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Turbine SectionModule 15.6

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LESSON PLANOperation and characteristics of different turbine blade types (3 Periods)Blade to disk attachment (2 Periods)Nozzle Guide Vanes (2 Periods)Causes and effect of turbine blade stress and creep (3 Periods)

FUNCTION OF THE TURBINETo drive the compressor and accessories and in case of turboprop to drive the propeller by extracting a portion of the pressure and kinetic energy from the high temperature combustion gases.In a typical jet engine 75% of the power produced internally is used to drive the compressor.Left is used to produce the thrust.

INTRODUCTIONThe turbine has the task of providing the power to drive the compressor and accessories and, in the case of engines which do not make use solely of a jet for propulsion, of providing shaft power for a propeller or rotor. It does this by extracting energy from the hot gases released from the combustion system and expanding them to a lower pressure and temperature.

INTRODUCTIONHigh stresses are involved in this process, and for efficient operation, the turbine blade tips may rotate at speeds over 1,500 feet per second. The continuous flow of gas to which the turbine is exposed may have an entry temperature between 850 and 1,700 C and may reach a velocity of over 2,500 feet per second in parts of the turbine

A TRIPLE STAGE TURBINE WITH A SINGLE SHAFT

INTRODUCTIONTo produce the driving torque, the turbine may consist of several stages each employing one row of stationary nozzle guide vanes and one row of moving blades. The number of stages depends upon the relationship between the power required from the gas flow, the rotational speed at which it must be produced and the diameter of turbine permitted.

INTRODUCTIONThe number of shafts, and therefore turbines, varies with the type of engine., high compression ratio engines usually have two shafts, driving high and low pressure compressors. On high by pass ratio fan engines that feature an intermediate pressure system, another turbine may be interposed between the high and low pressure turbines, thus forming triple-spool system. On some engines, driving torque is derived from a free-power turbine. This method allows the turbine to run at its optimum speed because it is mechanically independent of other turbine and compressor shafts.

A MULTI STAGE TURBINE DRIVING TWO SHAFTS

INTRODUCTIONThe mean blade speed of a turbine has considerable effect on the maximum efficiency possible for a given stage output. For a given output the gas velocities, deflections and hence losses are reduced in proportion to the square of higher mean blade speeds. Stress in the turbine disc increases as the square of the speed, therefore to maintain the same stress level at higher speed the sectional thickness, hence the weight, must be increased disproportionately.

INTRODUCTIONFor this reason, the final design is a compromise between efficiency and weight. Engines operating at higher turbine inlet temperatures are thermally more efficient and have an improved power to weight ratio. By-pass engines have a better propulsive efficiency and thus can have a smaller turbine for a given thrust.

A MULTI STAGE TURBINE DRIVING THREE SHAFTS

TURBINE CONSTRUCTIONThe basic components of the turbine are the combustion discharge nozzles, the nozzle guide vanes, the turbine discs and the turbine blades. The rotating assembly is carried on bearings mounted in the turbine casing and the turbine shaft may be common to the compressor shaft or connected to it by a self-aligning coupling.

TURBINE CONSTRUCTIONThe turbine wheel is one of the most highly stressed parts in the engines.

Not only must it operate at temperatures of approximately 1800F (982C) but it must do so under severe centrifugal loads imposed by high rotational speeds of over 60,000 rpm for small engines to 8000 rpm for the larger ones.

TURBINE CONSTRUCTIONConsequently the engine speed and turbine inlet temperature must be accurately controlled to keep the turbine within safe operating limits.The turbine assembly made out of two main parts, the disk and blades.The disk or wheel is statically and dynamically balanced unit of specially alloyed steel, usually containing large percentage of chromium, nickel and cobalt.

TURBINE DISCTurbine discs are usually manufactured from a machined forging with an integral shaft or with a flange onto which the shaft may be bolted. The disc also has, around its perimeter, provision for the attachment of the turbine blades.To limit the effect of heat conduction from the turbine blades to the disc a flow of cooling air is passed across both sides of each disc.

TURBINE CONSTRUCTIONAfter forging the disk is machined all over and carefully inspected using x-rays, sound waves and other inspection methods to ensure the structural integrity.

A FREE POWER TURBINE

ENERGY TRANSFER FROM GAS FLOW TO TURBINE

Turbine depends for its operation on the transfer of energy between the combustion gases and the turbine. This transfer is never 100 per cent because of thermodynamic and mechanical losses.When the gas is expanded by the combustion process, it forces its way into the discharge nozzles of the turbine where, because of their convergent shape, it is accelerated to about the speed of sound which, at the gas temperature, is about 2,500 feet per second.

ENERGY TRANSFER FROM GAS FLOW TO TURBINEAt the same time the gas flow is given a 'spin' or 'whirl' in the direction of rotation of the turbine blades by the nozzle guide vanes. On impact with the blades 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.

ENERGY TRANSFER FROM GAS FLOW TO TURBINEThe torque or turning power applied to the turbine is governed by the rate of gas flow and the energy change of the gas between the inlet and the outlet of the turbine blades. The design of the turbine is such that the whirl will be removed from the gas stream so that the flow at exit from the turbine will be substantially 'straightened out' to give an axial flow into the exhaust system.

ENERGY TRANSFER FROM GAS FLOW TO TURBINEExcessive residual whirl (spin) reduces the efficiency of the exhaust system and also tends to produce jet pipe vibration which has a detrimental effect on the exhaust cone supports and struts.

It will be seen that the nozzle guide vanes and blades of the turbine are 'twisted', the blades having a stagger angle that is greater at the tip than at the root. The reason for the twist is to make the gas flow from the combustion system 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. This results in certain changes in velocity, pressure and temperature occurring through the turbine.

TURBINE LOSSESThe losses which prevent the turbine from being 100 per cent efficient are due to a number of reasons. A typical un-cooled three-stage turbine would suffer a 3.5 per cent loss because of aerodynamic losses in the turbine blades. A further 4.5 per cent loss would be incurred by aerodynamic losses in the nozzle guide vanes, gas leakage over the turbine blade tips and exhaust system losses; these losses are of approximately equal proportions. The total losses result in an overall efficiency of approximately 92 per cent.

NGVStator section of turbine.Direct the air axially on to the blade of rotor section.The design of the nozzle guide vane and turbine blade passages is based broadly on aerodynamic considerations, and to obtain optimum efficiency, compatible with compressor and combustion design, the nozzle guide vanes and turbine blades are of a basic aerofoil shape.

Typical nozzle guide vanes showing their shape and location. (Courtesy: Rolls-Royce)

TYPES OF TURBINESAxial flow turbine

Radial inflow turbine

AXIAL FLOW TURBINEThe axial-flow turbine has two main elements: turbine rotors (or wheels, as they are sometimes called) and stationary vanes. The turbine blade themselves are of two basic types, the impulse and the reaction.The modern aircraft gas turbine engine utilizes blades that have both impulse and reaction sections.

AXIAL FLOW TURBINEThe stationary part of the assembly consists of a plane of contoured vanes, concentric with the axis of the turbine and set at an angle to form a series of small nozzles. These nozzles discharge the gases onto the blades in the turbine rotors. The stationary vane assembly of each stage in the turbine is usually referred to as the turbine nozzle guide vanes.

Single stage axial flow turbine wheelMulti stage axial flow turbine with turbine nozzles. First stage is not shown. For cooling purpose the blades may be solid or hollow on either nozzle and/or rotor.Axial Flow Turbine

TURBINE NOZZLE AREAThe turbine nozzle area is the most critical part of the turbine design. If the nozzle area is too large, the turbine will not operate at its best efficiency. If the area is too small the nozzle will have a tendency to lose efficiency under maximum thrust conditions. The turbine nozzle area is defined as the total cross-sectional area of the exhaust gas passages at their narrowest point through the turbine nozzle. It is calculated by measuring and adding the areas between individual nozzle guide vanes.

RADIAL INFLOW TURBINEHas the advantage of ruggedness and simplicity and is relatively inexpensive and easy to manufacture when compared with axial flow type. Inlet gas flows through peripheral nozzles to enter the wheel passages in an radial direction.The speeding gas exerts force on the wheel blades and then exhaust the air in an axial direction to the atmosphere.

RADIAL INFLOW TURBINEThese turbine wheels used for small engines, are well suited for lower range of specific speeds and work at relatively high efficiency

Turbine blades

The turbine blades are of an aerofoil shape, designed to provide passages between adjacent blades that give a steady acceleration of the flow up to the 'throat', where the area is smallest and the velocity reaches that required at exit to produce the required degree of reaction.

Turbine bladesThe actual area of each blade cross-section is fixed by the permitted stress in the material used and by the size of any holes which may be required for cooling purposes. High efficiency demands thin trailing edges to the sections, but a compromise has to be made so as to prevent the blades cracking due to the temperature changes during engine operation.

DIFFERENT TURBINE BLADE TYPESImpulse turbine Reaction turbine Reaction-Impulse turbine

IMPULSE TURBINEIn the IMPULSE type the total pressure drop across each stage occurs in the fixed nozzle guide vanes which, because of their convergent shape, increase the gas velocity whilst reducing the pressure. The gas is directed onto the turbine blades which experience an impulse force caused by the impact of the gas on the blades. All pressure energy of gas converted to kinetic energy.

IMPULSE TURBINEArea of the inlet and exit between the blades is equal.The impulse force does not act directly in the plane of rotation the turbine wheel but is resolved in to two components.

REACTION TURBINEIn the REACTION type the fixed nozzle guide vanes 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. Normally gas turbine engines do not use pure impulse or pure reaction turbine blades but the impulse-reaction combination.

REACTION TURBINEOn entering the first rotor stage, the gases see the rotor as a convergent passage (outlet area less than the inlet area).The change in the area produces an increase in the relative velocity with an accompanying pressure drop across the blades.The acceleration of gases generates a reaction force like that produce on a wing.It is from this feature of the reaction turbine that its name is derived.

REACTION TURBINEThe reaction force results from the acceleration of the gases across the blade.The direction in which the reaction force acts may be determined by considering the blade as an airfoil. The reaction force like lift may be drawn perpendicular to the relative wind.Both impulse and reaction forces are acting on the blade of a reaction turbine.

REACTION TURBINEImpulse turbine requires high velocity gas in order to obtain the maximum rate of momentum change.Reaction turbine causes its rate of momentum change by the nozzling action of the rotor blading and therefore does not require excessively high nozzle diaphragm exit velocities.Two forces (impulse + reaction) combine vectorically in to a resultant that acts in the plane of rotation to drive the turbine.

REACTION IMPULSE TURBINEIt is important to distribute the power load evenly from the base to the tip of the blade.An uneven workload will cause the gases to exit from the blade at different velocities and pressures.Obviously the blade tips will be travelling faster than the blade roots because they have a greater distance to travel in their larger circumference.

REACTION IMPULSE TURBINEIf all the gas velocity possible is made to impinge upon the blade roots, the difference in wheel speed at the roots and the tips will make the relative speed of the gases less at the tips, causing less power to be developed at the tips than at the roots.To cope with this problem in actual practice the turbine blading is a blending of the impulse type at the roots and the reaction type at the tips.

REACTION IMPULSE TURBINEMaking the blade impulse type at the roots and the reaction type at the tips the blade exit pressure can be held relatively constant.

REACTION IMPULSE TURBINERequired pressure drop for reaction is present at the tip and the gradually changes to the no pressure loss condition required for impulse at the root.Higher pressure at the tip will tend to make the gases flow toward the base of the blade which counteracts the centrifugal forces trying to throw the air towards the tip.

REACTION IMPULSE TURBINEAngle of the nozzle and the turbine blades are such that optimum performance is achieved only during a small range of engine RPM.To counteract the swirling of gases, straightening vanes are located immediately downstream of the turbine.These vanes also serve the function in many engines of providing one of the main structural components and they act as a passageway for oil, air and other lines.

METHOD OF TURBINE DISC ATTACHMENTThe method of attaching the blades to the turbine disc is of considerable importance, since the stress in the disc around the fixing or in the blade root has an important bearing on the limiting rim speed. The blades on the early Whittle engine were attached by the de Laval bulb root fixing, but this design was soon superseded by the 'fir-tree' fixing that is now used in the majority of gas turbine engines.

METHOD OF TURBINE DISC ATTACHMENTThis 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 turbine is stationary and is stiffened in the root by centrifugal loading when the turbine is rotating. Various methods of blade attachment are shown; however, the hollow blade and the de Laval bulb root types are not now generally used on gas turbine engines.