Turbine Assembly - Aircraft Gas Turbine Engines #10
Summary
TLDRThe video script delves into the intricacies of gas turbine engines, highlighting the turbine's role akin to an axial flow compressor in reverse. It discusses the conversion of thermal and kinetic energy from hot gases into mechanical energy, which powers the compressor and gearbox. The script explores materials like high-temperature steel, nickel-based alloys, and superalloys used in turbine blades, addressing challenges like creep and high-temperature operations. It also covers advanced manufacturing processes like single-crystal casting and the use of ceramics. The discussion extends to turbine stages, spool configurations, and the impact of rotational speed on efficiency and stress. The script concludes with insights on temperature monitoring and the losses incurred during turbine operation.
Takeaways
- 🔧 The turbine in a gas turbine engine functions similarly to an axial flow compressor in reverse, extracting energy from hot gases and converting it into mechanical energy to drive the compressor and other accessories.
- 💧 The turbine's first section, the nozzle guide vane, directs air onto the rotor blades, playing a crucial role in the energy conversion process.
- 🌡️ Modern engines can reach temperatures as high as 1700 degrees Celsius in the turbine, which is a significant factor in material selection and design.
- ⚙️ Gearboxes in gas turbine engines can be used to operate accessories or power propellers and rotors in engines not predominantly using jet propulsion for thrust.
- 🏋️♂️ The high rotational speeds of turbine blades can subject them to tensile loads exceeding two tonnes, leading to a phenomenon known as creep, where the metal stretches beyond its ability to return to its original shape.
- 🛠️ Early turbine blades were made from high-temperature steel, but advancements led to the use of nickel-based alloys, superalloys, and now single-crystal casting for improved heat and creep resistance.
- 🔬 Superalloys are complex mixtures of metals like chromium, cobalt, nickel, titanium, and tungsten, offering higher temperature limits and better performance than traditional steel.
- 🌀 The turbine assembly consists of one or more stages on a single shaft, which can be part of a spool when coupled with a compressor, affecting the engine's efficiency and operation.
- 🛑 Active clearance control and blade shrouding are used to minimize losses due to gas leakage and reduce blade vibration, enhancing the turbine's performance.
- ✈️ Free turbines, which are not connected to a compressor, can operate at their own optimum speed, offering benefits such as reduced noise pollution during taxiing and the ability to fit a parking brake.
Q & A
What is the function of the nozzle guide vane in a gas turbine engine?
-The nozzle guide vane in a gas turbine engine directs the air axially onto the blades of the rotor section, helping to convert the energy from the hot gases into mechanical energy.
How does a gas turbine engine utilize the energy available in the gas stream?
-A gas turbine engine utilizes the heat energy, potential pressure energy, and kinetic energy from the gas stream, converting these into mechanical energy to drive the compressor and other accessories.
What is the significance of the rotational speed of the turbine in a gas turbine engine?
-The rotational speed of the turbine is critical as it determines the blade tip velocity and the efficiency of energy conversion. High speeds can lead to blade tip velocities exceeding 1500 feet per second.
Why is the material of turbine blades important in gas turbine engines?
-The material of turbine blades is crucial because it must withstand high temperatures and tensile loading. Early blades were made of high-temperature steel, but advancements led to the use of nickel-based alloys, superalloys, and even single crystal casting for improved creep resistance.
What is creep, and how does it affect turbine blades?
-Creep is a phenomenon where the metal of the blade stretches beyond its ability to return to its original length due to the combined effects of high temperature and tensile loading. Over time, this can lead to blade failure.
How do turbine blades convert the energy from the gas stream into mechanical energy?
-Turbine blades convert the energy by extracting heat, pressure, and kinetic energy from the gas stream, which is then transformed into mechanical energy to drive the engine's compressor and other components.
What is the purpose of the divergent gas flow annulus in a turbine stage?
-The divergent gas flow annulus allows for longer blades to be fitted to each turbine stage, which helps control the gas stream velocity as it expands into the larger volume available at the rear of the engine.
Why are free turbines advantageous in certain gas turbine engine designs?
-Free turbines, which are not connected to a compressor, can operate at their own optimum design speed, leading to benefits such as reduced noise pollution during taxiing, less starting torque requirement, and the possibility of fitting a parking brake.
How does increasing the number of turbine stages affect the power output and turbine diameter?
-Increasing the number of stages in a turbine allows for an increase in power output without the need to increase the turbine diameter, which can help reduce drag and stress on the engine.
What are the different types of turbine blades used in gas turbine engines, and what determines their selection?
-Turbine blades can be impulse, reaction, or a combination of both, known as impulse-reaction. The selection of blade type depends on the specific design requirements of the engine, with the combination impulse-reaction type being more commonly used.
How is the turbine temperature monitored in a gas turbine engine, and why is this important?
-Turbine temperature is monitored using thermocouples placed in the gas flow within the turbine assembly. This monitoring is crucial to prevent exceeding the maximum turbine temperature, which could cause irreparable damage to the engine.
Outlines
🔥 Turbine Mechanics and Material Science
This paragraph discusses the functioning of a gas turbine engine's turbine, comparing it to an axial flow compressor operating in reverse. It explains how the turbine's nozzle guide vanes direct air onto the rotor blades, converting the energy from hot gases into mechanical energy to drive the compressor and other accessories. The paragraph also delves into the forms of energy present in the gas stream and how they are transformed into mechanical energy. It highlights the challenges faced by turbine blades due to high rotational speeds and temperatures, leading to creep, a deformation phenomenon. The evolution of materials used in turbine blades is traced from high-temperature steel to nickel-based alloys and superalloys, with a focus on their temperature limits and resistance to creep. Advanced manufacturing processes like powdered metallurgy and single-crystal casting are introduced as methods to improve blade strength and creep resistance.
🛠️ Turbine Stages and Efficiency Enhancements
The second paragraph focuses on the structure and operation of turbine stages, which consist of stationary nozzle guide vanes and rotating turbine blades. It describes how the turbine assembly is composed of multiple stages on a single shaft, forming a spool when coupled with a compressor. The paragraph emphasizes the design features like the divergent gas flow annulus and blade shrouds that optimize gas velocity control and reduce leakage and vibration. It also introduces the concept of free turbines, which are not connected to compressors and can operate at their own optimal speeds, providing benefits like reduced noise during taxiing and lower starting torque requirements. The paragraph further discusses strategies to increase turbine power output, such as adding more stages instead of increasing diameter, and the advantages of high bypass ratio engines in terms of efficiency and reduced stress on turbine blades.
🔧 Advanced Turbine Blade Design and Loss Analysis
This paragraph delves into the design and types of turbine blades, including impulse, reaction, and combination impulse-reaction blades. It explains the function of nozzle guide vanes in converting pressure energy to velocity energy and the role of turbine blades in this process. The discussion includes the challenges of balancing efficiency with the stresses imposed on blades due to increased rotational speeds. The paragraph introduces the concept of bypass engines and their three spool configurations, which allow for smaller turbine diameters and reduced stress issues. It also covers the importance of blade fixing methods, such as the fir-tree system, in withstanding the high centrifugal forces during operation. The paragraph concludes with an analysis of energy losses in turbines, detailing the sources and percentages of these losses, including aerodynamic losses and leakage.
🌡️ Turbine Temperature Management and Monitoring
The final paragraph emphasizes the importance of monitoring turbine temperature to prevent damage from exceeding temperature limits. It outlines various terms used to describe gas temperatures in the turbine zone, such as Exhaust Gas Temperature (EGT), Turbine Inlet Temperature (TIT), Turbine Gas Temperature (TGT), and Jet Pipe Temperature (JPT). The paragraph explains how these temperatures are measured using thermocouples and the significance of these measurements in controlling engine performance. It also touches on the use of thermocouple probes inside fixed nozzle guide vanes for more accurate temperature sensing. The paragraph concludes by discussing how changes in exhaust gas temperature are indicative of engine thrust adjustments and the necessity of managing these temperatures to ensure engine integrity.
Mindmap
Keywords
💡Gas Turbine Engine
💡Nozzle Guide Vane
💡Rotor Section
💡Energy Conversion
💡Creep
💡Superalloys
💡Powder Metallurgy
💡Single Crystal Casting
💡Ceramic Materials
💡Free Power Turbine
💡Turbine Efficiency
Highlights
The turbine of a gas turbine engine is similar to an axial flow compressor working in reverse.
The turbine's first part is a stationary section known as a nozzle guide vane.
The turbine extracts various forms of energy from hot gases to drive the compressor.
Gas stream energy includes heat, potential pressure, and kinetic energy.
The turbine blade tips can travel at speeds over 1500 feet per second.
Gas temperatures in modern engines can reach up to 1700 degrees Celsius.
Turbine blades experience tensile loading and high temperatures, leading to creep.
Early turbine blades were made from high-temperature steel, limiting engine power output.
Nickel-based alloys and superalloys were later used for improved temperature resistance.
Powder metallurgy and single crystal casting enhance blade creep resistance.
Ceramic materials are used to protect against corrosive conditions in turbine blades.
The turbine stage consists of stationary nozzle guide vanes and rotating turbine blades.
Turbine efficiency is affected by blade speed, with losses reducing as blade speed increases.
A free turbine is not connected to a compressor, allowing it to seek its own optimum speed.
Increasing the number of turbine stages can boost power output without increasing diameter.
Bypass engines with smaller diameter turbines can circumvent efficiency vs. stress issues.
Turbine blades can be impulse, reaction, or a combination of both.
The fir-tree fixing system is commonly used to attach blades to the turbine disk.
Turbine efficiency is high, but it still suffers losses, averaging about 8%.
Exhaust Gas Temperature (EGT) is a critical parameter monitored to prevent engine damage.
Transcripts
the turbine of a gas turbine engine can
be likened roughly to an axial flow
compressor working in Reverse
the first part of the turbine is a
status section which is called a nozzle
guide vein the nozzle guide vane directs
the air axially onto the blades of a
rotor section
the turbine extracts the different forms
of energy from the hot gases that flow
through it and converts that energy into
mechanical energy which it uses to drive
the compressor and gearbox is connected
to it
gear boxes can be used to operate
accessories or in the case of engines
that do not use predominantly jet
propulsion to generate their thrust to
power propellers or rotors
the energy available in the gas stream
flowing through the turbine takes
several forms there is the heat energy
the potential pressure energy
and finally the kinetic energy which
comes from the velocity of the gas
stream
the convolution of all these forms of
energy into mechanical energy means that
their values will be reduced as the gas
stream passes through the turbine
the less having said that the velocity
of the gas in the combustion chamber is
lower than the eventual velocity of the
gas when it reaches the exhaust unit
during normal operation of the engine
the rotational speed of the turbine may
be such that the blade tips travel at a
rate in excess of 1500 feet per second
at the same time the temperature of the
gas is driving the turbine can in a
modern engine reach as high as 1700
degrees Celsius
the speed of these gases is as high as
2500 feet per second that is close to
the speed of sound at these temperatures
these factors mean that a small turbine
blade weighing only two ounces when
stationary
can while it's rotating at its maximum
speed exert a load along its length
which exceeds two tonnes
this tensile loading coupled with the
tremendous heat causes a phenomenon
called creep which is C stretching of
the metal of the blade beyond its
ability to reform back to its original
length
whatever materials have been used to
produce the turbine and however
carefully the temperature and the
rotational speed limits of the engine
have been observed creep will
nevertheless cause the length of the
blade to increase over a period of time
and engine operating cycles a blade will
have a finite life before failure occurs
the turbine blades of early gas turbine
engines were manufactured from
high-temperature steel use of this
material imposed a stringent limit upon
the temperature at the rear of the
engine and because the gas turbine
engine is a heat engine it follows that
the power output of such engines was
limited as a consequence
the next advance in turbine technology
was the use of nickel-based alloys
these were subsequently superseded by
super alloys super alloys are a complex
mixture of many different metals
chromium cobalt nickel titanium tungsten
carbon etc
blades manufactured from superalloys
have a maximum temperature limit of
approximately 1,100 degrees Celsius
or if the blades are called internally
1425 degrees Celsius
traditional metal manufacturing
processes produce a crystal lattice or
grain in the material the boundaries of
the crystals create a weakness in the
structure and are usually the starting
point of any failure
some turbine blades are manufactured by
the process of powdered metallurgy in
which powdered super alloys are hot
pressed into a solid state the blades
thus produced have long slender crystals
and are very creep resistant
however in the search for even stronger
materials a procedure called single
crystal casting is now being used in the
most advanced engines this type of blade
has been directionally solidified via a
spiral selector which permits only one
crystal to grow in the blade
this process virtually eliminates
corrosion and creates an extremely creep
resistant blade
ceramic materials are also being used in
the production of turbine blades
originally the ceramic was applied as a
plasma spray the coating giving very
good protection against a corrosive
condition caused by a reaction between
the base metals of the blade the sodium
in the air and the sulphur in the fuel
we have seen earlier in the lesson on
compressors that the compressor added
energy to the gas stream by increasing
its pressure
that energy is extracted in the turbine
by reducing the pressure of the gases
flowing through it this drop in the
amount of the pressure energy occurs
both as its converted to kinetic energy
or a higher velocity in the nozzle
contains and also as its converted into
mechanical energy in the turbine blades
as is illustrated in this graph
we've already seen that the turbine
stage consists of two elements one row
of stationary nozzle guide vanes and one
row of rotating turbine blades
the complete turbine assembly comprises
one or more turbine stages on one shaft
which if coupled to a compressor forms a
spool
this picture shows across
action of a single shaft three-stage
turbine similar to that used on the
rolls-royce dart turboprop engine
there are certain features shown in this
diagram which are worthy of special note
the divergent gas flow annulus affords
longer and longer blades to be fitted to
each turbine stage moving backwards in
the engine this enables the velocity of
the gas stream to be controlled as the
gas expands into the large volume
available to it
the blade shroud shown here is fitted in
an attempt to minimize losses due to
leakage across the turbine blade tips it
also reduces the vibration of the blades
the clearance between the blade tips and
the turbine casing varies because of the
different rates of expansion and
contraction of the materials involved
in some engines and abrade herbal lining
is used in the turbine casing area to
reduce gas leakage through this
clearance between the blade tips and the
turbine
but active clearance control can be more
effective at maintaining minimum tip
clearance throughout the flight cycle
this picture shows its use on an
American engine
when a turbine is coupled to a
compressor to form a spool for peak
efficiency and to ensure that stall and
surge do not occur in the compressor the
spool must rotate at a speed which
conforms to the requirements of the
compressor we saw in the lesson on
compressors that the optimum speed of
the spool is that of its design point
a free turbine sometimes called a free
power turbine is a turbine which is not
connected to the compressor it's
connected only either to the propeller
or to the rotor reduction gearbox
the fact that it's not connected to a
compressor allows a free power turbine
to seek its own optimum design speed
rather than that of a compressor
the free power turbine has further
advantages the propeller of a free
powder by an engine can be held at low
rpm during taxiing thereby reducing
noise pollution and wear on the brakes
a free power turbine engine requires
less starting torque than does an engine
where the turbines and compressors are
coupled together
I wrote a parking brake can be fitted to
an engine with a free power turbine this
eliminates the dangers inherent in
having propellers rotating in windy
conditions on the ground
the power output of a turbine can be
increased by increasing its diameter
but this would have to detrimental
effects firstly it would increase the
drag factor by requiring that the engine
be designed to have a large diameter
and secondly that greatest stresses
would be imposed because of the greater
centrifugal forces created
a more effective method is illustrated
in this picture where an increase in the
number of stages which comprise the
turbine allows an increase in power
output with a reduction in turbine
diameter
it's a fact that the efficiency of a
turbine blade increases as its
rotational speed increases the losses
reducing proportion to the square of the
mean blade speed
unfortunately the stresses on the blade
increase in proportion to the square of
the plate speed it would seem then that
the engine designer is locked into a
vicious circle where any attempt to
increase engine efficiency by increasing
turbine speed would require stronger
blades this would mean making them
heavier which would create greater
stresses on them and so on ad nauseam
all is not lost however the advent of
the high ratio of bypass engine with its
much greater propulsive efficiency means
that for a given thrust it can have a
smaller diameter turbine this to some
extent circumvents the vicious circle
problems mentioned previously
bypass ratio type of engine shown here
consists of three spools firstly we have
the high-pressure spool driven by the
high-pressure turbine which rotates the
high-pressure spool at relatively high
speeds
then to the rear of the high-pressure
turbine is the intermediate pressure or
IP turbine driving the intermediate
pressure compressor through a shaft
which spins inside that of the high
pressure turbine
finally the rear most turbine is the
low-pressure or LP turbine it drives the
low-pressure compressor more commonly
called the fan through a shaft which
runs inside the high-pressure and the
intermediate pressure shafts
nozzle guidelines are of aerofoil shape
the space between two nozzle guide vanes
forms a convergent duct
within this convergent duct some of the
potential law pressure energy in the gas
stream is converted to kinetic or
velocity energy
the turbine blades themselves can be
impulse type which is similar in action
to a waterwheel
reaction type these blades rotate as a
reaction to the lift they create as the
gas stream flows over them
or a mixture of the two types which is
called impulse reaction
this picture shows three end on views of
the combination impulse reaction blade
it illustrates how the shape of the
blade changes from its base to its tip
the shape change is accomplished by the
blade having a greatest stagger angle at
its tip than at its base
this gives the blade a twist which
ensures that the gas flow does equal
work along the length of the blade and
also enables a gas flow to enter the
exhaust system with a uniform axial
velocity
normally gas turbine engines do not use
either pure impulse or the pure reaction
type of blades the proportion of each
type of blade utilized is dependent upon
the design requirements of the engine in
general the combination impulse reaction
is more commonly used
impulse type turbine blades are used in
air starter motors
it's very rare to find pure reaction
blades being used when they are the
novel guide vanes are designed to divert
the gas flow direction without altering
the pressure of the gas
the considerable stress imposed upon the
turbine blade and the turbine disk when
the engine is rotating at working speed
makes the method of fixing the blade to
the disk
extremely important
the fir-tree fixing is the most commonly
used system on modern engines the
serrations which form the fir-tree are
very accurately machined to ensure that
the enormous centrifugal load is shared
equally between them
the blade is free in the serrations
while the engine is not rotating but
during operation the centrifugal force
imposed holds it firmly in place
the turbine is a very efficient
mechanical device nevertheless it does
suffer losses during its operation on
average the energy loss in the turbine
is about 8%
this 8% is comprised of approximately
3.5 percent from aerodynamic losses in
the turbine blades
and 1.5% aerodynamic losses in the
nozzle guide vanes
divided equally between gas leakage over
the blade tips and exhaust system losses
the maximum temperature that the turbine
assembly can withstand limits the thrust
or power available
exceeding the maximum turbine
temperature will cause irreparable
damage to the engine
therefore it's imperative that the
turbine temperature is monitored closely
the temperature of the turbine is
measured by thermocouples which are
placed in the gas flow somewhere in the
turbine assembly typically after the
high or low pressure turbine the
temperature measured at this point would
be termed the exhaust gas temperature or
egt
other terms that are used to represent
the value of the gas temperature around
the turbine zone are turbine Inlet
temperature or ti t
turbine gas temperature or TGT
jet pipe temperature or jpt
the jetpack temperature is so named
because of the position of the
thermocouples they are actually placed
behind the turbine in the jet pipe
itself
in some modern engines the thermocouple
probes are fitted inside selected fixed
nozzle guide range to enable temperature
to be sensed without the probe being
battered by the high-velocity gas flow
is accelerated to produce more thrust or
more shaft horsepower the exhaust gas
temperature would increase in proportion
with the extra fuel flow and vice-versa
that completes the lesson on turbines
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