Gas Turbine Theory

ENGINEERING LESSON GUIDE 11

BRAYTON CYCLE

Brayton Cycle

COMPRESSOR

AIR

COMBUSTIONCHAMBER

EXHAUST

WORK

FUEL

TURBINE

Gas Turbine Engine

FOUR MAJOR COMPONENTS OF A GAS

TURBINE ENGINE

Compressor Combustor Turbine Accessory Drive Assembly

COMPRESSOR

Function: Provides required air mass at the appropriate pressure to burn the required amount of fuel and to control combustion temperature

Two types– Axial Flow– Centrifugal or Radial Flow

Radial Flow (Centrifugal) Compressor

Axial Flow Compressor

Axial flow v. Radial flow Centrifugal compressors are simple,

inexpensive, lightweight, and have a high pressure rise per stage

Centrifugal compressors experience large inter-stage losses and require a large frontal area; they are typically less efficient than multistage axial compressor

Multistage axial compressors can achieve larger compression ratios and are better suited for high-power marine applications

Compressor Stall

Occurs if for some reason air velocity decreases without a commensurate decrease in RPM or if RPM increases without the necessary air velocity increase

May occur for multistage axial compressors if some stages are operating inefficiently while others are overloaded at the same speed

Similar to wing stall for aircraft Can result in blade failure

Uses of Compressed Air PRIMARY AIR

– Typically 30% of all compressed air– Passed directly to combustor, mixed with fuel, and burned

SECONDARY AIR– Approximately 70%– Passes through holes in inner shell and mixes with combustion

gases– Two purposes

• Places an air film between the inner shell and combustion gases to prevent overheating of the inner shell

• Cools combustion gases to an acceptable inlet temperature for the turbine

FILM AIR– A small percentage of compressed air may be used to cool turbine

blades

COMBUSTION CHAMBER

Function: mixes fuel and air and burns this mixture to produce hot combustion gases

Consists of a casing, perforated inner shell, and fuel nozzles

Arrangement– Annular– Can or Tubular– Can-annular

TURBINE

Develops shaft rotational energy from the kinetic energy of the hot combustion gases entering through the vanes

Usually of axial flow design Drives the compressor and various engine

accessories The remaining useful energy can be used as

jet thrust or shaft mechanical work

Turbine Construction

STATOR– Stationary guide nozzles (vanes) discharge

gas at high velocity onto the moving blades– Attached to turbine casing

ROTOR– Consists of a shaft and bladed wheel (disc)– Attached to the main power-transmitting

shaft

Stator

Rotor

Film Cooling

High rotational speeds and high temperature combustion gases may cause a decrease in rotor and blade strength

In addition to secondary air, some turbines employ film cooling

Film cooling air can use approximately 5% of the compressed air

Interior and ExteriorCooling-air Circulation

Film Cooling and Impingement Convection

Cooling

ACCESSORY DRIVE ASSEMBLY

Provides the space for mounting and the motive force for driving the accessories required for the operation and control of the gas turbine engine

May be used to drive the fuel pump, lube oil pump, etc.

Advantages

Weight reduction of 70% when compared to a steam plant of comparable horsepower

Simplicity Reduced manning requirements with more highly

automated equipment Quicker response time Faster acceleration/deceleration Modular replacement More economical

Disadvantages

Many parts under high stress High pitched noise Needs large quantities of air Large heat source

Uses of Gas Turbine Engines

Aircraft Engines Main Propulsion

– Arleigh Burke, Oliver Hazard Perry, Ticonderoga, Spruance, LCAC

Auxiliary Applications– Electric generators

Questions?