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Old   January 12, 2009, 04:00
Default pump validation
  #1
Nitin Suryawanshi
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Dear friends i did cfd analysis for centrifugal pump, and for max flow to design point(operating point) flow range cfd head is acceptable with experimental head, but for the flow below design point cfd head is decreasing & it is less for low flow rate, but theorotically head should be higher at low flow rate Can anyone please guide me resons behind this????

Thank You Nitin Suryawanshi

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Old   January 12, 2009, 07:26
Default Re: pump validation
  #2
Mike
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I had the same problem trying to study a ducted fan. The problemn was overcomed decreasing the residues value from e-03 to e-04 and adding another convergence control such as the mass flow rate al the exit (sometimes the convergence criteria was reached, but the mass flow rate was still fluctuating and such fluctuation may be too large).

I hope be usefull

Mike
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Old   January 12, 2009, 08:27
Default Re: pump validation *NM*
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Nitin Suryawanshi
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Old   January 12, 2009, 08:32
Default Re: pump validation
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Nitin Suryawanshi
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Thanks for reply, i had iterated solution longer but u,v,w momentum residuals are in between e-3 to e-4 and continuty is below e-4 so is any other way to check it for low mass flow range

Thank You Nitin Suryawanshi
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Old   January 13, 2009, 02:33
Default Re: pump validation
  #5
zongtwi
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Can you explain more about your case? Mesh type, Y+ values, which turbulence model, which rotating model etc?

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Old   January 13, 2009, 11:24
Default Re: pump validation
  #6
Nitin Suryawanshi
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Its a rotor & stator assembly. with mass flow in & pressure Out bc's. I m maintaining Y+ below 3 as i m using sst model, & solving in MRF. But i m sorry, i forgeted to tell that i m using a solver CFX. reply plzzz

Thank You Regards Nitin Suryawanshi
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Old   January 14, 2009, 02:36
Default Re: pump validation
  #7
zongtwi
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Are you doing single passage analysis? or full 360degrees?

There are various reasons why this may happen. I've been involved in fan simulation to come up with a fan curve. Initially, when I compare the results to experiment data, I see the characteristics of my results change three times, even when using the same mesh and settings. At low flow rates, my results differ upto 20%, at mid range flow rates, my results closely match the experimental data, while at higher flow rates, the results differ upto 10%.

Discussing this with some other experts, they say that it is possible the flow characteristics at different flow rates differ significantly, and sometimes the MRF models (for CFX these are the Frozen Rotor, and Stage models) or even the turbulence models can't handle them all.

To try and sidestep this problem, try using the transient-rotor model, i.e. the sliding mesh model in CFX. This should give you much more accurate results. The assumptions for the MRF models can then be elimiated, and see how this affects your simulation. Then also try using someother turbulence models. If I'm not mistaken, the sst model doesn't handle flow reattachment that well (it's good for separation). And since you'de using CFX, you can try to see if the turbulent transition model can be used at lower flow rates to give more accurate prediction.

Hope that helps.
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Old   January 15, 2009, 01:22
Default Re: pump validation
  #8
Nitin Suryawanshi
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Thank You for reply yes i m modelling a single passage for analysis. I will try again according to your suggestions and i will get back to you.

Thanks & Regards Nitin Suryawanshi
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Old   April 1, 2009, 14:27
Default bondary condition
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nazareno mancinelli
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yochule is on a distinguished road
hi guy. i am studing the same issues about the rotor stator and rotating volumes.
how can you determine the flow rate or the rpm? i understood that the rpm is a bondary condition that we have to set. but i canīt set another bondary condition that isnīt connect with flow rate (velocity inlet, pressure inlet, etc, all are flow rate dependient).
so i wonder if can i set a bondary condition that not force the flow rate?
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