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Wednesday, August 30, 2017

Methods of Propulsion: Turbofan Engines

The turbofan jet engine is one of the most common type of propulsion used in aircraft. They are used in planes both commercial and military, big and small. They are also one of the most recognizable means of propulsion, found in airports around the world.

Working

A turbofan engine consists of five main parts: the fan, the compressor, the combustor, the turbine, and the exhaust nozzle.

The Fan
Image result for turbofan fanThe fan is one of the most iconic features of modern commercial airplanes. It is the large, bladed structure that is present at the front of the engine, and which can be seen from the front of the engine. The fan is usually made out of titanium.

The main job of the fan is to suck in huge quantities of air into the engine proper. In all turbofans, the air splits into two parts: one stream goes into the engine itself, while the second stream goes around the engine. This is known as the bypass airflow. (Above): The main fan of a turbofan engine.

Image result for turbofan cutawayThe Compressor
The compressor consists of a series of blades and aerofoils. It's main job is to compress the air, as the name suggests. The compressor's longer name is the axial flow compressor, a name it has received because it compresses air that flows parallel to the axle of the blades.

Inside the compressor, the air moves through sets of spinning blades which add energy to the air. In between each set of blades are static structures of fins known as stators or vanes. These stop the rotating air from spinning and transform their rotational energy into pressure before sending them on to the next set of blades. 

Several such series of blades and vanes may exist, and they compress the air to a great degree before passing it on to the next part in line. (Above left): In the picture above, the compressor is the series of small blades present behind the main fan.

The Combustor
The combustor is the region of the engine where the fuel is actually burned. It consists of many parts, all of which serve to maintain a steady rate of combustion in the face of high-speed airflows.

Once air has passed through the compressor, it is pushed into the diffuser, a structure which slows down the speeding air to make it easier to ignite. Further on, domes and swirlers add turbulence to the air, making it easier to mix fuel with. The air is then allowed into the combustion zone through liners, which are basically the perforated walls of the combustion zone. Finally, the fuel is injected into the sowed-down, turbulent air and then ignited with an igniter, which is quite similar to a spark plug.

Once the fire has started, the igniter is switched off. The flame is self sustaining, and keeps going for as long as there is fuel or until the pilot switches it off.

(Above left): In the picture in the Compressor section, the combustor can be seen right behind the compressor, in the middle of the picture.

The Turbine
Image result for turbofan turbineThe turbine is a set of fans present at the rear end of the combustion chamber. it is connected to the same axle that the main fan at the front and the compressor blades spin on. Hot exhaust from the combustion of fuel inside the combustion chamber spins the turbine and by proxy the main fan and compressor. The main fan and compressor blades are thus turned by the turbine, which is turned by the hot exhaust gases of the engine itself.

(Above left): In the picture in the Compressor section, the turbine can be seen near the very end of the engine.
(Above right): The turbines of this engine can be seen clearly.

The Nozzle
The nozzle is the final part of the engine. Through it, hot exhaust gases produced by the engine shoot out, pushing the engine forward, curtsy of the Third Law of Motion.

Some nozzles include a mixer, which mixes the bypass-air with the exhaust gases. This serves to make the engine quieter, which is a boon for commercial planes. Nozzles may also include chevrons, which are saw-tooth patterns found on the end of the nozzle. These serve to reduce engine noise by allowing a smooth mixing of hot and cold air. (Right): Chevrons on an engine.

Types

There are two main types of turbofan engines: high-bypass and low-bypass.
  1. A high-bypass engine sends more air around the engine than though the engine itself. This serves to make the engine quieter and more fuel efficient. These kind of engines are used mainly on commercial aircraft.
  2. A high-bypass engine sends more air into the engine than around it. These kind of engines are used mainly on military aircraft. In military aircraft, many times, an afterburner is attached to the end of the engine.

(Above): A high-bypass engine awaiting installation. Note how the main fan is much wider than the actual engine.



(Above): A Pratt and Whitney afterburning turbofan undergoing testing. Note here how the main intake is not much bigger than the engine itself.

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