Gas Turbine - Exhaust Flashcards
Components of exhausts
Jet pipe - outer casing for flow and guides to direct towards exit
Exhaust cone - protect turbine disc area from high temperature flow
Propelling nozzle - accelerate flow via divergent duct
Exhaust Cone
Reduce turbulence and back flow
Directs airflow
Exhaust Nozzle
Convergent to increase velocity of the air
Highest velocity within turbojet occurs here
Noise
High velocity of exhaust air impacts air and generates noise
Pressure Thrust Formula
Area of propelling nozzle x (exhaust static pressure - inlet static pressure)
De swirl vanes
Straighten
Principals of exhaust system
Straighten and accelerate for max thrust
Minimise noise
Minimise friction losses
Prevents hot gas from flowing across rear face of turbine
Struts reduce residual whirl
Turbo Fan vs Turbo Jet Noise
Turbo fans small acceleration to large mass therefore reducing exit velocity less noise
K energy extracted to fan slow velocity further
Mixing of gas streams in exhaust
Bypass ratio engines mixing slow cold stream with fast hot stream helps slow exhaust velocity and noise
Corrugated exhaust
Accessory Gearbox in engine
Driven by the high pressure compressor shaft via internal gearbox
- power for hydraulic pumps/electrical gens/fuel pumps/oil pumps
Second accessory gearbox will be connected to the
Intermediate compressor
Reverse Thrust provide what of forward thrust
45% of forward thrust
Types of reverse thrust systems
Clamshells/buckets- low bypass extend into core exhaust gases to reverse flow/redirect core air/pneumatically operated.
Cold stream reversal system - blocks and redirect bypass air/blocker doors block movement of air in bypass
Interlock Protection for reverse thrust
Three locks 1 in cockpit 2 controlled by engines
System prevented from operating until on ground (idle rpm)
How is EGT measured
Thermocouple via heat into electrical current
Hold/cold junction/voltage produced whenever temp differs
Propelling Nozzle is designed to
Increase the velocity and decrease the pressure of the gas stream
Tallis cone designed to
Minimise friction losses as well as protecting the back of the turbine disc
High Bypass turbofan reverses ops
Hydraulically
Only cold thrust of engine
Bucket Reverse systems are
Hydraulically operated
Reverse all the thrust
Clamshell Reverses are
Pnemuatically operated
Reverse all the thrust
Reverse doors deployment colours
Doors deployed - Amber (rev)
Door deployed and locked - Green (rev)
Exhaust P/V/T
Pressure increases
Velocity increases
Temperature increases
Purpose of a mixer unit
To reduce noise
Scarfing
Technique to reduce noise
N1 gauge showing REV in amber means
Doors unlocked
Total Thrust
Dynamic thrust + Pressure Thrust
Dynamic = M x (Vjet - Vflight)
Pressure Thrust = Acceleration x (P x Po)
P - nozzle static pressure in pascals
Po - Ambient static pressure in pascals
Exhaust Pressure ratio
The ratio between low pressure turbine exhaust and engine inlet pressure
Turbine outlet to compressor inlet
Specific Thrust
Thrust per unit air mass flowrate
Net thrust
—————-
Total intake airflow
Constant intake airflow increases with forward velocity which means specific thrust decreases
Engine tailpipe fire
Occurs at engine start or engine shutdown
Result of excess of fuel in combustion chamber/turbine or exhaust nozzle that ignites
Abnormally high EGT
Stop fuel flow and ventilate the engine
