length scales by

using Kolmogorov theory and it was revealed that the flow is not isotropic

everywhere.

Sheeba and Ganesan (2005) studied the pressure recovery in annular

diffuser with and without struts. They analysed experimentally and

numerically in an annular prediffuser of a marine gas turbine with a half cone

angle of 5°. They used diffuser with stream line centre body with circular

struts, diffuser with stream line centre body without struts and diffuser with

stream line centre body with aerofoil shape struts for their studies. They

concluded that the struts’ area and their wakes were interrupting the diffusion

of the diffuser. They also found that the presence of the struts increases the

pressure loss in diffuser. Further, the effectiveness of the diffuser with

circular strut is 3% lower than the diffuser with aerofoil shaped struts with

same blockage.

Cherry et al (2010) have made detailed three component velocity

measurements for two different annular diffusers with and without upstream

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wake disturbances. The first diffuser was designed to be far from separation

and the other diffuser was designed to operate near stall. The diffusers were

tested with and without a set of upstream airfoil - shaped struts to asses the

effect of inlet disturbances. The working fluid for all of the experiments was a

0.1 M copper sulphate solution in water. All velocity measurements were

made using Magnetic Resonance Velocimetry (MRV) using the phase-

contrast techniques. They reported the small - angle diffuser did not stall

while the larger –angle diffuser showed very small regions of reversed flow

both with and without the wake disturbances.

2.2.2 Conical Diffuser

So (1967) studied the behavior of rotating flow in a conical diffuser.

They found five distinct flow regimes representing three basic types of vortex

flow and two transitional phenomena. The first transition is characterised by

stagnant bubble which is followed by a one celled vortex flow. The second

transition relates a one celled vortex to a two celled vortex. A preliminary

analysis was performed in an attempt to predict the vortex decay in the

diffuser. The analysis achieved a qualitative agreement with the observed

vortex decay but there was a significant discrepancy between the theory and

experiment.

The effect of swirling inlet flow on pressure recovery in conical

diffusers was studied by McDonald et al (1971). They tested twenty four

different diffusers with total divergence angles from 4° to 31.2° and with area

ratio from 1.3 to 8.27. They concluded that the introduction of swirl has little

effect on unstalled for axial flow, addition of swirl improve the performance

of badly stalled diffuser. Further, they concluded that swirl improves the over-

all performance of a turbomachinery.

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Okwuobi and Azad (1973) conducted experiments in a conical

diffuser to study the structure of turbulence with a total divergence angle of 8°

and an area ratio of 4:1 with Reynolds number of 293000. The results showed

that the rate of turbulent energy production reaches a maximum value at the

edge of the wall layer extending to the point of maximum ‘u’ fluctuation.

They found that u12 varies linearly with the distance from the wall and linear

range grows with distance in the downstream direction. Further they

concluded that the energy convective diffusion due to kinetic and pressure

effects is comparable with the energy production

Azad and Kassab (1989) examined the turbulent flow in a conical

diffuser through determination of the mean pressures, mean strain rates,

energy, shear stress, triple products, length scale, balances of energy and

shear stress. They found some quantities to be more revealing than others in

pointing out the complexity of the flow subjected to an adverse pressure

gradient. They have found that the expanding wall region toward the exit has

a low mean velocity and very high turbulence intensity. They concluded that a

diffuser flow has two regions; a distinct initial region – characterised by extra

strain rate; and a distinct final region – characterised by different level and

different extent of equilibrium state.

Singh and Azad (1995) have investigated the flow through a conical

diffuser with high turbulence intensity. Pulse-wire anemometry was used to

measure the instantaneous reversal. They concluded that an increase in entry

Reynolds number decreases the size of near-wall instantaneous reversals

region. At higher Reynolds number the region of instantaneous reversals

moves slightly downstream of the turbomachiney. The wall-layer and the

central region of a conical diffuser were strongly influenced by initiation and

growth of instantaneous reversal. Further, they concluded that the cross-

stream pressure gradient is larger than the longitudinal pressure gradient

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which indicates the importance of the v-momentum equation in a conical

diffuser flow.

In their another study (1995), they studied the instantaneous flow

reversals and velocity filed in a conical diffuser. Experiments were conducted

using hot-wire anemometry and pulse wire technique. They conducted

experiments using the area ratio 4:1, total divergence angle 8° and entry

Reynolds number of 6.9 x 10 4 . They observed that the nature of the near –

wall flow filed is changed due to the initiation and growth of instantaneous

flow reversals. They concluded that in the near wall region of the end stages

of a conical diffuser the streamwise fluctuating velocities and wall shear

stress fluctuating were higher than the mean velocities and mean wall shear

stress respectively. They found that the pressure recovery and effectiveness of

the tested diffuser were 75% and 80% respectively.

Azad (1996) reviewed two conical diffusers namely a cast

aluminum and a plastic diffuser with total divergence angle of 8° and an area

ratio of 4:1. The focus was mainly on a conical diffuser flow experimentally

investigated since 1966. He used both hot-wire anemometer and a pulsed wire

anemometer. He presented the measurements of mean velocity, Reynolds

stress and the 4th order moment of turbulence fluctuations. He reported that

the turbulent flow is symmetrical for all mean values in the conical diffuser.

Further, he reported the diffuser flow has core and wall layers possessing

distinct dynamical behaviour.

Mahalakshmi et al (2007) investigated the flow through conical

diffusers with and without wake type velocity distortions at inlet. They tested

two conical diffusers of having half – cone angles 5° and 7°. They used two

wake producing bodies namely a bluff body and a streamlined body. They

found that for the 5°diffuser, there is a marginal increase in pressure recovery

with the presence of centre bodies. For the 7° diffuser, in the case of the

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streamline body, the wake grown under adverse pressure gradient conditions.

They also found that the pressure recovery is not affected when there is wake

at the diffuser inlet. In case of the bluff body, the wake decay rate is arrested

due to the interaction between boundary layer and wake.

Karunakaran and Ganesan (2009) studied flow through a conical

diffuser with and without inlet flow distortion. They used an axisymmetric 5°

conical diffuser. The inlet flow distortion is created by a bluff body placed at

the inlet of the diffuser. A five – hole pitot sphere of cobra type probe with

electronic digital manometer were used for the measurements. They found

that the near wall flow was decelerated and the core flow accelerated. They

concluded that the inlet distortion affects the pressure recovery and

effectiveness of the diffuser.

2.2.3 Other forms of Diffusers

Sovran and Klomp (1967) analysed over 100 geometries, nearly all

of which had conically diverging center bodies with an inlet radius ratio of

0.55 or 0.70. They developed a single correlation based on the area blockage

occurring in the internal flows due to the non-uniformities in velocity over the

cross stations of the system, which gives the quantitative effect for diffusers

of rectangular, conical and annular geometries. Their studies apply to axial

inlet / radial discharge diffusers. The mapping accomplished by these authors

is substantial and covers a very wide range of straight walled annular diffusers

with very low inlet blockage. Effects of systematic variation in inlet blockage,

inlet swirl and wall curvature were not included in their study. Their results

indicate significant dependence of diffuser pressure recovery on the inlet

turbulent boundary layer.

Adkins (1975) proposed a new design concept in vortex chamber

arrangement for gas turbine diffusers and presented its potential verified by

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experimental data. He obtained data for a series of research diffusers with area

ratios ranging from 1.9:1 to 3.2:1 and with diffuser entry Mach number,

velocity profile distortion levels which simulated those encountered in engine

systems. He predicted that, in engine systems, for moderate bleed rates, a

pressure recovery in excess of 80 percent could be obtained with diffuser

lengths only one third that required by conventional diffusers. He correlated

the minimum bleed requirements of the optimised diffusers with diffuser area

ratios and effective diameters. He concluded that the requirement for a bleed

off of a small fraction of the main air flow, being tolerable for most

applications. The vortex controlled diffusers has the potential to meet all the

requirements of the pre-combustor application. He found that with a short

length the total pressure loss is low and high static pressure recovery is

obtained and the flow field is independent of inlet Mach numbers.

Simpson et al (1981) studied the problem of turbulent boundary

layer separation due to adverse pressure gradient in devices like jet engines,

rocket engines. For measurements they used hot-wire anemometer in

upstream of separation and laser anemometer in separated zone. They

concluded that the turbulent shearing stress is low in reverse flow but the

turbulent velocities are comparable with the mean velocity. They also

reported that the mixing length and eddy viscosity models were physically

meaningless in the reverse flow. Further, they described three layers in fully

developed separation; the first one is viscous layer near the wall, the second

one is a overlap region between the viscous wall and outer regions and the

third one is the outer backflow region.

Chithambaran et al (1984) experimentally studied the turbulent

characteristics of incompressible flow in a two-dimensional diffuser with inlet

velocity distortion. The inlet velocity distortion was produced by NACA

airfoil placed at inlet of the diffuser. A DISA hot-wire anemometer was used

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to measure the turbulence level. They conducted three set of experiments with

the airfoil at incidence angle 0°, 4° and without aerofoil. They concluded that

the turbulence level in the wake region rapidly decreased and slightly

increases in the boundary layer region towards the exit of the diffuser. They

found that the maximum velocity fluctuation and the maximum Reynolds

shear stress move away from the wall in the streamwise direction. Further,

they found the relationship u’>w’>v’ is valid in the boundary layer region and

u’> v’ >w’ is valid for wake region.

Roach and Turner (1984) performed experiments to study the

secondary loss generation by gas turbine support struts. Experiments were

conducted on struts with circular and streamlined cross – section over a range

of Reynolds numbers, aspect ratio and thickness to chord ratio. They

concluded that the secondary drag co – efficient is a function of the maximum

thickness of the strut and the displacement thickness of the incident boundary

layer. They found that neither the aspect ratio nor the cross – sectional shape

influenced on the secondary flow loses with streamline stress model.

Sullerey et al (1990) analysed vortex controlled and hybrid diffusers

to achieve high pressure recovery in a short length. Investigation were

conducted on short diffusers with distorted inlet velocity for area ratios 2 and

2.5, divergence angles 30° and 45° and bleed off upto 7%. For each of the

above configurations, experiments were conducted by varying the fence

subtended angles from 0° to 30° and bleed rates varying from 0% to 7% for

vortex controlled diffuser and 0% to 4% for hybrid diffusers. The experiments

were carried out at a Reynolds number of 105. They found that the diffuser

effectiveness was improved with bleed – off. But optimum bleed off was

lower for hybrid diffuser. Further, they found a particular combination of

fence subtended and divergence angles gives optimum effectiveness of the

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diffuser. They concluded that the exit velocity profile can be controlled by

differential bleed.

Ganesan et al (1991) have done experimental and theoretical

investigations on mean and turbulent flow characteristics in a two

dimensional plane diffuser. They measured mean velocity, wall static pressure

and turbulence stresses. Prandtl’s mixing length model and k- model were

used to find the downstream velocity and the turbulent kinetic energy. They

compared the measured and predicted flow parameters and performance

parameters of the diffuser. They concluded that the simple Prandtl’s mixing

length model is quite enough to predict the mean flow parameters and to

evaluate the performance of the diffuser.

Kwong and Dowling (1994) studied experimentally and

theoretically the unsteady flow in both conical and rectangular diffusers. Two

conical diffusers with divergence angle of 16° and area ratio of 2.4 and 4.5

were used. They tested Reynolds number varying from 1.4 x 105 to 4 x 105.

The rectangular diffuser with length / inlet height ratio 7.5 and the inlet aspect

ratio 4 were used. The rectangular diffusers used were having total cone angle

was from 8° to 30°. They concluded that the mean pressure recovery suddenly

dropped with diffuser wall angle increases from 16° to 20° in rectangular

diffuser. They found that maximum pressure recovery is obtained when the

total cone angle was less than 16°. They found good agreement between the

measured frequencies with the predicted values.

Sonoda et al (1999) carried out experimental and numerical

investigations to study the flow characteristics within an annular S-shaped

duct with six struts, including the effect of inlet boundary layer on the flow.

They observed large differences of flow pattern at the S-shaped duct exit

without a remarkable change in the value of net total pressure loss. They

found a high pressure loss regions on either side of the strut wake near the

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hub that may act on a downstream compressor as a large inlet distortion and

strongly affect the downstream compressor performance. There was a much

distorted 3-D flow pattern at the exit of S-shaped duct.

Xia et al (1999) have investigated numerically and experimentally

the flow characteristics in a 180° bend annular diffuser. The diffuser

performance was studied with varying blow-off mass flow rates and the inlet

pressures. Ten struts were placed at bend in the diffuser and the Reynolds

number used was 1.13 x 105. They concluded that the pressure recovery co-

efficient increases with increasing blow-off mass flow rate and inlet pressure

but, it remains constant if the inlet pressure is greater than 10 bar. They found

that the numerical predictions were in good agreement with experiments.

Kibicho and Sayers (2009) investigated the mean flow field in wide

angled diffusers. They measured and presented the velocity and static

pressure fields and pressure recovery data for diffusers in the fully stalled

flow regime. The experiments were conducted with Reynolds number varying

from 1.07 x 105 to 2.14 x 105 and divergence angle between 30° to 50°. They

found that in 30° diffuser by increasing the velocity from 10 m/s to 20 m/s the

static pressure recovery increased by 8.31%. They concluded that the pressure

recovery is increased by increasing divergence angle and Reynolds number.

El-Askary and Nasr (2009) investigated experimentally and

numerically the turbulent flows in a combined bend – diffuser with a

rectangular cross section. The total divergence angle of 6°, 12°, 15° and 24°

were used for experimental studies. The low-Reynolds number k- turbulence

model were used in numerical studies. They concluded that the best

performance and minimum pressure loss achieved by an optimum diffuser

angle which depends on the inlet Reynolds number. They found that the side

load generated by the diffuser bend increases with increase in diffuser angle

and Reynolds number.

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2.3 NUMERICAL INVESTIGATIONS

In this section the literature on numerical investigations of annular,

conical and other forms of diffusers are reviewed.

2.3.1 Annular Diffuser

Ganesan and Murthy (1978) demonstrated the ability of a finite

difference scheme with marching integration technique to obtain the flow and

performance characteristics of straight core annular. They concluded 5°

diffuser was more effective compared to other diffusers.

Ganesan (1980) predicted the flow and boundary layer development

in straight core annular diffusers. Four different half cone angle diffusers have

been tested. He concluded that the half cone angle is an important parameter

for the boundary layer development when compared to the length of the

diffuser. The predicted results were in good agreement with the experimental

results.

Kanemoto and Toyokura (1983) theoretically analysed the flow in

annular diffusers. The boundary layer thickness and the performance of the

diffuser were calculated by experiments. Theoretically they found that the

axial flow increased near the hub in free vortex flow region. They concluded

that by increasing the boundary layer thickness the pressure recovery was

improved with a small whirl flow at entry of the diffuser. Further, they

concluded that the diffuser performance and the boundary layer thickness

decreases with increase in whirl at inlet of the diffuser.

Baskharone (1991) developed a finite element model of turbulent

flow field in the annular exhaust diffuser of a gas turbine. The analysis was

based on a modified version of Petrov-Galerkin weighted residual method,

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coupled with a highly accurate biquadratic finite element of lagrangian type.

Turbulence of the flow field was modeled using the two layer algebraic

turbulence closure of Baldwin and Lomax. He validated the computational

model using experimental data.

Yu Ji-jun et al (1992) numerically analysed the internal flow in an

annular diffuser with swirling flow by using three-dimensional momentum

integral equation. They concluded that the flow without inlet pre-swirl, the

development off the boundary layer near core is more rapid than near outer

casing. Further they reported that the flow with inlet pre-swirl, the

development of the boundary layer near outer casing is more rapid than near

inner core. The circumferential velocity gradually increased along the flow.

Mahalakshmi et al (1993) showed the ability of finite difference

scheme with marching integration technique in predicting the flow

characteristics of annular diffuser of equiangular and dump type. They used

two algebraic turbulence models namely Prandtl’s mixing length model and

Cebeci Smith model for the physical modeling and studied their relative

performance. They observed the Cebeci Smith model was performing better

than the other especially in the near wall region while both having good

predicting capabilities. They concluded that the geometry of the diffuser plays

an important role in the pressure recovery, development of flow and boundary

layer characteristics.

Djebedjan et al (1995) predicted turbulent flow in equiangular

annular diffuser with and without swirl. They used k- model and Reynolds-

Stress Model with the power – law differencing and the blended second order

upwind/central difference scheme. They concluded that the prediction of the

pressure recovery co-efficient improved with Reynolds – stress model with

blended second order upwind/central scheme.

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Shuja and Habib (1996) validated a numerical procedure developed

for the calculation of turbulent separated flow and heat transfer characteristics

in axisymmetric expanding ducts, with emphasis on annular diffuser. The

method is based on the fully conserved control volume representation of fully

elliptic momentum and energy equations in body fitted orthogonal curvilinear

co-ordinate systems. Turbulence was simulated with two equation (k- )

model. They validated the prediction through a systematic variation in

Reynolds number (6 x 103 – 6 x 105), outer wall half – cone angles (7o – 20o,

90o) and inlet swirl numbers (0.0 – 0.9). The results were reasonably well

predicted when compared to experimental data. They concluded that for high

anisotropic behaviour of turbulence downstream of diffusers, accurate

predictions require improved turbulence models that take into account the

anisotropic nature of the flow, effects of adverse pressure gradient, streamline

curvature and the wall effects.

Singh et al (2005) investigated the effect of inlet swirl on the

performance of annular diffusers having the same equivalent cone angles.

They tested a range of inlet swirl intensity for the best performance of annular

diffusers with different geometries namely parallel diverging hub and casing,

unequal diverging hub and casing, straight hub and diverging casing and

converging hub and diverging casing but having the same equivalent cone

angle. They concluded that the parallel diverging hub end casing annular

diffuser produces the best performance at high swirl intensities.

2.3.2 Conical Diffuser

Robertson and Fraser (1960) carried out theoretical analysis of

turbulent boundary layer based on the momentum based flow distortion

parameter of conical diffuser flow. They concluded that the separation

conditions were shown to depend on the initial momentum-thickness,

Reynolds number and the distance parameter involving the initial momentum-

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thickness, initial radius and diffuser length. They observed that the increase in

initial boundary layer thickness decrease the diffuser performance.

Shanmugam et al (1992) investigated numerically and

experimentally the flow through conical diffusers with two different

divergence angles. They found that the downstream of the diffuser, the wall

boundary layer thickness and maximum velocity in central core region

increase from station to station. The measured and predicted pressure

recovery co-efficient were below the ideal pressure recovery. They reported

that the numerical results were in good agreement with experimental results.

2.3.3 Other forms of Diffusers

Sagi and Johnstan (1967) developed the design of two-dimensional

curved diffusers and tested their performance. They tested various diffusers

with total cone angles between 30o and 90o. They concluded that the same

area ratio and length to width ratio the curved diffuser performance is lower

than that of the straight wall diffuser with similar flow pattern.

Jones and launder (1972) used a new model of turbulence in which

the local turbulent viscosity was determined from the solution of transport

equations of the turbulent kinetic energy and energy dissipation rate. This

model was applied to the prediction of wall boundary layer flows in which

streamwise accelerations are so severe that boundary layer reverts partially

towards laminar. The predicted values were in close agreement with the

measured values. The model was applied to prediction of a number of

strongly accelerated boundary layer flows. It has more predictive accuracy

than mixing length models. They concluded that specific model is needed

where surface mass fluxes and fluid property variation are substantial.

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Senoo and Nishi (1997) predicted the onset of separation in a

diffuser depends upon the local blockage factor. They formed a separation

limit relation for two dimensional diffuser which relates the shape factor of

the boundary layer and the total blockage factor at that section. Using this

relation, they found that it is possible to predict separation in a diffuser and to

evaluate the pressure recovery. In the case of decelerating flow, in their

another study (1977) found that the deceleration rate is an important

parameter to describe the development of turbulent boundary layer in an

adverse pressure gradient.

Issac and Wu (1989) conducted a numerical simulation of strongly

swirling axisymmetric flow field in sudden expansion geometry. The standard

k- model was used. The results were compared with the experimental results

of Dellenback et al (1988). The study was carried out with various Reynolds

number of 3 x 104, 6 x 104 and 1 x 105 and the swirl number from 0 to 1.2.

They concluded that the vortex core increases with increasing swirl number

but corner vortex shrink in the streamwise direction. They found that the

numerical results of axial and radial velocities were in good agreement with

experimental results.

Spall (1995) performed a numerical study of a prototypical vortex

controlled diffuser using the commercial code FLUENT. The incompressible

axisymmetric Reynolds averaged Navier Stokes equations were solved with

the effect of turbulence modeled using the renormalisation group (RNG)

based k- model. To understand the flow characteristics and mechanism by

which the vortex controlled diffuser operates, they analyzed the results for

vortex-controlled diffuser with and without bleed rates ranging from 1 to 7

percent. They found that the effectiveness increased and the distance required

for maximum diffusion to take place decreased as the bleed rate has increased.

They also identified the superiority of the RNG based k- model over the

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standard k- for predicting complex recirculating flows of this type. At low

bleed rates, the RNG based model appears to better model the physics of the

flow than the standard k- model.

Ganesan et al (2001) have investigated numerically in an inter-

casing duct with two rows of struts using PHOENICS software. The study has

been carried out for different swirl angles to simulate the design and off

design conditions and also to deswirl the flow before approaching the free

power turbine duct. They have found that at zero swirl, the flow diffuses

through out the duct without separation both in the hub and the tip station. As

the swirl angle increases to maximum, there is a separation in the hub and the

tip stations near the first row of struts. This separated flow leads to increase in

the total pressure loss in the system. They have observed that as the swirl

increases the total pressure loss also increases.

Ishizaka et al (2003) carried out CFD studies of industrial gas

turbine exhaust diffusers. They tested various Mach number from 0.4 to 0.77

and swirl angle from -10° to 20°. They found that the distortion of the static

pressure at the turbine exit change to the flat distribution at the inlet of first

strut but the distortion of total pressure disturbs the diffuser exit flow.

Johan Gullman Strand et al (2004) numerically and experimentally

studied the turbulent flow through a plane asymmetric diffuser. They used the

explicit algebraic Reynolds stress model with diffuser opening angles from 8°

to 10°. The experiments were conducted with 8.5° opening angle by using

PIV and LDV. They found that the predicted turbulent kinetic energy

dissipation is more than that of the experimental results.

Gopaliya et al (2010) numerically analysed the performance

characteristics of S – shaped diffusers with combined horizontal and vertical

offsets. They tested two different outlet configurations namely a semi-circular

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outlet and a rectangular outlet at Reynolds number 1.37 x 105. They used

standard k- model for the analysis. It was concluded that the S- shaped

diffusers with semi-circular outlet with 0.25 D vertical offsetting only fulfills

most of the requirements of the flow diffusion process efficiently and

effectively.

The list of various types of diffusers, Reynolds number and

dimensions studied by various researchers is presented in Table 2.1.

Table 2.1 Summarising table

S.No Author Types of

diffuser Reynolds No Area ratio

Inletdiameter

(mm)

Outlet diameter

(mm)

Half cone angle

1 So (1967) Conical 2.4 x 105 - 89 153 6°2 McDonald et

al (1971). Conical 1.5 x 105 1.3 to

8.27- - 4° to 31.2°

3 Kumar and Kumar (1980)

Annular 2.5 x 105 2.5 to 3.04

76 155 10° to 20°

4 Azad (1995) Conical 6.9 x 104

to 1.2 x 1051.25 101.6 202 8°

5 Singh and Azad (1995)

Conical 2.3 x 105 1.25 101.6 202 8°

6 Ubertini and Desideri (2000)

Annular 6 x 105 1.53 320 420 6.7°

7 Singh et al (2005)

Annular 2.5 x 105 3 111.8 180 7.5°

8 Mahalakshmi et al (2007)

Conical 1.6132 x 105 - 57 100 5° and 7°

9 Karunakaran and Ganesan (2009)

Conical 2.3 x 105 and 2.56 x 105

- 107 195 5°

2.4 SCOPE OF THE PRESENT WORK

Maximum pressure recovery is achieved in the case of a diffuser

without any obstacles at the inlet. However, in many practical situations the

flow at the inlet of the diffuser is distorted by the presence of centre bodies,

struts etc.

25

In the preceding sections it is seen that few investigations have been

made on annular diffusers with various strut designs. In the past (Fric et al

1996) a baseline and a tapered strut have been studied.

The present study focuses on the analysis of annular diffuser flow

with and without struts and with inlet guide vane wakes. Experimental and

numerical investigations have been carried out to study the effect of strut

shapes on the performance of an annular diffuser. For this purpose three strut

designs a baseline, tapered and dual tapered struts were considered.

Experiments were carried out only with baseline and tapered struts. The

numerical studies were carried out with dual tapered struts.