The Modern Gas Turbine

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The modern gas turbine how it works and how it’s built © 2000 Rolls-Royce plc TS22760 January 2000 Rolls-Royce plc PO Box 31 Derby DE24 8BJ England www. The gas turbine - how it works Jet propulsion Though the principle of jet propulsion had been demonstrated in the first century AD by Hero of Alexandria, the practical gas turbine, popularly called the ‘jet’ engine, had to wait for the 1930s and the genius of Frank Whittle. Originally designed for aircraft propulsion, the gas turbine is now adapted for marine propulsion, power generation and gas & oil pumping, all benefiting from its inherent high power and small size. How does a gas turbine work? Like the motor car engine, the gas turbine is an internal combustion engine. In both, air is compressed, fuel added, the mixture ignited, and the rapid expansion of the resultant hot gas produces the power. However, combustion in a motor car engine is intermittent and the expanding gas produces shaft power through a piston and crank, whereas in a jet engine combustion is continuous and its power results from expanding gas being forced out of the rear of the engine. One of Newton’s principles is that to every action there is an equal and opposite reaction. The expanding gas flow is an action which creates a reaction of equivalent force. This force - thrust is transmitted through the engine to the aircraft, propelling it through the air. An inflated party balloon can demonstrate this: with the neck closed, the air inside presses equally in all directions; open the neck, and the air is released as an action, creating a reaction on the opposite surface of the balloon which drives it forward. Thrust is measured in pounds force (lbf ), kilograms force (kgf ), or Newtons (N). 3 Layout of the gas turbine The gas turbine has three main sections: the compressors, the combustion system, and the turbines. Compressors Combustion system Turbines and exhaust Main types of gas turbine There are four main types of gas turbine: turbojet, turbofan, turboprop, and turboshaft. The turbojet and turbofan are both reaction engines which derive power from the reaction to the exhaust Cold-stream thrust stream. The turboprop and turboshaft operate differently by using the exhaust stream Hot-stream thrust to power an additional turbine which drives a propeller or output shaft. Turbojet The original concept, the turbojet, is the simplest form of gas turbine and relies Cold-stream thrust on the high velocity hot gas exhaust to provide the thrust. Its disadvantages Turbofan In the turbofan or ‘bypass’ engine the partly compressed airflow is divided, some into a central part - the gas generator or core - and some into a surrounding casing - the bypass duct. The gas generator acts like a turbojet whilst the larger mass of bypass air is accelerated relatively slowly down the duct to provide ‘cold stream’ thrust. The cold and hot streams mix to give better propulsive efficiency, lower noise levels, and improved fuel consumption. In the high bypass ratio turbofan, as much as seven or eight times as much air bypasses the core as passes through it. Turboprop As its name implies, a turboprop uses a propeller to transmit the power it produces. The propeller is driven through a reduction gear by a shaft from a power turbine, using the gas energy which would provide the thrust in a turbojet. Turboprop power is measured in total equivalent horsepower (tehp), or kilowatts (kW). Examples: Dart in BAe748 and Fokker F27; AE2100 in the Saab 2000. ¬ ¬ Air intake ¬ ¬ ¬ ¬ today are its relatively high noise levels The compressor The compressor draws air into the engine, pressurises it, and delivers it to the combustion chamber. It is driven from the turbine by a shaft. There are two types of compressor: the centrifugal flow impeller type, as used in Whittle’s designs, and the axial flow type which has several stages of alternate rotating and stationary aerofoil blades. The rotor blades are mounted on a drum and the stator vanes in the compressor casing. Axial compressors can achieve compression ratios in excess of 40:1. At full power the blades of the Trent 892 compressors rotate at 1000mph (1600kph) and take in 2600lb (1200kg) of air per second. The combustion system The combustion chamber receives air from the compressor which mixes with fuel sprayed from nozzles in the front of the chamber. The mixture is burned at temperatures up to 20000C to generate the maximum possible heat energy. The burning process is initiated by igniter plugs, isolated after start-up, and remains continuous until the fuel supply is shut off. At cruise the Trent 892 uses about 1000 gallons (4500 litres) of fuel per hour. The turbine Each turbine consists of one or more stages of alternate stationary and rotating aerofoil-section blades. The rotating turbine blades are carried on discs, which are connected by a shaft to the compressor. The stationary blades - nozzle guide vanes - are housed in the turbine casing. The turbine extracts energy from the hot exhaust gases to drive the compressor. In the Trent 892, the first turbine has to be air-cooled as it operates in a gas stream temperature of around 1500ºC - hotter than the melting point of the blade material. The total power generated by the engine is 250,000hp (200,000kW) and the exhaust gases exit at 1000mph (1600kph) and fuel consumption. Examples: Olympus 593 in the Concorde supersonic airliner; Viper in a variety of military aircraft. It achieves around 75% of its thrust from the bypass air and is ideal for subsonic transport aircraft. A low bypass ratio turbofan, where the air is divided approximately equally between the gas generator and the bypass duct, is wellsuited to high-speed military usage. Examples: in commercial usage - Trent in the Airbus A330; RB211-535 in the Boeing 757: in military usage - RB199 in the Tornado and EJ200 in the Typhoon.The vectoredthrust Pegasus in the Harrier is a variation of the turbofan. Turboshaft The turboshaft is a powerplant for helicopters. Like the turboprop, it also uses a power turbine and gearbox, though in this case the power is transmitted to the helicopter’s rotor system. This type of engine is also used in industrial and marine applications. Turboshaft power is measured in shaft horse power (shp), or kilowatts (kW). Examples: Allison 250 in the Jet Ranger; RTM322 in the Merlin. 4 5 Variations & additions Vectored thrust Thrust vectoring was developed for short take-off and vertical landing (STOVL) aircraft. The Pegasus turbofan, power for the Harrier ‘jump-jet’, is the unique example of this concept. The engine has four linked, swivelling nozzles to direct the gas stream from vertically downward for upward lift, through an arc to horizontally rearward for conventional forward flight. The bypass air is discharged through the two front nozzles and the hot gas exhaust through the two rear nozzles. Afterburning Afterburning, or reheat, increases engine thrust for short periods to improve aircraft take-off, climb and combat performance. Because the fuel in a gas turbine burns in an excess of air, sufficient oxygen remains in the exhaust to support further combustion, particularly in a turbofan. By injecting and burning additional fuel in the jet pipe, the exhaust velocity and consequently the engine thrust is increased Reverse thrust Thrust reversal, found on most commercial jets, is used to provide a braking force to add to the effect of an aircraft’s wheel-brakes when the aircraft lands. It is particularly useful in adverse weather conditions and is achieved by mechanically deflecting some or all of the exhaust stream of a gas turbine in a forward direction. Engine arrangement Following is a simplified look at some of the features of a large turbofan engine and the way it is put together. The example chosen is the Trent 800, which powers the Boeing 777 airliner. The RB211 family, to which the Trent belongs, features modular construction in its design. That is to say, it is built up from a number of large assemblies known as ‘modules’, each of which has its individual identity and service history. Module 02 Module 03 Module 04 Module 05 Module 06 Module 07 Module 08 The Trent 800 is built-up from eight basic modules: Module 01 LP compressor rotor, (to which the fan blades are fitted) IP compressor Intermediate case HP system IP turbine High speed gearbox LP compressor case LP turbine Various items and systems are then added to complete the engine. Significant benefits are gained from modular construction: Q Decreased turn-round time for repair Q Lower overall maintenance costs Q Reduced spare engine holdings Q Maximum life achieved from each module Q Savings on transport costs Q Ease of transport and storage Q On-wing test capability after any module change Modules 01, 02, 03, 04, 05 and 08 form the core engine which can be replaced as a complete assembly. Trent modular breakdown Module 07 Module 02 Module 03 Module 05 Module 08 Module 01 Module 06 Module 04 6 7 Module 01 Low Pressure (LP) Compressor Rotor Major components of this module are the disc and the shaft, which are connected by a curvic coupling. At the rear end of the shaft is a helical spline which transmits the drive from the LP turbine shaft. Dove-tail slots machined in the disc locate the 26 wide-chord, hollow fan blades (which are not modular parts). Disc - carries the fan blades Drive shaft Module 03 Intermediate Case The intermediate case is a fabricated, spoked structure housing the thrust bearings for all shafts, and forming the air path between the IP and HP compressors. Externally it carries the A-frame support arms which brace the fan case (Module 7), and its internal hollow struts provide access for services such as oil tubes, cooling air, and the radial drive-shaft to the accessory gearbox. Thrust bearings for all three shafts Curvic coupling Fabricated case Rear part of spinner Bevel drive to external gearbox Module 02 Intermediate Pressure (IP) Compressor This module comprises three casings bolted together, and the compressor rotor. First is the front bearing housing (FBH) which locates the roller bearings for both LP & IP compressors and a row each of fixed and variable vanes to control the airflow into the compressor. Next is the variable stator vane (VSV) casing containing a further two stages of variable vanes. The third casing is an assembly of bolted rings carrying six stages of fixed stator vanes. The compressor rotor is a welded drum assembly with eight stages of blades. A stub-shaft at the front of the drum locates in the IP roller bearing, and a curvic coupling extends from the stage 6 disc to connect to the IP shaft. Front bearing housing Variable stator vane case Module 04 High Pressure (HP) System The outer casing of this module links the intermediate case module and the IP turbine module. It contains the inner casing, HP compressor, combustion system and turbine, and also provides locations for fuel spray nozzles and igniter plugs. The compressor casing is made up of six flanged rings which, when bolted together, form slots into which the stator vanes are fitted during the bolting sequence. Two welded disc assemblies, bolted together, form the drum of the six-stage compressor into which the rotor blades are fitted. The first stage disc incorporates a curvic coupling to connect to the thrust bearing in the intermediate casing and the sixth disc extends into a drive cone and mini-disc which bolts to the turbine shaft. Within the inner case are the HP compressor outlet guide vanes,the annular combustion liner,and the air-cooled nozzle guide vane (NGV) assembly which directs the hot expanding gases into the turbine blades. Compressor Compressor rotor case and vanes The turbine is a single-stage unit.The hollow air-cooled blades are attached to the disc by fir-tree roots. A forward extension of the disc connects the turbine to the compressor mini-disc, and a stub-shaft for the turbine bearing is bolted to a rearward extension. Compressor Inner casing Outer casing Combustion system Turbine 8 9 Module 05 Intermediate Pressure (IP) Turbine Turbine Following the HP module is the IP turbine module. This assembly comprises turbine casing, blades and vanes, the turbine disc and shaft, and roller bearings for the HP and IP shafts. In the casing are mounted the IP NGVs and the first-stage LP NGVs. Both sets of NGVs are hollow, the IP vanes enclosing service tubes and struts for supporting the bearing housing, and the LP1 vanes containing thermo-couples for measurement of gas temperature. The IP vanes are air cooled. Fir-tree roots locate the IP blades in the disc of the single-stage turbine. Bolted to the turbine disc is the turbine shaft which has helical splines at its forward end. These connect to the IP compressor stub-shaft via the thrust bearings in the intermediate case. Shaft LP turbine first-stage NGVs HP & IP turbine roller bearings and housing Nozzle guide vanes Module 07 Low Pressure (LP) Compressor Case Largest of the modules, this is an assembly of forward and rear cylindrical casings and the fan outlet guide vane (OGV) ring. It is often referred to as the fan-case. The forward casing has to be able to contain a fan blade should it become detached during engine running. To meet this eventuality the specially-machined lightweight aluminium casing structure is wrapped with energy-absorbing Kevlar fabric. Internally, the casing contains acoustic panels, ice impact panels and the fan track lining. The titanium rear casing carries the fancase-mounted accessories and also contains acoustic linings. At their inner ends, the fan OGVs are secured to the torsion ring which locates the IP compressor module, whilst the outer ends are bolted to the front mounting ring. This assembly is welded to the titanium rear casing and bolted to the front casing. Front casing Fan outlet guide vane ring Engine front mount attachment Rear casing Module 06 High-Speed (HS) Gearbox The gearbox is mounted on the lower part of the LP compressor case. It is driven by a radial and angled drive shaft system from the intermediate case module and provides the drive for the accessories mounted on its front and rear faces. These accessories include oil, fuel and hydraulic pumps, electrical generators and the starter motor. During the starting sequence the air-driven starter motor drives back through the gearbox and drive shaft to provide initial rotation of the HP system. Faces for accessory mounting Drive input REAR Cast casing Module 08 Low Pressure (LP) Turbine Rearmost of the core engine modules is that of the LP turbine containing blades, vanes, discs, and the shaft and its roller bearing. The casing is made up of the turbine case and the turbine rear bearing support assembly. The latter also incorporates Drive input location bosses for the rear engine mount. Five bolted discs form the turbine rotor, with all blades located in fir-tree slots. The turbine shaft is bolted to the rotor via a curvic coupling and connects to the LP compressor shaft through helical splines at its forward end. The shaft extends rearwards into the roller bearing. Service tubes, electrical harnesses, and pressure sensing for the engine pressure ratio system are routed through the hollow vanes of the turbine bearing support assembly. Five-stage turbine Turbine case Shaft Turbine rear bearing support assembly Engine rear mount attachment FRONT 10 11