Gas turbine engine

Gas turbine engineTURBINE trade name COOLINGABSTRACT displayIt is well known from the thermodynamic analysis through literature take after that the performance of a fluid turbine engine is strongly influenced by the temperature at the inlet to the turbine. Figure 1 illustrates the relation between the specific motive output and turbine rotor inlet temperature. There is thus a growing tendency to use advanceder turbine inlet temperatures, implying increasing heat fill to the engine components. Engine manufacturers film recognised this for some time and have been continuously increasing turbine inlet temperature, especially during the last three decades. The swords atomic number 18 cooled by extracting air from the compressor stages.Modern gas turbine engines are designed to keep in line at inlet temperatures of 1800-2000K, which are far beyond the allowable alloy temperatures. Thus, to of importtain acceptable living and safety standards, the structural elements needs t o be protected against the severe thermal environment. This calls for the design of an efficient cooling formation for these elements. Rotor stigma of senior high school pressure gas turbine is such a critical element and hence the trade name metal temperature should not be allowed to exceed beyond a value at which the spirit or safety standards cant be met. It is required to cool the blade in such a right smart that the amount of heat transferred from the externally flow rate hot gas to the blade should be removed by an becharm cooling design to limit the real high temperature.STRESSES IN THE BLADERotor blades of gas turbine are subjected to very high rotational speeds of the suppose of several thousand rpm and withal are exposed to a variable thermal environment. Hence these blades are subjected to opposite types of linees of different magnitudes and directions. As it is known, that the medium is a forge of support and working temperature the cyberspace strivin g at any section of the blade should not exceed the supreme allowable value. The control on the blade metal temperature is the only way to sustain the filteres for the designed life of the blade for a specific operate condition and life requirement. Therefore to know just about the cooling requirement, assayes should be predicted correctly on the blades at different sections.There are in general four types of filtratees with that rotor blades are being subjectedCentrifugal tensile punctuateGas flexure mental strain andCentrifugal bending distort thermal stress1.1. Centrifugal tensile stressCentrifugal stress in the rotor blade is due to the rotation of the blade. It is tensile in nature. This is the largest in magnitude yet not necessarily the most important because it is nigh a steady stress. When the rotational speed of the blade is specified, the allowable motor(a) tensile stress places a limit on the annulus champaign but does not affect the choice of blade chord. This stress is the basic cause of the blade failure due to the creep.1.2. Centrifugal bending stressIf the blade design is such that the centroids of all the blade cross-sections at different radii, taken perpendicular to the radial direction, do not lie in the same radial plane, centrifugal stresses arising in the blade forget fork up to bend the blade. This type of stress arising due to the different directions of the centrifugal stresses in different blade sections is called as centrifugal bending stress. It will green goods compressive stress in one side of the blade whereas tensile stress in the opposite side. Any torsional stress arising from these centrifugal stresses is small lavish to be neglected. Thus this stress is very sensitive to manufacturing errors.1.3. Gas bending stressThe force arising from the change in angular pulsation of the gas in the tangential direction, which produces the useful torque, also tries to bend the blade about the axis of rotation of the blades. The stress arising due to this bending force is called as gas bending stress. There may be change of significationum in the axial direction and in reaction turbines there will certainly be a pressure force in the axial direction. All these two will produce a bending moment in the blade about the tangential direction. The gas bending stress will be tensile in the leading and trailing exhibits and compressive in the back of the blade and with tapered twisted blades either the leading or trailing edge suffers with the maximum value of this stress. This is a fluctuating stress and its value becomes maximum when the rotor blade passes through the leading edge of the stator.1.4. Gas bending stressTurbine blade is subjected to three-dimensional temperature gradients, along the blade height, along the blade profile and along the thickness of the blade.Due to these temperature gradients the blade fibres tend to deform unequally. This unequal deformation causes mainly two types of stresses to entrap up in the blade, compressive and tensile. As the blade considered is un-cooled therefore the contribution of the stress due to the temperature gradient along the thickness of the blade in net stress is not appreciable and can be neglected. Usually with the cooled blade this source of stress is main among all the sources of thermal stress.Again the thermal stress due to the temperature gradient along the blade height would not come in picture because the blade is free to expand along the height. provided the stress due to temperature gradient along the chord of the blade will contribute in net blade stress but its magnitude would not be much because the temperature gradient along the chord is not so high.BLADE MATERIAL AND STRENGTHGas turbine blades are exposed to a very severe thermal atmosphere. The temperature is so high that it is fairly much more than the melting points of the common high- durability seculars. Besides high temperature the requirement of du rability is also another factor, which makes common materials unsuitable for use. Only super alloys may be suitable for this purpose. But the current reduce of continuously increasing the turbine entry temperature attracted the concentration of the designers not only towards the in the buff materials with well-improved mechanical and thermal properties but also to restrict the temperature of the blade material by its proper cooling. So, the material should have sufficient strength to feel the operating situations.1.5. Strength of blade materialIn ordinary temperature conditions the strength of the material under changeless loads is estimated by tensile strength or yield strength. At high temperatures under action of constant loads in ordinary structural materials there appears the phenomenon of creep. It occurs as a result of prolonged exposure of materials to high stresses at high temperatures. This is peculiarly a acute problem on super stressed rotating turbine blades and i t occurs in the form of slowly and continuously developing waxy deformation. And excess of this plastic deformation causes the failure of the component. It is observed that at constant stress the high the temperature the more quickly proceeds the process of creep i.e. the lesser the life of the component. It means that at a particular stress lesser will be the temperature higher will be the life of component. Therefore life of the component is a function of working temperature and stress. Hence to maintain the life of the component at a desire value it is required to glare the temperature of the component.Gas turbines operate in conditions of high temperatures and therefore in highly stressed components like rotor blades there appears the phenomenon of creep. Therefore for these cases where creep is the main criterion behind component failure the ultimate tensile stress is defined as the stress at which the component fails at a certain working temperature after the expiry of a ce rtain period of time. It means that the strength of the material subjected at high temperatures is a function of this temperature and its operational life.PAST COOLINGThe technology of turbine cooling was recognised by some almost from the inception of the primary turbojet engine. Cooling studies were start performed in the 1940 and many investigations were carried on in the 1950s. Around 1960, turbine cooling was first used in a commercial aircraft engine. Since that time, there has been a very rapid rise in turbine inlet temperature that has placed an even greater emphasis on turbine cooling. A continuous improvement in high-temperature materials has also helped to increase the turbine inlet temperature. The cooling technique used during 1960s was angiotensin converting enzyme internal passage convection cooling. The air used for cooling was injected through the stemma of the blade and to the internal aerofoil.