lesson 11 gas turbine theory
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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?
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