29.3 turbine cooling

8/13/2019 29.3 Turbine Cooling

1/27

INTRODUCTION TO CONCEPT

OF TURBINE COOLING AND

BLADE MATERIALTECHNOLOGY


2/27

COOLING OF TURBINE BLADES

The efficiency of gas cycle depends upon the rangeof maximum temperature of the gas

If the maximum temperature increases, efficiency of thecycle also increases

Any increase in maximum temperature of the gas,results in an increase in blade temperature, therebyinducing more thermal stresses in the blade material

This limits the capacity of the turbine

To achieve greater power, the blades are cooledthrough the hollow passages


3/27

DIFFERENT METHODS OF BLADE COOLING

Internal Air Coo l ing It is done by supplying cold air throughhollow blade It implies an internal baffle or deflector to direct the

flow over the hotter portions on internal surface

Film Coo l ing It involves in supplying of thin film of cooled

compressed air through a narrow slit on the blade surface to form

boundary layer over the blades

Water Cooling Circulation of water is maintained through

hollow section of the blade from root towards tip

Disc Cool ing Disc is cooled by circulating cooling fluid andthus blade temperature is reduced by conduction


4/27

ADVANTAGES OF COOLING

The specific thrust of the jet engine is increased by

32 % for gas temperature increased from 800C to 1100C

at the expense of 7 % specific fuel consumption

If the maximum temperature is increased to 1600C

in place of present practice 900C then the specific fuel

consumption will be increased by 50 % for increase in a

specific power by 200 %


5/27


6/27

TURBINE COOLING

Because the limiting factor in most turbine designs is themaximum temperature that can be tolerated at the turbine inlet,

design engineers use every method at their command to

increase the allowable inlet temperature

On practically all large engines, one such method is to cool

the inlet guide-vanes of the first-stage turbine and the first-

stage rotor blades

The cooling is accomplished by directing compressor bleedair through as sage inside the engine to the turbine area where

the air (or coolant) is led to longitudinal holes, tubes, or

cavities in the vanes


7/27

TURBINE COOLING


8/27

TURBINE COOLING


9/27

TURBINE COOLING

After entering the passages in the vanes and blades, the air

(coolant) is distributed through holes at the leading and trailing

edges of the vanes and blades

The air impinges along the vane, blade surfaces, and then

passes out of the engine with the engine exhaust


10/27


11/27

THE INTERNAL COOLING SYSTEM OF AN

ADVANCED TURBINE

The PW4000 turbofan, an advanced engine, has acooling

system for the high-pressure turbine vanes and blades,

outer air seal, transition duct between the high-pressureand low-pressure turbines, and the high-pressure turbine

disks


12/27


13/27


ADVANCED TURBINE

This system must not be confused with the system thatuses fan air to cool the turbine cases to control blade tipclearance

The cooling system optimizes engine performance bybleeding and controlling twelfth-stage compressor air to thehigh-pressure turbine

The twelfth-stage cooling air is bled from the high-Pressure

compressor through four ducts

Two of the ducts continually inject cooling air


14/27


ADVANCED TURBINE

The air from the two remaining ducts is controlled by

valves; thus, fifty percent of the cooling air for the turbine

vanes can be shut off at lower power to increase engine

efficiency

The two valves also control the twelfth-stage cooling air

that is used to supplement fifteenth-stage air

The twelfth stage air is supplied from the valves to thediffuser case via two ducts


15/27


ADVANCED TURBINE

It flows through diffuser case struts and joins the modified

fifteenth-stage air flowing around the No 3 bearing

compartment

This combined coolant flows to and cook the high-pressureturbine disks

The electronic engine control controls the valves according

to a schedule determined by altitude and high-pressurerotor speed


16/27

Blade-Tip Clearance Control

Some modern jet engines, particularly those of Transports,

have a system that controls the clearance between theblade-tips and the outer case of the turbine,

It improves fuel efficiency and increases the life of the cases

Blade-tip clearance is controlled by scooping cool air from

the fan stream and distributing it through ducts

To spray over the turbine cases the hot cases are cooled by

the air, shrink, and thus lessen the gap around the rotating

turbine blades


17/27

BLADE-TIP CLEARANCE CONTROL

The reduced leakage of air past the tightly running blade

tips increases fuel efficiency

On the PW2000 and PW4000-series engines, both the low-

pressure and high-pressure turbine cases are cooled duringthe climb and cruise portions of flight

The FADEC commands the operation of the system

according to a schedule determined by altitude and high-pressure rotor speed

DIRECTIONAL SOLIDIFICATION PROCESS


18/27

DIRECTIONAL SOLIDIFICATION PROCESSDS-200

Close examination of a conventional turbine blade revealsa myriad of crystals that lie in all directions (equip-axed)

Improved service life can be obtained by aligning the

crystals to form columns along the blade length, produced

by a method known as DirectionalSolidification

Further advance of this technique is to make the blade out

of a single crystal

Each method extends the useful creep life of the blade, in

the case of the single crystal blade, the operating

temperature can be substantially increased


19/27


20/27


21/27


22/27

TURBINE MATERIALS

SINGLE CRYSTALS

Use of single crystals allows the raising of engine operating

temperatures, yielding increased power and fuel efficiency

The durability of single crystals in blades and vanes are

advances on directionally solidified, or columnar grain,

airfoils, which were themselves stronger than airfoils cast

by traditional means

The metal of conventional airfoils comprises crystals joinedat boundaries


23/27

TURBINE MATERIALS

SINGLE CRYSTALS

When high performance gas-turbine blades fail in service,

one of two modes of failure, rupture or fatigue, usually

predominates

The mechanism usually is failure along crystal boundaries,

with subsequent propagation of a crack

The inter crystal line cracking which leads to failure isinitiated principally at grain boundaries that are oriented

normal to the stress axis


24/27

TURBINE MATERIALS

SINGLE CRYSTALS

In the directional solidification process introduced by

Pratt& Whitney in 1969 grain boundaries are aligned in

columns parallel to the airfoil axis, providing improved high-

temperature performance

Single crystal material goes even further by eliminating all

grain boundaries,

It allowing maximum use of the natural strength of the

metal


25/27

COMPRESSOR-TURBINE

MATCHING

The flow characteristics of the turbine must be very

carefully matched with those of the compressor to obtain

the maximum efficiency and performance of the engine.

Nozzle guide vanes allowed too low a maximum flow, then

a back pressure would build up causing the compressor to

surge, too high a flow would cause the compressor to

choke.

In either condition a loss of efficiency would very rapidly

occur.


26/27

CONCEPT OF BLISK WITH REFERENCE

TO COMPRESSOR (DUAL ALLOY DISCS)

Very high stresses are imposed on the blade root fixing of

high work rate turbines, which make conventional methods

of blade attachment impractical

A dual alloy disc, or blisk has a ring of cast turbine blades

bonded to the disc

This type of turbine is suitable for small high power

helicopter engines


27/27

29.3 turbine cooling

Documents