Topic 03 - Sections of a Gas Turbine Engine Flashcards
five major sections of a gas turbine engine
- air intake
- compressor
- combustion chamber
- turbine
- exhaust
the five major sections of a gas turbine engine is grouped into which two sections?
hot and cold section
components included in the hot section
- combustion chamber
- turbine
- exhaust
components included in the cold section
- air intake
- compressor
if a gas turbine engine is mounted in the airframe, the air intake is ____________
built into the airframe itself
if a gas turbine engine is mounted on the wing or the rear fuselage, the air intake is __________________
the forward part of the nacelle installation
the air intake is designed to provide ___________________ to the first stage compressor of the engine, with the __________________ occurring through the inlet
turbulence-free supply of air, minimum energy loss
to enable the compressor to operate satisfactorily, the air must reach the compressor at a _____________, distributed evenly across the whole of the __________________
uniform pressure, face of the first stage
the design of the intake duct is vital to the performance of the engine if ____________ or _________ is to be avoided
compressor stall, surge
types of air intake
- pitot type intake
- secondary intake doors
- supersonic intake
- bellmouth intake
simplest form of intake
single entrance circular cross section, pitot type
how does air flow through a pitot type intake depending on where the engine is mounted?
- for wing mounted engines, the air flows directly through the intake
- for engines mounted at the bottom of the aircraft fin, the air forms an ‘S’ shaped duct
the pitot type intake maximizes the use of ___________ on the air, due to the aircraft ______________, and suffers only a __________________ of ram pressure as the aircraft attitude changes
ram effect, forward speed, minimum loss
how is the efficiency of a pitot type intake affected as the aircraft approaches sonic speed and why?
the efficiency reduces due to the formation of a shock wave at the engine intake lip
the shape of a subsonic intake usually takes the form of a _______________
divergent duct
how does the shape of a subsonic intake affect the air passing through?
- causes a reduction of airflow velocity
- causes an increase of air pressure at the inlet of the compressor
how does air behave in the air intake when the engine is running on a stationary aircraft?
- the pressure at the inlet of the compressor is below ambient pressure
- the compressor increases air velocity through the intake
how does air behave in the air intake as the aircraft begins to move?
- pressure within the inlet starts to rise
ram pressure recovery point of a pitot type intake
Mach 0.1 to Mach 0.2
what is ram pressure recovery?
pressure in the end of the intake returns to ambient pressure
how does air behave in the air intake as the aircraft speed increases even further?
more ram compression is produced, increasing the engine compression ratio
what is the effect of the increase in engine compression ratio?
generates more thrust without costing any increase in fuel low
another name for secondary intake doors
auxiliary inlet doors
why are secondary intake doors required on certain types o aircraft?
to allow a supplementary airflow to reach the compressor face during high power operation when the aircraft is either stationary or at low airspeeds
example of an aircraft the requires secondary intake doors
Harrier
high power operations at stationary or low airspeeds
vertical takeoff, hover, landing
how are auxiliary intake doors opened and closed
- when the air pressure in the intake is less than ambient, doors are opened by the depression in the intake
- when the air pressure in the intake is greater than ambient, the doors are closed by the pressure differential
type of intake required in a supersonic aircraft
supersonic intake
why is there a need for a special type of intake for supersonic aircraft?
because the engine compressor cannot handle supersonic flow
difference of supersonic intakes to subsonic ones
supersonic intakes are able to handle subsonic speeds normally but at supersonic speeds, the intake needs to slow it down to below sonic speed through shockwaves
which part of the supersonic intake controls the shockwave to efficiently reduce airflow velocity?
supersonic diffuser
what happens to supersonic air velocity as it enters a shockwave?
decreases
in some of the earliest types of supersonic aircraft, what is used to form a shock wave?
central shock cone or inlet cone
how does a convergent-divergent diffuser work in a supersonic intake
because supersonic flows slow down as they enter a constricted area
how does air at subsonic speeds behave when passing through a constriction?
air at subsonic speeds will accelerate toward the throat, maintaining constant density but decreases pressure
how does air at supersonic speeds behave when passing through a constriction?
air density and pressure increases while velocity decreases as it approaches a constriction
what is the objective of a convergent-divergent diffuser?
to slow down the airflow to Mach 1 just before the throat of the diffuser which would further slow down to well below the speed of sound through the divergent section
the convergent-divergent diffuser is only suitable for __________ of supersonic flight at less than _________
short bursts, Mach 2.0
for sustained speeds at or above Mach 2.0, the convergent-divergent diffuser was modified to use __________ to control the airflow into the engine
variable ramps
a type of diffuser with a sharp center body called a ‘spike’
center body design
what does the spike do in a center body design difuser?
strikes the airflow producing an oblique shockwave
why shouldn’t shockwaves enter the inlet?
their high pressure pulses could damage the engine
how should the spike be positioned according to aircraft speed
- spike should be extended when aircraft flies faster
- spike should be retracted when aircraft slows down
the center body design diffuser is suited for _____________ supersonic flight
sustained
example of an aircraft that effectively converts the engine from a conventional turbojet to a ramjet, in flight
SR71
for an SR71, what percentage of its thrust is from the ramjet operation? how much from the turbojet operation and by how much did it improve its specific fuel consumption?
80% (ramjet), 20% (turbojet), 10-15% (improvement)
_______________ have a convergent profile that is designed specifically for obtaining very high aerodynamic efficiency when stationary or in slow flight
bellmouth inlets
bellmouth inlets are typically used on what?
- helicopters
- some slow moving aircraft
- engines being run on ground test stands
a typical bellmouth is _________ in length and has ___________ shoulders offering very little ________________
short, rounded, air resistance
primary function of the compressor section
to supply a sufficient quantity of air to satisfy the requirements of the combustion burners
secondary function of the compressor section
to supply bleed air/customer bleed air for various purposes in the engine and aircraft
Bleeding air from the compressor does cause a ____________________________ which can be detected by observing the _______________ indicator
small but noticeable drop in engine power, engine pressure ratio (EPR)
compressor pressure ratio
outlet pressure divided by inlet pressure
three factors affecting the amount of air passing through the engine
- compressor speed (rpm)
- aircraft forward speed
- density of ambient air
to be effective, a modern compressor must increase the intake air pressure _________________ the ambient air pressure and move the air at a velocity of ___________________
20 to 30 times above, 400 to 500 feet per second
two types of compressor
- axial flow compressor
- centrifugal flow compressor
another name for centrifugal flow compressor
radial outflow compressor
one of the earliest compressor designs still used to day in some smaller engines and auxiliary power units (APUs)
centrifugal flow compressor or radial outflow compressor
a centrifugal compressor consists of
- impeller or rotor
- diffuser
- manifold
compressors with only one impeller
single-stage compressors
compressors having two impellers
double-stage compressors
when two impellers are mounted back-to-back, a ___________________ is created
double-sided or double-entry impeller
a single-stage, double-sided impeller allows a ____________ than that of a similar sized single-stage, single-sided impeller
higher mass airflow
engines with double-sided impellers typically have a ___________ overall diameter
smaller
the centrifugal compressor is ___________, and _____________________ than the axial flow compressor
more robust, easier and cheaper to manufacture
if we compare two compressors, one centrifugal and the other axial, each with the same frontal cross-sectional area, the ___________________ can take in far greater mass of air than the ____________________
axial flow compressor, centrifugal compressor
much higher compression ratios can be attained in an ___________ compressor
axial flow
how does the turbine assembly, attached to the compressor shaft work?
it converts the pressure, velocity, and heat of the gases passing through the turbine into mechanical energy, which is then used to drive the impeller of the compressor at high speed
how does a centrifugal compressor work?
- air is introduced to the eye or center of the impeller
- the centrifugal force causes the air to flow outwards across the impeller, towards the tip
- the air leaves the tip of the impeller and passes to the diffuser section
- the diffuser section then converts the kinetic energy of the airstream to potential energy which also decreases the velocity while increasing the pressure
what introduces air into the eye or center of the impeller?
rotating guide vanes
what is the effect of the divergence between the impeller blades?
- the pressure of the air increases as it flows outwards between them
- the velocity of the air also increases as mechanical energy is added into the equation
system of stationary divergent ducts designed to convert the kinetic energy of the airstream into potential energy
diffuser section
the compression ratio of a single-stage centrifugal compressor could be in the region of _____
4:1
how can you get greater engine compression ratio in centrifugal compressors?
by using two centrifugal compressors in series with each other
why is it not feasible to use more than two centrifugal compressor stages?
because excessive impeller tip speeds and excessive centrifugal loading prohibit efficient operation of a third stage
engine compression ratios higher than _____ are not considered possible using centrifugal compressors
12:1
advantages of centrifugal flow compressor
- simple manufacturing process
- relatively low cost
- low weight
- low starting power requirements
- operating efficiency over a wide range of rotational speed
disadvantages of centrifugal flow compressor
- large frontal area required for a given airflow increases aerodynamic drag
- practical limits on the number of stages restrict its usefulness when designing larger and more powerful engines
the axial flow compressor converts the ________________
of the airstream into _____________________
velocity (kinetic energy), pressure (potential energy)
what is a stage in an axial flow compressor?
one row of rotor blades fastened to a disk followed by one row of stator vanes
the rotor blades in an axial flow compressor have an ___________________ with a varying ________________, or ________
airfoil cross-section, angle of incidence, twist
what does the twist do in rotor blades used in axial flow compressors?
compensates for the blade velocity variation caused by its radius
the angle and airfoil contour of the blades force air _____________ in the same manner as a ____________
rearward, propeller
arranged in fixed rows between the rows of rotor blades and act as diffusers at each stage
stator vanes
what is the shape of stator vanes?
airfoil shape
the angle of attack of stator vanes can be _________ or ____________
fixed, variable
where are stator vanes fastened?
the compressor outer casing
the spaces between the rotor blades and the stator vanes form _____________
divergent passages
set of stator vanes immediately in front of the first stage rotor blades are called ___________
inlet guide vanes
what do inlet guide vanes do?
direct airflow into the first stage rotor blades at the best angle while imparting a swirling motion in the direction of engine rotation
what does the swirling motion caused by inlet guide vanes do?
improves the aerodynamics of the compressor by reducing the drag on the first stage rotor blades
the number of disks equates to the number of _________
stators
a number of disks are fastened together to form an _______________, which is driven by the _____________
integral rotor drum, turbine
each consecutive row of rotor blades and stator vanes constitutes a ______________
pressure stage
the number of pressure stages is determined by what?
the amount of air and total pressure rise required
a single pressure stage in an axial flow compressor is capable of producing a compressor pressure ratio of only __________
1.25:1
how are high compressor pressure ratios obtained in an axial flow compressor?
by adding more compressor stages
how can you add more compressor stages to an axial flow compressor?
by creating multi-spool compressors
the pressure rise across each stage in an axial compressor is only quite small, the ratio being about ______________
1.1 or 1.25:1
in order to achieve compression ratios demanded by more powerful engines, ___________________ may be fitted on ________, which is driven by its own ___________
many rotor stages, one shaft, turbine
each compressor stage of an axial flow compressor ____________ the pressure of the incoming air while the air’s velocity is _____________ as airflow proceeds through the compressor
raises, alternately increased then decreased
the rotor blades ____________ the airflow then the stator vanes _____________________
slightly accelerate, slows the air while increasing its pressure
what guides the compressed air into the diffuser to prepare it for combustion?
outlet guide vanes
the process of compression in an axial flow compressor will repeat depending on what?
the number of spools the compressor have
a number of compressor rotor stages on a single shaft driven by a turbine
spool
in larger, more modern engines, compressors may consist of up to _______ spools
three
space between the rotor drum and the compressor outer casing
air annulus
to maintain reasonably constant axial velocity as it passes through the compressor, the size of the air annulus must be ________
reduced
the gradual convergence of the annulus is achieved by _________________________, or in some cases, a ____________
either tapering the compressor outer casing or the rotor drum, combination of both
what gives us the angle of attack over the rotor blade, and determines the pressure zones on either side of the blade?
the vectorial relationship between the axial velocity of the air flowing through the compressor, and the rpm of that compressor
the vectorial relationship between the engine rpm and the airflow axial velocity will give an angle of attack over the rotor blade, and pressure zones, which are optimum that would occur at ____________
design point
the _______________ of the engine falls as the _________________ of the compressor falls and vice versa
compression ratio, speed of rotation
when the engine is running at low rpm, the air is ______________ as much as at the ______________, and the volume it occupies inside the engine becomes ____________
not compressed, design point, greater
an increased airflow velocity and decreased rpm will generate a ______ angle of attack over the rotor blade
low
a low angle of attack over the rotor blade will ____________ the size of the pressure zones
reduce
if the engine rotates faster than its design maximum, then its compression ratio will ___________________
increase accordingly
a decreased airflow velocity and increased rpm will generate a ______ angle of attack over the rotor blade
high
the reduction in axial velocity can reach a point where ______________ and a phenomenon called _________ may occur
turbulent airflow, stall
a partial breakdown of the airflow through the engine and is a progressive condition
stall
stall, if not checked may produce an event called __________
surge
a total breakdown of airflow through the engine, which in the worst case scenario, may cause the airflow through the engine to instantaneously reverse its direction of flow
surge
a compressor stall occurs when the compressor blades’ ______________ exceeds the ________________
angle of attack, critical angle of attack
can be described as an imbalance between the two vector quantities, inlet velocity, and compressor rotational speed
compressor stall
are fitted to engines which are prone to compressor stall at low rpm or during engine acceleration or deceleration
variable inlet guide vanes
where are variable inlet guide vanes fitted?
just in front of the first rotor stage
at low compressor speeds, the variable inlet guide vanes are angled to impart the ____________________ to the air, thereby correcting the relative airflow to obtain ___________________ over the rotor blade
greatest amount of swirl, optimum angle of attack
maintaining the optimum angle of attack allows a ______________________
smooth and rapid engine acceleration
at high compressor speeds, the vigvs _________ the swirl imparted to the airflow, thereby maintaining the _____________________ of the air flowing over the rotor blades
reduce, correct angle of attack
to minimize airflow problems which may occur after the first rotor stage, when the engine is operating at other than its optimum conditions, some engines are fitted with
variable stator vanes
variable stator vanes can be _______________________, so that as the compressor speed is reduced from the optimum design value, they maintain the airflow onto the following blades at an ____________________
progressively closed automatically, acceptable angle of attack
the last set of vanes the compressor air passes through
outlet vane assembly
these vanes straighten the airflow and eliminate any swirling motion or turbulence
outlet vane assembly
the air velocity of the compressed air must be __________ before it enters the combustion chamber
slowed
the divergent shape of a diffuser ______ compressor discharge while, at the same time, increasing air pressure to its ________________ in the engine
slows, highest value
in some engines, at low compressor rpm, the relationship of the rpm and axial airflow velocity may not be maintained to give the rotor blades the optimum angle of attack, unless ________________________ is allowed to escape from the ___________________ of the compressor
some of the excess volume of air, intermediate stages
If a compressor bleed valve is fitted to the _______________ at a position adjacent to the intermediate stages of the compressor, it can be opened at _________ and during ___________________, to allow some of the excess volume of air to escape.
compressor casing, low rpm, engine acceleration
effects of compressor bleed valve
- bringing the axial velocity of the air in the earlier stages of the compressor closer to the optimum value
- reduces the choking effects in the rear of the compressor
disadvantages of compressor bleeds
opening any compressor bleed decreases the mass airflow through the engine which causes a drop in thrust for a given throttle position, raising the engine’s specific fuel consumption while also raising the engine’s exhaust gas temperature, because the amount of cooling air available in the combustion chambers will have decreased.
why were compressors split?
to overcome the tendency of the compressor to stall at low rpm
initial two separate sections of the compressor
- high pressure (HP) compressor
- low pressure (LP) compressor
In later, more powerful engines, designers split the compressor into _____________, by adding an ______________________.
three sections, Intermediate Pressure (IP) compressor
Each compressor section is driven through a shaft by its _______________.
own turbine
axial flow compressor construction
- rotor assembly
- compressor casing
- stator vanes
- rotor blades
- fan blades
the rotor shaft is supported in bearings, and is coupled to the ______________ in a manner that will allow ___________________ to be catered for
turbine shaft, minor variations in alignment
the centrifugal load on the compressor dictates that the rotor blades are fixed to a _________, which itself is fitted around the ______________
disk, rotor shaft
the most common type of rotor blade fixing method
the root of the blade is shaped to form a dovetail joint, and is secured to the disk by a pin or locking tab
the compressor blades are quite ________ until seated by _______________ during engine operation
loose, centrifugal force
when the engine is windmilling on the ground, the blades rattle loosely and make a sound like a _____________________
bag of nails being shaken
blades integral to the disk, this type of blade and disk combination is called a ‘________’
blisk
materials used in compressor casing depending on the compressor section type
- aluminum alloy (LP)
- steel alloy (IP)
- nickel based alloy (HP)
rotor blades are normally made from ____________ and are machined to a ________________ before being attached to the rotor disk
nickel alloys, close tolerance
the rotor blades reduce in size from the front to the rear of the compressor, to accommodate the _______________________
convergent shape of the air annulus
some of the low pressure stages of the compressor, where the temperature is not too high, may have their rotor blades manufactured from ______________
titanium
the stator vanes are ________ shaped and are either fixed into _____________________, which are themselves fastened to the casing, or are fixed to the compressor casing directly as applied on _________________
airfoil, stator vanes retaining rings, shrouded vanes
the shrouding prevents the stator vanes from __________ which can be induced by the ___________ of the airflow over the vanes
vibrating, velocity
early engines used _____________ in the manufacture of stator vanes
aluminum alloys
what materials are now used in the manufacture of stator vanes?
steel or nickel based alloys
_____________ is occasionally used for the manufacture of the vanes in early stages of the compressor, but it is not suitable for the production of the smaller vanes further into the engine
titanium
where the rotor blades come into contact with the compressor casing
blade rub
what would happen to titanium if a blade rub would occur?
sufficient heat from friction may then be generated to ignite the titanium which could require expensive repairs or cause an airworthiness hazard
the high bypass ratio engine’s fan blades, were, in early engines, manufactured from ________________, because it combines the properties of _________________
solid titanium, strength and lightness
a ________________ is essential if the fan is to be able to withstand the out of balance forces which would occur if a fan blade failed
low blade-weight
despite the enormous strength of titanium, the fan blades had to have a ______________ incorporated into their design
snubber
a support which prevents aerodynamic instability
snubber
side effects of using a snubber
- adds weight
- interferes with some of the airflow
- creates additional aerodynamic drag
- reduces fan efficiency
which material was used as an experiment for fan blades but was discontinued due to loss of efficiency
carbon fiber
the greatest advancement for fan blades has been achieved by fabricating the blade from a ________________ sandwiched between ____________________________
honeycomb core, two outer skins of titanium
advantages of an axial flow compressor
- high peak efficiencies
- small frontal area
- straight through flow, allowing high ram efficiency
- increased pressure rise by increasing number of stages, with negligible losses
disadvantages of an axial flow compressor
- good efficiencies over only narrow rotational speed range
- difficulty of manufacture and high cost
- relatively high weight
- high starting power requirements
