How Do Compressor Blades Exceed the Speed of Sound Without Loss of Performance?

[Cliff_J]

Ok, so a prop is an airfoil that spins and as such is subject to limitations that severly hamper performance as it approaches the speed of sound correct?

But how do spinning compressor wheels like in a jet engine or a turbocharger not succumb to the same effects even when their blades would be slicing through the air at a rate faster than the speed of sound?

 

[enigma]

The airflow through the compressors in jet engines is subsonic.

There is a normal shock wave before the inlet to the engine which slows the flow down to subsonic speeds. The flow is then expanded in a diffuser slowing it down even more. It then enters the compressor.

SCRAMJets (Supersonic combusting ramjets), engines which transmit supersonic flow through the combustion chamber, have no compressors.

 

[Clausius2]

Ok, so a prop is an airfoil that spins and as such is subject to limitations that severly hamper performance as it approaches the speed of sound correct?

But how do spinning compressor wheels like in a jet engine or a turbocharger not succumb to the same effects even when their blades would be slicing through the air at a rate faster than the speed of sound?

Always I have calculated a turbocharger there is always the caution for not spinning faster than the sound speed. You have to look at blade radius $$R$$ in such a way that $$\omega R < c$$ where $$c$$ is the sound speed and $$\omega$$ angular velocity. Balancing the angular velocity and dimensions to not surpass this limit has been always for me a design constraint.

About jet engines, well I don't know very much of it. I read once that there are some engines which assumes it is going to be an slight supersonic flow at the compressor intake. Such losses of pressure and energy are then pre-assumed and blades are previously designed to deal with them. Also there are some turbines and compressors prepared to deal with supersonic flow. They take advantage of the theory of convergent-divergent nozzles, and their blade set is designed to behave like these nozzles.

In both cases, it is true that it is not welcome to have supersonic flow at blade's farthest extreme (Rmax), because there will be losses of energy in the case of a compressor or turbine, or horrible sound in the case of airplane's propellers.

 

[FredGarvin]

There are some compressors that are termed transonic in that only a portion of the compressor is at supersonic speeds. I have read that there are fully supersonic compressors (axial flow) that are reasearch models. I really don't have any information on how they work though.

The subsonic realm is definitely where most engines live. That's why it is so very important for airframers to have their inlet geometry right. In the same line as what Enigma mentioned, look at an F-14's inlet scheme. It's pretty complex and moveable geometry to ensure subsonic flow at the fan face.

 

[Clausius2]

There are some compressors that are termed transonic in that only a portion of the compressor is at supersonic speeds. I have read that there are fully supersonic compressors (axial flow) that are reasearch models. I really don't have any information on how they work though.

Remembering some class of Turbomachinery...
Watch the figure attached, Fred. In first figure it is drawn two compressor stages rotating at certain tangential speed U.


Such transonic compressors work as a converging nozzle. You know if we "cut" a portion of an axial compressor we take the usual blades diagram. Such blades are separated each other by a gap by which the air flows. In a subsonic compressor, the gap is becomes thicker as the flow pass through one compressor stage, in order to diffuse the flow and increase pressure.

In a supersonic compressor, there will be shock waves just at the entrance of each supersonic stages. Assuming the flow remains supersonic behind the oblique waves, the gap between blades must became thinner in order to diffuse the flow and increase pressure again.

There are supersonic turbines which accelerates the flow from subsonic to supersonic speed. It is done by means of a converging-diverging gap shape. See the second figure.

Anyway, a compressor-turbine working at supersonic speed must have some system of variable geometry in order to deal with shock waves and eventually a transition to subsonic (at least locally) when varying the speed.

Do you understand it, Fred?.

 

[FredGarvin]

Anyway, a compressor-turbine working at supersonic speed must have some system of variable geometry in order to deal with shock waves and eventually a transition to subsonic (at least locally) when varying the speed.

You've got me thinking about this now...I need to see if I can dig up some more info.

Thanks for the heads up.

 

[russ_watters]

You've got me thinking about this now...I need to see if I can dig up some more info.

Well, the moveable INLET CONE of the SR-71 is a good example.

The cone moves in and out about 3 feet, providing upwards of 70% of the compression at cruising speed.

 

[Cliff_J]

Thanks guys, awesome info.

I looked up a Garrett T66 turbo compressor map just for fun and if it has the compressor blades all the way out to the full 3" inlet diameter then the tips of the blades would be at 1005 ft/s at the full 102,400 RPM. So even if the diameter of the compressor is going to raise the velocity of the blades with respect to the air, since the air is actually traveling in a radial manner it is irrelevant to the airflow.

And I would venture to guess that some form of difussor/expansion is done on post-combustion airflow in the jets to prevent the same problems with the turbine section exceeding speed of sound?

I'd also venture to guess now that I think of it that the linear tip speed of the compressor blades themselves is a vector product since they have their speed with respect to their rotation and the airflow too in several hundred MPH. Hmm, need some paper to toy with this idea...switch frame of reference to that of an air molecule?

Good stuff like this would have changed my view of fluid dynamics instead of a bunch of calculus. At least calculus based physics is appropriately named and one prof made sure to keep it realistic to make applications memorable.

 

[FredGarvin]

Anyway, a compressor-turbine working at supersonic speed must have some system of variable geometry in order to deal with shock waves and eventually a transition to subsonic (at least locally) when varying the speed.

That makes sense. I would like to see what people have come up with. I would imagine it's a take off of IGV's in between stages.

Russ...The SR-71's engines still operated in the subsonic regime, didn't they?

 

[Clausius2]

That makes sense. I would like to see what people have come up with. I would imagine it's a take off of IGV's in between stages.

Russ...The SR-71's engines still operated in the subsonic regime, didn't they?

While wating Russ answer...

I think commercial jet engines works with subsonic regime. Every engine who has mobile parts such as compressor and turbine must operate in subsonic regime. The devices I have described above are somehow only experimental. Anyway it is not possible to produce a good combustion at supersonic regimes except in Scramjets, which hasn't got any mobile parts.

And I would venture to guess that some form of difussor/expansion is done on post-combustion airflow in the jets to prevent the same problems with the turbine section exceeding speed of sound?

It is not needed anything. Post combustion flow is always subsonic, and a turbine might operate at sonic regime as a maximum limit, any flow can be accelerated from subsonic to supersonic with a convergent nozzle. Although the section of the turbine increases with the axial coordinate, the duct between blades is a local convergent nozzle which extracts pressure in a local spinning reference frame. Never will be supersonic flow in a jet engine turbine.

Good stuff like this would have changed my view of fluid dynamics instead of a bunch of calculus.

Fluid dynamics is a field of science VERY well mathematically structured. I think it is with Electromagnetic Theory the field of Physics most deeply well structured. The challenge when learning Fluid Mechanics is that in order to understand this experimental applications reasonably well, you have to have dealt with all that bunch of calculus. Anyway, I am pride of this field, because doubt and prejuices hasn't got any place within it, every thing can be reasoned mathematically and physically. It is the maximum proof of an Science. Besides it has a lot of stuffs which remain as an unknown to nowadays engineers and physicists, like turbulence and combustion.

 

[enigma]

Russ...The SR-71's engines still operated in the subsonic regime, didn't they?

Yes,

The shaped inlets provided a lot of oblique shock waves for the air to travel through before it hit the final normal shock (just above Mach 1 at that point) right before the entrance to the engine.