What limits the minimum RPM for a jet engine?

[JoeFrickinFriday]

Gas turbine engines have maximum speed and power limits based on pressures, temperatures and mechanical loads on the parts spinning at ludicrous speed.

So...what parameters determine the MINImum RPM for a gas turbine? When that 747 is waiting for takeoff, all four engines are "idling," putting out the least thrust (and consuming the least fuel) possible. What exactly happens if they try to spin the engines at an even lower RPM?

 

[CH5OH]

you probably hit the first critical speed...
it keeps the maintenance technicians happy

 

[rb1957]

a gas turbine has a HP turbine disc which extracts power for the combustion gases and drive the compressor discs that feed the combustion chamber with compressed air that combust with the jet fuel to drive the HP turbine ... and so on.

Sooo, at idle you're still running the engine at some "ludicrous" speed, but somewhat less "ludicrous" than at take-off (which is when the poor things have to work the hardest, in-flight they're typically <50% max power), in order to sufficiently compress the ambient air; though they'd throttle back on the fuel flow since they don't need the power.

 

[byrdj]

when torque required to rotate the compressor stages exceed torque produced by the power stage you will stall.
when the air velocity in the burner can not support combustion, you'll have flame out

 

[berkshire]

In addition to byrdj's comments, if you idle a gas turbine engine too low, you will get a phenomenon called hanging where the turbine will run but not accelerate when additional fuel is added.
Most turbine Manufacturers will set the governors so that this condition is not encountered.
B.E.

 

[ajack1]

Volcanic ash seems to do the trick.

 

[Yagonyonok]

I'm not an aerospace guy, so take this for what its worth, but here is a video clip that shows what the others are talking about.

http://www.engineeringmotion.com/videos/334/how-a-jet-engine-works

Engineering is the art of modelling materials we do not wholly understand, into shapes we cannot precisely analyse so as to withstand forces we cannot properly assess, in such a way that the public has no reason to suspect the extent of our ignorance.
-A R Dykes

 

[JoeFrickinFriday]

>when torque required to rotate the compressor stages
>exceed torque produced by the power stage you will
>stall. when the air velocity in the burner can not
>support combustion, you'll have flame out

Makes sense in a "logic" kind of way. Presumably stable operation for a given fuel rate is defined by a kind of negative-feedback loop:

-if RPM rises due to disturbance, less torque is delivered to turbine, and compressor is demanding more torque; RPM settles back down to original level.

-if RPM dips due to disturbance, more torque is delivered to turbine, and compressor is demanding less torque; RPM rises back up to original level.

At extremely low RPM, does this negative-feedback loop get replaced by positive feedback? That is, reduced RPM leads to either reduced torque on the turbine and/or increased torque requirement at the compressor, resulting in a runaway decrease of RPM?

 

[berkshire]

Joe,
This may help.

http://web.mit.edu/16.unified/

http://www/FALL/thermodynamics/notes/node28.html

B.E.