Turbine and Nozzle

7/24/2019 Turbine and Nozzle

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Turbine and nozzle



Introduction to turbines

• The basic mechanical principle, that if a fluid with large

kinetic energy content is allowed to hit a set of blades free to

rotate, certain amount of shaft work can be extracted from the

passing fluid, has been known for long.

• This energy is available in the form of rotating shaft power and

can be used for various purposes.

• Wind turbines also use naturally available wind kinetic energy

to extract power out of it.



Introduction to turbines

• It was also understood that for a gas turbine engine to go on an

aircraft it must be compact in size and must weigh as less as

possible.

• This requirement translates to the need for a turbine which

produces high energy extraction per unit mass (energy density)

of gas it is working on.

• Thus the gas, if it contains sufficient residual energy after the

turbine, can be exited through a jet nozzle to create direct

engine jet thrust.



• It is well known that turbine operates on the momentum principle

• part of the energy of the gas during expansion is converted into

kinetic energy in the flow nozzle .

• The gas leaves these stationary nozzles at a relatively high velocity.

• Then it is made impinge on the blades over the turbine rotor or

wheel. momentum imparted to the blades turns the rotor.

There are two primary parts of turbines are

1.The stator nozzle

2.The turbine rotor blades



1.Flow comes from the combustor

with high internal energy (T01, P01,

Ca1), and is made to go through the

stator-where a large part of its

internal energy is converted to kinetic

energy.

2.The transfer of energy occurs in

rotor as the high kinetic energy flow,

impinging on the rotor blade, is also

made to take huge flow turning

through the passage between the

blades.





Turbine stage

The stage consists of a ring of fixed nozzle

blades followed by the rotor blade ring.Turbine stage is classified as

1.An impulse stage

2.A reaction stage



• An impulse turbine stage is characterized by the expansion ofthe gas which occurs only in the stator nozzles . the rotor

blades act as directional guide vanes to deflect the direction of

air flow.• Further they convert the kinetic energy of gas into work by

changing the momentum of the gas more or less at constant pressure.

• A reaction stage is one in which expansion of the gas takes place both in the stator and rotor . the function of the stator isas same as that in the impulse stage, but the function in therotor is two fold.

1.To convert the kinetic energy of gas into work

2.contributes a reaction force on the rotor blades.The reaction force is due to the increase in the velocity of

the gas relative to the blades . This results from the expansion ofthe gas during its passage through the rotor.



A SINGLE IMPULSE STAGE

•

Impulse machines are those in which there is no change of staticor pressure head of the fluid in the rotor.

• The rotor blades cause only energy transfer and there is no

energy transformation.

•

The energy transformation from pressure head to kinetic energyor vice versa takes place in fixed blades only.

• As can be seen from the fig that the rotor blade passages of an

impulse turbine there is no acceleration of the fluid. there is no

energy transformation.

• Hence the chances are greater for separation due to boundary

layer growth on the blade surface . due to this the rotor blade

suffers greater losses giving lower stage efficiency.

• Examples. Paddle wheel , pelton wheel and curtis steam turbine



A SINGLE REACTION STAGE

•

The reaction machines are those in which changes in static or pressure head occurs on both in the rotor and stator blade

passages.

• Here the energy transformation occurs both in the fixed as well

a moving blades.• The rotor experiences both energy transfer as well as energy

transformation.

• Therefore reaction turbines are considered to be more efficient.

•

This is mainly due to continuous acceleration of flow withlower losses.

• Examples : Hero’s turbine , lawn sprinkler , parson’s turbine





Turbine Velocity triangle



•

In the Impulse turbines described earlier,V2=V3

• In reaction turbinesV3>V2

• In the aircraft gas turbines the exit Machnumber from stator-nozzle M2 =1.0

• On the other hand, relative exit Mach numberfrom the rotor M3-rel= V3/a

3 < 1.0

• The work done in a gas turbines may beincreased by increasing entry temp T01.



Degree of reaction

• The degree of reaction of a turbomachine

stage may be defined as the ratio of static or

pressure head changes occurs in the rotor to

the total change across the stage.



Turbine blade cooling

• Various blade cooling techniques provide variousamounts of cooling

• Maximum cooling is normally applied to first HPstage stator, which faces the highest temperature

• Cooling is also applied to HP rotors. But thedetails of this technology is a little morecomplicated as the cooling has to be effectedwhen the blades are rotating at high speeds

• Modern LP stage stators are also cooled.

• Last stage blades do not require cooling as thegas temperature is already substantially reduced.



Methods of turbine blade cooling

• Cooling of components can be achieved by air or liquidcooling.

• Cooling of components can be achieved by air or liquidcooling. Liquid cooling seems to be more attractive

because of high specific heat capacity and chances ofevaporative cooling but there can be problem ofleakage, corrosion, choking, etc.

• On the other hand air cooling allows the discharged airinto main flow without any problem.

• Quantity of air required for this purpose is 1 –3% ofmain flow and blade temperature can be reduced by200 –300 °C.



Methods of turbine blade cooling

• Internal cooling

1. Convection cooling

2. Impingement cooling• External cooling

1.Film cooling

2. Cooling effusion











Turbine materials :Inconel, Monel









Coolant flow paths in a modern HP

turbine stator



Nozzles

• Nozzles form the exhaust system of gas turbine engines.

• It provides the thrust force required for all flight conditions.

• In turboprops, nozzles may generate part of the totalthrust.

• Main components: tail pipe or tail cone and the exhaustduct.

• Nozzles could be either of fixed geometry or variablegeometry configuration.



Cont.…

• Besides generating thrust, nozzles have other functions too.

• Variable area nozzles are used for adjusting the exit area fordifferent operating conditions of the engine.

• For thrust reversal: nozzle are deflected so as to generate a part ofthe thrust component in the forward direction resulting in braking.

• For thrust vectoring: vectoring the nozzles to carry out complexmanoeuvres.

• Exhaust noise control



ypes of nozzles

1.Convergent or Converging-diverging

2.Axisymmetric or two-dimensional

3.Fixed geometry or variable geometry

• Simplest is the fixed geometry convergent nozzle

Was used in subsonic commercial aircraft.

• Other nozzle geometries are complex and requiresophisticated control mechanisms.



Variable geometry nozzles

• Variable area nozzles or adjustable nozzles are

required for matched operation under all

operating conditions.



Exhaust nozzles: Fixed geometry

Subsonic convergent

Tail pipe Nozzle

Supersonic C-D Nozzle

Tail pipe C-D

Nozzle



Exhaust nozzles: Variable geometry

After burner

C-D Nozzle

Supersonic

subsonic



Requirements of Nozzle •

Be matched with other engine components

• Provide optimum expansion ratio

•

Have minimum losses at design and off-design

• Permit afterburner operation

•

Provide reversed thrust when necessary

• Provide necessary vectored thrust

•

Have minimal weight, cost and maintenance while satisfying theabove.



Nozzles

• Convergent nozzles are normally used in subsonic aircraft.

• These nozzles operate under choked condition, leading toincomplete expansion.

• This may lead to a pressure thrust.

• A C-D nozzle can expand fully to the ambient pressure anddevelop greater momentum thrust.

• However due to increased weight, geometric complexityand diameter, it is not used in subsonic transport aircraft.



Thrust vectoring

• Directing the thrust in a direction other than thatparallel to the vehicles’ longitudinal axis.

• This allows the aircraft to undergo maneuvers that

conventional control surfaces like ailerons or flapscannot provide.

• Used in modern day combat aircraft.

• Provides exceptional agility and maneuvering

capabilities.• The pilot can move, or vector, the nozzle up and down

by 0-90 degrees.





Thrust vectoring

• Thrust vectoring was originally developed as ameans for V/STOL (Vertical or Short Take Off andLanding).

• Thrust vectored aircraft have better climb rates,besides extreme maneuvers.

• Most of the modern day combat aircraft havethrust vectoring.

• Some of the latest aircraft also have axisymmetricnozzle thrust vectoring.



Thrust reversal • With increasing size and loads of modern day aircraft,

wheel brakes alone cannot brake and aircraft.

• Deflecting the exhaust stream to produce a component of

reverse thrust will provide an additional brakingmechanism.

• Most of the designs of thrust reversers have a dischargeangle of about 45o

• Therefore a component of the thrust will now have aforward direction and therefore contributes to braking.





Cont.….

• There are three types of thrust reversalmechanisms that are used

1. Clamshell type

2. External bucket type

3. Blocker doors

Clamshell type is normally pneumatically operated

system. When deployed, doors rotate and deflectthe primary jet through vanes . These are normallyused in non-afterburning engines.





Cont.…

• Bucket type system uses bucket type doors to deflectthe gas stream.

• In normal operation, the reverser door form part of the

convergent divergent nozzle.

• Blocker doors are normally used in high bypassturbofans.

• The cold bypass flow is deflected through cascadevanes to achieve the required flow deflection.









Noise control

• Jet exhaust noise is a major contributor to the overall noise

generated by an aircraft.

• Jet exhaust noise is caused by the turbulent mixing of the exhaust

gases with the lower velocity ambient air.

• Nozzle geometry can significantly influence the exhaust noise

characteristics.

• Better mixing between the jet exhaust and the ambient can be

achieved by properly contouring the nozzle exit.

•

Corrugations or lobes (multiple tubes) are some of the methods ofachievin lower et exhaust noise.



Noise control

Noise control using corrugations/serrations

at the nozzle exit



• REFER BOOK FOR FOLLOWING TOPICS.

1.NOZZLE COEFFICIENTS

2.NOZZLE PERFORMANCE.

3. Nozzle flows

(i) Under expansion (ii) Over expansion(iii) Optimal expansion

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Turbine and Nozzle