Chemical reaction UDF in Fluent

Londo · April 5, 2015, 07:29

Hello all,

Let's say that I have a reaction that goes as following:

C2H4 + 2O2 -> aCO + bH2O + cH + dOH

where a, b, c, d are not constants that you would usually have in a chemical mechanism, but are functions of concentrations and reaction rates.

This obviously has to go through UDF. The problem is that I can't seem to find the udf macro that allows to define non-constant stochiometric values. Can you help me with that?

CeesH · April 5, 2015, 08:14

Hi Londo,

I don't know of such a UDF either, but you can choose to skip the whole reaction formalism and go for source-UDFs. There, you can simply set the local source values (kg/m3-s) based on whatever equation you want, both positive and negative, hence it should not be difficult to combine with varying stoichiometry.

Best,
Cees

Londo · April 5, 2015, 08:52

Cees,

Thank you for you reply. That's an interesting idea. The question is whether Fluent will calculate temperatures in this case, or I'll have to write that as well?

CeesH · April 5, 2015, 09:27

You can include a sourceterm for energy in your UDF as well. It won't happen automatically indeed.

Londo · April 5, 2015, 09:38

The question is whether Fluent will be able to calculate the temperatures without it. Naturally, my aspiration is to keep the simulation as simple as possible, hence I wish to make as little changes as possible. Is it possible?

CeesH · April 5, 2015, 09:53

I'm not sure what you mean. If you use UDF sources instead of the built-in reaction model, there will be no calculation of reation enthalpy unless you explicitly add it to your source UDFs as an energy source. So unless there are other energy sources, your simulation will be isothermal. If temperature matters, you will need to add an enthalpy UDF as well. But as far as I know, that's the only way to do it. I know of UDFs to edit reaction rates, but not stoichiometry.

Londo · April 5, 2015, 10:10

Actually, this completely answers my question. Thank you very much!

CeesH · April 5, 2015, 10:14

Great! Glad to be of help

April 5, 2015, 07:29	Chemical reaction UDF in Fluent	#1
Londo New Member Londo Join Date: Feb 2015 Posts: 14 Rep Power: 11	Hello all, Let's say that I have a reaction that goes as following: C2H4 + 2O2 -> aCO + bH2O + cH + dOH where a, b, c, d are not constants that you would usually have in a chemical mechanism, but are functions of concentrations and reaction rates. This obviously has to go through UDF. The problem is that I can't seem to find the udf macro that allows to define non-constant stochiometric values. Can you help me with that?

April 5, 2015, 08:14		#2
CeesH Senior Member Cees Haringa Join Date: May 2013 Location: Delft Posts: 607 Rep Power: 0	Hi Londo, I don't know of such a UDF either, but you can choose to skip the whole reaction formalism and go for source-UDFs. There, you can simply set the local source values (kg/m3-s) based on whatever equation you want, both positive and negative, hence it should not be difficult to combine with varying stoichiometry. Best, Cees soheil_r7 likes this.

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April 5, 2015, 08:52		#3
Londo New Member Londo Join Date: Feb 2015 Posts: 14 Rep Power: 11	Cees, Thank you for you reply. That's an interesting idea. The question is whether Fluent will calculate temperatures in this case, or I'll have to write that as well?

April 5, 2015, 09:27		#4
CeesH Senior Member Cees Haringa Join Date: May 2013 Location: Delft Posts: 607 Rep Power: 0	You can include a sourceterm for energy in your UDF as well. It won't happen automatically indeed.

April 5, 2015, 09:38		#5
Londo New Member Londo Join Date: Feb 2015 Posts: 14 Rep Power: 11	The question is whether Fluent will be able to calculate the temperatures without it. Naturally, my aspiration is to keep the simulation as simple as possible, hence I wish to make as little changes as possible. Is it possible?

April 5, 2015, 09:53		#6
CeesH Senior Member Cees Haringa Join Date: May 2013 Location: Delft Posts: 607 Rep Power: 0	I'm not sure what you mean. If you use UDF sources instead of the built-in reaction model, there will be no calculation of reation enthalpy unless you explicitly add it to your source UDFs as an energy source. So unless there are other energy sources, your simulation will be isothermal. If temperature matters, you will need to add an enthalpy UDF as well. But as far as I know, that's the only way to do it. I know of UDFs to edit reaction rates, but not stoichiometry.

April 5, 2015, 10:10		#7
Londo New Member Londo Join Date: Feb 2015 Posts: 14 Rep Power: 11	Actually, this completely answers my question. Thank you very much!

April 5, 2015, 10:14		#8
CeesH Senior Member Cees Haringa Join Date: May 2013 Location: Delft Posts: 607 Rep Power: 0	Great! Glad to be of help