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mass fraction of the products in reactingFoam solver

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Old   October 5, 2013, 03:24
Default mass fraction of the products in reactingFoam solver
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sonu
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Hello ,
Various thread are there about regarding reactingFoam solver but nobody is replying there so i thought to open new thread .

i am using OF-2.2 and PasR model for my case with reactingFoam solver and at the end i noticed 2 things :

1. The temperature is bit on higher side.
2. The mass fraction of the products (CO2 and H2O ) is nearly double of what
of what should come .

so can anybody have any idea why this is happening ?
Thanks in advance
if needed then i can attach my case files .

Regards ,
Sonu
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Old   October 6, 2013, 01:51
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Hi sonu

Your question is so wide that I can not put anything here.
First of all what is your case exactly?

What BC are you exerting?

If you are using LES, What is your sub-grid scale? Is it suitable for the case or not?

........
........

And So on ...............

My suggestion for you is go further step by step and tell me exactly what are you doing. PLZ don't send anything.

Bobi
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Old   October 6, 2013, 10:43
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greetings Bobi ,
i am using OF- 2.2. , and want to simulate the global single step reaction of methane oxygen combustion using reactingFoam solver which uses PasR combustion model . This is the global one step reaction and other values i had given

Code:
CH4+2O2= CO2+2H2O      
pre- exponential factor = 5.2e16
beta=0
activation temperature=14906

My BC's are :

 fuel_inlet:
CH4: fixed value /1 

O2 : fixed value/ 0
N2 : calculated 
U: fixed value
p: zero gradient
k/eps/: value 



coflow_inlet:
CH4: fixed value / 0 

O2: fixed value / 0.23
N2 :calculated 

U: fixed value
p: zero gradient
k/eps/: value 


 outlet:
CH4:inlet outlet / 0
O2: zero gradient
N2: zero gradient 

 U: zero gradient
p: zero Gradient
k/eps: zero gradient

upper_walls:


CH4: zero Gradient 

O2: zero gradient
N2: zero gradient  U: fixed value
p: zero gradient
k/eps: uniform/1    



internal field:
CH4: 0
N2: 0.77
O2: 0.23
everything else seems to be ok when i checked results but 2 things paining me that temperature is bit on higher side and the mass fraction of the products are almost double of what should come .





so plz Bobi have a look and point out where i am doing wrong


Thank You

Sonu
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Old   October 6, 2013, 12:47
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Hi sonu

I think PaSR combustion model is typically employed with detailed chemistry (Inherent specifics of this model). If you still demand on employing global step reaction; try FSD combustion model which is developed for global step.


I hope you find my suggestion helpful.

Bobi
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Old   October 18, 2013, 13:52
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Thanks Bobi ,
Sori i was busy with some other work. ok i will try .

and one more doubt if u can suggest something about it. i am sending you some snapshot of the temperature contour. there in second pic some waviness started to come in the flow. initially its stable as seen from first pic but suddenly after some time it (waviness) comes into picture . So can you suggest any reasons for this to be happening ??

Thank You ,
Regards ,
sonu
Attached Images
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File Type: jpg cfd 2.jpg (13.7 KB, 21 views)
File Type: jpg cfd 3.jpg (14.1 KB, 21 views)
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Old   October 19, 2013, 03:36
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Hi Sonu

This issue (not always a problem) depends on some parameters:
1- maybe your grid is not fine enough to capture some unsteadiness in the flame.

2- some flames exhibit local extinctions that cause waviness structures.

So If you want me to help you better , specify your exact flame , otherwise try to fine your grid (be aware that the grid is more sensitive in cross sections resolution than axial resolution i.e. you must refine your mesh at the jet exit and the bluff body if it exists.) and compare the differences. If no difference exists, then it is local extinction.

Another issue: Why your Temp is so high?

What is your fuel? Nuclear one?

Bobi
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Old   October 19, 2013, 04:19
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Thanks Bobi for such a explained answer.

i will try with the more fine mesh.

actually this is the same case of methane-oxygen combustion which i tried to solve by using the reactingFoam solver, in which i have the problem with the temperature and mass fraction of the products (as posted in above post ). In which u suggested me to try FSD combustion model which is developed for global step. so i am looking for the reason of that waviness that comes in this case which you explained to me .

Thanks for help

Regards,
sonu
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