Diego
Topic Author
Posts: 135
Joined: Mon Apr 23, 2001 7:33 pm

### Axial Flow Compressor

Hi guys I have a doubt that keeps bugging me about the axial flow compressor: is it correct to say that in the axial flow compressor the mechanical energy applied to the associated turbine is firstly converted into kinetic energy under shape of centrifugal force and then into pressure or is there any other reason (ram effect?) while the air is rushed rerward along the longitudinal axis of the compressor? THANKS for the answer

FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

### RE: Axial Flow Compressor

There's not really any centrifugal force involved in the energy transformations, I'd say.

The exhaust gas exerts work on the turbine as it expands. This work is then transferred to the compressor, which converts it back into pressure.

If you view it as kinetic energy being converted into pressure at the compressor end of the gas generator spool while pressure is converted into kinetic energy at the same rate in the turbine end, keeping the kinetic energy (RPM) constant, or as pressure energy being transferred from the exhaust gases to the inlet gases more directly... that's getting close to the chicken or the egg discussion. Or perhaps the one about whether lift comes from downwash or from low pressure...

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.

Panman
Posts: 603
Joined: Mon Aug 09, 1999 8:25 pm

### RE: Axial Flow Compressor

If I understand you correctly you are asking how air is drawn into the engine while the aircraft is stationary?

Remember that each blade on each stage of the axial compressor is basically a very small aerofoil. Aerofoils create lift due to the low pressure area created on top of the wing (obviously while the aerofoil is being propelled through the air). Well on the compressor blades the 'top' surface is facing aft.

So once the compressor starts turning a low pressure area is created and this causes the air to be drawn into the engine.

Once the aircraft is moving ram effect is the main reason air enters the engine.

This is just a very simplistic view of propellor theory going to be revising props this weekend in preparation for my A&C oral exam in Gatwick on Tuesday. Wish me luck.

paNMan

FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

### RE: Axial Flow Compressor

Good luck, Panman. You're lucky this subject came up I think, it seems you have one of the concepts here backwards (pun unintentional).

Yes, each blade is an airfoil. However, what would be the top surface if it was a wing is facing front. That is just what is needed to get air moving into the engine. Think of the downwash generated by a wing, or even better that of a helicopter rotor, and you'll be able to see it.

It's the same for a propeller, BTW.

What's A&C? And hey, as you have an exam coming up, please to drop me a line if I added any confusion.

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.

Panman
Posts: 603
Joined: Mon Aug 09, 1999 8:25 pm

### RE: Axial Flow Compressor

FredT thanks for the correction. Just checked my notes and yes this is so.

Panman

Yikes!
Posts: 284
Joined: Sat Oct 13, 2001 4:51 pm

### RE: Axial Flow Compressor

FredT. Spot on.

wingscrubber
Posts: 809
Joined: Fri Sep 07, 2001 1:38 am

### RE: Axial Flow Compressor

The turbine is driven by the combusted gasses expanding rearwards out the exhaust, the energy applied to the turbine is transferred by the shaft to the compressor. So in energy transfer terms, it's pressure-kinetic-pressure. The compressor is driven by the turbine, the compressor itself doesn't really suck or blow air to any extent, it just compresses it, airliners use turbofan engines, the initial airflow is generated by the fan, and with turbojets (a jet without the fan stage) the first few rotor stages of the compressor will act as fans.

Pete
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