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Note: This discussion is about an older version of the COMSOL Multiphysics® software. The information provided may be out of date.

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Initial negative concentrations

Posted 2013年5月10日 GMT-4 13:19 Fluid & Heat, Computational Fluid Dynamics (CFD), Chemical Reaction Engineering, Studies & Solvers Version 4.3a 2 Replies

Romani Patel
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HI All,

I am trying to simulate a A+B-->C chemical reaction. I dont know why my reactants (A and B) concentration is going negative. How do I fix this?



I have attached my model as well as pic of the concentration profile.

I saw in the knowledge base they give you certain options, but they have not worked for me. I tried adaptive meshing as well introducing A>0, B>0 in my reaction terms in the variables part. This is what I have in my variables section.

A=1 mol/m^3
B=0.5 mol/m^3
C=0 mol/m^3
k1=0.01
rA=-k1*A*B*(A>0)*(B>0)
rB=-k1*B*A*(A>0)*(B>0)
rC=k1*A*B*(A>0)*(B>0)

If someone can guide me please.

Thanks,
Romani
2 Replies Last Post 2013年5月15日 GMT-4 09:55
Suresh Seetharam
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Posted: 1 decade ago 2013年5月15日 GMT-4 05:03
Hi,


I did not look at your model in detail, but out of curiosity why are you not defining your reaction as in your email?

You see what I mean in the attachment. Try this and see if you get negative.


Suresh


Hi, I did not look at your model in detail, but out of curiosity why are you not defining your reaction as in your email? You see what I mean in the attachment. Try this and see if you get negative. Suresh

Attachments:

Romani Patel
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Posted: 1 decade ago 2013年5月15日 GMT-4 09:55
Hi Suresh,

Thank you so much for replying. I tried running my model according to your suggestions. And in the plot I see that the concentrations are not as negative as before.

However when I click on the the surface graph of concentration at some places the concentration is negative. I have attached the pic for your reference highlighting the values in the surface concentration graph. Is there a way to fix this? I am also attaching my model.

I'd appreciate if you could guide me. I have tried refining mesh but still I see negative concentrations in the surface graph.

Thanks,
Romani




Hi,


I did not look at your model in detail, but out of curiosity why are you not defining your reaction as in your email?

You see what I mean in the attachment. Try this and see if you get negative.


Suresh


Hi Suresh, Thank you so much for replying. I tried running my model according to your suggestions. And in the plot I see that the concentrations are not as negative as before. However when I click on the the surface graph of concentration at some places the concentration is negative. I have attached the pic for your reference highlighting the values in the surface concentration graph. Is there a way to fix this? I am also attaching my model. I'd appreciate if you could guide me. I have tried refining mesh but still I see negative concentrations in the surface graph. Thanks, Romani [QUOTE] Hi, I did not look at your model in detail, but out of curiosity why are you not defining your reaction as in your email? You see what I mean in the attachment. Try this and see if you get negative. Suresh [/QUOTE]

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