Frequency testing on turbine wheels in a turbo

[NeilGriffin]

I am currently doing a study on the Natural frequency on Tubine wheels, The study consists on Hitting the blades of the wheels several times, recording the sound and using sigView software to show use the Natural Frequency.It is intended to find any harmfull blade resonant conditions when operating a max speed.

I am looking for ideas which maybe in the manufacturing stages of the wheel which may vary the frequency of the wheel. I have looking at the injection moulding pressure, temperture of the wheel, Blade Dampining( to find the individual frequency of each blade. My next step is going to be the tooling of part and position on the runner during manufacturing!

Does any one know any other areas which may cause this vary in frequency in turbine wheels in the turbo!

thanks
neil

 

[SMOKEY44211]

Hello Neil May I ask what style turbine you are considering? My guess is a single stage expansion axial flow common on most turbocharger applications. Most of the auto and truck units I've seen seem to rely on the mass of the blade to be robust enough to withstand the vibrations you are anticipating. I'm still in the learning curve myself so I can only share what I've learned so far. I think you would be ahead of the game to try to keep the tip speed subsonic. Difficult to calculate the forces when mach 1 is exceeded. The dynamics go down the blade as rpm increase. I've read that a prime # of blades is crucial to eliminate transient harmonic vibrations. I don't think this is a hard and fast rule as most units I've seen to not fit this criteria. Putting a ring around the perimiter of the blades would help dampen unwanted vibrations and properly designed should aid efficiency. Hopefully other more knowledgable members will respond to this thread and fill in the rest of the blanks.---------Phil

 

[GraviMan]

Interesting problem. Don't forget that there will be a large centrifugal force, trying to keep the blade "straight". This will have the effect of raising any modes, in the same way that tightening a guitar string does. An outer ring will increase this force.

Another major factor will be aerofoil shape, since most modes will be perpendicular to the chord line. Axial flow units go to great pains to ensure that the neutral axis in bending and shear lie on the same point throughout the blade, ideally with mass centre at the same point.

You could use Master Series, with a simulated centrifugal acceleration, then do Modal Analysis. Really form change is going to be the main factor in "tuning" blades. You can increase gap from stators to rotors, or even soften the leading edge - this will reduce the input into the blade. Gas turbines used to use mid span snubbers to help damp any modes, but this is now considered a clumsy solution.

Eccentric solutions (always worth rejecting): Use silicon fluid in a central cavity to mechanically damp blade. Coat blade in polymer (depending on temp). Go for composite blades. Get a Gas Turbine expert like Alstom or Rolls-Royce in.

Above all, let us know how you get on - Knowledge exchange is the main purpose of this forum...

Mart