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Slow convergence using mass flow BC

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Old   November 6, 2012, 05:11
Default Slow convergence using mass flow BC
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Vit Houst
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I have a problem, that when I use mass flow at outlet, the convergence is really slow. The change of the mass flow on the inlet is almost none and where pressure BC changed the mass flows in 50iterations, now it takes more than 300.
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Old   November 6, 2012, 06:35
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Have you read the boundary condition choice section in the CFX documentation? That describes what you need to know.

Otherwise, please post an image and your CCL file so we can understand what you are doing.
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Old   November 6, 2012, 07:40
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Hello ghorrocks,

yes I did. The purpose is to simulate a centrifugal compressor. For this reason I need to switch between different boundary conditions definitions. For choke region i use static pressure on the outlet. as the compression curves starts to be flat, i start to use mass flow BC.

The problem is not that it would not converge, but that it takes really long time to change mass flow. I found the expert parameters defining the underrelaxation of the values for these cases. Does anyone have experience with these?

For reference I attached pictures from solver, showing all iterations. You can notice the change in the speed when BC pressure was defined and when mass flow.
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File Type: jpg Obrázek 000.jpg (27.4 KB, 15 views)
File Type: jpg Obrázek 001.jpg (26.0 KB, 15 views)
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Old   November 6, 2012, 18:11
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I would not recommend fiddling with the under relaxation factors. It is highly unlikely they will help.

If the change is slow then you have to run a larger time step to push the change through the system. If you cannot run larger time steps by numeric stability then you need to improve mesh quality.
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Old   November 7, 2012, 11:22
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Use average static pressure condition as it is equivalent to specified mass flow rate boundary condition and at the same you are defining pressure which is more logical to define. Search for the Chima paper in Journal of turbomachinery for more explanation.

I can explain in more detail as discussion progresses.

Simple static pressure condition is also good if the boundary is not too close to the trailing edge. Moreover check the mesh quality.

Average static Pressure

Quote from above post

Quote:
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so you are ruling out the uniform static pressure boundary condition if it is placed too close?

And from your discussion I also infer that average pressure static with zero blend factor should work good regardless of location of outlet boundary?

Mass flow is not a option !!! because some times we have to work with new designs and it is very hard to make map near choke conditions.

These posts may be useful for you:

Comparison of fluent and CFX for turbomachinery

How to find a good time scale strategy?

Axial compressor map
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