Gas Turbine Engine: Nozzle Guide Vane Construction

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.