Turbofan tip speed 1

[MichaelkSA]

Good day all.

I know that with all prop-driven airplanes (whether it be piston or turbo-prop) you want the tip speed of the prop to be just fast enough to avoid the airflow over the prop of becoming supersonic in order to avoid the negative effects assosiated with supersonic flow.

What i want to know is what speeds the airflow over the fan of large turbofan engines can reach? Surely they are way above supersonic? And how are the advantages of these high speed flows justified compared to the wave-drag penalties?

Thank you for any response.

 

[btrueblood]

The intakes slow and compress the air, heating it...net result is the blades remain in subsonic flow. Realize that the fan in high-bypass engines does not turn at the same rpm as the core compressor/turbine - either the fan is driven by a seperate turbine (multi-spool designs) or by a gear-reduction drive.

There are designs for supersonic unducted fans (propfans), with the fan blades swept to reduce high mach effects. Mostly never used, due to high noise levels from the fans.

 

[MichaelkSA]

Thank you very much btrueblood, it makes complete sense.

The higher the temp, the higher M.

Any idea what temps are actually reached around the fan? Obviously very high temps are reached in the high pressure compressor and further downstream in the combustion chamber and turbines.



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[KENAT]

Hesitant to disagree with Btrue & the relevant brain cells must be rusty but I thought the tip speeds on high bypass fans are sometimes supersonic, something about the interaction with the walls of the 'duct' allow them to get away with it but I can't recall details.

I seem to remember that this was how the Harrier got away with simple intakes.

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[btrueblood]

The comment in that other thread was in regards to takeoff - max rpm, low altitude and (most important) near zero airspeed, so the inlet compression is minimal. Net effect there is lower temperatures, and tip speeds approaching Mach 1. I think, but am not certain, that the same thing applies to the Trent engine, i.e. the supersonic condition occurs during hover/low speed flight. But as a general rule, avoid operating above Mach 1 or the losses kill the efficiency.

 

[rb1957]

i thought there were some "tricks" too (to allow them to exceed sonic speed) ... the nacelle wall (creating a low flow area at the tip), supercritical airfoil for the fan.

but engines are truly a complicated piece of design ... produce max thrust at take-off (flow into the compressor is mostly tangential, at SL), produce max efficiency at cruise (flow into the compressor is mostly axial, at 36,000'), etc, etc, etc ... and then "oh yes, and the thing has to be able to swallow a bird"

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[GregLocock]

I found this,

http://www.pprune.org/questions/204270-q-turbofan-rpm-n1-100-cfm56-5b4.html

it says supersonic for an airbus

"Fan diameter is 67" or 5.6 feet. The state of Arkansas has legislated that pi=3, so I'll use that to find the circumference is 16.8 feet. And the blade tips travel 16.8 feet every revolution.

Speaking of revolutions, if we do 5200 in a minute, that's 83 in a second.

So - 16.8 feet in revolution, and 83 revolutions in a second, so it's about 1400 feet per second, without even considering the vector addition of incoming flow.

Sound travels 1100 feet per second on a standard day, so yes, the tips are supersonic. About 1.3 times the speed of sound."

Both noise and efficiency make me wonder if that can be true.

Cheers

Greg Locock


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[MichaelkSA]

Hi Greg.

I don't think the calc is so simple. (see btruebloods comment).

At standard atmospheric conditions the speed of sound in air might be about 1100 ft/s, but remember that as the air is drawn into the nacelle, it's speed is drastically reduced and it's temperature will be increased, thus also increasing the speed of sound in that medium.

However I remember flying in a B737 the other day and the pilot said we're at 38 000 feet and the outside air temp is -50 degrees C, so for the speed of sound to be higher than 1400ft/s, which it will need to be to remain sub-sonic, the air temp around the fan has to be higher than 180 degrees C, so that amounts to an increase of 230 degrees C, which sounds a bit steep, although I am not sure.




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[GregLocock]

"To optimize the cycle the
bypass flow has to be raised to a pressure of approximately 1.6 times the
inlet pressure. This is achieved in the fan by utilizing very high tip
speeds (1500 ft. per sec.) and airflow such that the bypass section of the
blades operate with a supersonic inlet air velocity of up to Mach 1.5 at the
tip" Quoted from "The Jet Engine" by Rolls Royce plc 5th edition 1996 page
26.



Cheers

Greg Locock


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[rb1957]

@greg ... 5000 rpm for the low pressure compressor (fan) feels a bit high

@michael ... speed of sound falls with altitude, refer "

http://www.aerospaceweb.org/question/atmosphere/q0112.shtml"

also, i agree inlet condition aren't the same as free stream; mind you greg was only calc'ing the tangental velocity component.

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[GregLocock]

But hang on, Rolls Royce are saying supersonic, admittedly not in a technical publication.

One huge argument against the flow being locally supersonic is that the radial location along the blade length where M=1 would move as N and other things change. Vastly different airfoil shapes are required below and above M=1 for efficient operation.

So I gather we have no turbofan designers here?





Cheers

Greg Locock


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[btrueblood]

Not a turbofan designer, just learned to do the analysis in university, and know lots of other people who work on the various parts. Does anybody do "engine design" - usually it's a whole department.

I hate to admit being wrong, but I like learning something new. Kenat's post prompted me to think twice, so I had a chat with an old colleague (he is ex-P&W) the other day (one more benefit of EngTips questions like this, it prompts me to pick up the phone and look up an old friend) who says yes, the newer fans with back-swept blade tips do operate with blade tip entrance relative Mach no.'s about 1.3 to 1.5 (the flow is slowed to subsonic speeds at the fan stators). The thrust efficiency of the fans takes a hit (i.e. are worse than fans running with subsonic tips), but the core flow for the engine also comes from center of the fan, thus there is a trade off - the core pressure ratio sets the thermal efficiency for the core, so you are trading one efficiency against the other. Running the numbers (and getting really well designed, very stiff but lightweight blade materials that won't flutter in the backswept configuration) shows that you need to run the fan at a high enough speed to get a useful pressure rise on the core flow, and that trading this for some loss of bypass flow thrust still gets you a better overall thrust efficiency, because you can process so much more airflow through the fan than for the smaller diameter subsonic fan (in general, the more bypass you can get the better the net thrust efficiency for simple 1-D trade analyses on turbofans). However, there are newer designs, with planetary reduction gearing for the fan (i.e. the 1st stage compressor wheel can run at high speed, and drive an outer spool wheel for the fan at its own optimal lower speed), which compete well in smaller sizes where the added gear weight penalty isn't as high.

So, yes, the fans on some (all?) of the latest, biggest turbofans, do operate at supersonic tip speed.

 

[MichaelkSA]

And there we have it.

Thanks btrueblood.

Isn't it amazing what a wealth of information we have at our fingertips with the internet and forums such as this.

Cheers

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[BernardRhodes]

Further to GregLocock's quote from 'The Jet Engine':

(i) These numbers (1500 ft/sec, 1.5 relative Mach number) are correct (regardless of whether or not the book is regarded as a technical publication).

(ii) They have applied for the last forty years, since the first RB211s. They are broadly typical of all large turbofans (CF6 etc., JT9D etc.)

Only the very latest engines (with bypass ratios much above 5 and hence lower-than-previously bypass pressure ratios) use lower tip speeds