Steam turbine blade design 1

[zamorac]

I am designing a low scale steam turbine (curtis) but I am dobious about the blade height, Do you know how to calculate the height of a blade?

I was considering just the flow of steam entering the blade wheel and its speed. With this, I just calculate the perpendicular area necessary for that speed and flow, and, because the blade step is fix, I calculate the height. That simple. However, the speed of the steam leaving the blade wheel is lower than the speed entering the blade wheel (same flow, though), so the blade height in the output tip of the wheel must be higher than the input tip, Is this correct? I normally see in pictures of turbines that the blade height is the same at both the input and output tips, so I am confused. Are my considerations wrong?

Thanks for your help.

 

[MikeHalloran]

Recognize that I understand very little about turbines.

That said..

I think it may be more instructive to consider the dimensions of the gas passages between the buckets than the dimensions of the buckets themselves.

Because the buckets are curved like an airfoil, the gas passage dimensions are different at the leading and trailing edges, even when the tips and roots sweep perfect right circular cylinders.

You also need to look at the velocity vectors of the gas flows and of the buckets, at the same time.

It all makes my head hurt, which is why I don't do turbomachinery..



Mike Halloran
Pembroke Pines, FL, USA

 

[unclesyd]

Blade height is very important due to fatigue of the blade proper and as stated by mikeHalloran it gets very complicated quickly.
The failures we experienced were in the blade proper, not the root as one would expect. We were fortunate in that the fatigue cracks were discovered during a yearly turn around instead breaking while running with the resulting catastrophic failure of the turbine. There was no quick resolution as we immediately removed the 9th rateau and ran for 18 months without the 9th stage. When we received the replacement blades they were almost one inch shorter.

 

[snoopnoon]

As far as your point on input height to exit height, consider the axial distance between the nozzle and the bucket. An angled tip is used to direct the steam to a diaphgram downstream that is of larger diameter. and angle ranging from 10-20 deg is usually about the normal.