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Convergence issue - combustion and wall heat transfer

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Old   October 9, 2014, 11:14
Unhappy Convergence issue - combustion and wall heat transfer
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Combustion community,

I am having a really troubling problem. I am simulating combustion in Fluent using the non-premixed model. I have two inlets fuel streams and a single oxidizer stream. When I model the combustion, I get spikes in my turbulence and continuity residuals every 10-20 iterations, but the outlet temperature converges well and the heat flux balance at the end of the run is close to zero.

Then I add in a heat flux through the walls, using an exterior heat transfer coefficient and bulk temperature. Once this happens, the outlet temperature has swings of up to 100 degrees C and the heat balance way off balance.

I have been working this model for days (long, 12+ hour days) with almost no progress. Has anyone seen this, or have any ideas why this might be occurring?

ANY ideas are welcome!

I have added some plots of outlet temperature fluctuations and residuals
Attached Images
File Type: jpg residuals.jpg (30.2 KB, 10 views)
File Type: png outlet temperature.png (32.3 KB, 12 views)

Last edited by MachZero; October 9, 2014 at 11:24. Reason: Added pictures
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Old   October 9, 2014, 15:32
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Update,

I was using the pseudotransient solver with low URFs on a poly mesh. I have seen to get a more consistent outlet temperature with a segregated solver, but the convergence is pretty poor when looking at residuals. There are a lot of spikes.
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Old   October 11, 2014, 14:52
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I would contact support...
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Old   October 11, 2014, 15:32
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No so fast on the support, lots of details are missing.

As a first guess, I think mesh and timestep can have a big influence on your case :

1. Ask on the ANSYS Meshing & Geometry forum how you could make a structured mesh.

and/or

2. Reduce the timestep and see if it affects the oscillations.
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Tags
combustion modelling, convection heat flux, convergence issues, heat transfer balance, turbulence modeling

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