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faivala August 28, 2012 19:23

Post - Angles alpha and beta in centrifugal compressor (solved)
Hi all,

I am trying to evaluate the angles alpha and beta at the impeller exit but I find it difficult to understand what CFX really does here. I found some threads about this but didn't help me further.

If I get my radial and circumferential velocities I should be able to calculate beta. I.e. I get 183 and 199 m/s. Therefore should my beta be arround 42. But CFX says my beta is arround 52. And something similar is happening to my alpha if I use the velocities in the stationary frame. My axial component is very small. If I add the circumferential velocities both relative and in stationary frame I come to almost my actual tip speed, 545 vs. 555 m/s.

I already tried using the functions average, mass flow average, area average. To calculate this I created a turbo-surface at the impeller exit. Is this a bad idea?

Is this because of the highly turbulent flow and CFX not calculating the angles analytically?

Thanks a lot!

edit: I just found out, that if I move my turbo surface some milimeters away from the actual exit (meridional from 0.72 to 0.74) my axial component almost disappears, my radial and meridional components start to be very similar and the calculations then work. Strange.

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