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positive feedback and negative species

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Old   March 10, 2005, 13:58
Default positive feedback and negative species
  #1
Andrew Garrard
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Hello everyone. I am attempting to write my own reaction model (because the kinetics in fluent do not cover what I need). I am using a UDS to model the species conc (in mol/m3 but could easily be mass fraction). The reaction rate is proportial to the species concentration like an arhenius rate. When I use the UDS however, I end up in a situation where the species concentration diverges. The reason is because the species concentration (modeled with UDS) goes negative. In a real life situation as the species concentration drops the reaction rate drops and hence the concentration can never go to zero.

My question is how does the FLUENT model stop the species mass fraction from going negative when the reaction rate may dictate that it should? The might be as a result of temperature, for example.

Any help deeply appreciated as I am at my wits end.
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Old   March 10, 2005, 20:29
Default Re: positive feedback and negative species
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cg
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The way Fluent handles this is:

if( source > 0 and conc > SMALL )

add source to S co-eff else

add source/conc to ap co-eff

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Old   March 11, 2005, 05:40
Default Re: positive feedback and negative species
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Andrew Garrard
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Thankyou for your help with this, but could your be a little bit more verbose with your descirptions. could you explain what S co-eff and ap co-eff are? BTW, I am working with a surface reaction, so there will be no source, just a boundary conditions, but I imagine the same same situation applies.
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Old   March 14, 2005, 21:05
Default Re: positive feedback and negative species
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cg
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For surface chemistry you are correct: you should set the species value at the boundary based on the cell center value (that is, solve reaction= diffusion at the boundary). I don't understand how this could be negative though, or cause Fluent to give negative species.

As for the ap and S co-efficients: Fluent sets up a linear system:

ap phi_p + Sum_nb ( a_nb phi_nb ) = S

where nb denoted neighbour.
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Old   March 16, 2005, 11:35
Default Re: positive feedback and negative species
  #5
Andrew Garrard
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I am setting the species flux at the boundary, not the species value. I am setting this using the set UDS flux function in the fluent GUI in the wall boundary condition panel.

Please allow me to illustrate why the values for species (modeled using a UDS) go negative. (sorry for long explanation)

The rate of reaction, consumption of species is dependant on the concentration of the species and another value, let's say temperature. Now say the consumption of species causes a temperature rise and the reaction is very sensative to temperature change. When the reaction proceds the tempreature goes up and cause a large reaction rate. Fluent then caluclates the species concenration at the boundary which is negative. The tempreature goes up alot. The rate of reaction is then reversed, but because the species concentration has such a large negative value, the reaction rate causes the value of concenration to go to an even larger positive value than when the calculation started. A divergence (or +ve feedback) loop exists. This could be avoided if the cocentration were not allowed to go below zero. In the fluent species model it isn't allowd to go to zero. How is this achieved.

Also, with your equation. I don't really understand what the terms are, what is ap, phi_p, and a and what is the equation refering to. Is there a refernce to this equation in the manual?

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