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negative concentration at surface

Posted Jul 26, 2017, 3:57 p.m. EDT 12 Replies

Jasper N
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i am modeling a laminar flow through a small channel with a surface reaction taking place on a small surface of one of the walls. This wall has an initial reactant B on it, which should be converted to product C through a reaction with reactant A introduced through the laminar flow.
The problem with this model is that the concentration of B on the surface can reach negative concentration. (which is equal to the concentration of C on the surface going over the initial concentration of B). even though i have set a number of maximum available sites on the surface, which is the same number as initial reactant B on this surface.
Is there a way to fix this problem?

i am using the chemistry, laminar flow, transport of diluted species and surface reaction modules for this.
concentrations are in the order of 10^-7. pressure is around 1 atm, fluid velocity is 2.5e-4 m/s.
I have tried different time steps for the solver, the danckwerts flux, ramp up the concentration from the initial start instead of a set concentration but none of these worked.
Only increasing my concentrations by a few orders of magnitude and decreasing my reaction rates to unrealistic values led me to a solution where the concentration of B didn't fall below zero and was limited to 0.
12 Replies Last Post Jul 27, 2017, 6:31 p.m. EDT
Kyle Minor
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Posted: 7 years ago Jul 26, 2017, 7:30 p.m. EDT
If your reaction rate is R, set R=max(eps^2,R). This should probably solve your issue.
If your reaction rate is R, set R=max(eps^2,R). This should probably solve your issue.
Peter Chang
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Posted: 7 years ago Jul 26, 2017, 9:38 p.m. EDT
Where is the negative concentration located?
Where is the negative concentration located?
Jasper N
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Posted: 7 years ago Jul 27, 2017, 1:41 a.m. EDT
Updated: 7 years ago Jul 27, 2017, 1:43 a.m. EDT

If your reaction rate is R, set R=max(eps^2,R). This should probably solve your issue.



where can i set this?
should it be inserted into the reaction rate equation or can it be set as a parameter?
[QUOTE] If your reaction rate is R, set R=max(eps^2,R). This should probably solve your issue. [/QUOTE] where can i set this? should it be inserted into the reaction rate equation or can it be set as a parameter?
Jasper N
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Posted: 7 years ago Jul 27, 2017, 1:42 a.m. EDT

Where is the negative concentration located?


the negative concentration is located on the surface where the reaction happens.
On the rest of the domain the concentration remains positive and physically feasible.
[QUOTE] Where is the negative concentration located? [/QUOTE] the negative concentration is located on the surface where the reaction happens. On the rest of the domain the concentration remains positive and physically feasible.
Peter Chang
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Posted: 7 years ago Jul 27, 2017, 8:53 a.m. EDT


Where is the negative concentration located?


the negative concentration is located on the surface where the reaction happens.
On the rest of the domain the concentration remains positive and physically feasible.


So the WHOLE surface has negative concentration?
[QUOTE] [QUOTE] Where is the negative concentration located? [/QUOTE] the negative concentration is located on the surface where the reaction happens. On the rest of the domain the concentration remains positive and physically feasible. [/QUOTE] So the WHOLE surface has negative concentration?
Jasper N
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Posted: 7 years ago Jul 27, 2017, 11:42 a.m. EDT



Where is the negative concentration located?


the negative concentration is located on the surface where the reaction happens.
On the rest of the domain the concentration remains positive and physically feasible.


So the WHOLE surface has negative concentration?

No,it starts at the edge of the reaction surface, but as time increases a greater portion of the surface reaches a negative concentration. I've included a picture of the domain i'm talking about.

i've tried refining the mesh from extra course to finer, and decreasing the initial step size of the solver to 1e-10 but it gave the same results.

[QUOTE] [QUOTE] [QUOTE] Where is the negative concentration located? [/QUOTE] the negative concentration is located on the surface where the reaction happens. On the rest of the domain the concentration remains positive and physically feasible. [/QUOTE] So the WHOLE surface has negative concentration? [/QUOTE] No,it starts at the edge of the reaction surface, but as time increases a greater portion of the surface reaches a negative concentration. I've included a picture of the domain i'm talking about. i've tried refining the mesh from extra course to finer, and decreasing the initial step size of the solver to 1e-10 but it gave the same results.

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Peter Chang
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Posted: 7 years ago Jul 27, 2017, 11:50 a.m. EDT




Where is the negative concentration located?


the negative concentration is located on the surface where the reaction happens.
On the rest of the domain the concentration remains positive and physically feasible.


So the WHOLE surface has negative concentration?

No,it starts at the edge of the reaction surface, but as time increases a greater portion of the surface reaches a negative concentration. I've included a picture of the domain i'm talking about.

i've tried refining the mesh from extra course to finer, and decreasing the initial step size of the solver to 1e-10 but it gave the same results.


Can you also plot the concentration of A on the surface as well?
[QUOTE] [QUOTE] [QUOTE] [QUOTE] Where is the negative concentration located? [/QUOTE] the negative concentration is located on the surface where the reaction happens. On the rest of the domain the concentration remains positive and physically feasible. [/QUOTE] So the WHOLE surface has negative concentration? [/QUOTE] No,it starts at the edge of the reaction surface, but as time increases a greater portion of the surface reaches a negative concentration. I've included a picture of the domain i'm talking about. i've tried refining the mesh from extra course to finer, and decreasing the initial step size of the solver to 1e-10 but it gave the same results. [/QUOTE] Can you also plot the concentration of A on the surface as well?
Jasper N
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Posted: 7 years ago Jul 27, 2017, 12:02 p.m. EDT
Updated: 7 years ago Jul 27, 2017, 12:09 p.m. EDT





Where is the negative concentration located?


the negative concentration is located on the surface where the reaction happens.
On the rest of the domain the concentration remains positive and physically feasible.


So the WHOLE surface has negative concentration?

No,it starts at the edge of the reaction surface, but as time increases a greater portion of the surface reaches a negative concentration. I've included a picture of the domain i'm talking about.

i've tried refining the mesh from extra course to finer, and decreasing the initial step size of the solver to 1e-10 but it gave the same results.


Can you also plot the concentration of A on the surface as well?


here are the plots of concentration AB on the surface. and Concentration A along the surface. (although last one is taken from the transport of diluted species data because A is not a surface species.) i also included the contour plot of the concentration of A inside the domain, which i never see falling below 0.
[QUOTE] [QUOTE] [QUOTE] [QUOTE] [QUOTE] Where is the negative concentration located? [/QUOTE] the negative concentration is located on the surface where the reaction happens. On the rest of the domain the concentration remains positive and physically feasible. [/QUOTE] So the WHOLE surface has negative concentration? [/QUOTE] No,it starts at the edge of the reaction surface, but as time increases a greater portion of the surface reaches a negative concentration. I've included a picture of the domain i'm talking about. i've tried refining the mesh from extra course to finer, and decreasing the initial step size of the solver to 1e-10 but it gave the same results. [/QUOTE] Can you also plot the concentration of A on the surface as well? [/QUOTE] here are the plots of concentration AB on the surface. and Concentration A along the surface. (although last one is taken from the transport of diluted species data because A is not a surface species.) i also included the contour plot of the concentration of A inside the domain, which i never see falling below 0.

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Peter Chang
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Posted: 7 years ago Jul 27, 2017, 12:32 p.m. EDT






Where is the negative concentration located?


the negative concentration is located on the surface where the reaction happens.
On the rest of the domain the concentration remains positive and physically feasible.


So the WHOLE surface has negative concentration?

No,it starts at the edge of the reaction surface, but as time increases a greater portion of the surface reaches a negative concentration. I've included a picture of the domain i'm talking about.

i've tried refining the mesh from extra course to finer, and decreasing the initial step size of the solver to 1e-10 but it gave the same results.


Can you also plot the concentration of A on the surface as well?


here are the plots of concentration AB on the surface. and Concentration A along the surface. (although last one is taken from the transport of diluted species data because A is not a surface species.) i also included the contour plot of the concentration of A inside the domain, which i never see falling below 0.


Sorry man, I am stumped. With your B concentration going negative, you should have a negative reaction rate, which should cause the generation of species A instead of consumption. But your A contour plot looks perfectly normal.
[QUOTE] [QUOTE] [QUOTE] [QUOTE] [QUOTE] [QUOTE] Where is the negative concentration located? [/QUOTE] the negative concentration is located on the surface where the reaction happens. On the rest of the domain the concentration remains positive and physically feasible. [/QUOTE] So the WHOLE surface has negative concentration? [/QUOTE] No,it starts at the edge of the reaction surface, but as time increases a greater portion of the surface reaches a negative concentration. I've included a picture of the domain i'm talking about. i've tried refining the mesh from extra course to finer, and decreasing the initial step size of the solver to 1e-10 but it gave the same results. [/QUOTE] Can you also plot the concentration of A on the surface as well? [/QUOTE] here are the plots of concentration AB on the surface. and Concentration A along the surface. (although last one is taken from the transport of diluted species data because A is not a surface species.) i also included the contour plot of the concentration of A inside the domain, which i never see falling below 0. [/QUOTE] Sorry man, I am stumped. With your B concentration going negative, you should have a negative reaction rate, which should cause the generation of species A instead of consumption. But your A contour plot looks perfectly normal.
Kyle Minor
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Posted: 7 years ago Jul 27, 2017, 5:19 p.m. EDT
Just put "max(eps^2, R)" where you would put the user defined reaction rate. With R being your actual reaction rate. This should fix the issue.
Just put "max(eps^2, R)" where you would put the user defined reaction rate. With R being your actual reaction rate. This should fix the issue.
Jasper N
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Posted: 7 years ago Jul 27, 2017, 5:53 p.m. EDT
Updated: 7 years ago Jul 27, 2017, 5:55 p.m. EDT

Just put "max(eps^2, R)" where you would put the user defined reaction rate. With R being your actual reaction rate. This should fix the issue.

thanks! i've put the term around my reaction rate term of the species which went negative in concentration. It seems to have fixed the issue!
[QUOTE] Just put "max(eps^2, R)" where you would put the user defined reaction rate. With R being your actual reaction rate. This should fix the issue. [/QUOTE] thanks! i've put the term around my reaction rate term of the species which went negative in concentration. It seems to have fixed the issue!
Kyle Minor
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Posted: 7 years ago Jul 27, 2017, 6:31 p.m. EDT
glad I could help
glad I could help

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