Gas Turbine Design Flashcards
Centrifugal compressor
Common early design
Heavy and inefficient
Noisy/large frontal area
Disadvantages of single spool axial flow
Inefficient unless at high speed
Noisy
Small mass of air exhausted
What is spool
Assembly comprising a compressor and turbine connected by a shaft
Axial flow
Passage of air through engines from front to rear along engines axis
Twin spool bypass design
No bypass duct
Smaller acceleration to a larger mass of air
Some air bypasses inner section (low pressure compressor)
Separating spools
+ reduces noise
+ bypass air is cold so engine more thermal efficient
+ greater propulsive efficiency
+ core smaller/less weight
Bypass ratio Calculation
Ratio of air entering bypass compared to air entering core
Calculate bypass flow
Divide bypass flow by core flow
High Bypass Ratio Turbo Fans
Accelerates a much larger mass of air through bypass duct
Twin spooled or tripled
Thermal/propulsive efficiency improved by accelerating large mass of cold unheated air
Twin Spool Design
Low pressure spool - drives the fan via LP turbine
High pressure spool (engine core) - Hp turbine and Hp compressor by HP shaft
Triple Spool Design
Third spool powers large diameter fan
IP compressor and HP compressor located in core
Cores drive the fans which product up to 85% of total thrust
N1/N2/N3 - rotational speed symbols for each spool
Turbo Propeller engines
Use turbofan engines with propeller for lower speed a/c
Require extra turbine stages to extract all energy in exhaust
Use centrifugal compressors
Bypass Ratio how it works and figure for each stream
Turbo fan a aerodynamic splitter divides incoming air into hot stream and a cold stream
Cold stream = not part of combustion but 85% of thrust comes from here
Hot stream = used for combustion accounts for 15% thrust
Accelerating air/combustion used to drive turbine
Bypass Ratio for turbo jet/turbo prop
Turbo jet considered to have zero BPR since all air is used in combustion
Turbo prop 100:1 since none of air passes through the combustion chamber provides thrust
Propulsive efficiency comparison between turbo-prop/turbojet/by pass engine
Turbo prop most efficient at low speeds
By pass engine most efficient at mid range speeds
Turbo jet most efficient at high speeds
Turbo Jet
Takes a small mass flow and imparts a large acceleration
Turbo fan
Takes a large mass flow and imparts a small acceleration
