Nozzle Guide Vanes Function & Quiz for Gas Turbines

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Questions and Answers

What is the primary purpose of the shrouds at the tips of the turbine blades?

To prevent leakage across the blade tips (correct)

To reduce the centrifugal forces during operation

To absorb energy from the exhaust gas

To increase the peripheral mass of the blade

What is the advantage of using shrouded turbines?

Improved compressor performance

Reduced centrifugal loading

Minimal loss of efficiency due to gas leakage (correct)

Increased peripheral mass of the blade

Why are shrouded blades better suited to low-speed turbines?

Because of the increased centrifugal loading (correct)

Because they can absorb more energy from the exhaust gas

Because they are less susceptible to blade creep

Because of the reduced peripheral mass of the blade

What happens to the blades during operation?

They expand due to heating and centrifugal forces Signup and view all the answers

What is the purpose of the turbine nozzle?

To impact the turbine blades Signup and view all the answers

Why do some engines use multiple turbine stages?

To absorb sufficient energy from the exhaust gas Signup and view all the answers

What is the primary function of the turbine rotor in a gas turbine engine?

To convert energy into torque or thrust Signup and view all the answers

Why are turbine blades twisted and have a stagger angle that increases from root to tip?

To ensure uniform axial velocity and equal work along the blade's length Signup and view all the answers

What is the primary reason for using shrouded tip blades in low-pressure turbines?

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

What is the result of energy absorption during the impact of gas flow with the turbine rotor and blades?

The turbine rotates at a high speed Signup and view all the answers

What is the purpose of the nozzle in the turbine stage?

To accelerate the gas flow Signup and view all the answers

What is the common design approach used in modern turbines?

Impulse/Reaction design Signup and view all the answers

What is the primary advantage of single-crystal blades in terms of strength and temperature resistance?

Their atomic structure is very uniform Signup and view all the answers

What is the benefit of 3D printing in producing turbine blades and vanes?

They are capable of handling more heat and are lighter Signup and view all the answers

What is the term for the velocity of the rim of the turbine disc?

Rim speed Signup and view all the answers

What type of fixing is commonly used to attach turbine blades to the turbine disc?

Fir tree fitting Signup and view all the answers

What is the primary purpose of thermal barrier coatings on turbine blades and vanes?

To protect against scaling type corrosion or erosion Signup and view all the answers

What happens to the blade in the serrations when the engine is stationary and cold?

It is free Signup and view all the answers

What is the condition caused by sodium (salt) in the air and sulphur in the fuel reacting chemically with the base metals?

Scaling Signup and view all the answers

What is responsible for stiffening the blade root when the turbine is rotating?

Centrifugal loading Signup and view all the answers

What is the process by which single-crystal blades are made?

Slow cooling of alloys Signup and view all the answers

What is the benefit of using ceramic and aluminium alloy thermal barrier coatings?

They give high surface strength and resistance to corrosion Signup and view all the answers

What is used to lock the fir tree blade in place?

All of the above Signup and view all the answers

What is the purpose of machining the attachment point accurately?

To ensure the loading is shared by all the serrations Signup and view all the answers

What is a primary requirement for nozzle guide vanes (NGVs)?

Extremely high heat resistance Signup and view all the answers

What material advancement has improved the efficiency of NGVs?

Nickel alloy with ceramic coatings Signup and view all the answers

Why are gaps left between shroud segments in NGV assemblies?

To allow for thermal expansion Signup and view all the answers

What is the purpose of nozzle guide vanes?

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

How are NGVs typically manufactured?

All of the above Signup and view all the answers

What type of turbine blade configuration is NOT commonly used?

Centrifugal Signup and view all the answers

Study Notes

Nozzle Guide Vanes (NGVs)

NGVs direct flow onto turbine blades and require extremely high heat resistance.
NGVs are 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 reduce the weight of NGVs by up to two thirds and increase the exhaust temperature, thereby increasing the engine's efficiency.

Turbine Blades

Three types of turbine blade and nozzle configurations are in common use: impulse, reaction, and 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 twisted, with a stagger angle that is greater at the tip than the root, 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.
Turbine blades are either open at the tip or fitted with interlocking or fixed shrouds.

Turbine Blade Materials

Most turbine blades are precision cast and finished by grinding to the desired shape.
These materials have very high temperature strength under centrifugal loads and are highly corrosion-resistant.
Most turbine blades today are cast as a single crystal, which gives them better strength and heat properties.
Single-crystal blades are made from alloys that are cooled very slowly to form large ‘grains’ or crystals, which enhances strength in all directions and temperature resistance.
The latest technology is producing turbine blades and vanes by 3D printing, which can be a combination of materials mixed together (nickel, titanium, aluminum, and ceramic).
These materials make them lighter (by up to 66%) and capable of handling more heat (currently 20% higher than what metal can tolerate).

Blade and Vane Thermal Barrier Coatings

Ceramic and aluminum alloy thermal barrier coating of super-alloy parts and some titanium parts are processes that give high surface strength and resistance to corrosion.
These coatings are generally referred to as plasma sprays and, when applied under high heat, melt into the surface of the base metal.
They are said to give the best protection against scaling type corrosion or erosion which occurs at high gas temperatures.

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, which involves 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.
Fir tree blades are locked in place by special tabs, pins, rivets, or locking plates.

Turbine Discs

Discs are machined forgings with an integral shaft or a flange onto which a shaft can be bolted.
The disc has provision for the attachment of blades around its circumference.

Turbine Sealing Methods

The most common method of sealing turbines is by an abradable shroud ring and knife edge tips.
The shrouds are small segments at the tips of the blades to prevent leakage across the tips.
The knife edge tips rotate within an abradable lining.
When shrouded tips are not used, a snug fit between the tips and the turbine casing is ensured by either abradable blade tips or an abradable lining fitted to the case.

Power Extraction

As the high-velocity gases pass through the turbine nozzle and impact the turbine blades, the turbine wheel rotates.
In some engines, a single turbine wheel cannot absorb sufficient energy from the exhaust gas to drive the compressor and accessories.
Therefore, many engines use multiple turbine stages, each stage consisting of a turbine nozzle and wheel.

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Description

This quiz covers the construction of nozzle guide vanes in gas turbine engines, including the materials used and their heat resistance properties. It is a part of the CASA Part 66 training program for aviation professionals.

Gas Turbine Engine: Nozzle Guide Vane Construction

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