Turbine blade
Encyclopedia
A turbine blade is the individual component which makes up the turbine
section of a gas turbine
. The blades are responsible for extracting energy from the high temperature, high pressure gas produced by the combustor
. The turbine blades are often the limiting component of gas turbines. To survive in this difficult environment, turbine blades often use exotic materials like superalloy
s and many different methods of cooling, such as internal air channels, boundary layer cooling, and thermal barrier coatings.
In a gas turbine engine, a single turbine section is made up of a disk or hub that holds many turbine blades. That turbine section is connected to a compressor section via a shaft (or "spool"), and that compressor section can either be axial
or centrifugal
. Air is compressed, raising the pressure and temperature, through the compressor stages of the engine. The pressure and temperature are then greatly increased by combustion of fuel inside the combustor, which sits between the compressor stages and the turbine stages. That high temperature and high pressure fuel then passes through the turbine stages. The turbine stages extract energy from this flow, lowering the pressure and temperature of the air, and transfer that energy to the compressor stages along the shaft. This is process is very similar to how an axial compressor works, only in reverse.
The number of turbine stages varies in different types of engines, with high thrust, high bypass ratio, engines tending to have the most turbine stages. The number of turbine stages can have a great effect on how the turbine blades are designed for each stage. Many gas turbine engines are two shaft designs, meaning that there is a high pressure shaft and a low pressure shaft. Other gas turbines used three shafts, adding an intermediate pressure shaft between the high and low pressure shafts. The high pressure turbine is exposed to the hottest, highest pressure, air, and the low pressure turbine is subjected to cooler, lower pressure air. That difference in conditions leads the design of high pressure and low pressure turbine blades to be significantly different in material and cooling choices even though the aerodynamic
and thermodynamic
principles are the same.
Turbine blades are subjected to stress from centrifugal force
(turbine stages can rotate at tens of thousands of revolutions per minute (RPM)) and fluid forces that can cause fracture
, yielding
, or creep
Creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to high levels of stress that are below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods, and near the melting point. Creep always increases with temperature. From Creep (deformation)
. failures. Additionally, the first stage (the stage directly following the combustor) of a modern turbine faces temperatures around 2500 °F (1,371.1 °C), up from temperatures around 1500 °F (815.6 °C) in early gas turbines. Modern military jet engines, like the Snecma M88
, can see turbine temperatures of 2900 °F (1,593.3 °C). Those high temperatures weaken the blades and make them more susceptible to creep failures. The high temperatures can also make the blades susceptible to corrosion
failures. Finally, vibrations from the engine and the turbine itself (see blade pass frequency) can cause fatigue
failures.
, used in the British Whittle
engines.
The development of superalloy
s in the 1940s and new processing methods such as vacuum induction melting
in the 1950s greatly increased the temperature capability of turbine blades. Further processing methods like hot isostatic pressing
improved the alloys used for turbine blades and increased turbine blade performance. Modern turbine blades often use nickel
-based superalloys that incorporate chromium
, cobalt
, and rhenium
.
Aside from alloy improvements, a major breakthrough was the development of directional solidification
(DS) and single crystal
(SC) production methods. These methods help greatly increase strength against fatigue and creep by aligning grain boundaries
in one direction (DS) or by eliminating grain boundaries all together (SC).
Another major improvement to turbine blade material technology was the development of thermal barrier coatings (TBC). Where DS and SC developments improved creep and fatigue resistance, TBCs improved corrosion and oxidation resistance, both of which become greater concerns as temperatures increased. The first TBCs, applied in the 1970s, were aluminide
coatings. Improved ceramic coatings became available in the 1980s. These coatings increased turbine blade capability by about 200°F (90°C). The coatings also improve blade life, almost doubling the life of turbine blades in some cases.
Most turbine blades are manufactured by investment casting
(or lost-wax processing). This process involves making a precise negative die of the blade shape that is filled with wax to form the blade shape. If the blade is hollow (i.e., it has internal cooling passages), a ceramic core in the shape of the passage is inserted into the middle. The wax blade is coated with a heat resistant material to make a shell, and then that shell is filled with the blade alloy. This step can be more complicated for DS or SC materials, but the process is similar. If there is a ceramic core in the middle of the blade, it is dissolved in a solution that leaves the blade hollow. The blades are coated with an TBC they will have, and then cooling holes are machined as needed, creating a complete turbine blade.
, film, and transpiration cooling. While all three methods have their differences, they all work by using cooler air (often bleed from the compressor) to remove heat from the turbine blades.
Convection cooling works by passing cooling air through passages internal to the blade. Heat is transferred by conduction through the blade, and then by convection into the air flowing inside of the blade. A large internal surface area is desirable for this method, so the cooling paths tend to be serpentine and full of small fins.
A variation of convection cooling, impingement cooling, works by hitting the inner surface of the blade with high velocity air. This allows more heat to be transferred by convection than regular convection cooling does. Impingement cooling is often used on certain areas of a turbine blade, like the leading edge, with standard convection cooling used in the rest of the blade.
The second major type of cooling is film cooling (also called thin film cooling). This type of cooling works by pumping cool air out of the blade through small holes in the blade. This air creates a thin layer (the film) of cool air on the surface of the blade, protecting it from the high temperature air. The air holes can be in many different blade locations, but they are most often along the leading edge. A United State Air Force program in the early 1970s funded the development of a turbine blade that was both film and convection cooled, and that method has become common in modern turbine blades.
One consideration with film cooling is that injecting the cooler bleed into the flow reduces turbine efficiency. That drop in efficiency also increases as the amount of cooling flow increases. The drop in efficiency, however, is usually mitigated by the increase in overall performance produced by the higher turbine temperature.
Transpiration cooling, the third major type of cooling, is similar to film cooling in that it creates a thin film of cooling air on the blade, but it is different in that that air is "leaked" through a porous shell rather than injected through holes. This type of cooling is effective at high temperatures as it uniformly covers the entire blade with cool air. Transpiration-cooled blades generally consist of a rigid strut with a porous shell. Air flows through internal channels of the strut and then passes through the porous shell to cool the blade. As with film cooling, increased cooling air decreases turbine efficiency, so that decrease has to be balanced with improved temperature performance.
Turbine
A turbine is a rotary engine that extracts energy from a fluid flow and converts it into useful work.The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they move and...
section of a gas turbine
Gas turbine
A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between....
. The blades are responsible for extracting energy from the high temperature, high pressure gas produced by the combustor
Combustor
A combustor is a component or area of a gas turbine, ramjet, or scramjet engine where combustion takes place. It is also known as a burner, combustion chamber or flame holder. In a gas turbine engine, the combustor or combustion chamber is fed high pressure air by the compression system. The...
. The turbine blades are often the limiting component of gas turbines. To survive in this difficult environment, turbine blades often use exotic materials like superalloy
Superalloy
A superalloy, or high-performance alloy, is an alloy that exhibits excellent mechanical strength and creep resistance at high temperatures, good surface stability, and corrosion and oxidation resistance. Superalloys typically have a matrix with an austenitic face-centered cubic crystal structure. ...
s and many different methods of cooling, such as internal air channels, boundary layer cooling, and thermal barrier coatings.
Introduction
Axial compressor
Axial compressors are rotating, airfoil-based compressors in which the working fluid principally flows parallel to the axis of rotation. This is in contrast with other rotating compressors such as centrifugal, axi-centrifugal and mixed-flow compressors where the air may enter axially but will have...
or centrifugal
Centrifugal compressor
Centrifugal compressors, sometimes termed radial compressors, are a sub-class of dynamic axisymmetric work-absorbing turbomachinery.The idealized compressive dynamic turbo-machine achieves a pressure rise by adding kinetic energy/velocity to a continuous flow of fluid through the rotor or impeller...
. Air is compressed, raising the pressure and temperature, through the compressor stages of the engine. The pressure and temperature are then greatly increased by combustion of fuel inside the combustor, which sits between the compressor stages and the turbine stages. That high temperature and high pressure fuel then passes through the turbine stages. The turbine stages extract energy from this flow, lowering the pressure and temperature of the air, and transfer that energy to the compressor stages along the shaft. This is process is very similar to how an axial compressor works, only in reverse.
The number of turbine stages varies in different types of engines, with high thrust, high bypass ratio, engines tending to have the most turbine stages. The number of turbine stages can have a great effect on how the turbine blades are designed for each stage. Many gas turbine engines are two shaft designs, meaning that there is a high pressure shaft and a low pressure shaft. Other gas turbines used three shafts, adding an intermediate pressure shaft between the high and low pressure shafts. The high pressure turbine is exposed to the hottest, highest pressure, air, and the low pressure turbine is subjected to cooler, lower pressure air. That difference in conditions leads the design of high pressure and low pressure turbine blades to be significantly different in material and cooling choices even though the aerodynamic
Aerodynamics
Aerodynamics is a branch of dynamics concerned with studying the motion of air, particularly when it interacts with a moving object. Aerodynamics is a subfield of fluid dynamics and gas dynamics, with much theory shared between them. Aerodynamics is often used synonymously with gas dynamics, with...
and thermodynamic
Thermodynamics
Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...
principles are the same.
Environment and failure modes
Turbine blades are subjected to very strenuous environments inside a gas turbine. They face high temperatures, high stresses, and a potentially high vibration environment. All three of these factors can lead to blade failures, which can destroy the engine, and turbine blades are carefully designed to resist those conditions.Turbine blades are subjected to stress from centrifugal force
Centrifugal force
Centrifugal force can generally be any force directed outward relative to some origin. More particularly, in classical mechanics, the centrifugal force is an outward force which arises when describing the motion of objects in a rotating reference frame...
(turbine stages can rotate at tens of thousands of revolutions per minute (RPM)) and fluid forces that can cause fracture
Fracture
A fracture is the separation of an object or material into two, or more, pieces under the action of stress.The word fracture is often applied to bones of living creatures , or to crystals or crystalline materials, such as gemstones or metal...
, yielding
Yield (engineering)
The yield strength or yield point of a material is defined in engineering and materials science as the stress at which a material begins to deform plastically. Prior to the yield point the material will deform elastically and will return to its original shape when the applied stress is removed...
, or creep
Creep (deformation)
In materials science, creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to high levels of stress that are below the yield strength of the material....
Creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to high levels of stress that are below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods, and near the melting point. Creep always increases with temperature. From Creep (deformation)
Creep (deformation)
In materials science, creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to high levels of stress that are below the yield strength of the material....
. failures. Additionally, the first stage (the stage directly following the combustor) of a modern turbine faces temperatures around 2500 °F (1,371.1 °C), up from temperatures around 1500 °F (815.6 °C) in early gas turbines. Modern military jet engines, like the Snecma M88
SNECMA M88
|-See also:-External links:* * Snecma M88's pdf* *...
, can see turbine temperatures of 2900 °F (1,593.3 °C). Those high temperatures weaken the blades and make them more susceptible to creep failures. The high temperatures can also make the blades susceptible to corrosion
Corrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
failures. Finally, vibrations from the engine and the turbine itself (see blade pass frequency) can cause fatigue
Fatigue (material)
'In materials science, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material.Fatigue occurs...
failures.
Materials
A key limiting factor in early jet engines was the performance of the materials available for the hot section (combustor and turbine) of the engine. The need for better materials spurred much research in the field of alloys and manufacturing techniques, and that research resulted in a long list of new materials and methods that make modern gas turbines possible.. One of the earliest of these was NimonicNimonic
Nimonic is a registered trademark of Special Metals Corporation that refers to a family of nickel-based superalloys. Nimonic alloys typically consist of more than 50% nickel and 20% chromium with additives such as titanium and aluminium. The main use is in gas turbine components and extremely high...
, used in the British Whittle
Frank Whittle
Air Commodore Sir Frank Whittle, OM, KBE, CB, FRS, Hon FRAeS was a British Royal Air Force engineer officer. He is credited with independently inventing the turbojet engine Air Commodore Sir Frank Whittle, OM, KBE, CB, FRS, Hon FRAeS (1 June 1907 – 9 August 1996) was a British Royal Air...
engines.
The development of superalloy
Superalloy
A superalloy, or high-performance alloy, is an alloy that exhibits excellent mechanical strength and creep resistance at high temperatures, good surface stability, and corrosion and oxidation resistance. Superalloys typically have a matrix with an austenitic face-centered cubic crystal structure. ...
s in the 1940s and new processing methods such as vacuum induction melting
Vacuum Induction Melting
Vacuum induction melting is a process for melting metal under vacuum conditions using electromagnetic induction. It works by creating electrical eddy currents in the metal which heats the "charge" to melt it...
in the 1950s greatly increased the temperature capability of turbine blades. Further processing methods like hot isostatic pressing
Hot isostatic pressing
Hot isostatic pressing is a manufacturing process used to reduce the porosity of metals and influence the density of many ceramic materials. This improves the material's mechanical properties and workability....
improved the alloys used for turbine blades and increased turbine blade performance. Modern turbine blades often use nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
-based superalloys that incorporate chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
, cobalt
Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....
, and rhenium
Rhenium
Rhenium is a chemical element with the symbol Re and atomic number 75. It is a silvery-white, heavy, third-row transition metal in group 7 of the periodic table. With an average concentration of 1 part per billion , rhenium is one of the rarest elements in the Earth's crust. The free element has...
.
Aside from alloy improvements, a major breakthrough was the development of directional solidification
Directional solidification
Directional solidification and progressive solidification describe types of solidification within castings. Directional solidification describes solidification that occurs from farthest end of the casting and works its way towards the sprue...
(DS) and single crystal
Single crystal
A single crystal or monocrystalline solid is a material in which the crystal lattice of the entire sample is continuous and unbroken to the edges of the sample, with no grain boundaries...
(SC) production methods. These methods help greatly increase strength against fatigue and creep by aligning grain boundaries
Grain boundary
A grain boundary is the interface between two grains, or crystallites, in a polycrystalline material. Grain boundaries are defects in the crystal structure, and tend to decrease the electrical and thermal conductivity of the material...
in one direction (DS) or by eliminating grain boundaries all together (SC).
Another major improvement to turbine blade material technology was the development of thermal barrier coatings (TBC). Where DS and SC developments improved creep and fatigue resistance, TBCs improved corrosion and oxidation resistance, both of which become greater concerns as temperatures increased. The first TBCs, applied in the 1970s, were aluminide
Aluminide
An aluminide is a compound that has aluminium with more electropositive elements. Since aluminium is near the nonmetals, it can bond with metals differently than other metals. The properties of an aluminide would be intermediate between a metal alloy and an ionic compound.-Examples:* magnesium...
coatings. Improved ceramic coatings became available in the 1980s. These coatings increased turbine blade capability by about 200°F (90°C). The coatings also improve blade life, almost doubling the life of turbine blades in some cases.
Most turbine blades are manufactured by investment casting
Investment casting
Investment casting is an industrial process based on and also called lost-wax casting, one of the oldest known metal-forming techniques. From 5,000 years ago, when beeswax formed the pattern, to today’s high-technology waxes, refractory materials and specialist alloys, the castings allow the...
(or lost-wax processing). This process involves making a precise negative die of the blade shape that is filled with wax to form the blade shape. If the blade is hollow (i.e., it has internal cooling passages), a ceramic core in the shape of the passage is inserted into the middle. The wax blade is coated with a heat resistant material to make a shell, and then that shell is filled with the blade alloy. This step can be more complicated for DS or SC materials, but the process is similar. If there is a ceramic core in the middle of the blade, it is dissolved in a solution that leaves the blade hollow. The blades are coated with an TBC they will have, and then cooling holes are machined as needed, creating a complete turbine blade.
List of turbine blade materials
Note: This list is not inclusive of all alloys used in turbine blades.- U-500 This material was used as a first stage (the most demanding stage) material in the 1960s, and is now used in later, less demanding, stages.
- Rene 77
- Rene N5
- Rene N6
- PWA1484
- CMSX-10
- InconelInconelInconel is a registered trademark of Special Metals Corporation that refers to a family of austenitic nickel-chromium-based superalloys. Inconel alloys are typically used in high temperature applications. It is often referred to in English as "Inco"...
- IN-738 - GE used IN-738 as a first stage blade material from 1971 until 1984, when it was replaced by GTD-111. It is now used as a second stage material. It was specifically designed for land-based turbines rather than aircraft gas turbines.
- GTD-111 Blades made from directionally solidified GTD-111 are being using in many GE Aviation gas turbines in the first stage. Blades made from equiaxed GTD-111 are being used in later stages.
- EPM-102 (MX4 (GE), PWA 1497 (P&W)) is a single crystal superalloy jointly developed by NASA, GE Aviation, and Pratt & Whitney for the High Speed Civil TransportHigh Speed Civil TransportThe High Speed Civil Transport , also known as High-Speed Research ,was a NASA project to design a supersonic transport. The aircraft was to be a future supersonic passenger aircraft, able to fly Mach 2, or twice the speed of sound. The project started in 1990 and ended during 1999...
(HCST). While the HCST program was canceled, the alloy is still being considered for use by GE and P&W.
Cooling
Another strategy to improving turbine blades and increasing their operating temperature, aside from better materials, is to cool the blades. There are three main types of cooling used in gas turbine blades; convectionConvection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
, film, and transpiration cooling. While all three methods have their differences, they all work by using cooler air (often bleed from the compressor) to remove heat from the turbine blades.
Convection cooling works by passing cooling air through passages internal to the blade. Heat is transferred by conduction through the blade, and then by convection into the air flowing inside of the blade. A large internal surface area is desirable for this method, so the cooling paths tend to be serpentine and full of small fins.
The second major type of cooling is film cooling (also called thin film cooling). This type of cooling works by pumping cool air out of the blade through small holes in the blade. This air creates a thin layer (the film) of cool air on the surface of the blade, protecting it from the high temperature air. The air holes can be in many different blade locations, but they are most often along the leading edge. A United State Air Force program in the early 1970s funded the development of a turbine blade that was both film and convection cooled, and that method has become common in modern turbine blades.
One consideration with film cooling is that injecting the cooler bleed into the flow reduces turbine efficiency. That drop in efficiency also increases as the amount of cooling flow increases. The drop in efficiency, however, is usually mitigated by the increase in overall performance produced by the higher turbine temperature.
Transpiration cooling, the third major type of cooling, is similar to film cooling in that it creates a thin film of cooling air on the blade, but it is different in that that air is "leaked" through a porous shell rather than injected through holes. This type of cooling is effective at high temperatures as it uniformly covers the entire blade with cool air. Transpiration-cooled blades generally consist of a rigid strut with a porous shell. Air flows through internal channels of the strut and then passes through the porous shell to cool the blade. As with film cooling, increased cooling air decreases turbine efficiency, so that decrease has to be balanced with improved temperature performance.