Why do Jet Engines Have a Compressor?


by Saladsamurai
Tags: compressor, engines
Saladsamurai
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#19
Oct23-08, 11:05 PM
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Hmmm.. haven't logged in in awhile. Had no idea this thread was generating so many replies! Great stuff guys! This is all great. I am taking a thermodynamics class and so far we are not too far into the text. So the problems we have had have merely told us that there is a compressor before the combustion chamber, but not WHY.

Thanks!!
Phrak
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#20
Oct25-08, 01:55 AM
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Quote Quote by Phrak View Post
A ratio of one gives zero, which makes no sense, or course.
Of course it makes sense, Prank. If the volume doesn't change it does no work.
Phrak
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#21
Oct25-08, 02:10 AM
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Nice graphs, Fred.

This equation has power in the numerator and energy in the denominator--it should be unitless.

[tex]\eta_p=\frac{T*u}{m\left[(1+f)(u_e^2/2)-u^2 /2\right]}[/tex]
zee24
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#22
Nov17-08, 04:10 AM
P: 1
did you know the formula PV=nRT, which n,R is constant and V(volume) we can say that constant too. so the temperature(T) based on pressure(P) and P=F/A, if we want to increase the thrust(F) we just have to decrease the area(A) or increase the pressure(P).
in ramjet case we just need incerase the volume of air and burn that to increase the thrust. thats why ramjet only effective in supersonic speed.
am i right?
redargon
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#23
Nov17-08, 06:09 AM
P: 348
not sure exactly how to answer your question, "am i right?", but I could point some things out.

1. PV=nRT is the ideal gas law and requires ideal gasses for it to be accurate, I think you can substitute in some conversion factors to apply it to certain gasses under certain conditions, but as far as I know, a fuel air mixture is not an ideal gas and shouldn't be modelled as such.
2. P=F/A, ie. F=PA, therefore to increase F you can either increase the A or increase the P or both, but I'm not quite sure how this relates to jet engines.
3. I'm not sure how you come to the ramjet conclusion. Ramjets can use the shockwave generated by supersonic flow to create the compression required for efficient combustion instead of using a mechanical compressor, but I'm not 100% sure you need to be going supersonic to use one. You could design it to have subsonic flow at the inlet and then cause it to go supersonic by using applicable geometries, but then choking could affect you mass flow rate, and as astronuc said earlier, having a large mass flow rate is part of the thrust generation.

Hope that helps.
deepthishan
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#24
Dec23-08, 08:58 PM
P: 38
if you think about it, if you compress the air entering, more volume can be burnt in the combustion chamber every minute
prost22
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#25
Mar19-09, 03:02 PM
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Isn't the answer as simple as this?
The compressor is needed to get you off the ground (ramjets are essentially turbojets with no compressor; you can't take off from the ground with a ramjet, without a booster of some sort). A jet engine is essentially a controlled explosion--if you had no compressor, and you were sitting in the ground, all you'd have is a fire in the combustor (the combustor injects fuel into the air stream, which starts on fire, heats up, goes through the turbine and out the back, through the nozzle); you'd get some 'thrust,' as the really hot air went through the turbine and through the back, but not enough to get you off the ground. Now, compress that air 20 times its original pressure, inject some fuel, and light it--voila! explosion! Air rushes out the back of the combustor through the turbine, turning the turbine, (which gives you enough energy to compress the air), the air blows out back through the nozzle.

deepthishan; since the combustion chamber has constant volume, I think you might have meant 'more mass, m' or 'more mass flow, m-dot' ("m" with a small dot on top of it, signifying dm/dt).
rbeale98
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#26
Mar24-09, 09:44 PM
P: 52
Quote Quote by prost22 View Post
Isn't the answer as simple as this?
The compressor is needed to get you off the ground (ramjets are essentially turbojets with no compressor; you can't take off from the ground with a ramjet, without a booster of some sort). A jet engine is essentially a controlled explosion--if you had no compressor, and you were sitting in the ground, all you'd have is a fire in the combustor (the combustor injects fuel into the air stream, which starts on fire, heats up, goes through the turbine and out the back, through the nozzle); you'd get some 'thrust,' as the really hot air went through the turbine and through the back, but not enough to get you off the ground. Now, compress that air 20 times its original pressure, inject some fuel, and light it--voila! explosion! Air rushes out the back of the combustor through the turbine, turning the turbine, (which gives you enough energy to compress the air), the air blows out back through the nozzle.

deepthishan; since the combustion chamber has constant volume, I think you might have meant 'more mass, m' or 'more mass flow, m-dot' ("m" with a small dot on top of it, signifying dm/dt).

in the case of sitting on the ground with no compressor, the burning fuel/air would have no preferred direction to flow. I agree with your general point however that the basic purpose of the compressor is to make the air more explosive. The amount of energy spent driving the compressor is well worth the energy you get by blowing up a stream of compressed air mixed with fuel.

Another important point is that the compressor slows down the incoming flow as it compresses. this makes it easier to hold the flame. an earlier poster said theres no reason why uncompressed air would not burn. true, but uncompressed air would be fast enough to blow the flame out the back.
Cyrus
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#27
Mar24-09, 09:50 PM
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Quote Quote by rbeale98 View Post
in the case of sitting on the ground with no compressor, the burning fuel/air would have no preferred direction to flow. I agree with your general point however that the basic purpose of the compressor is to make the air more explosive. The amount of energy spent driving the compressor is well worth the energy you get by blowing up a stream of compressed air mixed with fuel.

Another important point is that the compressor slows down the incoming flow as it compresses. this makes it easier to hold the flame. an earlier poster said theres no reason why uncompressed air would not burn. true, but uncompressed air would be fast enough to blow the flame out the back.
It also makes more drag than the entire airframe at top speeds.


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