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How can I write Eddy-dissipation model in DEFINE_VR_RATE udf?

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Old   September 20, 2017, 05:55
Default How can I write Eddy-dissipation model in DEFINE_VR_RATE udf?
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Sangwoo Kim
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Hi.

I have three reactions and want to use finite-rate model to two reactions and eddy-dissipation model to one reaction
So I should use UDF, DEFINE_VR_RATE. In UDF manual, it gives an example about finite-rate model. But, there is no example about eddy-dissipation model. and I have no idea that how I implement the eddy-dissipation model.

As you may know, DEFINE_VR_RATE macro returns only two values: rr for laminar and rr_t for turbulent.
They may be used to calculate source and sink term of each species due to the reaction.
But, the reaction rate of eddy-dissipation model is defined for "each species" in Theory Guide manual.
What values should I return as a result of UDF in this case?

Briefly, can anyone let me know how I write DEFINE_VR_RATE udf for implementing eddy-dissipation model?

Thank you.
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Old   September 20, 2017, 08:39
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The UDF manual says "Note that rr and rr_t are conversion rates in kmol/m3-s. These rates (rr and rr_t), when multiplied by the respective stoichiometric coefficients, yield the production/consumption rates of the individual chemical components"

First image that I attached is the definition of the reaction rate (in kg/m3-s) of Eddy-dissipation model.
I guess rr_t is kind of 'universal' conversion rate that is applied to all species.
According to the UDF manual, rr_t will be multiplied by the stoichiometric coefficients. So, the stoichiometric coefficients should not exist in the formula of rr_t
And the conversion rate that is returned is in kmol/m3-s. So, molecular weight should not exist in the formula of rr_t
Then, maybe I have to return the second image (minimum value, of course) as rr_t
Am I understanding correctly??

I have only a few of experience of using UDF so any comments will be really helpful.

Thank you
Attached Images
File Type: png rxn rates.png (6.5 KB, 58 views)
File Type: png rates.png (10.8 KB, 66 views)
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Old   September 25, 2017, 22:49
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Sangwoo Kim
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leave this thread just for someone's information.
I checked several cases with my udf and concluded that my guess was correct.
you can use the formula shown in the second image in order to model some chemical reaction with eddy dissipation model
if you let the value from eddy dissipation to 'rr_t' in DEFINE_VR_RATE, and the value from finite rate to 'rr', then you can use finite-rate/eddy-dissipation model, using udf

But my calculation and result were not checked by other people. So if my conclusion is actually wrong, please leave a comment.
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Old   February 25, 2020, 17:31
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Samot Cevardaz
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Dear swtbkim,

could you kindly help me with the implementation of rr_t equations in a udf?

Kind regards,

Thank you
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