Aircraft Gas Turbine Engines #03 - Introduction Part 3

Summary

This script delves into the intricacies of gas turbine engines, detailing how air velocity and pressure adjustments are crucial for engine efficiency. It explains the role of duct shapes in managing kinetic and pressure energy transformations, and the significance of compressor and turbine design in various engine types like turbojets, turboprops, and turboshafts. The script also touches on bypass ratios and their impact on propulsive efficiency, highlighting the evolution of engine technology to meet the demands of modern aviation.

Takeaways

• 🌬️ The efficiency of an engine is influenced by how well it manages air's velocity and pressure changes through ducts of different shapes.
• 🔥 At the exhaust nozzle, pressure is reduced to ambient levels, leading to a significant increase in gas velocity.
• 📦 Divergent ducts are used post-compressor to increase air pressure without additional energy expenditure and to decrease velocity for easier combustion.
• ⏫ A convergent duct accelerates gas towards the turbine blades, enhancing torque transfer and engine efficiency.
• 🔩 In a spool setup, a compressor and turbine are on the same shaft, common in fighter aircraft for high-speed operations.
• 🔄 Turboprop engines convert gas stream energy into mechanical energy to drive both the compressor and the propeller, with minimal residual thrust.
• 🚀 Turboshaft engines, used in helicopters and power supply units, feature a free power turbine for a broader operating speed range.
• ⏏️ Modular construction in engines allows for easier maintenance by replacing engine sections rather than entire units.
• 🌀 Bypass engines, like the fan jet, offer high propulsive efficiency by using a large mass of air at lower velocities.
• 📉 Specific fuel consumption is lower in low and high bypass ratio engines compared to pure turbojets, making them more efficient for transport aircraft.

Q & A

What adjustments are necessary as air passes through the engine?

As air passes through the engine, adjustments to its velocity and pressure must be made. This includes compressing the air during the compression stage without significantly increasing its velocity, and dropping the pressure to ambient levels at the exhaust nozzle to increase velocity.

How do different duct shapes affect the air within an engine?

Different duct shapes are used to change the pressure and velocity of air within an engine. For instance, a divergent duct increases pressure without adding energy, while a convergent duct accelerates the gas to transfer more torque to the turbine.

What is the significance of the design of ducts in an engine?

The design of ducts is crucial as it affects the efficiency of energy conversion between kinetic and pressure energy, which in turn reflects on the overall engine efficiency.

How does a divergent duct benefit the engine's operation?

A divergent duct provides two benefits: it increases air pressure without the need for additional compressor energy, and it decreases air velocity, making combustion in the chamber more manageable.

What is the purpose of a convergent duct in the exhaust nozzle?

A convergent duct is used to accelerate the gas, which increases the rate of gas flow into the turbine, thereby increasing the torque applied to the turbine blade.

What is a spool in the context of gas turbine engines?

A spool refers to a unit where a compressor and turbine are joined on one shaft, such as in a single spool axial flow compressor turbojet engine.

How does a turboprop engine differ from a turbojet in terms of power handling?

In a turboprop engine, almost all the energy in the gas stream is converted into mechanical energy to drive the compressor and propeller, with only a small amount of jet thrust available. In contrast, a turbojet uses virtually all remaining energy after the compressor for thrust.

What is the role of a centrifugal compressor in a turboshaft engine?

In a turboshaft engine, a centrifugal compressor is used in the high-pressure stage to throw air out radially so it can enter the combustion chamber correctly, allowing for a shorter, stiffer, and lighter engine design.

What is the function of a free power turbine in a turboshaft engine?

A free power turbine is a turbine not connected to any compressors, allowing it to operate at a wider speed range. It converts any remaining energy in the gas stream to mechanical energy for the attached load.

What is the significance of bypass ratio in engine design?

The bypass ratio is the ratio of air bypassed around the engine's hot core to the air passing through it. Engines with higher bypass ratios are more efficient at lower speeds, offering better propulsive efficiency and lower specific fuel consumption.

How does the propulsive efficiency of different engine types compare?

At lower speeds, turboprops have the highest propulsive efficiency, but this drops off above 350 mph. Turbojets have lower efficiency at low speeds but improve beyond 800 mph, where turboprops cannot match. Bypass engines, both low and high ratio, offer efficiencies between turboprops and pure turbojets at speeds typical for jet transport aircraft.

Outlines

00:00

🚀 Engine Dynamics and Efficiency

This paragraph discusses the critical role of air velocity and pressure adjustments in a jet engine's performance. It explains how different duct shapes manage these changes, impacting the engine's overall efficiency. The use of a divergent duct after the compressor and before the combustion chamber is highlighted for its dual benefits of increasing pressure without energy expenditure and decreasing velocity to facilitate combustion. The paragraph also introduces the concept of a convergent duct accelerating gas towards the turbine blades to enhance torque transfer. The discussion on spool, axial flow compressor turbojet engines, and their applications in fighter aircraft precedes an explanation of how air passes through the engine, with a focus on energy transformations and the balance between thrust and efficiency in turbojet and turboprop engines.

05:03

🌀 Turboshaft and Bypass Engines

The second paragraph delves into the workings of the turboshaft engine, which can power helicopter rotors or generate electrical power. It contrasts the turboprop and turboshaft engines, emphasizing the latter's free power turbine that allows for a broader operating speed range. The concept of bypass ratio is introduced, defining low and high bypass ratio engines and their impact on propulsive efficiency and specific fuel consumption. The paragraph also describes the twin spool low bypass ratio engine, explaining how air is split between the bypass duct and the hot core, and how this split enhances efficiency. The historical development of the fan jet engine, exemplified by the Rolls-Royce RB211, is also covered, detailing the airflow through the engine and the significance of the fan in thrust generation.

10:09

🛫 Propulsive Efficiency and Engine Design

The final paragraph focuses on the propulsive efficiency of different gas turbine engines, comparing turboprops, pure turbojets, and bypass engines. It discusses how the choice between low and high acceleration of air masses affects engine efficiency and thrust. The graph mentioned illustrates the varying efficiencies of these engines at different speeds, showing the turboprop's dominance at lower speeds and the turbojet's superiority at higher speeds. The paragraph also addresses the practical considerations of using large aircraft, where engine reliability is crucial to avoid significant operational costs. It concludes with a mention of modular engine construction as a strategy to minimize downtime and costs associated with engine failures.

Keywords

💡Compression Stage

The compression stage is a critical part of a gas turbine engine where air is compressed to increase its pressure before it enters the combustion chamber. This is essential for the efficient operation of the engine as it prepares the air for mixing with fuel and combustion. In the script, it is mentioned that during this stage, the air must be compressed without a significant increase in velocity, highlighting the delicate balance of forces within the engine.

💡Exhaust Nozzle

The exhaust nozzle is the component at the end of a jet engine where the gas pressure is reduced to ambient levels, resulting in a substantial increase in velocity. This change in pressure and velocity is crucial for the generation of thrust, which propels the aircraft forward. The script explains how the design of the nozzle, particularly the use of a divergent duct, can influence the engine's efficiency by managing these changes.

💡Ducts

Ducts, or shaped passages, are integral to the design of a gas turbine engine as they guide the airflow and facilitate changes in pressure and velocity. The script emphasizes the importance of duct design, noting that the efficiency of energy conversion between kinetic and pressure energy is reflected in the overall performance of the engine. The use of divergent and convergent ducts is highlighted to illustrate how they can increase pressure or accelerate gas, respectively.

💡Kinetic Energy

Kinetic energy is the energy an object possesses due to its motion. In the context of the video, kinetic energy is a form of energy that the gas in the engine possesses as it moves. The script discusses how the engine's ducts are designed to convert kinetic energy to pressure energy and vice versa, which is crucial for the engine's efficiency and performance.

💡Spool

A spool refers to a unit where a compressor and turbine are joined on one shaft. The script mentions single spool axial flow compressor turbojet engines, which were considered useful for aircraft requiring a small frontal area and high forward speed, such as fighter jets. The term 'spool' is indicative of the mechanical arrangement within the engine that links the compressor to the turbine.

💡Combustion Chambers

Combustion chambers are where fuel is added to the compressed air and ignited, resulting in a significant increase in temperature and volume. This process is vital for generating the energy required to drive the compressor and ultimately produce thrust. The script describes how the air, after being compressed, is directed into these chambers for the combustion process.

💡Turbine

A turbine is a device that extracts energy from a flow of fluid, in this case, the gas stream in a jet engine. The script explains that about 50% of the energy in the gas stream is used to drive the compressor, and the turbine plays a key role in this process. The efficiency of the turbine直接影响s the overall efficiency of the engine.

💡Turboprop Engine

A turboprop engine is a type of gas turbine engine where the output is the sum of the shaft horsepower developed at the turbine and the residual jet thrust. The script contrasts the turboprop with the turbojet, explaining that in a turboprop, almost all the energy in the gas stream is converted into mechanical energy to drive the compressor and the propeller, with only a small amount of jet thrust available.

💡Turboshaft Engine

A turboshaft engine is similar to a turboprop engine but is designed to drive a shaft, such as a helicopter rotor or an alternator for electrical power. The script mentions that turboshaft engines can be used in applications requiring a compact supply of electrical power, and they often incorporate a free power turbine for a wider operating speed range.

💡Bypass Ratio

Bypass ratio refers to the ratio of the amount of air bypassed around the hot core of the engine to the amount of air passing through the hot core. The script explains that engines with different bypass ratios have different propulsive efficiencies and specific fuel consumptions. Low bypass ratio engines are compared with high bypass ratio engines, with the latter being more efficient at the speeds typical of jet transport aircraft.

💡Propulsive Efficiency

Propulsive efficiency is a measure of how effectively an engine converts fuel energy into thrust. The script discusses how the propulsive efficiency of low and high bypass ratio engines is much greater than that of pure turbojets at the speeds associated with jet transport aircraft, making them more efficient for this application.

💡Modular Construction

Modular construction in the context of gas turbine engines refers to the design approach where the engine is split into several modules for easier maintenance and repair. The script mentions that engine manufacturers use this method to minimize the financial burden on users in the event of engine failure, allowing for quicker and more cost-effective repairs.

Highlights

The necessity of adjusting air velocity and pressure as it passes through the engine.

Compression stage requires air compression without significant velocity increase.

At the exhaust nozzle, pressure is reduced to ambient, leading to a velocity increase.

Efficient design of ducts is critical for the engine's overall efficiency.

Divergent ducts increase air pressure without energy expenditure post-compressor.

Decreased velocity in divergent ducts aids combustion chamber flame maintenance.

Convergent ducts accelerate gas, enhancing turbine blade torque.

Converting pressure energy to kinetic energy improves turbine efficiency.

A compressor and turbine on one shaft form a spool, common in fighter aircraft engines.

Controlling smooth airflow through the engine is challenging across rotational speeds.

In turboprop engines, output is the sum of shaft horsepower and residual jet thrust (ESHP).

Turboprops convert gas stream energy into mechanical energy for the compressor and propeller.

Turboshaft engines, used in helicopters and power supply, feature a free power turbine for wider speed range.

Bypass ratio defines the proportion of air bypassing the engine's hot core to that passing through it.

Low and high bypass ratio engines offer greater propulsive efficiency and lower fuel consumption than pure turbojets.

Fan jet engines, like the Rolls-Royce RB211, rely on bypass duct air for thrust, offering high propulsive efficiency.

Thrust can be achieved by low acceleration of large air mass or high acceleration of small air mass, with the former being more efficient.

Bypass engines, both low and high ratio, find a niche in mid-speed ranges for jet transport aircraft.

Modular construction in engine manufacturing facilitates easier maintenance and reduces financial burden on users.

Transcripts

As the air passes through the engine various adjustments must be made to its velocity and

pressure. For example throughout the compression stage the air must be compressed, but no appreciable

increase in its velocity can be easily tolerated.

Another example which we've already seen occurs at the exhaust nozzle where the pressure of

the gas is dropped to that of ambient and a considerable increase in its velocity results.

These changes in pressure and velocity are accomplished by the different shaped passages

or ducts through which the air must pass before it exits the engine. The design of these ducts

is extremely important because the efficiency with which the changes from kinetic energy

to pressure energy and vice-versa occur are reflected in the overall efficiency of the

engine. In this example of the use of different duct shapes within the engine. It can be seen

that the use of a divergent duct will increase the pressure of the air after it leaves the

final stage of the compressor and before it enters the combustion chamber.

This air sometimes called compressor delivery air is the highest pressure air in the engine.

Using a divergent duct here gives a two-fold advantage. First, an increase in pressure

has been achieved with no expenditure of energy in driving the compressor. Secondly, a decrease

in velocity has been contrived which will serve to make the task of the combustion chamber

in keeping the flame burning less difficult.

This example which we've already experienced shows how a convergent duct is used to accelerate

the gas as it passes through the nozzle guide vanes on its way to the turbine blades. The

torque which is applied to the turbine blade is dependent amongst other things upon the

rate of gas flow into it. It follows then that the faster we can make the gas flow into

the turbine the more torque we can transfer to the turbine.

Logically therefore if we convert some of the considerable pressure energy of the gas

stream into kinetic energy it will be more efficient in imparting a turning effect upon

the turbine and its shaft.

When a compressor and turbine are joined on one shaft the unit is called a spool. This

diagram shows a single spool axial flow compressor turbojet engine. This type of engine was for

a long time considered to be the most useful where an engine with a small frontal area

was required such as in fighter aircraft. Where a high forward speed was the main criterion.

There are however problems with the control of the smooth flow of air through the engine

throughout its rotational speed range, more of this later.

As it flows from the compressor the air is fed directly into the combustion chambers

and fuel is added in the resulting mixture ignited. the resultant increase in temperature

will cause the substantial increase in volume which is required. The energy required to

drive the compressor approximately 50% of the energy in the gas stream is now extracted

from the stream as it passes through the turbine. The remaining energy in the gas stream acts

as thrust as the gas is passed atmosphere by the end of the jet pipe.

This diagram illustrates both a centrifugal compressor turboprop engine and an axial flow

compressor turboprop engine. the output from a turboprop engine is the sum of the shaft

horsepower developed at the turbine and the residual jet thrust. This is called equivalent

shaft horsepower or ESHP. The major difference between the turbojet and the turboprop in

how they handle the generated power is that in the turbojet virtually all the energy that

remains after the compressor has been powered is used as thrust. Whereas in the turbo problem

almost all the energy in the gas stream is converted into mechanical energy to drive

both the compressor and the propeller. Only a small amount of jet thrust is available

from the exhaust system of a turboprop with an efficient turbine it can fairly be described

as residual thrust only.

Apart from this difference the airflow through the engines is virtually the same in either

case. The compressor passes the air to the combustion chambers, where the fuel is added

and the mixture is burnt. And because of the temperature rise a substantial increase in

the volume of the air is obtained at a nominally constant pressure. The gas is now expanded

in the turbines where are dropping the pressure and temperature is converted into the kinetic

energy which is exchanged for the torque used to drive the compressor or compressors and

the propeller through its reduction gear.

The turboshaft engine can be thought of as a turboprop engine where the propeller has

been replaced by a shaft turboshaft engines can be used to drive helicopter rotors, they

can also be used in applications.

Where a compact supply of electrical power is required their output shaft being attached

to an alternator. This is the type of engine normally used to see Auxiliary Power Unit

or APU on most modern transport aircraft.

Most if not all turboshaft engines incorporate a free power turbine. A free power turbine

is a turbine that is not connected to any of the compressors. This frees the turbine

from the constraint of having to rotate at a speed that suits the compressor and this

gives the turbine a much wider operating speed range. The single spool turboshaft engine

illustrated here has a reverse flow combustion chamber system. This allows the engine to

be much shorter stiffer and lighter than it otherwise would be. But does add the requirement

for a centrifugal compressor to be used in the high-pressure stage this allows for the

air to be thrown out radially in order that you can enter the combustion chamber in the

correct direction. Other than this deviation the airflow is similar to that previously

described for the turbojet engine, up to the point where it leaves the first stage turbine.

The first stage turbine having converted sufficient energy from the gas stream to drive the compressor,

the free power turbine converts any remaining energy to mechanical energy which is utilized

to drive whatever is attached to its shaft.

The bypass ratio of an engine is due find out the ratio of the amount of air which is

bypassed around the hot core of the engine to the amount of air which passes through

the hot core. An engine with a bypass ratio in the region of about 1 or 2 to 1 would be

considered to be a low bypass ratio engine. Whereas an engine with a bypass ratio of around

5 to 1 would be considered to be a high bypass ratio engine.

The engine shown here is a twin spool low bypass ratio engine. The airflow as far as

the end of the low-pressure compressor is identical to that of a pure turbojet. But

Then the airflow splits into two. An amount depending on the bypass ratio will flow down

the bypass duct and the remainder continues into what is the start of the hot core of

the engine the high-pressure compressor.

From the high-pressure compressor the air follows the usual path through the combustion

chambers and into the turbine. Before it leaves the hot core and rejoins the bypass air in

the mixer unit of the exhaust system.

The propulsive efficiency of both the low and high bypass ratio engines is much greater

than that of the pure turbojet at the speeds normally associated with jet transport aircraft.

The term propulsive efficiency is explained later in this lesson.

The specific fuel consumption of both the low and high bypass ratio engines is also

much lower than that of the pure turbojet.

The experience which was gained by many and operating the low-bypass ratio type of engine

proved that engines which dealt with a larger comparative air mass flow and lower jet velocities

could deliver propulsive efficiencies comparable to those of turboprops. And indeed a higher

propulsive efficiency than turbo jets operating at normal cruising speeds. The advent of the

fan jet engine had arrived

This model of a triple spool front fan turbojet engine shown here represents probably the

most successful early example of this type of engine the Rolls-Royce RB211.

The air enters the intake and passes immediately into the low-pressure compressor more commonly

called the fan. Here its pressure is raised before it splits to go either through the

bypass duct or into the intermediate pressure compressor. The amount going into either depending

upon the bypass ratio. The thrust of this type of engine is almost completely dependent

on the relatively cold air going through the bypass duct which has a high mass and relatively

low velocity, hence its high propulsive efficiency.

The air which passes through to the hot core which consists initially of the intermediate

and high pressure compressors has a great deal of energy added in the combustion chambers.

But this energy is required to drive all the compressors which includes the fan. In fact

it is the rear most turbine or the low-pressure turbine which is responsible for driving the

front fan, by extracting virtually all of the energy that remains in the gas stream.

If it's efficient in doing its job then there should be only residual thrust remaining where

the hot gases emerge from the turbine

We previously explained that thrust is the product of mass times acceleration. It can

be demonstrated that the same amount of thrust can be provided either by imparting a low

acceleration to a large mass of air or by giving a small mass of air a large acceleration.

In practice the former is preferred since it's been found that the losses due to turbulence

are much lower and the propulsive efficiency is higher. In this graph the levels of propulsive

efficiency for several different types of gas turbine engine will be shown.

The highest propulsive efficiency at lower speeds is offered by the turbo propeller combination.

However, above about 350 miles per hour the propellers efficiency drops off quite rapidly

due to the disturbance of the airflow at the tips of the blades.

In comparison with the turbo problem the propulsive efficiency of the pure turbojet appears quite

poor at the lower air speeds. As the airspeed increases in excess of 800 miles per hour

however the propulsive efficiency of the turbojet starts to improve beyond the capability of

the turboprop engine to match it and from then on there is no comparison. The eventual

outcome being a propulsive efficiency close to 90%.

Cruising speeds in the order of 800 miles per hour are at present out of the reach of

most transport aircraft. And this fact means that in the mid speed range where most of

the world's jet transport aircraft operate. There is a niche for the bypass engine both

low ratio and high ratio.

The low and high bypass ratio engines which includes the ducted fan or turbofan engine

have a propulsive efficiency which fits neatly between that of the turboprop and the pure

turbojet.

By dealing with comparatively larger mass air flows at lower jet velocities the bypass

engine attains a propulsive efficiency which exceeds that of both the turboprop and the

pure turbojet at the speeds normally associated with jet transport aircraft.

The use of larger and larger aircraft has meant that more passengers can be accommodated

in each flight thus air travel has become less and less expensive for the individual.

This concept of using large aircraft works well. As long as the aircraft themselves remain

serviceable. If however, one restricting component such as an engine becomes unserviceable on

a large aircraft then the expense involved in keeping three or four hundred passengers

fed, accommodated and happy becomes exorbitant.

Engine manufacturers in an attempt to minimize the financial burden imposed upon the users

of their equipment in the event of its failure. Have started to use modular construction methods,

which facilitate changing sections of an engine rather than a whole engine. This diagram shows

how the engine is split into several modules. This concludes the introduction to the gas

turbine engine.