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-   -   Fluctuation problem - Plz help! (http://www.cfd-online.com/Forums/fluent/75537-fluctuation-problem-plz-help.html)

 andres555 April 28, 2010 08:55

Fluctuation problem - Plz help!

Hi guys,

I am doing a simulation of a industrial furnace(10 MW) with fluent and i´m getting fluctuation on the local velocity of some part of my furnace (near to burner, fluctuation Vel.Magni= 0.5 m/s between iter).

My velocity residuals have small fluctuating too, they are at the order 1e-3.

My energy balance is fluctuating from 83000 w to 81000 w.

it is a stable state problem.

i will apreciate if you give me an advise and if you can refer a book or paper that talk about this kind of problem.

Thanks for your help and sorry for my English.

 andres555 April 29, 2010 21:04

 leonardo.morita April 30, 2010 04:24

hmmm...without further details, I can only guess some basic points, such as: bad quality mesh (check it), inappropriate discretization scheme (what are you using?) or even transient nature of your problem (sure it is steady?)
Another possible reason for this oscillations is the number of cells in your domain. I don't know if it's the case, but I've seen that it's difficult for large systems to converge to a "very steady" solution, oscillations being frequent.

 andres555 April 30, 2010 09:56

I checked the simulation with 5 mesh of diferent quality.
The maximun skew is 0.83 and there are no more than 5 elements.

I have a big domain with 490000 hibrid cells (Tetrahedral+Hexahedral). (Furnace 10 MW - 55 premixed burners - methane combustion)
I am using, P-V coupling=SIMPLE, and second order Upwind for all equations, Pressure is second order. my turbulence model is k-e standard

The simulation requiere at least 18000 iterations for get a error on energy balance of 110000 w, this value is lower than my convergence criteria (5%). Any way the velocity fluctuation occurrs ... so i continued with the iterations until the oscilation in the energy balance start (83000-81000).

I will apreciate if you can refer me a paper or book that talk about this problem or if you know how could be solve it.

Now i am doing a transient simulation but i´m not sure is the problem will be solved.

 leonardo.morita April 30, 2010 10:31

Concerning your mesh, 0.83 is not a very very high value, but we cannot discard it as a source of oscillations. You need to see if your mesh size and quality is appropriate to your case, try to check wall spacings etc.
Discretization schemes look reasonable, but you can try PISO with skewness correctors instead of SIMPLE. Search any recommendations for coupling schemes for your specific case.
Are you modeling combustion as well? Or just fluid flow? I know nothing about combustion, but high oscillations near a burner wouldn't see that strange to me...
Finally, I didn't understand very well which velocity oscillates...is it a surface-monitor you defined? Where is located this surface? The same for the energy balance...

 andres555 April 30, 2010 10:49

The energy balance is the one that is saw on report--> fluxes, heat transfer. Thats mean global energy balance. The velocities are measure in a line in front of the burner plane (distance is 0.6m) so is a local velocity.

yep i am modelling combustion.

I really really thank you answer, now i am doing what you say

 leonardo.morita April 30, 2010 11:02

Another tip: you should also look for combustion topics in the forum, since this kind of problem may be frequent...but honestly I know nothing about it

 andres555 May 4, 2010 10:19

Hi again,

I tried all the solver (coupled and segregate). Tried with all discretization schemes (second order, MUSCl and presto). My best answer was got it with presto, anyway fluctuation for the global energy flux and local velocity is there.

The global energy flux is lower than my convergence criteria (just 80kw from a furnace of 10 MW), but my velocity not. What should i do ? is the result that i got acceptable?

This is a master thesis.