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1
Turbine Engine
© Devinder K Yadav
Gas Turbine Theory 1
2
Gas Turbine Cycles
• Closed circuit gas turbine powerplant
• Open circuit gas turbine powerplant
3
Closed circuit gas turbine powerplant
4
Open circuit gas turbine powerplant
5
Basic Gas Turbines Engines
The turbine engine produces thrust by increasing the
velocity of the air flowing through the engine. It
consists of:
• air inlet,
• compressor,
• combustion chambers,
• turbine section,
• exhaust section,
• accessory section.
6
7
Basic Gas Turbines Engines
Basic Gas Turbines Engines
Turbine engine advantages over a piston
engine:
• less vibration
• increased aircraft performance
• reliability
• ease of operation.
8
9
Piston Engines v Turbine Engines
10
11
How Turbine engine works
Physics applicable to jet engines
•Newton’s Third Law of Motion
•Charles’ First Gas Law
•Charles’ Second Gas Law
•Pascal’s Law
•Bernoulli’s Theorem
•First Law of Thermodynamics
•Second Law of Thermodynamics
12
For every action there is an equal and
opposite reaction
• Turbine engines are known as reaction
engine
Newton’s Third Law of Motion
13
14
15
• When the pressure of a gas remains
constant, the volume of the gas will
increase as it’s temperature is increased
Charles’ First Gas Law
16
17
Charles’ Second Gas Law
• When the volume of a gas is held
constant, the pressure of the gas will
increase as it’s temperature is increased
18
Pascal’s Law
• Pressure always acts at right angles to
any confining surface, undiminished
throughout the fluid regardless of shape
and size of the container
19
20
• The sum of all energies in a perfect fluid
must remain constant
• If kinetic energy increases then potential
energy must decrease, ie:- velocity is
inversely proportional to pressure
Bernoulli’s Theorem
21
22
• Energy can neither be created nor
destroyed
The First Law of Thermodynamics
23
24
• Energy will always flow from an area of
higher potential to an area of lower
potential
Second Law of Thermodynamics
25
26
A convergent duct
27
A divergent duct
28
The Turbine Engine
29
The Brayton Cycle
A B C D
30
Pressure vs Temperature
Temperature
Pressure
Atmospheric
Pressure
31
Enthalpy vs Entropy
Entropy
Usability of heat energy
Enthalpy
Total
energy of
the gas
Atmospheric
Pressure
A
B C
D
A B C D
32
Temperature, pressure and velocity
33
Force (F) = ma = (weight ÷ gravity) × acceleration
Thrust (T) = ma + (pressure × area)
T =
Where,
Wa - weight of air
V1 – velocity of airplane
V2 – velocity of air at jet nozzle
Wf – weight of fuel
Aj – area of jet nozzle
Pj – static pressure of jet nozzle
Pam – ambient static pressure
The Jet Engine Equation
Pam)Aj(PjVfg
WfV1)(V2
g
Wa
34
35
• A common method of determining engine
thrust
• EPR is the ratio between the total
pressure in the exhaust duct and the total
pressure at the inlet to the engine
Engine Pressure Ratio (EPR)
36
A larger EPR = more thrust
Typical EPR values (Boeing 727):
NB. EPR is only useful as a measure of
thrust on those engines with fixed area
exhaust nozzles 37
Thrust versus horsepower
• Recall: power = rate of doing work
• in other words:
– Lift up a one pound weight through 550 feet in one
second and you have 1 horsepower
• Mathematically:
– Power = Force x Distance
Time
• Propeller torque and RPM are used to
calculate horsepower
38
Thrust versus horsepower • Power harder to measure in a jet engine (time
and distance elements not always involved)
• Once a jet engine is moving forward then a comparison can be made
• At an airspeed of 375 mph (325 kts), one lb of thrust = 1 HP
• THP = thrust x TAS (kts)
325
• So a B777 engine produces 90,000lbs of thrust – On take off (100kts) = 27690 HP
– During climb (300kts) = 83070 HP (assuming full power)
39
Methods of Jet Propulsion
40
A Ram Jet Engine
41
A Pulse Jet Engine
42
A Rocket Engine
43
Gas Turbine Engine
44
Turbojet:
Turbofan
Turbojet
Turboshaft
Gas Turbine Engine Types
45
46
Turbojet engines Also called as the pure jet.
The compressor section passes inlet air at a high rate of
speed to the combustion chamber.
The combustion chamber contains the fuel inlet and igniters
for combustion. The expanding air drives a turbine, which is
connected by a shaft to the compressor, sustaining engine
operation.
The accelerated exhaust gases from the engine provide
thrust.
Turbojet engines are limited on range and endurance. They
are also slow to respond to throttle applications at slow
compressor speeds.
47
Turboprop engines
A turboprop engine is a turbine engine that drives a
propeller through a reduction gear. The exhaust gases
drive a power turbine connected by a shaft that drives the
reduction gear assembly.
Turboprop engines are most efficient at speeds between
250 and 400 m.p.h. and altitudes between 18,000 and
30,000 feet. They also perform well at the slow airspeeds
required for takeoff and landing, and are fuel efficient. The
48
Turbofan engines
Turbofan engines are designed to create additional thrust by diverting a
secondary airflow around the combustion chamber.
The turbofan bypass air generates increased thrust, cools the engine, and
aids in exhaust noise suppression. This provides turbojet-type cruise
speed and lower fuel consumption.
The inlet air that passes through a turbofan engine is usually divided into
two separate streams of air. One stream passes through the engine core,
while a second stream bypasses the engine core. It is this bypass stream
of air that is responsible for the term “bypass engine.” A turbofan’s bypass
ratio refers to the ratio of the mass airflow that passes through the fan
divided by the mass airflow that passes through the engine core.
49
Turboshaft engines
It delivers power to a shaft that drives something other than
a propeller.
The biggest difference between a turbojet and turboshaft
engine is that on a turboshaft engine, most of the energy
produced by the expanding gases is used to drive a turbine
rather than produce thrust.
Many helicopters use a turboshaft gas turbine engine. In
addition, turboshaft engines are widely used as auxiliary
power units on large aircraft.
Turbojet
50
Turbojet
51
Turboprop
52
Turboprop
53
Turbofan
54
55
Turbofan
56
Turboshaft engine
57
Turboshaft engine
High bypass ratio turbofan
58
Low bypass ratio turbofan
59
Fan Bypass Ratio
It is the ratio of airflow through the fan
duct to the airflow through the engine
core
For example, if a turbofan has a bypass
ratio of 6 to 1, 7 units of air are entering
the intake duct with 1 unit entering the
engine core and 6 units going through the
fan section only
60
61
Thrust versus A/C speed & drag
Propulsive Efficiency
Compares the work done by the engine on the
air mass with the work done by the engine on
the aircraft.
62
Propulsive Efficiency
Thrust (force) = mass x acceleration
A turbojet gives a large acceleration to a
small mass of air
A turboprop gives a small acceleration to a
large mass of air
63
Propulsive Efficiency
Ratio of exhaust gas velocity to aircraft speed 64
• The turbofan has replaced the turbojet for
commercially operated aircraft
• For a turbojet and turbofan of the same
rated thrust the turbofan will burn less fuel
• The turbofan has less wasted kinetic
energy after exiting the exhaust (exhaust
velocity is closer to aircraft speed)
Propulsive Efficiency
65
Propulsive Efficiency
66
Effect of aircraft speed on jet thrust
Thrust = M(V2 – V1)
Airspeed
Ram effect
Resultant thrust
250 kts
67
Effect of engine RPM on thrust
% Engine RPM
68
Effect of air temperature on thrust
Air Temperature
69
Effect of air pressure on thrust
Air Pressure
70
Effect of altitude on thrust
Altitude
Stratosphere
71
1
Turbine Engine
© Devinder K Yadav
Gas Turbine Theory 2
2
Turbine Engine
Design and Construction
Entrance Ducts (Intake)
Compressor Section
Compressor-Diffuser Section
Combustion Section
Turbine Section
Exhaust Section 3
Turbine Engine Entrance Ducts
Properties
Must furnish a uniform supply of air to the
compressor in all conditions
Contributes to stall-free compressor
performance
Must create minimal drag
4
Turbine Engine Entrance Ducts
5
Gas Turbine Entrance Ducts
A divergent duct from front to back
Increased static pressure
to the compressor
Designed to be efficient at the cruise but
must still operate effectively when the
aircraft is stationary and before RAM
pressure recovery occurs
6
Turbojet inlet duct
Single entrance duct
7
Turbojet inlet duct
Divided entrance duct
8
Turbojet inlet duct Variable geometry ducts
Divergent subsonic inlet duct
Supersonic inlet duct
9
Turboprop inlets
10
Turbofan engine inlets
11
Inlet Guide Vanes
Direct intake duct air onto the first
compressor stage rotor at the correct
angle of attack
Both stationary and variable angle inlet
guide vanes may be used
12
Inlet Guide Vanes
13
Compressor Section
It’s function is to supply air in sufficient
quantity to satisfy the needs of the
combustor
Compressors operate on the principle of
acceleration of air followed by diffusion to
convert the acquired kinetic energy into a
pressure rise
14
Compressor Section
A secondary purpose of the compressor
section is to supply bleed air for use by
the engine and aircraft systems
Common bleed air uses are
Cabin pressurisation
Air Conditioning
Aircraft pneumatic systems
Anti icing, inflating door seals, suction 15
Compressor Section
There are two types of compressors
Centrifugal flow
Axial flow
16
Centrifugal Compressor
It consists of an impeller (rotor), a diffuser (stator) and a
manifold.
17
The principal differences between the two types of impellers are
size and ducting arrangement.
The double-entry type has a smaller diameter but is usually
operated at a higher rotational speed to ensure enough airflow.
The single-entry impeller permits convenient ducting directly to
the impeller eye (inducer vanes) as opposed to the more
complicated ducting necessary to reach the rear side of the
double-entry type.
Plenum Chamber
This chamber is necessary for a double-entry compressor because
air must enter the engine at almost right angles to the engine axis.
To give a positive flow, air must surround the engine compressor at
a positive pressure before entering the compressor.
Single & double entry impellers
18
Centrifugal Compressor
Impellers
19
Centrifugal Compressor
20
Centrifugal Compressor
21
Centrifugal Compressor
Air enters the impeller at the hub and then
flows outward through impeller blades
The impeller imparts rotational and
outward velocity to the air which then
flows into the diffuser where divergent
ducts convert velocity into pressure
22
Centrifugal Compressor
23
Centrifugal Compressor
24
Advantages of Centrifugal Compressor
High pressure rise (10:1)
Good efficiency over a wide
rotational speed range
Robust
Low cost
25
Disadvantages of Centrifugal Compressor
Large frontal area
More than two stages is not practical
because of the energy losses between
stages
26
Two stage centrifugal compressors
Single stage
27
Two stage centrifugal compressor
28
Centrifugal Compressor
Most common in rotorcraft and
turboprop aircraft because of their
robustness – more reliable on gravel
runways
29
Axial Compressor
The airflow and compression occur parallel
to the rotational axis of the compressor
Air Flow
30
Axial Compressor
31
Axial Compressor
32
Axial Compressor
33
34
Axial Compressor
35
Axial Compressor
36
Axial Compressor
37
Axial Compressor
38
Variable stator
vanes operation:
They are
operated by fuel
pressure and
scheduling is
done by main
engine control
(fuel control
unit).
39
Axial Compressor
The air flows axially through a number of
rotating rotor blades and fixed
intervening stator vanes
Each set of rotating blades and stator vanes
is known as a compressor stage
40
41
42
Vector Diagram – complete engine
43
44
Axial compressor roots and tips
Vibration is a problem with any rotational
machinery
The root of the compressor disk is often
only loosely fitted to the hub
As the compressor rotates centrifugal
loading locks the blade in its correct position
and the air stream over the airfoil provides a
shock mounting or cushioning effect 45
To avoid energy losses (including shock
waves) over the tips of the rotor blades,
the clearance between the rotor and the
surrounding shroud must be kept to a
minimum
Newer engines are designed to rotate within
a shroud strip of abradable material
Sometimes during coastdown a high
pitched noise can be heard if the blade
tip and shroud strip are touching 46
Advantages of Axial Flow Compressors
Higher compression available by
addition of compression stages
Small frontal area and lower drag
47
Disadvantages of Axial Compressors
High cost of manufacture
Relatively high weight
Higher starting power requirements
Lower pressure rise per stage
Good compression in the cruise and
take off power settings only 48
Combination Compressors
Popular in many small turbine engines
(Pratt and Whitney PT 6) 49
Axial Compressor
There are three designs of axial flow
compressors
Single spool
Double spool
Triple spool
50
Axial Compressor
(N1)
(N2)
Spools are not mechanically linked together 51
Multi Spool Compressors
For any given power setting the high
pressure compressor speed is held
constant by the fuel control unit
The low pressure compressor(s) will speed
up and slow down with changes in engine
inlet conditions resulting from atmospheric
changes
52
Trent 900 triple spool compressor
53
Advantages of multi-spool axial
compressors
Less power required for starting
Less prone to compressor stalling
Quicker acceleration
54
Compressor Stall & Surge
Compressor blades, being aerofoils, can
stall at too high an angle of attack
the close proximity of blades in different
stages means that if one stage stalls, so
may the next
55
Angle of Attack and compressor stall
Compressor stalls cause air flowing
through the compressor to slow down,
stagnate or reverse direction
this is then know as an engine surge
Any change to the design airflow will have
an effect to all other sections of the gas
turbine engine
56
57
58
59
Angle of Attack and compressor stall
Causes
Excessive fuel flow changes
Turbulent air
Contaminated or damaged compressors
Contaminated or damaged turbine blades
Engine operation outside design RPM
Too rapid movement of throttles 60
Angle of Attack and compressor stall
Can occur during a cross wind take-off
Can occur during a steep turn
Detected by
Audible noise and/or vibration
Fluctuating RPM
Increased EGT 61
Angle of Attack and compressor stall
Reverse air flow may result in the compressor
blades bending and contacting the stator
vanes
Sophisticated engines use:
bleed air to reduce the possibility of
compressor stall, or
variable incidence guide vanes 62
63
Turbine Engine
© Devinder K Yadav
1
Gas Turbine Theory 3
2
The Combustion Section
The combustion process must ideally be
able to efficiently convert chemical energy
to heat energy under all operating
situations from engine start to engine shut
down
A chemically correct (stoichiometric)
mixture is approximately 15:1 air/fuel
3
The Combustion Process
The temperature of the gases released by
combustion can be well in excess of 15000C
which will destroy the combustion chamber
and turbine section
About 60% of the air entering the
combustion chamber is used for cooling only
4
The Combustion Process
5
6
The Combustion Process
To function properly the combustion
chamber must
1. Provide a proper environment for the
mix of air and fuel
2. Cool the hot gases to a temperature
the turbine section components can
withstand
To accomplish this the airflow through
the combustor is divided into primary
and secondary paths 7
The Combustion Process
Air from the compressor may enter
the combustion chamber in excess of
500 feet per second (300 knots)
The axial flow of the primary airflow
must be reduced to about 5 feet per
second (3 knots)
Because of the slow flame propagation
rate of jet fuels if the primary velocity
were too high it would blow the flame
out (flame out) 8
The Combustion Process
The reduction in axial velocity is achieved
by swirl vanes which create radial motion
and retard axial motion
The air from the swirl vanes and secondary
air holes interact and create a region of low
velocity circulation
This forms a toroidal vortex similar to a
smoke ring stabilising and anchoring the
flame 9
The Combustion Process
10
The Combustion Process
11
The Combustion Process
The combustion process is complete in
the first one third of the combustion
liner
In the remaining two thirds of the
combustor length the combusted and
uncombusted gas is mixed to provide an
even heat distribution at the turbine
nozzle 12
Flame Out
Although uncommon in modern engines
they still occur
Some common causes are
Turbulent weather
High altitude
Violent flight maneuvers
13
The Combustion Process
** Be careful when quoting air/fuel ratios**
14
Flame Out
Flame out (lean) Usually occurs at low
fuel pressures at low engine speeds in
high altitude flight
Flame out (rich) Usually occurs during
fast engine acceleration in which an over
rich mixture causes combustion pressure
to increase until compressor flow stagnates
Turbulent inlet conditions can also cause stalls 15
Flame Out
To minimise the possibility of flame out it
is essential to have a correct matching of
compression ratio, mass airflow and engine
speed
16
Combustion Chamber Types
The various combustion chambers in
use include
Multiple can
Can Annular
Annular reverse flow
Annular
17
Multiple Can
18
Multiple Can
This type of combustion chamber is more
common with centrifugal flow
compressors and earlier types of axial
flow compressors
The separate flame tubes are
interconnected to allow a constant
pressure and also propagate
combustion around the flame tubes
during starting 19
Can Annular
20
Annular
21
Annular Reverse Flow
22
Annular Reverse Flow
Common in turboprop engines as this
arrangement provides shorter engine
length and also a weight reduction
23
Garrett TPE 331 Reverse Flow
Combustion Chamber
24
Fuel Supply
Fuel is supplied to the combustion chamber
by one of two methods
The most common is the injection of a
fine atomised spray into the re-circulating
airstream through spray nozzles
25
The Combustion Process
26
Fuel Supply
The second fuel supply method is based on
the pre-vaporisation of the fuel before it
enters the combustion zone
The fuel/air mix is carried in a vaporising
tube which passes through the primary
flame area of the combustion chamber
More common in low power engines
27
The Turbine Section
The turbine section is bolted onto the
combuster and contains nozzle guide vanes,
turbine rotors and turbine stators
The turbine functions to transform a
portion of the kinetic and heat energy in
the exhaust gases into mechanical work to
drive the compressor, propeller, fan and
accessories 28
The turbine section
29
The turbine section
Turbine Stator Turbine Rotor 30
The turbine section
• Since energy is extracted from the airflow through a turbine section, pressure decreases across the turbine section
• Hence the boundary layer is much more likely to remain attached than in the compressor section
• Each stage of the turbine section can support several stages of compressor
31
The turbine section
32
The turbine
section
33
The turbine
section
34
The turbine
section
35
The turbine
section
36
The turbine
section
37
The turbine
section
38
The turbine
section
39
The turbine
section
40
41
42
Turbine Blades
Turbine blades extract energy from the
gas stream in two ways
Reaction
Impulse
43
Reaction turbine blades
Reaction drives the blades via an
aerodynamic reaction
The gas stream is accelerated by
convergent nozzle guide vanes and directed
to flow over the turbine blades producing
an aerodynamic reaction
44
Impulse turbine blades
Impulse turbine blades rotate via impact
of high velocity gas on the blades
The blades of a pure impulse turbine are
bucket shaped to maximise the conversion
of kinetic energy to mechanical energy
45
46
Turbine Blades
Most turbine blades combine both
impulse and reaction principles
The degree of reaction depends on the
type of engine
47
Turbine Blades
Turbojets require high exhaust velocities
to produce thrust so they use high reaction
turbine blades to produce maximum
acceleration
Turboprops and APUs use impulse turbine
blades because they are concerned with
power extraction and not thrust
48
Turbine Blades
Turbofans use reaction/impulse blades to
extract energy to drive the fan while
maintaining reasonably high exhaust
velocity for core engine thrust
49
Turbine Blades
• Higher entry
pressure at the blade
tips means that, to
create a uniform
exit flow, blade
profiles are adjusted
to a reaction profile
at the tip
50
51
Turbine Blade Creep
Turbine blades are subject to enormous
stress loads
A blade weighing only 8 grams may
have to resist a centrifugal force of over
2000 kg
This causes turbine blades to lengthen with
continued use – known as creep 52
53
54
Turbine Blade Creep
If manufacturer’s temperature or rpm
limits are exceeded the creep rate increases
and blade life is drastically reduced
Overhauls are timed to ensure that blades
are replaced before tertiary creep begins
55
Turbine Temperature Measurement
Ideally temperature probes should be placed
in the turbine inlet to measure turbine inlet
temperature (TIT)
The temperature at the turbine inlet is
usually too hot to place temperature probes
56
Turbine Temperature Measurement
Temperature probes are usually placed in an
intermediate stage (ITT) or at the turbine
outlet stage (TOT)
ITT and TOT readings are often
compensated to give an indication of the
temperature at the most critical point – the
turbine inlet
57
Turbine blade cooling
• Cooled by internal air cooling system
58
Exhaust Section
The exhaust section is located behind the
turbine section and usually consists of a
convergent cone to convert pressure
energy to kinetic energy
59
Exhausts
60
Exhausts
61
Exhausts
62
Engine Exhausts
63
Exhausts
64
Engine exhausts
• Convergent exhaust duct
65
Exhausts
66
Exhausts
67
Exhausts
68
Exhausts
69
Exhausts
70
Exhausts
71
Accessory Section
The primary function is to provide space for the
mounting of accessories necessary for operation
and control of the engine. It also includes
accessories concerned with the aircraft, such as
electric generators and fluid power pumps.
Secondary functions include acting as an oil
reservoir and/or oil sump, and housing the
accessory drive gears and reduction gears.
Accessories are usually mounted on common
pads either ahead of or adjacent to the
compressor section. 72
Accessory
Section
73
Accessory
Section
Accessory
Section
74
Accessory
Section
75
Accessory
Section
76
Accessory
Section
77
Accessory
Section
78
Accessory Section
79
Auxiliary Power Units
80
Auxiliary Power Units
A gas turbine powerplant
Supplies the aircraft with
Bleed air
Electrical power
Hydraulic power
81
Auxiliary Power Units
Used mainly during ground operations,
take-off and landing
Most can be used in flight as a back up
supply source but usually have an
operating altitude limit
82
Auxiliary Power Units
APUs have the following features:
Operate at a constant RPM
Start sequence is fully automatic
Vital parameters are automatically
monitored
Automatic shutdown with any faults 83
Auxiliary Power Units
A typical cockpit panel consists of:
Start and stop button
Turbine temperature indicator (EGT)
RPM indicator
Control switches for bleed air,
hydraulic and electrical generation 84
85
Many turbofan engines have two or more spools to
A. improve the cooling of the combustion chamber
walls resulting in a lower turbine temperature
B. assist the compressor sections to rotate closer to
their ideal RPM
C. reduce vibration within the engine core
D. increase spool up time required when compared
to a single spool
And….. the answer is……….. 86
An ideal jet intake delivers air to the compressor
in which state?
A. No turbulence and pressure lower than ambient
B. Increased radial velocity and temperature higher
than ambient
C. Increased temperature and velocity compared
ambient conditions
D. No turbulence and pressure higher than ambient
And….. the answer is……….. 87
The row of stator blades after each row of
compressor blades in n axial flow compressor is
designed to
A. longitudinally balance the engine
B. convert axial flow to radial flow before the next
rotating compressor section
C. convert kinetic energy to pressure energy
D. Convert pressure energy to pressure energy
And….. the answer is……….. 88
Of a turbofan’s total air passing through the
intake 21% goes through the engine core. The
bypass ratio is closest to
A. 4:1
B. 5:1
C. 1:5
D. 1:4
And….. the answer is……….. 89
Which of the following would increase the
maximum possible performance of a jet engine?
A. introduce the air into the engine at a lower speed
B. introduce the air into the engine at a lower
temperature
C. introduce the air into the engine at a higher
temperature
D. introduce the air into the engine at a lower
pressure
And….. the answer is……….. 90
Gas decreases in velocity and increases in pressure
when
A. flowing through a convergent duct
B. it is within the last two thirds of the combustion
chamber
C. it is within the nozzle guide vanes prior to the first
turbine rotor section
D. flowing through a divergent duct
And….. the answer is……….. 91
What is meant by tertiary creep in a turbine blade
of a gas turbine engine?
A. blade creep experienced on the test bench by the
manufacturer
B. normal blade creep during the acceptable working
life of the turbine blade section
C. blade creep that could be detrimental to
continued use of the turbine section
D. an unruly university aviation student
And….. the answer is……….. 92
For a given engine RPM, thrust output from a gas
turbine engine will be greatest
A. At MSL in ISA conditions
B. At high altitude in ISA conditions
C. At high altitude in ISA + conditions
D. At MSL in ISA + conditions
And….. the answer is……….. 93
94
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