Jet Propulsion

Recap Quiz

• What does the compressor do?• What is the difference between Rotor Blades and

Stator Blades?• How is the Turbine driven?• Name the 3 types of Combustion Chambers?• Briefly explain what happens to the temperature

and pressure as the air goes through the engine?• Name 2 different types of jet engines?• What is a Compressor Surge and how is it solved?

• What does the compressor do?The function of the compressor in a jet engine is to

increase the pressure of airflow through the engine.

• What is the difference between Rotor Blades and Stator Blades?

Rotor blades push the air back to the Stator blades. These then compress the air increasing its pressure.

• How is the Compressor driven?By the Turbine

• Name the 3 types of Combustion Chambers?Can, Cannular and Annular.

• Briefly explain what happens to the temperature and pressure as the air goes through the engine

Pressure: As the air goes through the compressors, it increases. Once it is ignited, the pressure drops and continues to drop as the air goes out the exhaust.

Temperature: As the air goes through the compressors the temperature increases slowly. Once it is ignited in the combustion chamber, there is a rapid increase. IT then slowly decreases as it leaves the exhaust.

• Name 2 different types of jet engines?Turbojet, Turboshaft, Turboprop, Turbofan,

• What is a Compressor Surge and how is it solved?A compressor surge (stall) is a situation of abnormal

airflow resulting from a stall of the airfoil within the compressor of a jet engine. The compressors are unable to cope with the amount of air entering the engine.

This is solved by having a bleed valve.A double shaft.

Intakes

• An engine’s air inlet duct is normally considered an airframe part, and not a part of the engine. However, the duct, itself, is so important to engine performance.

The inlet duct has two engine functions:

• It must be able to recover as much of the total pressure of the free air-stream as possible and deliver this pressure to the front of the engine with minimum loss.

• The duct must deliver air to the compressor inlet under all flight conditions with as little turbulence and pressure variation as possible.

• Most inlet ducts on subsonic airplanes are of the divergent type. Air flowing into a divergent duct expands slightly and converts some of its velocity energy into pressure.

• Rectangular inlets are designed to absorb the shockwaves produced.

CompressorsCentrifugal flow

• Centrifugal compressors were used on many of the earliest gas turbine engines because of their ruggedness, light weight and high pressure ratio for each stage of compression.

• A typical centrifugal compressor consists of three components: the impeller, the diffuser, and the manifold.

Eye of the impeller

ImpellerDiffuser

Manifold

• Air enters the eye of the fast-rotating impeller and is accelerated to a high velocity as it is slung to the outer edge by centrifugal force. The high-velocity air then flows into the diffuser, which fits closely around the outside edge of the impeller.

• There it flows through divergent ducts where some of the velocity energy is changed into pressure energy.

• The air, which has slowed down and has had its pressure increased, flows into the manifold through a series of turning vanes.

• From the manifold, the air flows into the combustion section of the engine.

Axial flow compressor

• Axial-flow are compressors in which the air passes axially or straight through the compressor.

• They are heavier than a centrifugal compressor, but they are capable of a much higher overall compression ratio, and they have a smaller cross-sectional area, which makes them easier to streamline.

• Axial-flow compressors have therefore become the standard for large gas turbine engines and are also used on many small engines.

• Axial-flow compressors are made up of a number of stages of rotor blades that are driven by the turbine, and that rotate between stages of fixed stator vanes.

• Both the rotor blades and stator vanes have airfoil shapes and are mounted so that they form a series of divergent ducts through which the air flows.

CombustionAir distribution

• Approximately 82% of the air from the compressor passes around the outside of the inner flame tube.

• This air is then passes into the flame tube via a number of large and small ‘dilution’ holes into the inside of the flame tube where it is heated by the combustion flame

• Approximately 18% of the air passes immediately into the flame tube where it is split into another two main flows: -

• A. Approximately half of the air passes around and through the end baffle plate and swirls into the flame tube. It can be seen that this air flow produces a forward flow right at the point where the fuel is sprayed in.

• B. Approximately half the airflow passes through swirl vanes situated around the fuel jet. This swirl air hits the forward flow to produce what is known as a ‘Re-Circulating Vortex’.

F u e l F e e d

O u t e r C o m b u s t i o n C a s i n g

1 s t S t a g e N G V ’ s

1 8 % V o r t e x F l o w

R e c i r c u l a t i n g V o r t e x

I n n e r F l a m e T u b e

8 2 % C o o l i n g F l o w

J E T E N G I N E M E C H A N I C A L A R R A N G E M E N TJ E T E N G I N E M E C H A N I C A L A R R A N G E M E N T

Afterburners

• An afterburner is an additional component on some jet engines. Its purpose is to provide an increase in thrust.

• Afterburning is achieved by injecting additional fuel into the jet pipe downstream of the turbine. It is burnt using the remaining oxygen.

• This heats and expands the exhaust gases further, and can increase the thrust of a jet engine by 50% or more.

Reverse Thrust

• Modern aircraft are normally so heavy to land at such a high speed that the aircraft brakes cannot be depended upon for complete speed control.

• Thrust reversal is the temporary diversion of an aircraft engine's exhaust so that the exhaust produced is directed forward, rather than rearwards.

• This acts against the forward travel of the aircraft, providing deceleration

CLAM SHELL DOORS

FORWARD THRUSTREVERSE THRUST

TARGET DOORS

FORWARD THRUSTREVERSE THRUST