Graeme Marion

I have read of small turbines exceeding 160,000rpm.
Larger turbines are sometimes quoted at 120,000rpm. plus.

Is it possible that some of the internal parts are going beyond the speed of sound?

Has anyone done the maths? Just wondering
Regards from Sunny Queensland.[8D]

Wayne22

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I happen to have a 54mm compressor nearby, so I figured it would be spinning at 453m per second at the tips (at 160,000rpm) .. The speed of sound is approximately 350 mps..if my figures are right that looks like mach 1.25 ........

sonic bang

nony, you are right. It does go above mach 1 but just to be precise, speed of sound is approx. 331.46 mps and mach number for 54mm compressor comes out to approx mach 1.365

BMT

Mach nrs are dependant on air density. If you knew the air density at any given point of the compressor only then can you calculate the mach nr. Good turbine design means not going supersonic. You will get massive choking and stall.
Andre Baird

Miniflyer

BMT, that is incorrect. Mach number is absolutely only dependant on air temperature. The higher the temp, the higher the mach number.
At EGT temperatures you will not have supersonic tips. (Not at the compressor either, the compressed air exits the compressor at roughly 100°-120° C, rising the mach number enough not to penetrate the sound barrier...)

At 600 degrees Celcius Mach 1 will be reached at 592.8 m/s

BMT

You are assuming a single state gas. Air density is temperature dependant and air itself changes composition day to day and from one altitude to another. My definition of air density includes the concept of a variable gas and temperature. For this reason your assumption cannot work for a turbine wheel which has another gas as medium. In the textbook sense where the gas is a constant you can reduce the effect of mach number to temperature alone.
http://www.grc.nasa.gov/WWW/K-12/airplane/sound.html

Andre Baird

Miniflyer

BMT, at the point of the turbine wheel, we are encountering a single state gas. That gas is regular "atmospheric" air, superheatet to around 700 to 800 deg C at the point where it hits the wheel.
Mach numbers change with the medium and temperature solemly, not the specific density of the medium. So if you were to change to water, or another medium, you would notice a change in speed of sound. Not if you heat air and accellerate it to around 800m/s...

As your own link clearly states:

BMT

I suggest you study the differences in the medium. The gas at a turbine wheel is considerably different. Unless you have the perfect "green" turbine the others consume hydrocarbons and combine with oxygen to form amongs others, CO2.
Air density is not a constant but can be measured. Reading my first post and then using some lateral thinking you will realize to know air density you have to know its exact composition, pressure and TEMPERATURE. Then you can calculate the mach numer once you have all the information. Just knowing the temperature will never be enough.
Most might say this is splitting hairs but since you insist on textbook bound facts you may want to go to the next level of textbook. This may take you some time so further explanations I will leave for that time.

Miniflyer

I am not going strictly textbook. Of course "air" changes in the chamber, but only a small part (remember 80% nitrogen that hardly changes at all, only a fraction turned into NO or NO2). The remaining 20% will change somewhat, however the difference in CO2 and pure O2 concerning speed of sound difference is fractional.
For in-the-ballpark figures it is therefore safe to presume that the gas constant does not change in the burning process. Remaining variable remains temperature.
(While you now are also bringin pressure into the game: compressor exit and nvg exip pressure is at or near atmospheric. We are talking high-velocity, low pressure gas streams here. Pressure builds up in or after the diffusor, and rapidly falls in the accelleration process. While case pressure will be around 3 bar with "slow" moving air, entry and exit pressures are somewhere close to 1 bar, thereby falling out of your calculation also.).

Yes, i do consider this splitting hairs, because no matter how you turn it, your speed of sound at the turbine wheel will be around 600m/s (plus minus whatever), no matter what other factors you want to bring in (humidity, weather, battery level or the mood of the wife).

Can we at least agree on that the turbine wheel cannot break the sound barrier because else boundary layer seperation would occur, thus "stalling" the turbine wheel?

Red B.

Hi all!
Is this a five minute argument or is it the full half hour ? :-)

Miniflyer wrote:
In real life the simple Laplace formula for the speed of sound in gases (speed of sound is proportional to the square root of the absolute temperature) need to be corrected to account for three real-gas effects: molecular degrees of freedom which in equilibrium are not fully excited, deviations from the ideal-gas law, and dispersion due to relaxation processes. These are called the specific-heat, virial, and relaxation corrections, respectively. Thus the composition of the medium has an effect.

Miniflyer is correct about supersonic flow inside a gas turbine:
Compressor fans will not work well on supersonic air. Therefore, for a supersonic aircraft, the airflow must be slowed to subsonic speed before it enters the compressor section of the engine. To do this the air inlet has often been designed to incorporate a simple convergent divergent diffuser (there are other solutions as well).
The principle is simple: A supersonic flow will slow down as it enters a constricted area. (N.B., this is the opposite of the response of a subsonic flow entering a constricted area!).
If the convergent part of the diffuser is designed to slow the airflow down to slightly below subsonic, the flow will then slow down even more as it moves through the divergent section of the inlet before finally entering the engine.

/Red B.

Gazzer

Without butting in with ignornant comments what a lot of knowledge and stuff I have learnt in a few moments

But......

Was not the original question posed about the tip speed of the compressor, meaning that it travels at a speed tha equates to or at least in comparative terms, a speed faster than the speed of sound.....

I got the feeling it does!!!!

However, I am beginning to understand why there is such a measurement called mach........

BMT

Supersonic air is a bad thing but apparently can be done or at least have been attempted in the region of compressor to diffusor interface according to advanced research that I dont have any insight into. For the regular gas turbine is is a no-no, so yes I agree.
Its not really an argument but when you define anything to be a hard and fast fact I am reminded of too many projects that failed due to not thinking beyond the textbook. I certainly do not pretend to know all there is to know .
Andre Baird

Gazzer

My ignorance would then wonder how fast military jets perform in air digestion when going supersonic.

Again, through ignorance I am assuming that the air gets buffered up the "Ram" effect which slows it down prior to reaching the compressor.

The reason it is bad I wonder, is it just so turbulent and random. Thinking further this is also the reason for all flying tailplanes on supersonic jets, i.e. the air is bad and unstable.

Or have I just wandered pointlessly into the lions lair!!!!

BMT engines are rather good so you must have at the very least quite adequate knowledge André!!!!

Gazzer

Gordon Mc

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Well Andre, maybe you don't quite know everything there is to know, but in my book you & Bob Parks are right up there on the same level of the pedestall ... I have yet to encounter anyone with a greater combination of knowledge and humility as you guys. It's quite often the really quiet guys that we need to listen to the most.

Gordon

TREADSTONE.

Correct ... The tip speeds of those scimitar shaped Rolls-Royce T900 (A380 Airbus engine) fan blades is over 1.5 times the speed of sound.

Miniflyer

Red B., i did not say this calculation produces on the spot results, all i was saying is that for these purposes, the ideal gas theorem puts us in the ballpark and sets us a good idea what we are dealing with here. Of course there are factors playing into all this, but their effects do not throw off that theorem by far. Serious engineers at GE or RR will be going a lot more into the fluid flow theory because they have to produce highly efficient designs. For simple proof of supersonic or subsonic flow, this is more than close enough

I fully second you on your explanation on the intake design and function.


Gazzer, the reason for flying tailplanes on supersonic designs is the shockwave separation on these surfaces. Regular tail surfaces become ineffective and "blanked out".
Neat story to this is the X-1 Testflight program, in which elevator authority was lost on breaking the sound barrier. Mr. Yeager found out that he could remain in control when flying with the trim, as this moved the entire surface. The plane was then modified to full flying surface. Funny how some small ideas make so much possible....

Red B.

Miniflyer wrote:
I agree with you on that point :-)

/Red B.

Gazzer

Thanks guys,

Again I have learnt something and explained so simply.

This site is good isn't it!!!!!

GAzzer

BMT

Nice to learn from people in the know. I have been fortunate to know a lot of people that know one hell of a lot more than I do.
Gordon,
But to give credit where credit is due, its the hobby of aeromodelling that taught me the most about stuff. Model or not its a challenge to understand the principles of flight, the electronics, the composites structure all that stuff.
Its a great hobby!
Andre

Rupurt

Andre, are you selling parts at the moment.

Graeme Marion

Then the answer is yes?

Miniflyer

The answer is no in the case of our turbines.
About the fan tip speed of the T900: i'm not sure, i'll do some asking....if they are designed for supersonic airflow then this might be the case, however they would be less than efficient at anything below mach1, so i seriously doubt that since cruise power is usually quite a bit less than takeoff power (and inner fan would be spinning below sos, even at high power). Fan RPM on airline engines is usually pretty low anyway, and at that diameter such a high-revving fan would not be needed to produve the thrust that is achieved anyway...
I'll see if i can dig up some facts and numbers

S_Ellzey

Thought I might toss my 2 cents worth into the discussion.

Axial flow compressors and prop blades have a different concern as far as going supersonic than does a centrifugal compressor. If a blade, be it a prop or an axial compressor gets to fast you run into all the compressibility problems (shocks and the like), but for these cases you have a blade moving through the air. For a centrifugal compressor (like our models have) the air goes through the blades. The eye of the compressor is too small to reach sonic speeds. The velocity though the remainder of the compressor, by design, will not go sonic relative to the wheel itself. So, would it even matter if the tangential velocity of the wheel where to exceed the local speed of sound, since the motion of the air and the wheel relative to each other is very much subsonic?

Steven

bcovish

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Huh[sm=confused.gif][sm=confused.gif]

seanreit

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Bob, just push the stick forward, listen for the pop, hear the spool up, bring back to idle, taxi out and go fly! The only supersonic we need to worry about is how fast I can get you a beer if you ever ask for one again

LOL, can't wait to catch up with you guys at Austin, I have a new top secret project I'm working on Don't know if it will be ready, but it's gonna be FAST!