Gas Turbine - Exhaust

Question 1

Components of exhausts

Answer 1

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

Question 2

Exhaust Cone

Answer 2

Reduce turbulence and back flow
Directs airflow

Question 3

Exhaust Nozzle

Answer 3

Convergent to increase velocity of the air
Highest velocity within turbojet occurs here

Question 4

Noise

Answer 4

High velocity of exhaust air impacts air and generates noise

Question 5

Pressure Thrust Formula

Answer 5

Area of propelling nozzle x (exhaust static pressure - inlet static pressure)

Question 6

De swirl vanes

Answer 6

Straighten

Question 7

Principals of exhaust system

Answer 7

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

Question 8

Turbo Fan vs Turbo Jet Noise

Answer 8

Turbo fans small acceleration to large mass therefore reducing exit velocity less noise

K energy extracted to fan slow velocity further

Question 9

Mixing of gas streams in exhaust

Answer 9

Bypass ratio engines mixing slow cold stream with fast hot stream helps slow exhaust velocity and noise

Corrugated exhaust

Question 10

Accessory Gearbox in engine

Answer 10

Driven by the high pressure compressor shaft via internal gearbox

• power for hydraulic pumps/electrical gens/fuel pumps/oil pumps

Question 11

Second accessory gearbox will be connected to the

Answer 11

Intermediate compressor

Question 12

Reverse Thrust provide what of forward thrust

Answer 12

45% of forward thrust

Question 13

Types of reverse thrust systems

Answer 13

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

Question 14

Interlock Protection for reverse thrust

Answer 14

Three locks 1 in cockpit 2 controlled by engines
System prevented from operating until on ground (idle rpm)

Question 15

How is EGT measured

Answer 15

Thermocouple via heat into electrical current

Hold/cold junction/voltage produced whenever temp differs

Question 16

Propelling Nozzle is designed to

Answer 16

Increase the velocity and decrease the pressure of the gas stream

Question 17

Tallis cone designed to

Answer 17

Minimise friction losses as well as protecting the back of the turbine disc

Question 18

High Bypass turbofan reverses ops

Answer 18

Hydraulically
Only cold thrust of engine

Question 19

Bucket Reverse systems are

Answer 19

Hydraulically operated
Reverse all the thrust

Question 20

Clamshell Reverses are

Answer 20

Pnemuatically operated
Reverse all the thrust

Question 21

Reverse doors deployment colours

Answer 21

Doors deployed - Amber (rev)
Door deployed and locked - Green (rev)

Question 22

Exhaust P/V/T

Answer 22

Pressure increases
Velocity increases
Temperature increases

Question 23

Purpose of a mixer unit

Answer 23

To reduce noise

Question 24

Scarfing

Answer 24

Technique to reduce noise

Question 25

N1 gauge showing REV in amber means

Answer 25

Doors unlocked

Question 26

Total Thrust

Answer 26

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

Question 27

Exhaust Pressure ratio

Answer 27

The ratio between low pressure turbine exhaust and engine inlet pressure

Turbine outlet to compressor inlet

Question 28

Specific Thrust

Answer 28

Thrust per unit air mass flowrate

Net thrust
—————-
Total intake airflow

Constant intake airflow increases with forward velocity which means specific thrust decreases

Question 29

Engine tailpipe fire

Answer 29

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