Convergence issue - combustion and wall heat transfer

MachZero · October 9, 2014, 11:14

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

MachZero · October 9, 2014, 15:32

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.

Industrial_CFD · October 11, 2014, 14:52

I would contact support...

macfly · October 11, 2014, 15:32

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.

October 9, 2014, 15:32	update	#2
MachZero Member Join Date: Aug 2011 Posts: 89 Rep Power: 15	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.

October 11, 2014, 14:52	reply	#3
Industrial_CFD Member Anonymous Join Date: Jan 2012 Location: Canada Posts: 65 Rep Power: 14	I would contact support...

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October 11, 2014, 15:32		#4
macfly Senior Member François Grégoire Join Date: Jan 2010 Location: Canada Posts: 392 Rep Power: 17	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.