03 turbomachinery relations

@ M.S.Ramaiah School of Advanced Studies, Bengaluru

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Propulsion System Components and Turbomachinery Relations

Session delivered by:

Mr. Subbaramu S.


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This session is intended to discuss the following: Construction and working principle of aircraft propulsion

system components Axial and radial turbomachines Euler turbine equation Velocity triangles Absolute and relative flow parameters

Session Objectives


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Introduction to Turbomachinery

A turbomachine is basically a rotating machine

The rotating wheel is called a rotor / runner / impeller

The rotor will be immersed in a fluid continuum

The fluid medium can be gas / steam / water / air

Energy transfer takes place either from rotor to fluid, or from fluid to rotor

Compressor rotor (imparts energy to the fluid)

Turbine rotor (extracts energy from the fluid)


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Turbomachine - Definition

A turbomachine is a device where mechanical energy in the form of shaft work, is transferred either to or from a continuously flowing fluid by the dynamic action of rotating blade rows.

The interaction between the fluid and the turbomachine blades also results in fluid dynamic lift.

A turbomachine produces change in enthalpy of the fluid passing through it.

Intake Exhaust

Compressor Combustor Turbine

PresenterPresentation NotesA Turbomachine is a machine in which the main element (rotor/runner/impeller) causing energy transfer is rotating and the element is immersed in working fluid which is continuously flowingA Rotating Machine is one in which the main element (rotor/runner/impeller) causing energy transfer is rotating whether the fluid flow is continuous or intermittentThe transfer of energy between element and fluid is continuous and is a result of the rate of change of angular momentum. The rotor/runner/impeller consists of a number of vanes or blades and there is a transfer of energy between fluid and this rotor/runner/impeller


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Turbomachine - Classifications

Turbomachines

Power producing Power absorbing

rotor

fluid with high energy

fluid with low energy

work impeller

fluid with high energy

fluid with low energy

work


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Turbomachines may also be classified as:

Turbines, compressors, pumps, fans , blowers Incompressible or compressible Axial-flow, mixed-flow or radial-flow geometry Single stage or multi-stage Turbo-pump, turbo-compressor or torque-converter Impulse, reaction or impulse-reaction

Turbomachine - Classification


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Propellers A propeller is a device which transmits power by converting

it into thrust for propulsion of a vehicle though a fluid byrotating two or more twisted blades about a central shaft, in amanner analogous to rotating a screw through a solid.

The blades of a propeller act as rotating wings and produceforce through application of Newton's third law, generating adifference in pressure between the forward and rear surfacesof the airfoil-shaped blades.

Air propeller Marine propeller


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Propellers

Froude analysis of propeller

Slip Stream Theory

444111 VAVAm ==Continuity equation

1 VmT = Thrust generated

Power required

1TVP =

m = mass flow rate in kg/sT = thrust in NP = power in WA = area in mV = velocity in m/s = density in kg/m3


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Ducted Fan A ducted fan is a propulsion arrangement, whereby a propeller is mounted within

a cylindrical shroud or duct. The duct prevents losses in thrust from the tips of the propeller and if the duct

has an airfoil cross-section, it can provide additional thrust of its own. Ducted fan propulsion is used in aircraft, airboats and hovercraft. In aircraft application, ducted fans normally have shorter and more number of

blades than propellers, and thus, can operate at higher rotational speeds. The operating speed of an unshrouded propeller is limited since tip speeds

approach the sound barrier at lower speeds than an equivalent ducted propeller.

PresenterPresentation NotesThe higher rotational speed of a ducted fan may require a gearbox when used with piston engines, which adds weight and negates some of the advantages. Instead, electric or Wankel rotary engines are the preferred method of power, and efficient home-made examples exist for both. A turbine can also be used to power the fan; in this configuration the ducted fan is referred to as a turbofan. Ducted fans usually have an odd number of blades to prevent resonance in the duct.


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Ducted FanDuct Shapes

decelerating shroud noise reduction. accelerating shroud low speed heavily loaded propellers (improves efficiency)

Ducted fans are favoured in VTOL and other low-speed designs for their high thrust-to-weight ratio.

Accelerating shroud Decelerating shroud


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Centrifugal Compressor The flow enters a three dimensional impeller axially through an inlet duct.

The impeller may be preceded by a row of inlet guide vanes. The impeller, through its blades, imparts velocity and pressure to the gas,

which flows in radial direction. The rise in pressure takes place due to the centrifugal action of the impeller

and diverging passages of the downstream diffuser and / or volute. Vaned or vaneless diffuser with volute are provided to convert kinetic energy

at impeller exit into static pressure at compressor discharge. Centrifugal compressors are used to produce large pressure ratios. A single stage centrifugal compressor may have typical pressure ratio of about

4:1. Some test compressors are designed for pressure ratio up to 8:1. Centrifugal compressors are suitable for low specific speed, high pressure ratio

per stage and low mass flow rate applications. Based on application, the centrifugal compressors can be either single stage or

multistage type.


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Components of a centrifugal compressor Impeller Diffuser Casing Shaft

Application of centrifugal compressor

Gas turbine Turbocharger Process industry

Gas compression Oxygen plants Instrument air

Centrifugal Compressor

Inducer

Diffuser vane


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Types of Centrifugal Compressor Impeller

Radial exit impellerBack swept impeller

Forward swept impeller02 90= 02 90=

Forward sweep V < U2Radial exit V = U2Backward sweep V > U2

Impeller with splitter blades Impeller with diffuser

PresenterPresentation NotesForward sweep impeller -http://images.google.co.in/imgres?imgurl=http://www.pfspumps.com/images/cat_image/Compressor%2520Impeller.gif&imgrefurl=http://www.pfspumps.com/product_catalogue.asp&h=211&w=280&sz=418&hl=en&start=1&tbnid=O5phwZhOVXKUZM:&tbnh=86&tbnw=114&prev=/images%3Fq%3Dcompressor%2Bimpeller%26svnum%3D10%26hl%3Den%26lr%3D%26client%3Dfirefox-a%26rls%3Dorg.mozilla:en-US:official%26sa%3DG


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Radial Turbine

Flow enters the impeller radially and exits axially. These machines are termed as inward flow turbines.

A radial turbine stage consists of volute, nozzle guide vanes and impeller. High pressure gas passes through the volute and / or nozzle guide vanes,

increasing its kinetic energy. The high velocity gas transfers its energy to the impeller shaft by flowing radially inward through the impeller.

The nozzles with adjustable vanes provide highest efficiency. Radial turbines employ a relatively higher pressure drop per stage with low

mass flow rate.

The specific speed and power range of the radial turbines are low. Since rotors / impellers are made of single piece construction, they are

mechanically strong and are more reliable.


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Radial Turbine

Impeller

Impeller and nozzle

Applications of Radial Turbine


InletExit

ExducerNozzle vane


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Radial turbine impeller

Radial Turbine Component

Radial turbine volute casing


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Axial Compressor An axial compressor consists of a row of rotor blades followed by a row of

stator blades and the working fluid traverses through these without significant change in radius.

The energy level of the fluid flowing through it is increased by the action of the rotor blades, which exert a torque on the fluid supplied by an external source.

An axial compressor is a relatively low pressure ratio turbomachine with higher mass flow rate as compared to a centrifugal compressor.

The flow stream lines passing through the bladings are nearly parallel to the shaft axis.

Flow enters axially and discharges almost axially. The blade passages diverge from inlet to exit, and hence the flow decelerates Due to density variation from inlet to exit, the compressor end walls have flare

with flow area reducing from inlet to exit.


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Single Stage Axial CompressorComponents of Axial Compressor

Rotor Stator Casing Shaft


Applications of axial compressor

Rotor Stator

Flow


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Multistage Axial Compressor


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Transonic and Subsonic Axial Compressors Transonic Compressor Subsonic Compressor

Flow is supersonic in some part of rotor blade span and subsonic elsewhere.

Inlet Mach numbers are high. The blades are thin. Max. thickness

to chord ratio is about 4%. The blade profiles are designed /

generated and seldom chosen from a standard family of profiles. Multiple Circular Arc (MCA), Arbitrary Mean Camber Line (AMCL) and Controlled Diffusion (CD) bladings are used.

Shock waves are formed at leading end or within the blade passages.

Flow throughout the rotor blade span is subsonic.

Inlet Mach numbers are low. The blades are relatively thicker.

Max. thickness to chord ratio lies between 5% to 15%.

Blade leading edge is thicker than the trailing edge.

Standard blade profiles, such as NACA-65 or C series, are available. Double Circular Arc Aerofoils (DCA) need to be generated.

Shock waves are not formed during normal operation.


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Axial Turbine The kinetic energy of combustion gas is converted to mechanical power by the

its impulse or reaction with a series of blades arranged around the circum-ference of a wheel or cylinder.

Stationary blades / guide blades act as nozzles and they convert fluid pressure into kinetic energy. The following rotating blades convert kinetic energy into useful work.

Axial turbines have low pressure drop per stage and higher mass flow rate compared to radial turbines.

The flow stream lines through the bladings are nearly parallel to the shaft axis. Flow enters axially and discharges almost axially. The blade passages converge from inlet to exit, and hence the flow accelerates. Blade profile is thicker at the inlet and thinner at the exit. Due to density variation from inlet to exit, the turbine end walls have flare with

flow area increasing from inlet to exit.


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Axial Turbine

Axial Turbine Stage

Stator Rotor


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Multistage Axial Turbine

A series of stages form multistage turbine.

The energy transfer in a stage is limited by the blade speed.

If more energy transfer per unit mass is required, then more number of stages are arranged one after the other.


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Combustor Combustion takes place in a combustor, which is located between the

compressor and the turbine.

Combustor design requires low velocity airflow in the combustion zone, where the fuel and air are mixed and ignited. The flame holder in the combustion zone allows a stable flame front to be established and maintained.


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Types of CombustorThere are three types of combustors:1. Annular2. Can3. Can-annular

1

2

3


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Gas Turbine


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Turbomachinery Relations


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Compression and Expansion

Ideal adiabatic change in stagnation conditions across a turbomachine

Compressor Turbine


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Control volume for a generalised turbomachine. Swirling fluid enters the control volume at radius r1 with tangential velocity Cw1 and leaves at radius r2 with tangential velocity Cw2.

By Newtons second law of motion,

Torque on the rotor = Rate of change of angular

momentum)( 1122 rCrCmT ww = N-m

= TW

Power = Torque x Angular speed

= )( 1122 rCrCm ww

)( 1122 ww CUCUm =

Euler Turbine Equation

T,

Cw2

Cw1


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Euler Turbine EquationFor a compressor, the work is done on the fluid. Hence, the specific work is given by

0)( 1122 >== wwcc CUCUmWW

watts

For a turbine, the fluid does work on the rotor. Hence, the specific work is given by

0)( 2211 >== wwtt CUCUmWW

watts

Specific work can also be related to the change in total enthalpy:

( ) ( )01021122 )( hhCUCUmWW wwcc ===

( ) ( )02012211 )( hhCUCUmWW wwtt ===

Compressor

Turbine


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Velocity Triangles (1)Axial Compressor

Direction of rotation

Flow

Inlet

Exit


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Centrifugal compressor stage and velocity diagrams at impeller entry and exit

Vaneless space

Centrifugal Compressor

Velocity Triangles (2)


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Velocity Triangles (3)Axial Turbine


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Velocity Triangles (4)

Radial turbine with radial inlet flow and axial outlet flow

Radial Turbine


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Velocity Triangles (5)Centrifugal Pump


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Components of Energy TransferConsidering the inlet velocity triangle

112

12

12

12

12

1

211

21

21

21

21

21

2

or)(

and

WWW

Wa

Wa

CUCUWCC

CUWCCCC

+=

=

=

( )21212111 21or WUCCU W +=

( )22222222 21 WUCCU W +=Similarly, from outlet velocity

triangle, we can obtain

Representative velocity diagrams at inlet


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)( 1122 UCUCmW ww =

Substituting in Power Equation

( ) ( )

++= 21

21

21

22

22

22 2

121 WUCWUCmW

Thus the specific work is given by,

222

21

22

22

21

21

22 UUWWCCW ++=

Components of Energy Transfer

or


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222

21

22

22

21

21

22 UUWWCCW ++=Specific Work,

(C22- C12)/2 : This term represents the dynamic enthalpy change in the turbomachine. It appears from the expression that for maximum specific work, the exit velocity of fluid C2 needs to be maximum. However, in a multistage turbomachine, converting this large dynamic head into useful static head by the next stage stator would lead to higher pressure losses.

(W12- W22)/2: This term represents the static enthalpy change. Hence W2 must be much less than W1 for this term to be more positive. Hence in a power absorbing machine, the rotor blade-to-blade passage will be diverging.

(U12- U22)/2: This term also represents the static enthalpy change. It represents the centrifugal head created in the blade passage. Higher U2 would be possible if the exit of the fluid occurs at higher radius, leading to maximum work derived.

Components of Energy Transfer


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2

2

0Cpp +=

Static pressure

Dynamic pressure

2

2

0Vpp r

+=

Static pressure

Dynamic pressure

p0 remains unchanged across stator

p0r remain unchanged across rotor, if there is no radius change along the streamline

Absolute and Relative Total Pressure


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2

2

0Chh +=

Static enthalpy

Kinetic energy

pcCTT2

2

0 +=Static temperature

Dynamic temperature

2

2

0Vhh r +=

Static enthalpy

Kinetic energy

pr c

VTT2

2

0 +=Static temperature

Dynamic temperature

h0 and T0 remain unchanged across stator

h0r and T0r remain unchanged across rotor, (if there is no radius change along the streamline)

Absolute and Relative Total Enthalpy / Temperature


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RothalpyThe Euler work can be rewritten as

where the function I, termed as Rotahlpy, is a constant along the streamlines through a turbomachine. It is a contraction of rotational stagnation enthalpy and is a fluid mechanical property used in the study of flow within rotating systems. The rothalpy can also be written in terms of the static enthalpy as

The Euler equation can also be written in terms of relative quantities for a rotating frame of reference to produce the following relation

Defining a relative stagnation enthalpy as

Hence, for rotating blade rows, the h0,rel is constant through the blades provided the blade speed is constant. In other words, h0,rel = constant, if the radius of a streamline passing through the blades stays the same.


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Compression Process on h-s Diagram

Axial compressor Centrifugal compressor

Rothalpy


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Session Summary

The construction and working of different components of aircraft propulsion system are briefly explained.

Euler turbine equation is derived and the significance of each term is discussed.

Method of drawing velocity triangles for turbine and compressor stages is explained and their importance in design and performance analysis of turbomachinery is highlighted.

Absolute and relative quantities and concept of rothalpy are explained.


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Thank you

Slide Number 1Slide Number 2Slide Number 3Slide Number 4Turbomachine - Classifications Slide Number 6Slide Number 7Slide Number 8Slide Number 9Slide Number 10Slide Number 11Slide Number 12Slide Number 13Slide Number 14Slide Number 15Slide Number 16Slide Number 17Slide Number 18Slide Number 19Slide Number 20Slide Number 21Slide Number 22Slide Number 23Slide Number 24Slide Number 25Gas TurbineSlide Number 27Slide Number 28Slide Number 29Slide Number 30Slide Number 31Slide Number 32Slide Number 33Slide Number 34Slide Number 35Slide Number 36Slide Number 37Slide Number 38Slide Number 39Slide Number 40Slide Number 41Slide Number 42Session SummarySlide Number 44

03 turbomachinery relations

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