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Faro
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Where Is The Thrust Applied Inside An Engine?

Mon May 11, 2009 4:32 pm

Apart from the fan, what parts of a turbofan engine bear the greatest forward force on them that generates thrust? Is there as much forward force bearing on the compressor stages as there is on the turbines? And what about afterburners, where does the excess thrust they produce apply itself on in the engine's insides?

Faro
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RE: Where Is The Thrust Applied Inside An Engine?

Mon May 11, 2009 5:34 pm



Quoting Faro (Thread starter):
Apart from the fan, what parts of a turbofan engine bear the greatest forward force on them that generates thrust?

In a high by-pass fan engine something like 80% or more comes from the by-pass air. The other remaining 20% or so is exhaust....
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tdscanuck
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RE: Where Is The Thrust Applied Inside An Engine?

Tue May 12, 2009 4:35 am



Quoting Faro (Thread starter):
Apart from the fan, what parts of a turbofan engine bear the greatest forward force on them that generates thrust?

Next biggest after the fan is probably the inside surface of the nozzle, for most engines.

Quoting Faro (Thread starter):
Is there as much forward force bearing on the compressor stages as there is on the turbines?

There should be more forward force on the compressor than rearward force on the turbine, since the pressure drop across the turbine is usually lower than the rise across the compressor. The compressors, to first order, can be thought of as small fans.

Quoting Faro (Thread starter):
And what about afterburners, where does the excess thrust they produce apply itself on in the engine's insides?

Primarily through the compressor stages.

Tom.
 
JoeCanuck
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RE: Where Is The Thrust Applied Inside An Engine?

Tue May 12, 2009 5:31 am

I always liked the high school balloon theory of thrust. Equal pressure on all walls in a sealed balloon. When the air is released, the side wall pressures balance out and the higher pressure on the front/top of the balloon is higher than the pressure of the opening and it flies the direction opposite the opening.

I've never thought about what part of a jet engine would equal the front of the balloon.
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Faro
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RE: Where Is The Thrust Applied Inside An Engine?

Tue May 12, 2009 6:39 pm



Quoting Tdscanuck (Reply 2):
Next biggest after the fan is probably the inside surface of the nozzle, for most engines.

But the nozzle has no surface on which to have thrust "applied", its walls are substantially parallel to the exhaust gases. There are the flameholders in afterburning engines of course but those look seem too flimsy to support the thrust application and conduct it to the casing.

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JoeCanuck
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RE: Where Is The Thrust Applied Inside An Engine?

Tue May 12, 2009 7:08 pm

To make it more confusing, how does the process work with a ramjet, where there are no moving parts and nothing solid to push against?
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RE: Where Is The Thrust Applied Inside An Engine?

Tue May 12, 2009 9:39 pm



Quoting JoeCanuck (Reply 5):
To make it more confusing, how does the process work with a ramjet, where there are no moving parts and nothing solid to push against?

jets/Ramjets/scramjets/rockets/etc all push against nozzles at one point or another too:

http://www.grc.nasa.gov/WWW/K-12/airplane/Images/ramth.gif


And a bit of trivia: jet engines also produce reverse thrust inside the core itself even with the thrust reversers off, but the total forward net thrust component far overcomes the reverse thrust. I just don't remember which sections of the engine produce that small internal reverse thrust.
 
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RE: Where Is The Thrust Applied Inside An Engine?

Wed May 13, 2009 2:56 am



Quoting Faro (Reply 4):
But the nozzle has no surface on which to have thrust "applied", its walls are substantially parallel to the exhaust gases.



Quoting JoeCanuck (Reply 5):

The thrust force actually comes from the acceleration of the exhaust gases in the nozzle. This acceleration requires a force to achieve, which for a flowing gas, manifests as a pressure drop along the nozzle.

Quoting FLY2HMO (Reply 6):
I just don't remember which sections of the engine produce that small internal reverse thrust.

I'd say you are thinking of the primary zone in the combustion chamber. This zone features a strongly re-circulating flow, which acts to stabilise the flame, and prevent it from extiguishing



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Fly2HMO
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RE: Where Is The Thrust Applied Inside An Engine?

Wed May 13, 2009 5:21 am



Quoting JetMech (Reply 7):
I'd say you are thinking of the primary zone in the combustion chamber. This zone features a strongly re-circulating flow, which acts to stabilise the flame, and prevent it from extiguishing

Hmm not quite. What I was remembering was seeing a diagram in my engines class with vectors over each different section of the engine (compressor, combustor, turbine, nozzle, IIRC) showing in which direction the thrust from those vectors was "pushing" and with what magnitude (lbs). I went through both of my RR Jet Engine books and cant seem to find a similar diagram.
 
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RE: Where Is The Thrust Applied Inside An Engine?

Wed May 13, 2009 5:35 am



Quoting JetMech (Reply 7):
The thrust force actually comes from the acceleration of the exhaust gases in the nozzle. This acceleration requires a force to achieve, which for a flowing gas, manifests as a pressure drop along the nozzle.

What he was struggling with is that this force must be reacted somewhere by the physical structure of the engine, in order to be transmitted to the airframe. The question was where is it reacted?  Big grin
 
JoeCanuck
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RE: Where Is The Thrust Applied Inside An Engine?

Wed May 13, 2009 6:00 am

The expanding gases will take the route of least resistance out of the engine. At the front, highly compressed air is preventing the exit that way so it goes out the back.
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RE: Where Is The Thrust Applied Inside An Engine?

Wed May 13, 2009 6:39 am



Quoting WingedMigrator (Reply 9):
The question was where is it reacted?

Well that would be against the bearings and the engine torque mounts, right?
 
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RE: Where Is The Thrust Applied Inside An Engine?

Wed May 13, 2009 7:47 am



Quoting Faro (Reply 4):
But the nozzle has no surface on which to have thrust "applied", its walls are substantially parallel to the exhaust gases.

Although the nozzle walls are parallel to the exhaust gas path, they're not generally parallel to the freestream. That means the pressure acting on the nozzle walls has a fore/aft component...thrust.

Quoting Faro (Reply 4):
There are the flameholders in afterburning engines of course but those look seem too flimsy to support the thrust application and conduct it to the casing.

Flameholders actually are a drag (they want to go out the back of the engine). The afterburner works by increasing mass flow through the engine...all the force is picked up by the same bits of the engine as it always was.

Quoting JoeCanuck (Reply 5):
To make it more confusing, how does the process work with a ramjet, where there are no moving parts and nothing solid to push against?

In the simplest ramjets (an inlet followed by a straight-duct burner), the thrust is actually picked up in the diffuser (inlet)...this seems tremendously weird, but it's true. I can dig up the pictures if anyone would like. Even if you put in a convergent nozzle, you still get all your thrust from the inlet. It's only if you install a convergent/divergent nozzle that you can get any thrust from the nozzle.

Quoting FLY2HMO (Reply 8):
What I was remembering was seeing a diagram in my engines class with vectors over each different section of the engine (compressor, combustor, turbine, nozzle, IIRC) showing in which direction the thrust from those vectors was "pushing" and with what magnitude (lbs).

You may be thinking of the turbine...it wants to go backwards out of the engine, and is hence pulling back on the entire engine (drag).

Quoting WingedMigrator (Reply 9):

What he was struggling with is that this force must be reacted somewhere by the physical structure of the engine, in order to be transmitted to the airframe. The question was where is it reacted?

It goes form the fluid to the engine via pressure against the internal surfaces, especially those that aren't parallel to the freestream. From the rotating bits it starts as pressure on the blades and moves as stress in the blade to the hub to the shaft to the bearings to the engine case to the engine mount. For the non-rotating bits, it goes from the blades straight to the engine case, then to the mounts.

Tom.
 
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Faro
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RE: Where Is The Thrust Applied Inside An Engine?

Wed May 13, 2009 10:00 am

Quoting Tdscanuck (Reply 12):
Although the nozzle walls are parallel to the exhaust gas path, they're not generally parallel to the freestream. That means the pressure acting on the nozzle walls has a fore/aft component...thrust.

What is the freestream as opposed to exhaust gas path?

Also, with a convergent/divergent nozzle, thrust cannot be applied when the nozzle is fully closed since there is no surface component facing the back on which the exhaust gas stream thrust can "push", is there?.

Faro

[Edited 2009-05-13 03:09:15]

[Edited 2009-05-13 03:09:44]
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tdscanuck
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RE: Where Is The Thrust Applied Inside An Engine?

Thu May 14, 2009 2:47 am



Quoting Faro (Reply 13):

What is the freestream as opposed to exhaust gas path?

Freestream is the outside air...it's (usually) parallel to the axis of the engine. The gas path may be quite bent relative to the freestream, like in the back end of most turbofans...it's got some significant radial component.

Quoting Faro (Reply 13):
Also, with a convergent/divergent nozzle, thrust cannot be applied when the nozzle is fully closed since there is no surface component facing the back on which the exhaust gas stream thrust can "push", is there?.

I think you're describing a simple convergent nozzle. In that case, you're right, the nozzle itself isn't contributing thrust (it's actually contributing drag). A full con/di nozzle is one that necks down then opens up again. You see this in the most extreme case in rockets, but it's present in any jet that's got a sonic or supersonic exhaust (most fighter engines and practical ramjets). It's sometimes very difficult to see on fighter jets at afterburner because the expansion/contraction, relative to the overall duct diameter, is pretty small.

Tom.

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