Aircraft Gas Turbine Engine Turbine Section

The turbine transforms a portion of thekinetic (velocity) energy of the exhaust gases into mechanical energy to drivethe gas generator compressor and accessories. The sole purpose of the gasgenerator turbine is to absorb approximately 60 to 70 percent of the totalpressure energy from the exhaust gases. The exact amount of energy absorptionat the turbine is determined by the load the turbine is driving (i.e.,compressor size and type, number of accessories, and the load applied by theother turbine stages). These turbine stages can be used to drive a low-pressurecompressor (fan), propeller, and shaft. The turbine section of a gas turbineengine is located aft, or downstream, of the combustion chamber. Specifically,it is directly behind the combustion chamber outlet.

The turbine assembly consists of two basicelements: turbine inlet guide vanes and turbine blades.

The stator element is known by a variety ofnames, of which turbine inlet nozzle vanes, turbine inlet guide vanes, andnozzle diaphragm are three of the most commonly used. The turbine inlet nozzlevanes are located directly aft of the combustion chambers and immediatelyforward of the turbine wheel. This is the highest or hottest temperature thatcomes in contact with metal components in the engine. The turbine inlettemperature must be controlled or damage will occur to the turbine inlet vanes.

After the combustion chamber has introducedthe heat energy into the mass airflow and delivered it evenly to the turbineinlet nozzles, the nozzles must prepare the mass air flow to drive the turbinerotor. The stationary vanes of the turbine inlet nozzles are contoured and setat such an angle that they form a number of small nozzles discharging gas atextremely high speed; thus, the nozzle converts a varying portion of the heatand pressure energy to velocity energy that can then be converted to mechanicalenergy through the turbine blades.

There are three types of turbine blades:the impulse turbine blade, reaction turbine blade, and the reaction-impulseturbine blade. The impulse turbine blade is also referred to as a bucket. Thisis because as the stream of air strikes the center of the blade it changes thedirection of the energy as it causes the blades to rotate the disk and rotorshaft. The turbine nozzle guide vanes can usually be adjusted during engineoverhaul and assembly in order to increase the efficiency of the air streamstriking the blades or buckets of the turbine.

Reaction turbine blades cause the disk torotate by the aerodynamic action of the airstream directed to flow past theblade at a particular angle in order to develop the most efficient power fromthe turbine engine. [Figure 3]

The reaction-impulse turbine blade combinesthe action of both the impulse and reaction blades designs. The blade has moreof the bucket shape of the impulse blade at the blade root and it also has moreof an airfoil shape of the reaction blade on the second half of the bladetoward the outer end of the blade.

The second purpose of the turbine inletnozzle is to deflect the gases to a specific angle in the direction of turbine wheelrotation. Since the gas flow from the nozzle must enter the turbine bladepassageway while it is still rotating, it is essential to aim the gas in thegeneral direction of turbine rotation.

The turbine inlet nozzle assembly consistsof an inner shroud and an outer shroud between which the nozzle vanes arefixed. The number and size of inlet vanes employed vary with different typesand sizes of engines. Figure 4 illustrates typical turbine inlet nozzlesfeaturing loose and welded vanes. The vanes of the turbine inlet nozzle may beassembled between the outer and inner shrouds or rings in a variety of ways.Although the actual elements may vary slightly in configuration andconstruction features, there is one characteristic peculiar to all turbine inletnozzles: the nozzle vanes must be constructed to allow thermal expansion.Otherwise, there would be severe distortion or warping of the metal componentsbecause of rapid temperature changes. The thermal expansion of turbine nozzlesis accomplished by one of several methods. One method necessitates looseassembly of the supporting inner and outer vane shrouds.

Each vane fits into a contoured slot in theshrouds, which conforms to the airfoil shape of the vane. These slots areslightly larger than the vanes to give a loose fit. For further support, theinner and outer shrouds are encased by inner and outer support rings, whichprovide increased strength and rigidity. These support rings also facilitateremoval of the nozzle vanes as a unit. Without the rings, the vanes could fallout as the shrouds were removed.

Another method of thermal expansionconstruction is to fit the vanes into inner and outer shrouds; however, in thismethod the vanes are welded or riveted into position. Some means must beprovided to allow thermal expansion; therefore, either the inner or the outershroud ring is cut into segments. The saw cuts separating the segments allowsufficient expansion to prevent stress and warping of the vanes.

The rotor element of the turbine sectionconsists essentially of a shaft and a wheel. [Figure 5] The turbine wheel is adynamically balanced unit consisting of blades attached to a rotating disk. Thedisk, in turn, is attached to the main power-transmitting shaft of the engine.The exhaust gases leaving the turbine inlet nozzle vanes act on the blades ofthe turbine wheel, causing the assembly to rotate at a very high rate of speed.The high rotational speed imposes severe centrifugal loads on the turbinewheel, and at the same time the elevated temperatures result in a lowering ofthe strength of the material. Consequently, the engine speed and temperaturemust be controlled to keep turbine operation within safe limits.

The turbine disk is referred to as suchwithout blades. When the turbine blades are installed, the disk then becomesthe turbine wheel. The disk acts as an anchoring component for the turbineblades. Since the disk is bolted or welded to the shaft, the blades cantransmit to the rotor shaft the energy they extract from the exhaust gases.

The disk rim is exposed to the hot gasespassing through the blades and absorbs considerable heat from these gases. Inaddition, the rim also absorbs heat from the turbine blades by conduction.Hence, disk rim temperatures are normally high and well above the temperaturesof the more remote inner portion of the disk. As a result of these temperaturegradients, thermal stresses are added to the rotational stresses. There arevarious methods to relieve, at least partially, the aforementioned stresses.One such method is to bleed cooling air back onto the face of the disk.

Another method of relieving the thermalstresses of the disk is incidental to blade installation. A series of groovesor notches, conforming to the blade root design, are broached in the rim of thedisk. These grooves allow attachment of the turbine blades to the disk; at thesame time, space is provided by the notches for thermal expansion of the disk.Sufficient clearance exists between the blade root and the notch to permitmovement of the turbine blade when the disk is cold. During engine operation, expansionof the disk decreases the clearance. This causes the blade root to fit tightlyin the disk rim.

The turbine shaft is usually fabricatedfrom alloy steel. [Figure 5] It must be capable of absorbing the high torqueloads that are exerted on it.

The methods of connecting the shaft to theturbine disk vary. In one method, the shaft is welded to the disk, which has abutt or protrusion provided for the joint. Another method is by bolting. Thismethod requires that the shaft have a hub that fits a machined surface on thedisk face. Then, the bolts are inserted through holes in the shaft hub andanchored in tapped holes in the disk. Of the two connection methods, bolting ismore common.

The turbine shaft must have some means forattachment to the compressor rotor hub. This is usually accomplished by aspline cut on the forward end of the shaft. The spline fits into a couplingdevice between the compressor and turbine shafts. If a coupling is not used,the splined end of the turbine shaft may fit into a splined recess in thecompressor rotor hub. This splined coupling arrangement is used almostexclusively with centrifugal compressor engines, while axial compressor enginesmay use either of these described methods.

There are various ways of attaching turbineblades, some similar to compressor blade attachment. The most satisfactorymethod utilizes the fir-tree design.

The blades are retained in their respectivegrooves by a variety of methods, the more common of which are peening, welding,lock tabs, and riveting. Figure 7 shows a typical turbine wheel using rivetsfor blade retention.

The peening method of blade retention isused frequently in various ways. One of the most common applications of peeningrequires a small notch to be ground in the edge of the blade fir-tree rootprior to the blade installation. After the blade is inserted into the disk, thenotch is filled by the disk metal, which is “flowed” into it by a smallpunch-mark made in the disk adjacent to the notch. The tool used for this jobis similar to a center punch.

Another method of blade retention is toconstruct the root of the blade so that it contains all the elements necessaryfor its retention. This method uses the blade root as a stop made on one end ofthe root so that the blade can be inserted and removed in one direction only,while on the opposite end is a tang. This tang is bent to secure the blade inthe disk.

Turbine blades may be either forged orcast, depending on the composition of the alloys. Most blades are precisioncast and finish ground to the desired shape. Many turbine blades are cast as asingle crystal, which gives the blades better strength and heat properties.Heat barrier coating, such as ceramic coating, and air flow cooling help keepthe turbine blades and inlet nozzles cooler. This allows the exhausttemperature to be raised, increasing the efficiency of the engine. Figure 8shows a turbine blade with air holes for cooling purposes.

Most turbines are open at the outerperimeter of the blades; however, a second type called the shrouded turbine issometimes used. The shrouded turbine blades, in effect, form a band around theouter perimeter of the turbine wheel. This improves efficiency and vibrationcharacteristics, and permits lighter stage weights. On the other hand, itlimits turbine speed and requires more blades.

In turbine rotor construction, itoccasionally becomes necessary to utilize turbines of more than one stage. Asingle turbine wheel often cannot absorb enough power from the exhaust gases todrive the components dependent on the turbine for rotative power; thus, it isnecessary to add additional turbine stages.

A turbine stage consists of a row ofstationary vanes or nozzles, followed by a row of rotating blades. In somemodels of turboprop engine, as many as five turbine stages have been utilizedsuccessfully. It should be remembered that, regardless of the number of wheelsnecessary for driving engine components, there is always a turbine nozzlepreceding each wheel.

As was brought out in the precedingdiscussion of turbine stages, the occasional use of more than one turbine wheelis warranted in cases of heavy rotational loads. It should also be pointed outthat the same loads that necessitate multistage turbines often make itadvantageous to incorporate multiple compressor rotors.