Turbine PDF

Turbines Turbine Section The turbine has the task of providing the power to drive the compressor and accessories. In engines that do not use solely a jet for propulsion, it also provides shaft power for a propeller or rotor. The turbine carries out the task of extracting energy from...

Turbines Turbine Section The turbine has the task of providing the power to drive the compressor and accessories. In engines that do not use solely a jet for propulsion, it also provides shaft power for a propeller or rotor. The turbine carries out the task of extracting energy from the hot gases released from the combustion system and expanding them to a lower pressure and temperature. The energy is extracted by passing the airflow over a set of aerofoil-shaped blades. High stresses are involved in this process, and for efficient operation, the turbine blade tips may rotate at speeds up to 1500 ft/s. The continuous flow of gas to which the turbine is exposed may have an entry temperature between 850 and 1700 °C and may reach a velocity of 2500 ft/s in parts of the turbine. Image by Jeff Dahl licensed under Creative Commons Licence. Adapted for training use by Aviation Australia. Turbine engine 2022-08-24 B1-15a Gas Turbine Engine Page 143 of 244 CASA Part 66 - Training Materials Only  Radial Inflow Turbine Radial inflow turbines are sometimes used in ancillary equipment such as gas turbine compressors (GTCs), Auxiliary Power Units (APUs), air turbine motors (ATMs) and some types of turbo superchargers. The design and construction of the turbine centre on a disc having radially disposed vanes machined into its face and are similar to those of the impellers used in centrifugal compressors. The main difference is the fitting of a set of vanes to the rear face of the turbine wheel. These vanes are known as exducer vanes, as illustrated below. Their purpose is to increase the length of the turbine vanes and, by doing so, increase the efficiency of the turbine. The gas flow through the turbine first enters through a ring of stationary nozzle guide vanes, which apportion it equally around the turbine and direct it onto the periphery of the turbine wheel at the optimum angle of attack. It then flows to the eye of the turbine and across the exducer vanes (to extract the remaining energy from the gas flow) and exits at atmospheric pressure through the jet pipe. Aviation Australia Radial inflow turbine 2022-08-24 B1-15a Gas Turbine Engine Page 144 of 244 CASA Part 66 - Training Materials Only  Axial Flow Turbine The basic components of the turbine are: Turbine case Stator, also called the nozzle Shroud Rotor. Components of a turbine stage A disc and a number of turbine blades make up a turbine rotor. A nozzle and a rotor make up a turbine stage. The rotating assembly is supported on bearings mounted in the turbine casing. A turbine shaft may be common to the compressor shaft or connected via a self-aligning coupling. The location of a typical turbine assembly is labelled in the illustration below. 2022-08-24 B1-15a Gas Turbine Engine Page 145 of 244 CASA Part 66 - Training Materials Only  Turbine stages within the gas turbine engine unit 2022-08-24 B1-15a Gas Turbine Engine Page 146 of 244 CASA Part 66 - Training Materials Only  Turbine Case The turbine casing encloses the turbine rotor and stator assembly, giving either direct or indirect support to the stator elements. A typical case has flanges on both ends that provide a means of attaching the turbine section to the combustion section and the exhaust assembly. The perimeter of some turbine cases is encircled by several tubes, or passages. These passages are used to route cooling air around the turbine case to control thermal expansion. This, in turn, decreases the clearance between the case and the turbine blades, making the turbine section more efficient. It is known as active tip clearance control, or ACC (discussed in 15.12). Turbine casings are typically made from nickel-based alloys such as Inconel. Turbine casing 2022-08-24 B1-15a Gas Turbine Engine Page 147 of 244 CASA Part 66 - Training Materials Only  Turbine Stator A stator element is known by a variety of names, of which turbine inlet nozzle, nozzle guide vane (NGV), and nozzle diaphragm are three of the most common. The nozzle guide vanes are located directly aft of the combustion section and immediately forward of the turbine wheel. Because of their location, nozzle guide vanes are typically exposed to the highest temperatures in a gas turbine engine. Turbine stator The design of NGVs and turbine blades is based broadly on aerodynamic considerations. To obtain optimum efficiency compatible with compressor and combustor design, they have a basic aerofoil shape. The purpose of NGVs is to direct the combustion gases onto the turbine blades at the optimum angle of attack for efficient operation and to convert pressure energy into increased velocity of the gases flowing onto the turbine blades. 2022-08-24 B1-15a Gas Turbine Engine Page 148 of 244 CASA Part 66 - Training Materials Only  Aviation Australia Nozzle guide vanes direct flow onto turbine blades NGVs require extremely high heat resistance and are generally constructed from alloys of cobalt, nickel and chrome. Aviation Australia Nozzle guide vanes (NGVs) 2022-08-24 B1-15a Gas Turbine Engine Page 149 of 244 CASA Part 66 - Training Materials Only  Advances in NGV designs include nickel alloy with ceramic coatings, Ceramic Matrix Composites (CMCs) and adaptive manufacturing (3D printing). CMCs and adaptive manufacturing are reducing the weight of NGVs by up to two thirds and increasing the exhaust temperature, thereby increasing the efficiency of the engine. Relevant Youtube link: CMC Manufacturing (Video) A typical NGV assembly may, depending on the size of the engine, be manufactured as a complete ring of vanes, segments of a ring or individual vanes as shown below. When vanes are riveted or welded into segmented shrouds, the gaps between shroud segments allow for thermal expansion. Nozzle guide vane types are matched to the turbine blade types. Ring of vanes (complete unit) 2022-08-24 B1-15a Gas Turbine Engine Page 150 of 244 CASA Part 66 - Training Materials Only  Turbine Blades Turbine Blade Types Three types of turbine blade and nozzle configurations are in common use today: Impulse Reaction Impulse/reaction. Impulse Turbine and Nozzle In the impulse type of blades, as shown in the diagram, the total pressure drop across each stage of turbine occurs in the nozzle guide vanes, which, because of their convergent shape, increase the gas velocity while reducing the pressure. The gas is then directed onto the blades, which experience an impulse force caused by the impact of the gas on the blade. Impulse blades are commonly used on cartridge and air starters, but not in aircraft engines. The diagram shows the bucket‑like shape of the turbine blades. © Aviation Australia Impulse turbine and nozzle 2022-08-24 B1-15a Gas Turbine Engine Page 151 of 244 CASA Part 66 - Training Materials Only  Reaction Turbine and Nozzle In the reaction type blades pictured here, the fixed NGVs are designed to alter the gas flow direction without changing the pressure. The converging blade passages experience a reaction force resulting from the expansion and acceleration of the gas, and the total pressure drop occurs through the turbine blade passage. Normally gas turbine engines do not use pure impulse or pure reaction blades. © Aviation Australia Reaction turbine and nozzle 2022-08-24 B1-15a Gas Turbine Engine Page 152 of 244 CASA Part 66 - Training Materials Only  Impulse/Reaction Turbine and Nozzle Impulse/reaction blades are a combination of the impulse and the reaction blade types as shown in the diagram below. Normally, gas turbine engines do not use either pure impulse or pure reaction turbine blades. With the impulse/reaction turbine, the proportion of each principle incorporated in its design is therefore largely dependent on the type of engine in which the turbine is to operate, but in general it is about 50% impulse and 50% reaction, resulting in a pressure drop across both the nozzle and turbine. The turbine is driven by the impulse of the gas flow and its subsequent reaction as it accelerates through the converging blade passage. © Aviation Australia Impulse/reaction turbine and nozzle 2022-08-24 B1-15a Gas Turbine Engine Page 153 of 244 CASA Part 66 - Training Materials Only  Impulse/Reaction Turbine Blades To more evenly distribute the workload along the length of the blade, most modern turbine engines incorporate impulse/reaction turbine blades. With this type of blade, the blade base is impulse- shaped, while the blade tip is reaction-shaped. This design creates a uniform velocity and pressure drop across the entire blade length. © Aviation Australia Impulse/reaction turbine blade 2022-08-24 B1-15a Gas Turbine Engine Page 154 of 244 CASA Part 66 - Training Materials Only  Gas Path Through the Turbine For its operation, the turbine depends on the transfer of energy between the combustion gases and the turbine. The transfer is never 100% because of thermodynamic and mechanical losses. The table below shows airflow through the turbine stator is dependent on the design of the stator. Where the shape is parallel (reaction), there is no change in pressure or velocity. However, in the impulse and impulse/reaction designs, the stator is convergent, so the pressure decreases and the velocity increases. On impact with the turbine rotor and during the subsequent reaction through the blades, energy is absorbed, causing the turbine to rotate at high speed and so provide the power for driving the turbine shaft and compressor. The modern turbines all use the impulse/reaction design for optimum energy conversion to torque, to the propeller or to thrust from the fan. © Aviation Australia Air flow through a turbine stage 2022-08-24 B1-15a Gas Turbine Engine Page 155 of 244 CASA Part 66 - Training Materials Only  Energy Transfer The nozzle and blades of the turbine are twisted, the blades having a stagger angle that is greater at the tip than the root, as shown in the illustration. The reasons for this twist are to make the gas flow from the combustion section do equal work at all positions along the length of the blade and to ensure that the flow enters the exhaust system with a uniform axial velocity. © Aviation Australia Turbine blades having a stagger angle that is greater at the tip than the root 2022-08-24 B1-15a Gas Turbine Engine Page 156 of 244 CASA Part 66 - Training Materials Only  Turbine Blades Turbine blades are either open at the tip or fitted with interlocking or fixed shrouds. It is common to see both types in one engine, with the high-speed wheel containing open tip blades and the lower speed wheel shrouded tip blades (see the photo below). Tip loading from rotational forces often limits the use of shrouds to lower speed locations, such as low-pressure turbines in turbofan engines. Turbine blade tips can be open or fitted with interlocking shrouds This is also true of LP turbines in turboshaft engines, where all the energy is designed to be absorbed by the turbine blades, and energy remaining in the tailpipe as a result of tip losses would be completely lost to the engine. 2022-08-24 B1-15a Gas Turbine Engine Page 157 of 244 CASA Part 66 - Training Materials Only  Turbines (no shroud and with fixed shroud) Turbine Shrouds and Seals With shrouded blades, a shroud is attached to the tip of each blade. Once installed, the shrouds of the blades contact each other, thereby providing support. This added support reduces vibration substantially. The shrouds also prevent air from escaping over the blade tips, making the turbine more efficient. However, because of the added weight, shrouded turbine blades are more susceptible to blade growth. Aviation Australia Shrouded blades 2022-08-24 B1-15a Gas Turbine Engine Page 158 of 244 CASA Part 66 - Training Materials Only  Knife edge seals are often mounted on the tip shrouds. They also reduce air losses across the tips and keep the airflow in an axial direction to maximise the impact force of the flowing gases onto the blades. The seals fit in close tolerance to the shroud rings mounted in the turbine case. Knife edge seal 2022-08-24 B1-15a Gas Turbine Engine Page 159 of 244 CASA Part 66 - Training Materials Only  Turbine Construction Blade Material and Construction Turbine blades may be either forged or cast, depending on the composition of the alloys. Most blades are precision cast and finished by grinding to the desired shape. These materials have very high temperature strength under centrifugal loads and are highly corrosion-resistant. Most turbine blades today are cast as a single crystal, which gives them better strength and heat properties. Single-crystal blades are made from alloys that are cooled very slowly to form large ‘grains’ or crystals (large enough to make a blade). Therefore, the atomic structure of a single crystal blade is very uniform – which enhances strength in all directions and temperature resistance. With only one grain and no grain boundaries, corrosion due to expansion is all but eliminated. The latest technology is producing turbine blades and vanes by 3D printing. They can be a combination of materials which are mixed together. The main materials are nickel, titanium, aluminium and ceramic, which make them lighter (by up to 66%) and capable of handling more heat (currently 20% higher than what metal can tolerate). Blade and Vane Thermal Barrier Coatings Ceramic and aluminium alloy thermal barrier coating of super-alloy parts and some titanium parts are also processes which give high surface strength and resistance to corrosion. These coatings are generally referred to as plasma sprays and, when applied under high heat, melt into the surface of the base metal. They are said to give the best protection against scaling type corrosion or erosion which occurs at high gas temperatures. Scaling is a condition caused by sodium (salt) in the air and sulphur in the fuel reacting chemically with the base metals. Blade and NGV thermal barrier coatings 2022-08-24 B1-15a Gas Turbine Engine Page 160 of 244 CASA Part 66 - Training Materials Only  Blade Attachment Methods The method of attaching turbine blades to the turbine disc is of considerable importance since the stress in the disc around the attachment point or in the blade root greatly affects the limiting rim speed. Rim speed is the velocity of the rim of the disc. The design of the attachment point must be such that the accumulation of stresses does not lead to failure in the blade or disc. © Aviation Australia Blade fixed to disc (fir tree fitting) Fir tree fixing is now used on the majority of gas turbine engines. This type of fixing involves very accurate machining to ensure that the loading is shared by all the serrations. The blade is free in the serrations when the engine is stationary and cold and is stiffened in the root by: Thermal expansion of the disc and blades Centrifugal loading when the turbine is rotating. Fir tree blades are locked in place by special tabs, pins, rivets or locking plates. 2022-08-24 B1-15a Gas Turbine Engine Page 161 of 244 CASA Part 66 - Training Materials Only  Fir tree fixing Turbine Discs Discs are machined forgings with an integral shaft or a flange onto which a shaft can be bolted. The disc has provision for the attachment of blades around its circumference. Turbine discs 2022-08-24 B1-15a Gas Turbine Engine Page 162 of 244 CASA Part 66 - Training Materials Only  Turbine Sealing Methods The most common method of sealing turbines is by an abradable shroud ring and knife edge tips as shown in the illustration. The shrouds are small segments at the tips of the blades to prevent leakage across the tips. The knife edge tips rotate within an abradable lining. When shrouded tips are not used, a snug fit between the tips and the turbine casing is ensured by either abradable blade tips or an abradable lining fitted to the case. Abradable shroud ring and knife edge tips The rotor assembly fits easily into the turbine casing when assembled, but expansion due to heating and centrifugal forces during operation causes the blades to cut their own seat and ensure the best possible fit. Shrouded turbines have the advantages of extra strength and minimal loss of efficiency due to gas leakage across the blade tip. However, they suffer the disadvantage of susceptibility to blade creep due to the increased centrifugal loading caused by the increase in peripheral mass of the blade. Because of the extra weight, shrouded blades are better suited to low-speed turbines. Relevant Youtube link: Video - Turbine Assembly 2022-08-24 B1-15a Gas Turbine Engine Page 163 of 244 CASA Part 66 - Training Materials Only  Power Extraction As the high-velocity gases pass through the turbine nozzle and impact the turbine blades, the turbine wheel rotates. In some engines, a single turbine wheel cannot absorb sufficient energy from the exhaust gas to drive the compressor and accessories. Therefore, many engines use multiple turbine stages, each stage consisting of a turbine nozzle and wheel. Once the turbine has extracted enough power to run the compressors, the exhaust gas uses its remaining energy to add to the reactive force of thrust. Dual-spool turbofan engine Turbine Loads and Stresses To fully utilise the power available from the combustion gases, turbines are operated at the highest tolerable temperature and blade tip speed. The high tip speeds (up to 1500 ft/s) impart high centrifugal loads on the blades and discs. Temperatures in the turbine can also be between 850 and 1700 °C, and gas velocity may be around 2500 ft/s. The turbine environment is extremely hostile. This hostile environment can lead to design and longevity problems. A 60-g turbine blade, under normal operating conditions, may be subjected to a load of 2 T. 2022-08-24 B1-15a Gas Turbine Engine Page 164 of 244 CASA Part 66 - Training Materials Only  Blade Creep ‘Creep’ occurs when high stresses are applied to the turbine at a high temperature. It is a gradual increase in blade length or disc diameter with time, leading eventually to a failure of the blade or rubbing of the blade tip against the casing. The time elapsed before failure depends on the load applied and the temperature. The graph indicates typical variations in the creep strain of a turbine blade. Blade creep is divided into three stages: Primary creep – a fairly rapid initial increase in length occurring when new blades experience operational stresses for the first time. Secondary creep – a long period during which the increase is approximately linear with time, happening over the normal operating life of the engine. Tertiary creep – a period during which the increase in blade length is rapid, leading to total blade failure. This last condition should never be experienced in the life of an engine. However, in the event of severe over-speeding or high temperature, this excessive creep can occur. If high temperatures or over-speeding have occurred, blade creep checks are performed to verify that excessive blade creep has not occurred. Aviation Australia Blade creep graph 2022-08-24 B1-15a Gas Turbine Engine Page 165 of 244 CASA Part 66 - Training Materials Only  Creep Checks At intervals laid down in the maintenance manual, or when an engine is operated outside its working limits (i.e. over-temperature or over-speeding), a turbine check is required. There are two methods of carrying out a creep check: Measure each blade individually Measure the distance between the turbine blade tip and the shroud ring. The measurements obtained are then compared to the last creep check measurements, and an indication of blade creep can be ascertained. Of the two methods of carrying out turbine blade checks, measuring each blade individually is the most accurate as the measurement is not affected by such variables as turbine disc growth or turbine shroud warpage. However, to perform this check, the turbine assembly must be removed from the engine to give the fitter access to the blades. Although it is not as accurate, measuring the distance from the tip of the turbine blade to the shroud ring is a more convenient and practical method of ascertaining the turbine condition at short notice. If the measurements obtained during this check are out of tolerance, then the engine is removed for a more detailed inspection. 2022-08-24 B1-15a Gas Turbine Engine Page 166 of 244 CASA Part 66 - Training Materials Only