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TURBINE BLADES Overview
TURBINE BLADES 3/20/2013
Submitted by:Itisha Ghalay Sukanya Prabhakaran Fansurah Bhanu Great Chayran Anand Singh Gill Sujith P
TURBINE BLADES Overview
Overview The number of commercial flights has escalated rapidly since the middle of last century. As the cost of fuel is getting higher, operating cost of an aircraft is increasing each day. Mostly gas turbine engines power these flights as it gives better efficiency at high speed. In order to improve efficiency of these engines we need to increase the TET (turbine entry temperature) of turbine blades.
TURBINE BLADES History
History The graph below shows the development of TET with time i.e. the TET has risen by nearly 800 degree celcius.Sir Frank Whittle designed and built the first jet engine in 1941 and had TET of around 800 degree celcius. Rolls-Royce produced the Dart, the first gas turbine in civil flights and had TET of around 895 degree celcius and Conway, the first Bypass engine and had TET of 1040 degree celcius.The RB family of engines improved the TET from around 1270 to 1500 degree celcius.
TURBINE BLADES Materials
Materials It is the advancement in the materials for high temperature which has contributed in increasing the efficiency of the jet engine. To find out the materials we need to know which materials can retain their mechanical properties like specific strength, required for the turbine blades at the operating temperature of engine. The graph below shows how well nickel alloys maintain their mechanical properties with increasing temperature when compared with the alternatives.
Figure 1: Strength v/s temperature for different aero engine materials. From the graph we can see that only materials which fit the bill are Ni based alloy. So the selection of Ni based alloys for turbine was done in the early 1940’s and this trend has not changed and still there is no other element to replace it in the near future.
Development of alloys In 1940’s manufacturers used wrought alloys such as Nimonic alloys. With further advancement in Ni alloys the conventional cast alloys came into existence in 1960’s
Generation of alloys and their application in turbine blades of various aircrafts: 1st Gen alloy: CONVENTIONAL CASTING(CC) in Gloster Meteor & Mig15 2nd Gen alloy: DIRECTIONAL SOLIDIFICATION(DS) in Mirage 2000 3rd Gen alloy: SINGLE CRYSTAL(SC) in F-22 Raptor & Sukhoi Pak
Creep At elevated temperature the major problem encountered in turbine blade was creep, so its study was intensified. Creep is a time-dependent permanent deformation of solids at high temperature under stress.
Creep Curve: 1. Primary Creep-10 creep is decreasing creep rate because of work hardening process resulting from deformation. 2. Secondary Creep-20 creep is deformation continuous at an approximately constant rate. 3. Tertiary Creep-In 30 creep if the temperature increases, there occur a creep rate accelerates until fracture part.
Mechanism of Creep: Microscopic structure of blade, consist of grains and grain boundaries. At higher temperature atomic bonding starts to fail, causing the movements of atom through grain boundaries. As the temperature increases atom which are impeded by the small barrier will be able to excite through grain boundaries by the process of thermal activation in short time. And these atoms don’t return to their initial position even after the engine has stopped. As a result, next time the blades rotate the process of thermal activation continues along with the other atoms. This deposition of atoms over the surface causes restructuring of atom thereby leading to elongation of blades called creep.
TURBINE BLADES Single Crystal Blades
Single Crystal Blades The blades are able to operate at high temperatures due to the single crystal structure and the composition of the nickel based superalloy.
MANUFACTURING OF SINGLE CRYSTAL TURBINE BLADES:
Making of the wax model-A wax model of the casting is prepared by injecting molten wax into a metallic mould. It is arranged in clusters connected by wax replicas of runners and risers. Development of the Investment shellCoating- Dipping the model into ceramic slurries. Stuccoing- Stuccoing with larger particles of these same materials. Hardening- The coating is allowed to harden. Dewaxing-The investment is then allowed to completely dry, which can take 16 to 48 hours. It is then turned upside-down and placed in a furnace or autoclave to melt out or vaporize the wax. Burnout and preheating-The mold is then subjected to a burnout, which heats the mold between 870 °C and 1095 °C to remove any moisture and residual wax. The mold is preheated to allow the metal to stay liquid longer to fill any details and to increase dimensional accuracy. Pouring- The molten superalloy is poured into the ceramic mold, which is water cooled at the bottom end. After solidification is complete, the investment shell is removed.
Removal of grain boundaries- This is done by adding a grain selector at the very base of the wax mould. The cross section of the grain selector allows only one grain to successfully pass through to the top.
TURBINE BLADES Thermal Barrier Coatings
Thermal Barrier Coatings Introduction:Thermal-barrier coatings (TBCs) are refractory-oxide ceramic coatings applied to the surfaces of metallic parts in the hottest part of gas-turbine engines (figure 1); enabling modern engines to operate at significantly higher gas temperatures than their predecessors.
Figure 1 cut section of turbine engine and a microscope image of TBC So, as to cater for the increase in airline passenger in next 20 years and meet the challenges of environment together these developments require continuous innovation in the field of gas turbine and high temperature engine materials including TBCs and associated technologies. With the introduction of TBCs a major increase in temperature and engine efficiency is achieved. Typical composition of a TBC is ~7%Y2O3~ stabilized ZrO2.These coatings are applied on all the metallic parts like combustors, stationary guide vanes, rotating blades, blade outer air-seals, and shrouds in the high-pressure section behind the combustor; and afterburners in the tail section of jet engines. Today the gas-temperature increase facilitated by the use of TBCs, in conjunction with innovative air-cooling approaches, has been much greater than that enabled by earlier materials development, including the development of single-crystal Ni-based superalloys. Originally TBCs were first introduced in 1980s to rotor turbine blades to extent the useful life of them. The ceramic coating was not considered in the design of the temperature capability of the underlying metal parts. Today the ceramic coatings are critical component of turbine blades because of the gas temperature which are significantly higher than the melting point of superalloys.
TURBINE BLADES Thermal Barrier Coatings
Figure 2- Progression of temperature capabilities of Ni-based Superalloys and thermal-barrier coating (TBC) materials over the past 50 years. The red lines indicate progression of Maximum allowable gas temperatures in engines, with the large Increase gained from employing TBCs.
TURBINE BLADES Thermal Barrier Coatings
What is TBC and its intended work? TBC include three layers, a metallic bondcoat layer that is more oxidation resistant than the superalloy, and a thin, thermally grown oxide (TGO) layer that forms between the topcoat and the bond coat as result of bond-coat oxidation in-service (figure 3).The bondcoat is designed in such a way that results in TGO made of AL2O3-A mechanically robust protection form oxygen diffusion.
Figure 3 TBC illustration During its service TBC is very active and there is a microstructural and phase change even in the topcoat itself. TBCs have to perform many tasks apart from insulating superalloys. They are used to protect the superalloys from oxidation and have strain compliance to minimize thermal-expansion-mismatch stresses with the superalloy parts on heating and cooling, and must also reflect much of the radiant heat from the hot gas, preventing it from reaching the metal alloy. They have to maintain this protection for prolong period of time and being cycled numerous times between ~13000 C and room temperature. TBC need to do this without separating from turbine blades while withstanding extreme thermal gradients. These coatings are of intermediate thickness (100 μ m to 1 mm), and they must be deposited at a high rate to incorporate porosity. Because porosity further enhances the high strain compliance and reduced thermal conductivity. Yet the TBC need to resist fracture, erosion, FOD.
TURBINE BLADES Processing of TBC
Processing of TBC
TBC need to be deposited on complicated surfaces with very complex curves. Till now two process are formulated to do this and they are APS (air plasma spraying) and by EBPVD (electron beam physical vapour deposition). Typically APS a low cost process is used to coat stationary part of the engine and EBPVD is used for most demanding parts like blades and vanes (figure1 & 4). Each offer different advantage in terms of properties and performance.
Figure 4 TBC coating of combustor and microscopic view of TBC deposited by process APS
TURBINE BLADES Future of TBC
Future of TBC There are a lot of possibilities in the improvements and unresolved problems which are needed to be solved.
Improvements Top coat
The ceramics currently used in topcoat were selected because of its low thermal conductivity, resistance to sintering, capability to deposit with constant composition. Despite of these advantages there is a worldwide search of oxides having superior properties and most important property looked for is low thermal conductivity. Bondcoat
Today most of the focus is on this layer of TBC as they have to supply AL to form TGO and maintain cohesion without reacting with it and be elastic at highest temperature of operation and resistance to cavitation resulted by thermal cycle. Operate at highest temperature to minimize the cooling air required for blades and vanes and also have resistance to react with the superalloy. The problems needed to be solved are like to minimize the deformation at operating temperature and intermediate temperature. To minimize interdiffusion with the underlying superalloy to prevent the formation of brittle intermetals, and how to deliver critical elements in addition to Al, such as Hf and Y, to the growing TGO to minimize its inelastic plastic deformation under thermal cycling. Processing
A critical aspect of ceramic TBCs, in addition to the material, is the coating defect architecture facilitated by processing. To meet the future demands of the industry new processing techniques are to be developed.
Problems There are problems during the operations of TBC such as “Attack by molten deposits and its mitigation”. These are the silicates sucked by the engine and at high temperature melts and gets deposited at the Top coat this leads to the degradation of 7YSZ TBCs. This directly affects the higher temperature operations and reduced Efficiency. It appears that wetting of TBCs by the molten CMAS (calcium-magnesium-alumina-silicate) glass and dissolution of YSZ grain in that glass.