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Axial Flow Compressor  
User currently offlineDiego From Italy, joined Apr 2001, 135 posts, RR: 0
Posted (12 years 7 months 3 weeks 2 days 8 hours ago) and read 1453 times:

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

6 replies: All unread, jump to last
 
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 1, posted (12 years 7 months 3 weeks 2 days 7 hours ago) and read 1407 times:

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...  Smile

Cheers,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlinePanman From Trinidad and Tobago, joined Aug 1999, 790 posts, RR: 0
Reply 2, posted (12 years 7 months 3 weeks 2 days 7 hours ago) and read 1388 times:

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


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 3, posted (12 years 7 months 3 weeks 2 days 5 hours ago) and read 1375 times:

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

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.  Big grin

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.
User currently offlinePanman From Trinidad and Tobago, joined Aug 1999, 790 posts, RR: 0
Reply 4, posted (12 years 7 months 3 weeks 1 day 23 hours ago) and read 1364 times:

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

Panman


User currently offlineYikes! From Canada, joined Oct 2001, 284 posts, RR: 1
Reply 5, posted (12 years 7 months 3 weeks 1 day 18 hours ago) and read 1344 times:

FredT. Spot on.



User currently offlineWingscrubber From UK - England, joined Sep 2001, 852 posts, RR: 0
Reply 6, posted (12 years 7 months 2 weeks 6 days 16 hours ago) and read 1316 times:

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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