Gas Turbine Theory and Construction

IntroductionComprehend the thermodynamic processes occurring in a gas turbineComprehend the basic components of gas turbine engines and their basic operationComprehend the support systems associated with gas turbine engines

BackgroundAircraft turbojet/turbofan engines are precursors to gas turbinesInstalled for propulsion in: FFGsDDsDDGsCGsM-1 tanks Also used for electrical generation & auxiliary applications

Advantages of GTEsWeight reduction of 70%SimplicityReduced manning requirementsQuicker response timeFaster Acceleration/decelerationModular replacementLess vibrationsMore economical

Disadvantages of GTEsMany parts under high stressHigh pitched noiseNeeds large quantities of airLarge quantities of hot exhaust (target)Cannot be repaired in place

Brayton CycleUnlike diesels, operate on STEADY-FLOW cycleOpen cycle, unheated engine1-2: Compression

2-3: Combustion

3-4: Expansion through Turbine and Exhaust Nozzle

(4-1: Atmospheric Pressure)

Basic Components

Basic Components

Basic ComponentsCompressorDraws in air & compresses itCombustion ChamberFuel pumped in and ignited to burn with compressed airTurbineHot gases converted to workCan drive compressor & external load

Basic ComponentsCompressorDraws in air & compresses itCombustion ChamberFuel pumped in and ignited to burn with compressed airTurbineHot gases converted to workCan drive compressor & external load

Basic ComponentsCompressorDraws in air & compresses itCombustion ChamberFuel pumped in and ignited to burn with compressed airTurbineHot gases converted to workCan drive compressor & external load

CompressorSupplies high pressure air for combustion processCompressor typesRadial/centrifugal flow compressorAxial flow compressor

CompressorRadial/centrifugal flowAdv: simple design, good for low compression ratios (5:1)Disadvantage: Difficult to stage, less efficientAxial flow Good for high compression ratios (20:1) - Most commonly used

CompressorControlling Load on CompressorTo ensure maximum efficiency and allow for flexibility, compressor can be split into HP & LP sectionsVane control: inlet vanes/nozzle angles can be varied to control air flowCompressor StallInterruption of air flow due to turbulence

Use of Compressed AirPrimary Air (30%)Passes directly to combustor for combustion processSecondary Air (65%)Passes through holes in perforated inner shell & mixes with combustion gasesFilm Cooling Air (5%)Insulates/cools turbine blades

Combustion ChambersWhere air & fuel are mixed, ignited, and burnedSpark plugs used to ignite fuelTypesCan: for small, centrifugal compressorsAnnular: for larger, axial compressors (LM 2500)Can-annular: for really large turbines

TurbinesConsists of one or more stages designed to develop rotational energyUses sets of nozzles & blades

TurbinesSplit ShaftGas generator turbine drives compressorPower turbine separate from gas generator turbinePower turbine driven by exhaust from gas generator turbinePower turbine drives power coupling

Single ShaftEfficiently operates at constant speedsUsed in GTGS (gas turbine generator systems)Single shaftPower coupling on same shaft as turbineSame shaft drives rotor of compressor and power components*Primarily used for electric power because of constant speed, regardless of load.

Split ShaftBest where speeds and loads varyUsed in LM-2500Power shaft is decoupled from compressorAllows both to operate at efficient speeds (not the same)*More suitable for main propulsion applications due to the fact that the gas generator turbine and power turbine operate near their most efficient speeds throughout a RANGE of load demands.

Accessory Drive AssemblyPurpose - Provides motive force for driving the accessories required for operation and control of engineAttached Accessory EquipmentFuel oil pumpLube oil pumpStarter (pneumatic, electric, hydraulic)

Gas Turbine SystemsAir SystemAir intakes are located high up & multiple filtersExhaust discharged out stacksFuel SystemUses either DFM or JP-5Lubrication SystemSupply bearings and gears with oil

Gas Turbine SystemsStarting SystemTo get compressor initially rotated, HP air used (can use electrical also)Once at certain RPM, fuel injected and spark ignitedPower Transmission SystemReduction gears used to transfer torqueWith split shaft, turbines can run @ different speeds

Questions?

*********Air is drawn into the front of the compressor. Each succeeding stage is smaller increasing velocity (recall Bernoullis equation). Between each rotating stage is a stationary stage or stator. The stator partially converts the high velocity to pressure and directs the air to the next set of rotating blades. The rotor imparts velocity to the air (like a fan). Each stage consists of a rotor and stator and results in a pressure increase. Air exits the compressor and enters the diffuser. Suddenly, the air moves from a narrow passage into a wide area. By bernoulli, the air loses velocity and expands in volume and increases pressure. Now, the air is slow moving and high pressure, usually about 19:1. Fuel is injected here and the mixture is ignited by a spark. The spark causes a rapid increase in the volume of the air an combustion gases (at constant pressure).The combustion mixture goes rearward to a nozzle which directs the gas onto the turbine blades and accelerates the gases. The gases are now high velocity, high temperature, low pressure and are used to turn the turbine. The kinetic and thermal energy of the gases are transferred the turbine blades. The turbine is multistaged to remove as much of the energy from the gas as possible.

****Radial flow or centrifugal compressor- compressor draws in the entering air at the hub of the impeller and accelerates it radially outward by centrifugal force through the impeller. Reasonably efficient for high pressure ratios developed in a single stage.

Axial flow- Rotor has fixed blades which force the air rearward much like an aircraft propeller. The stator directs the air rearward to the next stage. Very much like a turbine used in reverse. Used in multistage arrangements and pressure ratios increase with the number of stages.

****Can Type- Individual liners and cases mounted around the engine each with its own fuel nozzle.

*Annular type- Liner consists of an undivided circular shroud extending all the way around the outside of the turbine shaft housing. The dome of the liner has small slots and holes to admit primary air. There are also holes in the dome for the fuel nozzles to extend through into the combustion area. The combustion space is formed by the inner and outer liners. The inner liner prevents flame from contacting the turbine shaft housing.

*Can-annular type- Designed to deal with split spool compressor. Individual cans are placed inside an annular case. Combines the strength of annular design with the convenience of maintenance of the can. Also keeps high temperatures in the inner can.*Turbines, like compressors, consist of stator and rotor elements. Stators prepare the mass flow for harnessing of power through the turbine rotor. The nozzles take the high pressure, high-energy mixture and give it velocity for driving the rotor. Also deflects the gases to a specific angle in the direction of the turbine wheel rotation.

Rotors consist of a shaft and bladed wheel. Turbine operates at high speed

***Single- Aircraft and electric power- constant speed independent of load**Split- allows the compressor to run at different speed that power turbine maximizing the efficiency of operation.

*Provides the space for mounting and the motive force for driving the accessories required for the operation and control of the engineDiscuss the following accessory equipment attached at the assembly:Fuel oil pumpLube oil pumpStarter (pneumatic, electric, hydraulic)

***