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The problem of Stiff Chemistry Solver in Multiphase model

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Old   July 2, 2015, 09:08
Default The problem of Stiff Chemistry Solver in Multiphase model
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Yu-sen Niu
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Hello everyone!
I'm working on a project using ANSYS Fluent 13.0. In order to simulate the chemistry reactions between species, the Laminar Finite-Rate model is turned on. The Stiff Chemistry Solver and ISAT are used to make computation stable and fast. It works very well just in sing phase model. After that I bulid a mixture multiphase model which contains liquid water, then I find the Stiff Chemistry Solver disapper in Species Model Dialog, and I can't use ISAT also. This is bad, because the computation is very unstable without Stiff Chemistry Solver and ISAT, actually it diverges in a few iterations. I have tried Eulerian multiphase model, but it also doesn't work.
I find that the reactions can be created in 'Phase Interaction'->'Reactions' dialog. But the options that can be used is not too much, the 'Third-Body' can not be added into the reaction. And the Arrhenius React Rate can't be used in Mixture Multiphase, so I have to write udfs for the reaction rate. Although it is very complicated, I have fininshed this job finally. The udfs are compiled and loaded into the model. And the 'Heterogeneous Stiff Chemistry Solver' is used. But when I run the simulation, the Fluent reports these errors at the beginning.
ODE Solver failed in 14520 cells.
Mass_Diffusivity invalid(zero) diffusivity.
Please check material properties.
I have tried again and again, it's all the same. Then I turn off 'Heterogeneous Stiff Chemistry Solver', these errors are gone, and the simulation can go on, but the residual curve is very bad.
So does anyone have the same problem? And is it possilbe to use Stiff Chemistry Solver and ISAT in Mixture Multiphase model?
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Old   January 11, 2016, 02:52
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I think you can first check the material properties, especially the properties of the mixture.
The mixing law you chose would be significant.
After recheck and modification, you can try to initialize and check the density or viscosity.

Heterogeneous Stiff Chemistry Solver in "Phase→Phase interaction→Reactions" should be checked.
This solver allowed the user to include inter-phase reaction and in-phase reaction.
The "Third-Body" can not be added into the reaction in "Phase→Phase interaction→Reactions," but you can add these reaction that using third body into Reactions and Mechanisms in Material property use "Species transport" Model.

However, some things need more clearification :
1. Could one add the same reaction in "Species transport" model and "Phase→Phase interaction→Reactions."
It seems no conflict? would the reaction rate be doubled?
2. The contour of Heterogeneous reaction rate is zero, but the reaction heat is not zero. I don't know why. Is the reaction rate too large? or the time step is too large? Confusing.
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