Gas Turbine Theory and Construction

Introduction Comprehend the thermodynamic processes occurring in a gas turbine Comprehend the basic components of gas turbine engines and their basic operation Comprehend the support systems associated with gas turbine engines

Background Aircraft turbojet/turbofan engines are precursors to gas turbines Installed for propulsion in: FFG’s DD’s DDG’s CG’s M-1 tanks Also used for electrical generation & auxiliary applications

Advantages of GTE’s Weight reduction of 70% Simplicity Reduced manning requirements Quicker response time Faster Acceleration/deceleration Modular replacement Less vibrations More economical

Disadvantages of GTE’s Many parts under high stress High pitched noise Needs large quantities of air Large quantities of hot exhaust (target) Cannot be repaired in place

Brayton Cycle Unlike diesels, operate on STEADY-FLOW cycle Open cycle, unheated engine 1-2: Compression 2-3: Combustion 3-4: Expansion through Turbine and Exhaust Nozzle (4-1: Atmospheric Pressure)

Basic Components

Basic Components

Basic Components Compressor Combustion Chamber Turbine Draws in air & compresses it Combustion Chamber Fuel pumped in and ignited to burn with compressed air Turbine Hot gases converted to work Can drive compressor & external load 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.

Basic Components Compressor Combustion Chamber Turbine Draws in air & compresses it Combustion Chamber Fuel pumped in and ignited to burn with compressed air Turbine Hot gases converted to work Can drive compressor & external load

Basic Components Compressor Combustion Chamber Turbine Draws in air & compresses it Combustion Chamber Fuel pumped in and ignited to burn with compressed air Turbine Hot gases converted to work Can drive compressor & external load

Compressor Supplies high pressure air for combustion process Compressor types Radial/centrifugal flow compressor Axial flow compressor

Compressor Radial/centrifugal flow Axial flow Adv: simple design, good for low compression ratios (5:1) Disadv: Difficult to stage, less efficient Axial flow Good for high compression ratios (20:1) Most commonly used 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.

Compressor Controlling Load on Compressor Compressor Stall To ensure maximum efficiency and allow for flexibility, compressor can be split into HP & LP sections Vane control: inlet vanes/nozzle angles can be varied to control air flow Compressor Stall Interruption of air flow due to turbulence

Use of Compressed Air Primary Air (30%) Secondary Air (65%) Passes directly to combustor for combustion process Secondary Air (65%) Passes through holes in perforated inner shell & mixes with combustion gases Film Cooling Air (5%) Insulates/cools turbine blades

Combustion Chambers Where air & fuel are mixed, ignited, and burned Spark plugs used to ignite fuel Types Can: for small, centrifugal compressors Annular: for larger, axial compressors (LM 2500) Can-annular: for really large turbines

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 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 Consists of one or more stages designed to develop rotational energy Uses sets of nozzles & blades 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

Turbines Split Shaft Gas generator turbine drives compressor Power turbine separate from gas generator turbine Power turbine driven by exhaust from gas generator turbine Power turbine drives power coupling

Single Shaft Efficiently operates at constant speeds Used in GTGS (gas turbine generator systems) Single shaft Power coupling on same shaft as turbine Same shaft drives rotor of compressor and power components *Primarily used for electric power because of constant speed, regardless of load.

Single- Aircraft and electric power- constant speed independent of load

Split Shaft Best where speeds and loads vary Used in LM-2500 Power shaft is decoupled from compressor Allows 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.

Split- allows the compressor to run at different speed that power turbine maximizing the efficiency of operation.

Accessory Drive Assembly Purpose - Provides motive force for driving the accessories required for operation and control of engine Attached Accessory Equipment Fuel oil pump Lube oil pump Starter (pneumatic, electric, hydraulic) Provides the space for mounting and the motive force for driving the accessories required for the operation and control of the engine Discuss the following accessory equipment attached at the assembly: Fuel oil pump Lube oil pump Starter (pneumatic, electric, hydraulic)

Gas Turbine Systems Air System Fuel System Lubrication System Air intakes are located high up & multiple filters Exhaust discharged out stacks Fuel System Uses either DFM or JP-5 Lubrication System Supply bearings and gears with oil

Gas Turbine Systems Starting System Power Transmission System To get compressor initially rotated, HP air used (can use electrical also) Once at certain RPM, fuel injected and spark ignited Power Transmission System Reduction gears used to transfer torque With split shaft, turbines can run @ different speeds

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