A turbine blade is the individual component which makes up the turbine section of a gas turbine or steam 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.
Blade fatigue is a major source of failure in steam turbines and gas turbines.
Fatigue is caused by the stress induced by vibration and resonance within the operating range of machinery. To protect blades from these high dynamic stresses, friction dampers are used.
Blades of wind turbines and water turbines are designed to operate in different conditions, which typically involve lower rotational speeds and temperatures. 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 temperature is then greatly increased by combustion of fuel inside the combustor, which sits between the compressor stages and the turbine stages. The high-temperature and high-pressure exhaust gases then pass through the turbine stages.
The turbine stages extract energy from this flow, lowering the pressure and temperature of the air and transfer the kinetic energy to the compressor stages along the spool.
This 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-bypass-ratio engines tending to have the most turbine stages. Many gas turbine engines are twin-spool designs, meaning that there is a high-pressure spool and a low-pressure spool. Other gas turbines use three spools, adding an intermediate-pressure spool between the high- and low-pressure spool.
The high-pressure turbine is exposed to the hottest, highest-pressure air, and the low-pressure turbine is subjected to cooler, lower-pressure air. The difference in conditions leads to the design of high-pressure and low-pressure turbine blades that are significantly different in material and cooling choices even though the aerodynamic and thermodynamic principles are the same. Steam turbine blades are critical components in power plants which convert the linear motion of high-temperature and high-pressure steam flowing down a pressure gradient into a rotary motion of the turbine shaft.
Turbine blades are subjected to very strenuous environments inside a gas turbine. They face high temperatures, high stresses, and a potential environment of high vibration.
Gas turbine blade failure pdf download
All three of these factors can lead to blade failures, potentially destroying the engine, therefore turbine blades are carefully designed to resist these conditions.
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 [nb 1] failures.
The high temperatures can also make the blades susceptible to corrosion failures. 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.
The development of superalloys in the s and new processing methods such as vacuum induction melting in the s 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. 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 altogether SC.
SC research began in the s with Pratt and Whitney and took about 10 years to be implemented. 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 became greater concerns as temperatures increased. The first TBCs, applied in the s, were aluminide coatings. Improved ceramic coatings became available in the s.
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. The wax blade is coated with a heat-resistant material to make a shell, and then that shell is filled with the blade alloy.
Vibration analysis for detecting failure mode and crack location in first stage gas turbine blade
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 a TBC, and then any cooling holes are machined. Ceramic matrix composites CMC , where fibers are embedded in a matrix of polymer derived ceramics , are being developed for use in turbine blades.
Note: This list is not inclusive of all alloys used in turbine blades. At a constant pressure ratio, thermal efficiency of the engine increases as the turbine entry temperature TET increases.
However, high temperatures can damage the turbine, as the blades are under large centrifugal stresses and materials are weaker at high temperature.
So, turbine blade cooling is essential. Cooling of components can be achieved by air or liquid cooling. Liquid cooling seems to be more attractive because of high specific heat capacity and chances of evaporative cooling but there can be leakage, corrosion, choking and other problems. While all methods have their differences, they all work by using cooler air often bled from the compressor to remove heat from the turbine blades.
It 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 internal passages in the blade may be circular or elliptical in shape. Cooling is achieved by passing the air through these passages from hub towards the blade tip.
This cooling air comes from an air compressor. In case of gas turbine the fluid outside is relatively hot which passes through the cooling passage and mixes with the main stream at the blade tip. 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 used in the regions of greatest heat loads.
In case of turbine blades, the leading edge has maximum temperature and thus heat load. Impingement cooling is also used in mid chord of the vane. Blades are hollow with a core. Cooling air enters from the leading edge region and turns towards the trailing edge. Film cooling also called thin film cooling , a widely used type, allows for higher cooling effectiveness than either convection and impingement cooling.
A thin layer the film of cooling air is then created on the external surface of the blade, reducing the heat transfer from main flow, whose temperature — kelvins can exceed the melting point of the blade material — kelvins.
Higher cooling effectiveness with maximum value of one indicates that the blade material temperature is closer to the coolant temperature. In locations where the blade temperature approaches the hot gas temperature, the cooling effectiveness approaches to zero.
The cooling effectiveness is mainly affected by the coolant flow parameters and the injection geometry. Coolant flow parameters include the velocity, density, blowing and momentum ratios which are calculated using the coolant and mainstream flow characteristics. Injection geometry parameters consist of hole or slot geometry i.
A United States Air Force program in the early s funded the development of a turbine blade that was both film and convection cooled, and that method has become common in modern turbine blades. The blade surface is made of porous material which means having a large number of small orifices on the surface. Cooling air is forced through these porous holes which forms a film or cooler boundary layer.
Investigation of blade failure in a gas turbine
Besides this uniform cooling is caused by effusion of the coolant over the entire blade surface. In the narrow trailing edge film cooling is used to enhance heat transfer from the blade. There is an array of pin fins on the blade surface.
Heat transfer takes place from this array and through the side walls.
As the coolant flows across the fins with high velocity, the flow separates and wakes are formed. Many factors contribute towards heat transfer rate among which the type of pin fin and the spacing between fins are the most significant.
This is similar to film cooling in that it creates a thin film of cooling air on the blade, but it is different in 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.
Air flows through internal channels of the strut and then passes through the porous shell to cool the blade. From Wikipedia, the free encyclopedia. Individual component which makes up the turbine section of a gas turbine or steam turbine.
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. Journal of Propulsion and Power.
The Evolution of Jet Engine Turbine Blades
Journal of Mechanical Science and Technology. Fractographic investigations of the failure of L-1 low pressure steam turbine blade. Case Studies in Engineering Failure Analysis, 1 2 , pp. AIAA United States Geological Survey. Retrieved 25 November GE Reports. Retrieved 2 November