# [Help!] impeller torque calculation: pressure and viscous moments

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October 21, 2013, 06:52
[Help!] impeller torque calculation: pressure and viscous moments
#1
Senior Member

Daniele
Join Date: Oct 2010
Location: Italy
Posts: 992
Rep Power: 17
Hi all!
I simulated a vessel+impeller with multiple reference frame and I want to calculate the power needed to drive the impeller: as I know the power P is:

P [Watt]=2*pigreco*N*Torque

N=revolution per seconds (12 rpm)
Torque in N*m

Impeller axis rotation is Z and the vessel is centered in X,Y; the domain is in +Z.
Impeller rotates counterclockwise.

Rotating frame is all the cylinder zone, which rotates at 12 rpm, because impeller 007/impeller 007-shadow are very closed to the wall. Bottom conical zone is stationary zone.
Vessel walls have absolute 0 rotational velocity.
Impeller/impeller 007/impeller 007-shadow have relative velocity to adjacent fluid zone of 0.

If I calculate the pressure and viscous moments (moment center: 0,0,0 moment axis 0,0,1) I have:

Impeller zone (all in N*m)
Pressure: -225
Viscous: +103
Total: -122

Impeller 007 zone (all in N*m)
Pressure: -109
Viscous: -8
Total: -117

Impeller 007-shadow zone (all in N*m)
Pressure: -107
Viscous: -8
Total: -115

So total moment is: -354

and P should be:
P=abs(total moment)*N*2*pigreco=354*12/60*2*pigreco=445 W

Is it right?

What I don't understand is why in the impeller zone viscous moment has opposite sign in respect to pressure moment.
Can anybody explain to me please?

I'm attaching some pictures.

Thank you
Daniele
Attached Images
 Agitatore+vessel.jpg (25.7 KB, 53 views) Vettori-velocita-SST.jpg (33.4 KB, 50 views)

 March 27, 2014, 03:37 #2 Member   Pekka Pasanen Join Date: Feb 2012 Location: Finland Posts: 77 Rep Power: 6 Did you ever figure this out? I'm having the same problem with my turbomachinery case, the pressure moment seems to be in the right direction but the viscous moment has opposite sign, which doesn't make any sense to me.

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