Why air is compressed in Gas turbine engine ?

[gkraju]

i understood brayton cycle etc.. but why is air compressed in the GT engine ? iS is anything related related to following reasons ?
1. FOr better combustion of fuel, if so why ?
2 IS it to increase the temperature of air, if so why ?
3 any other reason

 

[mugaliens]

"As with all cyclic heat engines, higher combustion temperature means greater efficiency. The limiting factor is the ability of the steel, nickel, ceramic, or other materials that make up the engine to withstand heat and pressure." - http://en.wikipedia.org/wiki/Gas_turbine_engine#Theory_of_operation"

Additional restrictions against infinite compression include various cycle losses which, combined with materials limitations, will trend towards an optimal solution involving high, but not impossibly high temperatures.

 

[xxChrisxx]

It's not just about efficiency.

Power output is a function of fuel burnt. The more fuel you can burn in a given time the more power you make. Compression allows lots of air to be combust in a small space.

 

[russ_watters]

It's more direct than about output or efficiency: if you don't compress the air, it won't move through the engine. The exit velocity of the exhaust gases is a function of pressure and if the combustion chamber had a higher pressure than ambient and there was no compressor, the exhaust gases would go out the back and the front of the engine.

 

[sorter]

Comapring b/w Turbojet & Turbofan which has high exit temperature

 

[boneh3ad]

If you are just talking exit temperature from the core, they should be roughly the same. If you are talking about average exit temperature, a turbojet would be higher because it doesn't have all that ambient bypass air to mix with.

 

[minger]

To understand why we compress air in a jet engine, one must first understand the goal. For an aviation gas turbine, the goal is thrust. The purpose of the turbine stage is to power the fan and the compressor. The thrust comes from moving air through the engine.

Now, we know why the fan is there. It's there to move the air. We get thrust from that. Now, the compressor's main purpose is to assist the turbine in energy extraction. By compressing the air, not only does it assist in combustion, it also helps in energy extraction by the turbine stage(s).

As for the temperatures, a turbojet will have significantly higher higher temperatures, as 100% of the flow goes through the turbine stage. As we mentioned, for a thrust-producing aviation engine, the goal of the turbine is to extract 'just' enough energy to power the fan/compressor stages. So, the flow still has a ton of energy, in the form of velocity and temperature. Exhaust temperatures can be well over 1000 degrees.

In a turbofan, as mentioned, some, if not almost all (in the case of the bigger engines) of the flow goes through the bypass. So, while the inner core flow will be hot, after the mixing region, it is relatively mild.

 

[sorter]

If you are just talking exit temperature from the core, they should be roughly the same. If you are talking about average exit temperature, a turbojet would be higher because it doesn't have all that ambient bypass air to mix with.

To understand why we compress air in a jet engine, one must first understand the goal. For an aviation gas turbine, the goal is thrust. The purpose of the turbine stage is to power the fan and the compressor. The thrust comes from moving air through the engine.

Now, we know why the fan is there. It's there to move the air. We get thrust from that. Now, the compressor's main purpose is to assist the turbine in energy extraction. By compressing the air, not only does it assist in combustion, it also helps in energy extraction by the turbine stage(s).

As for the temperatures, a turbojet will have significantly higher higher temperatures, as 100% of the flow goes through the turbine stage. As we mentioned, for a thrust-producing aviation engine, the goal of the turbine is to extract 'just' enough energy to power the fan/compressor stages. So, the flow still has a ton of energy, in the form of velocity and temperature. Exhaust temperatures can be well over 1000 degrees.

In a turbofan, as mentioned, some, if not almost all (in the case of the bigger engines) of the flow goes through the bypass. So, while the inner core flow will be hot, after the mixing region, it is relatively mild.

Thnx for replies.
If such be the case


Why TURBOJETs are preferred over Turbofans in Antiship missiles.(provided turbojets be more prone to IR detection against relatively cold sea)??

With the exception of (as far as I know )
RGM/UGM-109B Tomahawk Anti Ship Missile (TASM) - radar guided anti-shipping variant
Kh-35E Uran (SS-N-25 'Switchblade', 3M24)

& (not yet confirmed)

The YJ-62 is likely to become the primary PLA-N anti-shipping missile of
domestic origin, as it provides a large airframe with considerable long term
growth potential, including significant range improvements with a future
turbofan engine.

http://www.ausairpower.net/DT-Regional-ASCM-1207.pdf

 

[JaredJames]

Why TURBOJETs are preferred over Turbofans in Antiship missiles.(provided turbojets be more prone to IR detection against relatively cold sea)??

Gonna stick my neck out here and say speed.

It's harder to shoot down a fast moving object. I don't know what the latest turbofans are capable of, but these certainly would give you a good attack speed that would make it difficult to shoot down.

 

[dtango]

Thnx for replies.
Why TURBOJETs are preferred over Turbofans in Antiship missiles.(provided turbojets be more prone to IR detection against relatively cold sea)??

1) Turbojets produce more thrust at high mach numbers
2) Turbojets have a higher thrust-to-weight ratio compared to turbofans

These two factors lend to design choices when you want a missile to get to the target as fast as possible. The trade off is range where turbofans give you higher efficiency thus longer range (e.g. like Tomahawks..).

------------------
As to the OP, I think Minger's response is the best answer so far. Essentially the compressor is there to compress the air to increase mass flow. Without the increased mass flow we wouldn't get any meaningful amount of output from the GT to do anything useful with it.

 

[mugaliens]

Gonna stick my neck out here and say speed.

It's harder to shoot down a fast moving object. I don't know what the latest turbofans are capable of, but these certainly would give you a good attack speed that would make it difficult to shoot down.

High sub-mach for high-bypass turbofans, although even the F-22 uses a turbofan engine.

 

[TheFerruccio]

It's more direct than about output or efficiency: if you don't compress the air, it won't move through the engine. The exit velocity of the exhaust gases is a function of pressure and if the combustion chamber had a higher pressure than ambient and there was no compressor, the exhaust gases would go out the back and the front of the engine.

As an addendum showing just how much energy is being directed one way thanks to the compressor, the following video shows exactly what happens when the compressor stops:

http://www.youtube.com/watch?v=Ek6adm4iV4Y#t=0m27s

The ignited gases shooting out the front is a direct result of the compressor no longer holding that stuff back as everything slows down abruptly.

 

[mheslep]

Thnx for replies.
If such be the case Why TURBOJETs are preferred over Turbofans in Antiship missiles.(provided turbojets be more prone to IR detection against relatively cold sea)??

One advantage to jets over fans is a fast response to a power increase. With the fan, response time is limited by the comparatively large angular moment of the fan. I'm told this is occasionally disconcerting to former military pilots who, new to commercial turbo fans, wait a bit over long to increase power when needed.