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Somebody Help PLS: non-premixed combustion

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Old   June 4, 2008, 11:42
Default Somebody Help PLS: non-premixed combustion
  #1
Abba H
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Im modelling a non premixed combustion in fluent and Im having problem of convergence. The simulation have been running for almost 8 hours now and still no convergence. The furnace is 500 MW coal furnace with 900,000 mesh volumes generated. It is also showing me reverse flow from the begining of the iteration. Is there away to avoid getting that? How do I run a successful simulation of FLUENT using the non-premixed combustion model. Can somebody Help please
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Old   June 4, 2008, 15:45
Default Re: Somebody Help PLS: non-premixed combustion
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Allan Walsh
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Convergence is a relative term. Do you mean the residuals are getting larger or are you just waiting for a message to pop up that the solution has converged. What about parameters like temperature, heat fluxes, oxygen, etc? Are they changing? Is your outlet boundary at a place where reverse flow is likely? You can tolerate reverse flow in some cells without divergence and get reasonable results if you set the backflow conditions appropriately.

Good luck.
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Old   June 4, 2008, 15:56
Default Re: Somebody Help PLS: non-premixed combustion
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Abba H
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Thanks Allan

Actually the residuals are not getting larger, they are diminishing, however, at some point they started fluctuating about a constant value. And my outlet is not a boundary where a reverse flow is likely to occur. Right now, Im just resign to faith, and c what will happen after the iteration is complete, wether the solution will reach convergence or not. However, if I could reasonably get some help, I have a lot of question to ask about the Fluent models itself, especially when I was defining the boundary conditions. First, the Furnace have 24 burners, and the burners are place on the back wall, so the inlet is in the negative direction of the Z-axis (furnace wall). So when I was defining the velocity, I defined them as negatives velocities. Could that be one of the problems of my not getting convergence and the backflow?

Will appreciate ur more of ur help. Thanks for ur time
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Old   June 4, 2008, 23:07
Default Re: Somebody Help PLS: non-premixed combustion
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Raghu
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Hi,

Reduce ur Under relaxation factor for combustion, and for turbulence too, try this it may work out.......

With regards, Raghu
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Old   June 5, 2008, 12:21
Default Re: Somebody Help PLS: non-premixed combustion
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Abba H
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Hello

I reduced the under relaxation factors it attained convergence but the problem is there was no combustion at all. What could be the problem?
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Old   June 6, 2008, 06:53
Default Re: Somebody Help PLS: non-premixed combustion
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Raghu
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Ok then before reducing the under relaxation factors, did u able to see the combustion effect on it..... Otherwise u must have solved ur problem without enbaling the combustion equation please check it..... Whether its enabled or not.....

With regards.... Raghu
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Old   June 6, 2008, 10:05
Default Re: Somebody Help PLS: non-premixed combustion
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Abba H
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I have enabled the combustion equation but still Im not getting combustion. Im going over the the whole process and see if I made a mistake somewhere. Im really grateful for u help.
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Old   June 6, 2008, 11:17
Default Re: Somebody Help PLS: non-premixed combustion
  #8
Raghu
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Ok then did u give the correct injection point in ur domain otherwise it will inject some were else and check ur PDF curve too, check its mean temperature and enthalpy values tell me how much its showing.....
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Old   June 6, 2008, 11:37
Default Re: Somebody Help PLS: non-premixed combustion
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Abba H
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Hi Thanks for your response I think the injection points are ok. I have checked that. The Maximum Value of the mean temperature is 2.5569e+3 and occurs at mean fraction of 9.84375e-02 and the Minimum Mean Temperature occurs at 4.00e+2 at mean Mixture fraction of 1.000. However there is no enthalpy function in the option of the drop the down list and the Mole fraction of C, both the maximum and minumum values are zero Kind Regards
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Old   June 6, 2008, 12:57
Default Re: Somebody Help PLS: non-premixed combustion
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Raghu
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Ok then can u send me ur temperature result image file in jpeg format so that i can tell my view, if u dont mind abba.....
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Old   June 6, 2008, 13:39
Default Re: Somebody Help PLS: non-premixed combustion
  #11
Abba H
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yeh, I can send it, I dont mind, but how? This is my email abbahassan@hotmail.co.uk. I cant find a place to attached it here
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Old   June 6, 2008, 13:49
Default Re: Somebody Help PLS: non-premixed combustion
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Raghu
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Ok then i will send an email to ur email by tomo morn abba... So that u can send me that image file.....
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Old   June 6, 2008, 13:50
Default Re: Somebody Help PLS: non-premixed combustion
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Abba H
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Alright, thanks, Im really grateful

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Old   June 6, 2008, 13:54
Default Re: Somebody Help PLS: non-premixed combustion
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Raghu
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Hey please no formalities ya........
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Old   June 11, 2008, 14:47
Default Re: Somebody Help PLS: non-premixed combustion
  #15
leo
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what do you mean with combustion equation? the energy equation?... i think you can also simulate combustion without enabling the energy equation
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