The Gas Turbine Blade Engineering Essay
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Published: Mon, 5 Dec 2016
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 limiting component of the gas turbines. To survive in this difficult environment, turbines blades are made up of super alloys and also use many methods of cooling, such as internal air channels, boundary layer cooling and thermal barrier coatings. The turbine blades are fixed on a disk or a hub in the gas turbine engines. The section which holds the blades is called as the turbine section.
Component function – Function of the gas turbine blade.
The system which houses the blade is the gas turbine engine. A gas turbine is an internal combustion engine, which is designed to convert latent energy in a fuel (kerosene, jet fuel, diesel, gasoline) to a mechanical energy or kinetic energy to do the work. This energy can be used in the form of a thrust, or a mechanical horsepower to drive a propeller, a rotor or just about anything else. The turbine is the main part of the gas turbine engine which contains a hub on which blades are fixed. The turbines are of two types a) Radial turbine and b) Axial flow turbine.
a) Radial turbine: It works in the reverse direction. High velocity gas flows towards the center of the turbine wheel, impending upon the vanes/blades and causing the wheel to rotate.
b) Axial flow turbine: In an axial flow turbine there are two types of axial turbine blades.
I) Concave U-shaped blade: An impulse turbine has concave u-shaped blades. High velocity gas pass through the stationary set of blades which redirects the gas flows so that it strikes the concave section directly.
Impulse Turbine blade.
II) Reaction blade: The other type of blades is the reaction blade. They are designed so that when the gas strikes them the gas molecules are deflected in a particular direction. This deflection of gas causes a reaction jet effect which tends to turn the turbine wheel in the opposite direction to the gas flow.
Reaction turbine Blade
The main function of the gas turbine blades is that it acts as a mean for extracting energy from the high temperature, high pressure gas produced by the combustor. These blades get red hot while carrying the centrifugal loads due to rotation at high speeds. Therefore these blades have small holes on it to pass cool air through them for the cooling of the blades.
In service conditions- Conditions that the gas turbine blades experience during their operation in the Gas turbine engine.
During the operation of the gas turbine engines the turbine blades are subjected to enormous stresses. Turbine blades can be a part of a power plant or jet engines. They experience high temperatures, vibrations and large stresses during their normal operations also. The stresses experienced by the turbine blades are even larger in the gas turbine engines. The thermal stresses affect the life of the blade. As the temperature increases in operation the material strength and creep resistance start decreasing. There are several stages in turbines during their operations and which can have an effect on how the blades are designed to for each stage. The turbine blades have a defines life term through which it can perform well but after a certain amount of time the life of the blades decrease which leads to decreased aerodynamic efficiency , more fuel consumption, impeded air flow and etc. The life of the blades depend on the material selection, the better the material selection the better will be the
life of the blades. The blades continue to lose their fatigue strength during continuous cyclic loading in operation. Corrosion, deterioration of coatings on the blade, degradation of the component. The total service life of the blades is determined by engineering analysis and by component testing by various methods. Doing an analysis on the blade after operation shows the intense wear out of the contact surfaces of the flanges is also found like
Wear with regular shape.
Wear with formation of crater.
Transfer of metal to surface of the flange.
Transverse marks on the contact surfaces and non working surface of the flange.
Operational requirements – General characteristics of the turbine blade in order to function and withstand in service conditions for a desired service life.
The general characteristics that the blade must possess are that the blades must be made of very durable and ductile materials having good properties so that the blade can perform well in the operating conditions. The blades should not deteriorate under extreme heating conditions, neither the coatings should affect the performance of the blades. There should be minimal vibrations affecting the blades. The fatigue strength of the blades should be very high to sustain the extreme heat so that the service life of the blades is not affected. There should be means provided for the cooling of the blades by drilling holes on them to establish a protection layer on the edge of the blades by which the hot flue gases do not affect them. There also should be a relationship between the power required from the gas, the rotational speeds at which it must be produced. The design of the blade passages and nozzle guide vanes are based on aerodynamic considerations to obtain optimum efficiency. The nozzle guide vanes and the turbine blades should be in aerofoil shape. The blades while in operation should be strong enough to carry centrifugal loads due to rotation at high speeds. To operate under these conditions cool air is forced out of many small holes in the blade so that the air remains close to the blade and prevents it from melting. A protective layer is essential over the blades which resists high temperature and hot corrosion. Finally the blades should improve the aerodynamic efficiency of the gas turbine engines and less fuel consumption during its operation.
Material characteristics: Material selection and the properties it should posses to manufacture the turbine blade.
Modern gas turbines and turbine blades have most advanced technology in all aspects. In the world today majority of the gas turbine blades are made from nickel based alloys for the reason that they have a melting point of (1200-1315 deg). The temperature at which the gas turbine runs is much more than the melting point of nickel based alloys (1350 deg). Therefore some advancement like cooling of the blades and low thermal conductivity coatings on the surface has been used now days. The blades made from nickel need protection from melting at high temperature and also to prevent their corrosion. A coating on the surface of the blades will protect the blades from getting corroded and melting at high temperature. The coatings like NiCrAlY and MCrAlY have many desirable properties like protecting the blade from getting corroded and oxidation and help in providing the blade maximum life in the corrosive environments. The reason for the improved ductility of coatings is due to formation of a thick, protective coating and chemically stable alumina scale on the surface upon exposure to the environment. Yttrium when added to the coating increases the adherence of the oxide layer nickel alloy. Zirconia (ceramic) is also used as a coating material because it has a low thermal conduction and low thermal expansion and improved oxidation resistance. Rolles-Royce and Jhonson matthey have developed platinum aluminide diffusion coating that allows some advantages over commercial systems.
Uncoated and NiAl coated blade turbine blade
This report explains what turbine blade is and it gives a brief description of turbine blades and what their role is in the gas turbine engines and its importance in the operation of the engines. It also explains the operating conditions of the blade and many factors which are experienced by the blade. The in service conditions of the blade while in operation and the factors which affect the service life of the blade. The report also explains the material selection phase which is the most important phase in manufacturing the turbine blade, the different types of coatings and the properties of the material which help in getting a better desired service life of the blade.
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