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Why doesn't my two-step chemical model work?[For H2+air reaction][finite rate model]

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Old   November 9, 2019, 13:28
Question Why doesn't my two-step chemical model work?[For H2+air reaction][finite rate model]
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Roh
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Guys,
I'm trying to use the two-step chemical model in my simulation. Here is the chemical mechanism:




Here is the reference of the mechanism:


https://www.researchgate.net/publica...-O2_combustion


I wrote this CHEMKIN file:


Code:
ELEMENTS                                                                        
O  H  N                                                                           
END                                                                             
SPECIES                                                                         
OH  H2  O2  H2O  N2                     
END                                                                             

REACTIONS                                                                       
H2+O2<=>2OH                     2.3E+22    0.000  10202           
2OH+H2<=>2H2O                    1.83E+27    0.000  21992 
END

I imported the CHEMKIN file and it's ok but the solution and the residuals doesn't look ok. See:








As you see the residuals of OH and H2O have oscilation. It should be noted that the solution is ok for one-step Hydrogen-air mechanism of Fluent.


What's the matter with this mechanism? What's your opinion? How to solve it?
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File Type: png screen-shot-residual.png (62.9 KB, 27 views)
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Old   December 1, 2019, 09:28
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No idea??????
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Old   December 2, 2019, 01:01
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Hi Sir ,
I want to clarify how do you calculate activation energy value from the reference file you attached and the values you entered in chemkin file looks different???
Looks forward for the reply


Many Thanks
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Old   December 2, 2019, 02:24
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Check your units ,that may vary.
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Old   December 2, 2019, 08:30
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Quote:
Originally Posted by Sowmiya View Post
Hi Sir ,
I want to clarify how do you calculate activation energy value from the reference file you attached and the values you entered in chemkin file looks different???
Looks forward for the reply


Many Thanks
Sowmiya
Quote:
Originally Posted by Jegan View Post
Check your units ,that may vary.

Thank you Sowmiya and Jegan for reply,

Ok, let me tell you how I've written this CHEMKIN file. As it is said in this file (page No. 17):

https://www.et.byu.edu/~tom/classes/...dentmanual.pdf

The default units for A and activation energy are in terms of cm, sec, K, and moles. And also the Arrhenius equation is defined as(for CHEMKIN):

https://www.researchgate.net/profile...image/ar-1.png

As what is mentioned in the Article:

https://www.researchgate.net/publica...-O2_combustion

I attached a screenshot of the mechanism (table 1) that is mentioned in the article.

http://uupload.ir/files/2ru4_dyfuhjklkk.png



As you see in the screenshot, There are three parameters in the table; A, T, and n. I think that the authors of the paper didn't know that there are several forms of the Arrhenius equation.
I attached the screenshots of three forms. Here are the images:

https://www.researchgate.net/profile...image/ar-1.png


https://www.researchgate.net/profile...image/ar-2.png


https://www.researchgate.net/profile...image/ar-3.png


Then I think the authors have made a mistake and the form of the Arrhenius equation for two-steps mechanism is this:

https://www.researchgate.net/profile...image/ar-3.png

Thus, T should be Tar. The unit of this parameter is K but for CHEMKIN file we need to convert it to cal/mole (convert Tar to Ea). Ok, Therfore we multiply Tar by the gas constant which is 1.98720425864083 (cal⋅K−1⋅mol−1). For example for the first reaction mechanism we have:

5134*1.98720425864083=10202 cal/mole

For A, the units should be in terms of cm3, sec, and moles. Then we multiply A by 10e+6 to change the unit from m3.mol-1.s-1 to cm3.mol-1.s-1. Thus, e.g. we have:

2.3*10e+16*10e+6= 2.3*10e+22


This is the procedure that I've done to convert them.


Do you see any problem?
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Old   January 12, 2020, 01:40
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Dear Sir,
Thank you for clarifying my doubts clearly. Also I want to know when you convert Ea in chemkin file to cal⋅K−1⋅mol−1 but the fluent should have the unit of Ea in J/ kgmol. How you are correlating the units? Looking forward for the reply.

Many Thanks
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Old   January 12, 2020, 02:50
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Convert your cal/mol value into J/kmol

or in mechanism file we can write REACTIONS CAL/MOLE

This may work
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