27-11-2012, 02:15 PM
Jet Propulsion
Jet Propulsion.ppt (Size: 3.23 MB / Downloads: 453)
Lesson Objectives
After this lesson students should be able to:
Define what a jet engine is
Describe how Newton’s laws apply to jet or rocket engines
List examples of jet engine applications
List some key points in the history of jet propulsion
List advantages and disadvantages of jet engines
Definition of a Jet Engine
An engine that burns fuel and uses the expanding exhaust gases to turn a turbine and/or produce thrust
The concept of thrust is based on the principle of Newton’s Third Law
Newton’s Third Law
For every action there is an equal and opposite reaction
An example of this is a spray nozzle on a garden hose
Newton’s Second LawF=M x A
Newton’s second law states - The force of an object is equal to its mass times its acceleration
The force of the spray nozzle is equal to the mass of the water multiplied by the acceleration of the water when it comes through the nozzle
This is the same principle used in rocket and jet engines
Turbofan
Some of the thrust is produced by gasses expelled from the exhaust nozzle just like a turbojet engine
Most of the thrust is produced from the large inlet fan
The Bypass ratio of a turbofan is typically 8:1 (eight times more air is bypassed than passes through the compressor and combustion chamber)
If one wanted to increase thrust you would either have to increase the speed of the air being moved or increase the mass of the air being moved (Thrust = Mass x Acceleration) ... However…
It is more efficient to accelerate a larger mass of air to a lower velocity
Due to this principle the turbofan is more efficient than the turbojet
Due to the lower velocity the turbofan is also significantly quieter than a turbojet
Almost all modern commercial aircraft use turbofan engines (excluding the Concord)