# Mass flow not matching with analytical value

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January 27, 2017, 07:17
Mass flow not matching with analytical value
#1
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Arvind
Join Date: May 2016
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I am simulating an axial flow turbine with velocity inlet and pressure outlet BC. The simulation is giving good result with respect to torque and pressure drop matching with the experimental results. I have also checked with different mesh counts for grid independence study. The mass flow from inlet and outlet is also matching.

However the massflow rate is not matching with experimental or analytical result ( i.e A*V* density) and as a result the efficiency is not matching with the experimental results. The difference is in the order of 30%.

Any clue in this regard will be very helpful.
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 January 27, 2017, 15:51 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,789 Rep Power: 143 Aren't you defining the mass flow rate at your inlet boundary? Does this mean your inlet boundary condition is wrong?

 January 27, 2017, 18:50 #3 New Member   Arvind Join Date: May 2016 Posts: 26 Rep Power: 10 I am defining the velocity at the inlet, i.e normal velocity to the inlet. The mass flow rate should be calculated from CFD.

 January 27, 2017, 19:03 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,789 Rep Power: 143 1) How are you calculating the mass flow rate? 2) Is the flow incompressible or compressible? 3) Steady state or transient? 4) If the mass flow rate is wrong then can you adjust the inlet boundary velocity to get the correct mass flow rate?

 January 27, 2017, 21:41 #5 New Member   Arvind Join Date: May 2016 Posts: 26 Rep Power: 10 1. I am calculating mass flow from the function calculator in cfx post: massflow()@inlet 2. The flow is incompressible 3. i am using SST k-omega model for steady state 4. I cant change the inlet velocity because that is my inlet BC, i have to find out the torque and efficiency for particular inlet flow velocity okinugraha likes this.

 January 28, 2017, 00:44 #6 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,789 Rep Power: 143 Then it appears your desired mass flow rate is inconsistent with your inlet velocity. Or are you saying the mass flow() calculation is wrong?

 January 28, 2017, 07:36 #7 Member   Peter Join Date: Sep 2011 Location: Germany Posts: 39 Rep Power: 14 Just to ensure: You are using periodic BC, are you multiplying the calculated massflow with the number of segments?

 January 28, 2017, 07:46 #8 New Member   Arvind Join Date: May 2016 Posts: 26 Rep Power: 10 Yes i am multiplying the torque and massflow with the number of segments. I am checking the physics again and re-doing the simulation; i will get back after getting the result. Thanks for your suggestions.

 January 29, 2017, 08:35 #9 Senior Member   Join Date: Jun 2009 Posts: 174 Rep Power: 17 I see your model is an axial rotor with a lot extended up/downstreams cut in the axial direction which means the relative frame of reference only. How do you know the inlet velocity BC value flowing normal to the boundary?

 February 1, 2017, 06:36 #10 Member   Join Date: Jan 2015 Posts: 63 Rep Power: 11 Are you sure that your simulation is converged? Have you monitored the mass flow rate to see if it is still changing? Also, have you tried to run the full model, just to understand if it is the periodic BC making mess?

February 1, 2017, 22:13
#11
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Arvind
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Quote:
 Originally Posted by highorder_cfd Are you sure that your simulation is converged? Have you monitored the mass flow rate to see if it is still changing? Also, have you tried to run the full model, just to understand if it is the periodic BC making mess?
I have monitored the mass flow rate and it is not changing after certain iterations.

No i have not tried running the full model, i will check it. Thanks for the suggestion @highorder_cfd

February 1, 2017, 22:20
#12
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Arvind
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Quote:
 Originally Posted by turbo I see your model is an axial rotor with a lot extended up/downstreams cut in the axial direction which means the relative frame of reference only. How do you know the inlet velocity BC value flowing normal to the boundary?
From the physical model i know that inlet velocity value is normal to the inlet boundary. Is that what you mean @turbo?

 February 2, 2017, 04:34 #13 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,789 Rep Power: 143 So if you know the inlet velocity and that it is perpendicular to the inlet boundary, then the mass flow rate = density*area*normal velocity at the inlet, so you know the mass flow rate without doing any CFD. If this mass flow rate does not match your desired mass flow rate then you have a problem with the inlet boundary condition being inconsistent with the desired mass flow rate, don't you? If I have missed something please explain it to me, I will be keen to hear it. highorder_cfd likes this.

 February 4, 2017, 09:18 #14 New Member   Arvind Join Date: May 2016 Posts: 26 Rep Power: 10 Thank you @ghorrocks and @highorder_cfd for your inputs. The mass flow rate calculated using the formula is matching with the mass flow rate calculated from cfd. I guess my problem is with the mesh or the domain, as a result the efficiency is not matching with the experimental result.

February 5, 2017, 16:40
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By the way, could you post here a pic of your mesh? It might be too coarse where you are prescribing the Boundary Condition.

Quote:
 Originally Posted by adilarvind Thank you @ghorrocks and @highorder_cfd for your inputs. The mass flow rate calculated using the formula is matching with the mass flow rate calculated from cfd. I guess my problem is with the mesh or the domain, as a result the efficiency is not matching with the experimental result.

 July 23, 2024, 10:49 #16 New Member   Daryn Bertelson Join Date: Aug 2017 Posts: 3 Rep Power: 9 Hello, did you ever find out if the issue was with your mesh? I am also having issues with "massflow rate is not matching with experimental or analytical result ( i.e A*V* density)".

 July 23, 2024, 18:35 #17 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,789 Rep Power: 143 There are many reasons why this problem could come about. If you describe what you are doing, show some images and attach your output file we will try to help you. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

July 24, 2024, 13:31
flow rate vs velocity integrals at pressure outlet
#18
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Daryn Bertelson
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Quote:
 Originally Posted by ghorrocks There are many reasons why this problem could come about. If you describe what you are doing, show some images and attach your output file we will try to help you.
I'm not sure if you we responding to original poster, but if it is OK for me to continue on this thread, I might be having similar questions. The main question is: Can you trust the mass flow rate surface integral in Fluent? It does appear to be better for measuring flow rate as compared to converting the velocity. Please see the attached results worksheet snips. The difference between inlet and outlet flow rates are much closer using the mass flow rate surface integrals.

The red line on the velocity contour is where I'm taking measurements. The blue arrow shows the side of the distributor near the inlet. I can't show more in a public forum.

2024-07-25 - removed images, since this the wrong forum anyway (question was for Fluent)

Last edited by pocketprotector; July 25, 2024 at 09:35. Reason: wrong forum

 July 24, 2024, 18:33 #19 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,789 Rep Power: 143 For how Fluent does this try the Fluent forum. This is the CFX forum. There are many reasons why there could be a difference in mass flow rates, and they require the details of how you are measuring it, your geometry, what boundary conditions you are using and so on. If you cannot show more on the forum then we will not be able to help you much. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.