Monday, April 18, 2016

Jet Turbine Basics

Have you ever wondered how a turbofan engine works? Well no need to wonder anymore, there are 4 simple steps in a jet turbine: suck, squeeze, bang, and blow.
The biggest part of the jet engine is the fan which sucks the air into the system. A Pratt & Whitney fan can suck 2600 pounds of air per second or enough to empty a 4 bedroom house in less than half a second. The intake air is separated into 2 streams, primary air and bypass air. The primary air is about 15% of the total intake from the fan, this goes into the next step, squeeze.
The primary air enters the compressor which increases the pressure by 30 times and increases the temperature by over 1100 degrees after the second stage of compression.
After compression the next step, bang, or combustion takes place. This combustion is a fuel and compressed air mixture which then expands and increases the temperature even greater for the next stage, blow. Below is the Pratt & Whitney F100 turbofan engine for the F-15E

During this stage the turbine takes the energy from the combustion and converts it to mechanical rotational energy that drives the shaft of the compressors and fan. The turbine also pushes out the exhaust creating thrust. However, the majority of the thrust produced in a turbojet engine comes from the 85% of air that did not go through all those stages, the bypass air. In the PW4084 engine the bypass air accounts for 90% of the thrust produced. This bypass air only needs a small amount of compression to create a large amount of thrust, it also greatly helps to cool the engine.
A turbojet on the other hand has all the air from the fan go through the compressors and turbine. At subsonic speeds the turbofan is more efficient than the turbo jet but since it has a larger fan it has a larger frontal surface area and creates more drag.

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