This document provides information about turbine sections in gas turbine engines. It discusses the operation of different turbine blade types and how blades are attached to disks. It describes nozzle guide vanes and the causes and effects of turbine blade stress and creep. It explains the functions of the turbine to drive compressors and accessories. It also summarizes the types of turbines, including axial flow and radial inflow turbines, and describes impulse, reaction and reaction-impulse turbine blades.

1. Turbine Section
Module 15.6

2. LESSON PLAN
 Operation and characteristics of
different turbine blade types (3
Periods)
 Blade to disk attachment (2 Periods)
 Nozzle Guide Vanes (2 Periods)
 Causes and effect of turbine blade
stress and creep (3 Periods)

5. FUNCTION OF THE TURBINE
 To drive the compressor and accessories
and in case of turboprop to drive the
propeller by extracting a portion of the
pressure and kinetic energy from the high
temperature combustion gases.
 In a typical jet engine 75% of the power
produced internally is used to drive the
compressor.
 Left is used to produce the thrust.

6. INTRODUCTION
 The turbine has the task of providing
the power to drive the compressor and
accessories and, in the case of engines
which do not make use solely of a jet for
propulsion, of providing shaft power for
a propeller or rotor.
 It does this by extracting energy from
the hot gases released from the
combustion system and expanding
them to a lower pressure and
temperature.

7. INTRODUCTION
 High stresses are involved in this
process, and for efficient operation, the
turbine blade tips may rotate at speeds
over 1,500 feet per second.
 The continuous flow of gas to which the
turbine is exposed may have an entry
temperature between 850 and 1,700 ºC
and may reach a velocity of over 2,500
feet per second in parts of the turbine

8. A TRIPLE STAGE TURBINE WITH A
SINGLE SHAFT

9. INTRODUCTION
 To produce the driving torque, the turbine
may consist of several stages each
employing one row of stationary nozzle
guide vanes and one row of moving
blades.
 The number of stages depends upon the
relationship between the power required
from the gas flow, the rotational speed at
which it must be produced and the
diameter of turbine permitted.

10. INTRODUCTION
 The number of shafts, and therefore turbines,
varies with the type of engine., high compression
ratio engines usually have two shafts, driving high
and low pressure compressors.
 On high by pass ratio fan engines that feature an
intermediate pressure system, another turbine
may be interposed between the high and low
pressure turbines, thus forming triple-spool
system.
 On some engines, driving torque is derived from a
free-power turbine. This method allows the
turbine to run at its optimum speed because it is
mechanically independent of other turbine and
compressor shafts.

11. A MULTI STAGE TURBINE DRIVING TWO SHAFTS

12. INTRODUCTION
 The mean blade speed of a turbine has
considerable effect on the maximum
efficiency possible for a given stage output.
For a given output the gas velocities,
deflections and hence losses are reduced
in proportion to the square of higher mean
blade speeds.
 Stress in the turbine disc increases as the
square of the speed, therefore to maintain
the same stress level at higher speed the
sectional thickness, hence the weight, must
be increased disproportionately.

13. INTRODUCTION
 For this reason, the final design is a
compromise between efficiency and
weight.
 Engines operating at higher turbine inlet
temperatures are thermally more efficient
and have an improved power to weight
ratio.
 By-pass engines have a better propulsive
efficiency and thus can have a smaller
turbine for a given thrust.

14. A MULTI STAGE TURBINE
DRIVING THREE SHAFTS

16. TURBINE CONSTRUCTION
 The basic components of the turbine are
the combustion discharge nozzles, the
nozzle guide vanes, the turbine discs
and the turbine blades.
 The rotating assembly is carried on
bearings mounted in the turbine casing
and the turbine shaft may be common to
the compressor shaft or connected to it
by a self-aligning coupling.

18. TURBINE CONSTRUCTION
 The turbine wheel is one of the most
highly stressed parts in the engines.
 Not only must it operate at temperatures
of approximately 1800ºF (982ºC) but it
must do so under severe centrifugal
loads imposed by high rotational speeds
of over 60,000 rpm for small engines to
8000 rpm for the larger ones.

19. TURBINE CONSTRUCTION
 Consequently the engine speed and
turbine inlet temperature must be
accurately controlled to keep the turbine
within safe operating limits.
 The turbine assembly made out of two
main parts, the disk and blades.
 The disk or wheel is statically and
dynamically balanced unit of specially
alloyed steel, usually containing large
percentage of chromium, nickel and
cobalt.

20. TURBINE DISC
 Turbine discs are usually manufactured
from a machined forging with an integral
shaft or with a flange onto which the shaft
may be bolted.
 The disc also has, around its perimeter,
provision for the attachment of the turbine
blades.
 To limit the effect of heat conduction from
the turbine blades to the disc a flow of
cooling air is passed across both sides of
each disc.

22. TURBINE CONSTRUCTION
 After forging the disk is machined all over
and carefully inspected using x-rays,
sound waves and other inspection
methods to ensure the structural integrity.

23. A FREE POWER TURBINE

24. ENERGY TRANSFER FROM GAS
FLOW TO TURBINE
 Turbine depends for its operation on the
transfer of energy between the combustion
gases and the turbine. This transfer is never
100 per cent because of thermodynamic and
mechanical losses.
 When the gas is expanded by the
combustion process, it forces its way into the
discharge nozzles of the turbine where,
because of their convergent shape, it is
accelerated to about the speed of sound
which, at the gas temperature, is about
2,500 feet per second.

25. ENERGY TRANSFER FROM
GAS FLOW TO TURBINE
 At the same time the gas flow is given a
'spin' or 'whirl' in the direction of rotation
of the turbine blades by the nozzle guide
vanes.
 On impact with the blades and during
the subsequent reaction through the
blades, energy is absorbed, causing the
turbine to rotate at high speed and so
provide the power for driving the turbine
shaft and compressor.

26. ENERGY TRANSFER FROM GAS
FLOW TO TURBINE
 The torque or turning power applied to
the turbine is governed by the rate of gas
flow and the energy change of the gas
between the inlet and the outlet of the
turbine blades.
 The design of the turbine is such that the
whirl will be removed from the gas stream
so that the flow at exit from the turbine
will be substantially 'straightened out' to
give an axial flow into the exhaust
system.

27. ENERGY TRANSFER FROM GAS
FLOW TO TURBINE
 Excessive residual
whirl (spin) reduces
the efficiency of the
exhaust system and
also tends to
produce jet pipe
vibration which has
a detrimental effect
on the exhaust cone
supports and struts.

29.  It will be seen that the nozzle guide
vanes and blades of the turbine are
'twisted', the blades having a stagger
angle that is greater at the tip than at the
root.
 The reason for the twist is to make the
gas flow from the combustion system do
equal work at all positions along the
length of the blade and to ensure that
the flow enters the exhaust system with
a uniform axial velocity.
 This results in certain changes in
velocity, pressure and temperature

30. TURBINE LOSSES
 The losses which prevent the turbine from
being 100 per cent efficient are due to a
number of reasons.
 A typical un-cooled three-stage turbine
would suffer a 3.5 per cent loss because of
aerodynamic losses in the turbine blades.
 A further 4.5 per cent loss would be
incurred by aerodynamic losses in the
nozzle guide vanes, gas leakage over the
turbine blade tips and exhaust system
losses; these losses are of approximately
equal proportions.
 The total losses result in an overall
efficiency of approximately 92 per cent.

31. NGV
 Stator section of turbine.
 Direct the air axially on to the blade of
rotor section.
 The design of the nozzle guide vane
and turbine blade passages is based
broadly on aerodynamic
considerations, and to obtain optimum
efficiency, compatible with compressor
and combustion design, the nozzle
guide vanes and turbine blades are of a
basic aerofoil shape.

32. Typical nozzle guide vanes showing their shape and
location. (Courtesy: Rolls-Royce)

33. TYPES OF TURBINES
 Axial flow turbine
 Radial inflow turbine

34. AXIAL FLOW TURBINE
 The axial-flow turbine has two main
elements: turbine rotors (or wheels, as
they are sometimes called) and
stationary vanes.
 The turbine blade themselves are of two
basic types, the impulse and the
reaction.
 The modern aircraft gas turbine engine
utilizes blades that have both impulse
and reaction sections.

35. AXIAL FLOW TURBINE
 The stationary part of the assembly
consists of a plane of contoured vanes,
concentric with the axis of the turbine
and set at an angle to form a series of
small nozzles.
 These nozzles discharge the gases onto
the blades in the turbine rotors.
 The stationary vane assembly of each
stage in the turbine is usually referred to
as the turbine nozzle guide vanes.

36. Single stage axial flow turbine
wheel
Multi stage axial flow turbine with
turbine nozzles. First stage is not
shown. For cooling purpose the
blades may be solid or hollow on
either nozzle and/or rotor.
Axial Flow Turbine

37. TURBINE NOZZLE AREA
 The turbine nozzle area is the most critical
part of the turbine design. If the nozzle area
is too large, the turbine will not operate at its
best efficiency.
 If the area is too small the nozzle will have a
tendency to lose efficiency under maximum
thrust conditions.
 The turbine nozzle area is defined as the
total cross-sectional area of the exhaust gas
passages at their narrowest point through
the turbine nozzle.
 It is calculated by measuring and adding the
areas between individual nozzle guide
vanes.

38. RADIAL INFLOW TURBINE
 Has the advantage of ruggedness and
simplicity and is relatively inexpensive
and easy to manufacture when compared
with axial flow type.
 Inlet gas flows through peripheral nozzles
to enter the wheel passages in an radial
direction.
 The speeding gas exerts force on the
wheel blades and then exhaust the air in
an axial direction to the atmosphere.

39. RADIAL INFLOW TURBINE
 These turbine wheels used for small
engines, are well suited for lower range
of specific speeds and work at relatively
high efficiency

42. TURBINE BLADES
 The turbine blades are of an aerofoil
shape, designed to provide passages
between adjacent blades that give a
steady acceleration of the flow up to the
'throat', where the area is smallest and the
velocity reaches that required at exit to
produce the required degree of reaction.

43. TURBINE BLADES
 The actual area of each blade cross-
section is fixed by the permitted stress in
the material used and by the size of any
holes which may be required for cooling
purposes.
 High efficiency demands thin trailing
edges to the sections, but a compromise
has to be made so as to prevent the
blades cracking due to the temperature
changes during engine operation.

44. DIFFERENT TURBINE BLADE
TYPES
Impulse turbine
Reaction turbine
Reaction-Impulse turbine

45. IMPULSE TURBINE
 In the IMPULSE type the total pressure
drop across each stage occurs in the
fixed nozzle guide vanes which, because
of their convergent shape, increase the
gas velocity whilst reducing the pressure.
 The gas is directed onto the turbine
blades which experience an impulse force
caused by the impact of the gas on the
blades.
 All pressure energy of gas converted to
kinetic energy.

46. IMPULSE TURBINE
 Area of the inlet and exit between the
blades is equal.
 The impulse force does not act directly in
the plane of rotation the turbine wheel but
is resolved in to two components.

47. REACTION TURBINE
 In the REACTION type the fixed nozzle
guide vanes are designed to alter the
gas flow direction without changing the
pressure.
 The converging blade passages
experience a reaction force resulting
from the expansion and acceleration of
the gas.
 Normally gas turbine engines do not use
pure impulse or pure reaction turbine
blades but the impulse-reaction
combination.

48. REACTION TURBINE
 On entering the first rotor stage, the gases
see the rotor as a convergent passage
(outlet area less than the inlet area).
 The change in the area produces an
increase in the relative velocity with an
accompanying pressure drop across the
blades.
 The acceleration of gases generates a
reaction force like that produce on a wing.
 It is from this feature of the reaction
turbine that its name is derived.

49. REACTION TURBINE
 The reaction force results from the
acceleration of the gases across the
blade.
 The direction in which the reaction force
acts may be determined by considering
the blade as an airfoil.
 The reaction force like lift may be drawn
perpendicular to the relative wind.
 Both impulse and reaction forces are
acting on the blade of a reaction turbine.

50. REACTION TURBINE
 Impulse turbine requires high velocity gas
in order to obtain the maximum rate of
momentum change.
 Reaction turbine causes its rate of
momentum change by the nozzling
action of the rotor blading and therefore
does not require excessively high nozzle
diaphragm exit velocities.
 Two forces (impulse + reaction) combine
vectorically in to a resultant that acts in
the plane of rotation to drive the turbine.

56. REACTION – IMPULSE
TURBINE
 It is important to distribute the power load
evenly from the base to the tip of the
blade.
 An uneven workload will cause the gases
to exit from the blade at different velocities
and pressures.
 Obviously the blade tips will be travelling
faster than the blade roots because they
have a greater distance to travel in their
larger circumference.

57. REACTION – IMPULSE
TURBINE
 If all the gas velocity possible is made to
impinge upon the blade roots, the
difference in wheel speed at the roots and
the tips will make the relative speed of the
gases less at the tips, causing less power
to be developed at the tips than at the
roots.
 To cope with this problem in actual
practice the turbine blading is a blending
of the impulse type at the roots and the
reaction type at the tips.

58. REACTION – IMPULSE
TURBINE
 Making the blade impulse type at the
roots and the reaction type at the tips the
blade exit pressure can be held relatively
constant.

59. REACTION – IMPULSE
TURBINE
 Required pressure drop for reaction is
present at the tip and the gradually
changes to the ‘no pressure loss’
condition required for impulse at the root.
 Higher pressure at the tip will tend to
make the gases flow toward the base of
the blade which counteracts the
centrifugal forces trying to throw the air
towards the tip.

60. REACTION – IMPULSE
TURBINE
 Angle of the nozzle and the turbine blades
are such that optimum performance is
achieved only during a small range of engine
RPM.
 To counteract the swirling of gases,
straightening vanes are located immediately
downstream of the turbine.
 These vanes also serve the function in many
engines of providing one of the main
structural components and they act as a
passageway for oil, air and other lines.

61. METHOD OF TURBINE DISC
ATTACHMENT
 The method of attaching the blades to
the turbine disc is of considerable
importance, since the stress in the disc
around the fixing or in the blade root has
an important bearing on the limiting rim
speed.
 The blades on the early Whittle engine
were attached by the de Laval bulb root
fixing, but this design was soon
superseded by the 'fir-tree' fixing that is
now used in the majority of gas turbine
engines.

63. METHOD OF TURBINE DISC
ATTACHMENT
 This type of fixing involves very accurate
machining to ensure that the loading is
shared by all the serration’s.
 The blade is free in the serration’s when
the turbine is stationary and is stiffened in
the root by centrifugal loading when the
turbine is rotating.
 Various methods of blade attachment are
shown; however, the hollow blade and the
de Laval bulb root types are not now
generally used on gas turbine engines.