Turbine Section in Jet Engines

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Questions and Answers

What is the primary task of the turbine in a gas turbine engine?

To compress air

To generate electricity

To provide power to drive the compressor and accessories (correct)

To cool the engine

What is the maximum speed that turbine blade tips may rotate at?

1500 ft/s (correct)

500 ft/s

1000 ft/s

2000 ft/s

What is the typical entry temperature range of the gas to which the turbine is exposed?

1000-2000 °C

500-1000 °C

1500-2500 °C

850-1700 °C (correct)

What is the purpose of exducer vanes in a radial inflow turbine?

To increase the length of the turbine vanes and increase efficiency Signup and view all the answers

What type of engines are radial inflow turbines sometimes used in?

Gas turbine compressors (GTCs), Auxiliary Power Units (APUs), air turbine motors (ATMs) and some types of turbo superchargers Signup and view all the answers

What is the shape of the blades in a turbine?

Aerofoil-shaped Signup and view all the answers

What is the main function of the nozzle guide vanes in the turbine?

To apportion the gas flow equally around the turbine Signup and view all the answers

What is the name of the vanes that extract the remaining energy from the gas flow?

Exducer vanes Signup and view all the answers

What is the purpose of the turbine casing?

To enclose the turbine rotor and stator assembly Signup and view all the answers

What is a turbine stage composed of?

A nozzle and a rotor Signup and view all the answers

What is the function of the shroud in a turbine?

Not mentioned in the content Signup and view all the answers

What is the typical location of a turbine assembly?

After the combustion section and before the exhaust assembly Signup and view all the answers

What is the primary function of nozzle guide vanes in a gas turbine engine?

To direct flow onto turbine blades Signup and view all the answers

What materials are commonly used to construct nozzle guide vanes?

Alloys of cobalt, nickel and chrome Signup and view all the answers

What is the purpose of the gaps between shroud segments in a nozzle guide vane assembly?

To allow for thermal expansion Signup and view all the answers

What type of manufacturing is being used to reduce the weight of nozzle guide vanes?

Adaptive manufacturing (3D printing) Signup and view all the answers

What is the relationship between nozzle guide vane types and turbine blade types?

Nozzle guide vane types are matched to the turbine blade types Signup and view all the answers

How many types of turbine blade and nozzle configurations are in common use today?

Three Signup and view all the answers

What is the main reason for twisting the nozzle and blades of the turbine?

To make the gas flow do equal work at all positions along the length of the blade Signup and view all the answers

What is the purpose of the stagger angle in the turbine blades?

To ensure the flow enters the exhaust system with a uniform axial velocity Signup and view all the answers

Why are shrouds often used in lower speed turbine locations?

To reduce tip loading from rotational forces Signup and view all the answers

What is the result of the energy absorption during the reaction through the turbine blades?

The turbine shaft and compressor are driven Signup and view all the answers

What type of turbine design is used in modern turbines for optimum energy conversion to torque?

Impulse/reaction design Signup and view all the answers

Why are open tip blades often used in high-speed turbine wheels?

Because tip loading from rotational forces is not a concern Signup and view all the answers

What is the main reason why the method of attaching turbine blades to the turbine disc is of considerable importance?

To minimize the stress in the disc around the attachment point Signup and view all the answers

What type of machining is required for fir tree fixing to ensure that the loading is shared by all the serrations?

Accurate machining Signup and view all the answers

What stiffens the blade root when the engine is stationary and cold?

Thermal expansion of the disc and blades Signup and view all the answers

What is the purpose of the special tabs, pins, rivets, or locking plates in fir tree fixing?

To lock the blades in place Signup and view all the answers

What is the component of the turbine engine that has provision for the attachment of blades around its circumference?

Turbine disc Signup and view all the answers

What is the result of the centrifugal loading when the turbine is rotating?

The blade is stiffened in the root Signup and view all the answers

Study Notes

Turbine Section

The turbine provides power to drive the compressor and accessories, and also provides shaft power for a propeller or rotor in engines that don't use solely a jet for propulsion.
The turbine extracts energy from the hot gases released from the combustion system and expands them to a lower pressure and temperature.
Energy is extracted by passing the airflow over a set of aerofoil-shaped blades.
High stresses are involved in this process, and for efficient operation, the turbine blade tips may rotate at speeds up to 1500 ft/s.
The continuous flow of gas to which the turbine is exposed may have an entry temperature between 850 and 1700 °C and may reach a velocity of 2500 ft/s in parts of the turbine.

Radial Inflow Turbine

Radial inflow turbines are used in ancillary equipment such as gas turbine compressors, Auxiliary Power Units (APUs), air turbine motors, and some types of turbo superchargers.
The design and construction of the turbine center on a disc having radially disposed vanes machined into its face.
The main difference is the fitting of a set of vanes to the rear face of the turbine wheel, known as exducer vanes, which increase the length of the turbine vanes and improve efficiency.
Gas flow through the turbine first enters through a ring of stationary nozzle guide vanes, then flows to the eye of the turbine and across the exducer vanes, and exits at atmospheric pressure through the jet pipe.

Axial Flow Turbine

The basic components of the turbine are:
- Turbine case
- Stator (also called the nozzle)
- Shroud
- Rotor
A disc and a number of turbine blades make up a turbine rotor, and a nozzle and a rotor make up a turbine stage.
The rotating assembly is supported on bearings mounted in the turbine casing.

Turbine Case

The turbine casing encloses the turbine rotor and stator assembly, giving either direct or indirect support to the stator elements.
A typical case has flanges on both ends that provide a means of attaching the turbine section to the combustion section and the exhaust assembly.

Nozzle Guide Vanes (NGVs)

NGVs direct flow onto turbine blades and require extremely high heat resistance.
They are generally constructed from alloys of cobalt, nickel, and chrome.
Advances in NGV designs include nickel alloy with ceramic coatings, Ceramic Matrix Composites (CMCs), and adaptive manufacturing (3D printing).
CMCs and adaptive manufacturing are reducing the weight of NGVs by up to two-thirds and increasing the exhaust temperature, thereby increasing the efficiency of the engine.

Turbine Blades

Three types of turbine blade and nozzle configurations are in common use today:
- Impulse
- Reaction
- Impulse/reaction
The modern turbines all use the impulse/reaction design for optimum energy conversion to torque, to the propeller or to thrust from the fan.
Turbine blades are either open at the tip or fitted with interlocking or fixed shrouds.
Blade and NGV thermal barrier coatings are used to improve efficiency.

Energy Transfer

The nozzle and blades of the turbine are twisted, with the blades having a stagger angle that is greater at the tip than the root.
The reasons for this twist are to make the gas flow from the combustion section do equal work at all positions along the length of the blade and to ensure that the flow enters the exhaust system with a uniform axial velocity.

Blade Attachment Methods

The method of attaching turbine blades to the turbine disc is of considerable importance since the stress in the disc around the attachment point or in the blade root greatly affects the limiting rim speed.
Fir tree fixing is now used on the majority of gas turbine engines, involving very accurate machining to ensure that the loading is shared by all the serrations.
The blade is free in the serrations when the engine is stationary and cold and is stiffened in the root by thermal expansion of the disc and blades and centrifugal loading when the turbine is rotating.

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Description

Learn about the turbine's role in jet engines, including how it generates power and extracts energy from hot gases. Understand the process of energy extraction and the stresses involved.