Turbine Assembly - Aircraft Gas Turbine Engines #10

Summary

The 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

00:00

🔥 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.

05:02

🛠️ 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.

10:02

🔧 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.

15:05

🌡️ 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.

Keywords

💡Gas Turbine Engine

A gas turbine engine is a type of internal combustion engine that uses the heat energy from burning fuel to produce mechanical work. In the context of the video, it's likened to an axial flow compressor working in reverse, where the turbine extracts energy from hot gases to drive the compressor and other accessories. The script explains how the turbine converts different forms of energy in the gas stream into mechanical energy, which is crucial for the operation of the engine.

💡Nozzle Guide Vane

Nozzle guide vanes are stationary components in a turbine that direct the flow of gases onto the rotor blades. They play a critical role in the conversion of potential pressure energy in the gas stream into kinetic energy. The script mentions that these vanes are part of the first section of the turbine and are essential for the efficient operation of the engine by ensuring the proper direction and velocity of the gas flow.

💡Rotor Section

The rotor section of a turbine is the part that contains the rotating blades. It is where the mechanical energy is extracted from the hot gases. The script describes how the turbine's rotor section works in conjunction with the nozzle guide vanes to harness the energy from the gas stream, converting it into the mechanical energy needed to drive the engine's components.

💡Energy Conversion

Energy conversion in the context of the video refers to the process by which the turbine extracts various forms of energy from the hot gases and converts them into mechanical energy. The script explains that this includes the conversion of heat energy, potential pressure energy, and kinetic energy, which is vital for the functioning of the gas turbine engine.

💡Creep

Creep is a phenomenon where the metal of the turbine blade stretches beyond its ability to return to its original shape due to prolonged exposure to high temperatures and tensile loading. The script highlights the significance of creep in the context of turbine blade design, as it can lead to blade failure over time, necessitating the use of materials and manufacturing processes that resist this effect.

💡Superalloys

Superalloys are a class of materials used in the construction of turbine blades due to their high-temperature resistance and strength. The script mentions that superalloys, which are complex mixtures of metals like chromium, cobalt, nickel, and titanium, have replaced nickel-based alloys and high-temperature steel in early gas turbine engines, allowing for higher operating temperatures and thus greater power output.

💡Powder Metallurgy

Powder metallurgy is a manufacturing process mentioned in the script where powdered superalloys are hot-pressed into a solid state to create turbine blades. This process results in long, slender crystals that are very creep resistant, enhancing the durability and performance of the blades under the extreme conditions they experience in a gas turbine engine.

💡Single Crystal Casting

Single crystal casting is an advanced manufacturing technique used to produce turbine blades with a single, unbroken crystal structure. The script explains that this method virtually eliminates corrosion and creates an extremely creep-resistant blade, which is essential for withstanding the high temperatures and stresses in the turbine.

💡Ceramic Materials

Ceramic materials are used in the production of turbine blades to provide protection against corrosive conditions caused by reactions between the base metals of the blade, sodium in the air, and sulfur in the fuel. The script describes how ceramic coatings, such as plasma spray, can enhance the durability and performance of the blades in the harsh environment of a gas turbine engine.

💡Free Power Turbine

A free power turbine is a type of turbine that is not connected to the compressor but is connected to the propeller or rotor reduction gearbox. The script highlights the advantages of a free power turbine, such as the ability to operate at its own optimum speed, reduced noise pollution during taxiing, and the possibility of fitting a parking brake, which enhances safety and efficiency.

💡Turbine Efficiency

Turbine efficiency refers to the percentage of energy extracted from the gas stream that is converted into mechanical energy. The script discusses the various losses that occur in a turbine, such as aerodynamic losses and gas leakage, and how these affect overall efficiency. It also touches on the design challenges in increasing turbine speed to improve efficiency while managing the resulting stresses on the blades.

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