Non-reacting species still showing reaction rate

sdsuaztec · October 30, 2022, 04:17

Version of Fluent is 2022 R2, but issue has been encountered in earlier versions as well.

I am modeling a hydrocarbon combustion reaction using the Species Transport model with volumetric reactions. The form is Hydrocarbon + O2 -> CO2 + H2O(v). There is N2 in the modeling because the oxidizer is air, but no N2 reactions have been defined. After running calculations, if I plot N2 reaction rate, there is a non-negligible rate of N2 being consumed and/or generated, depending on the case being run. The reaction rate does reduce and change location with iterations, though I have never run enough iterations to eliminate the effect altogether.

I thought this might be caused by the Fluent feature where the last component listed in the mixture material is used to fill in any imbalances, so I added Argon as a final species and ran per usual. For that case, Argon did indeed have significant reactions. However, there were also Nitrogen reactions, although they appear to be an order of magnitude lower and likely balanced between generation and consumption of N2.

I am looking for confirmation that this would just be a matter of mass and energy imbalances that should resolve with more iterations, and that it is just a matter of Fluent including reaction terms in the calculations for these non-reacting species.

alainislas · October 30, 2022, 06:58

Whate are your mixture species? Yes, in this case N2 should be the last specie defined in the species list, so Fluent treats it as the bulk.

Which TCI model you using? Can you post a picture of your reactions panel?

October 30, 2022, 04:17	Non-reacting species still showing reaction rate	#1
sdsuaztec New Member Tanner Join Date: Oct 2022 Posts: 1 Rep Power: 0	Version of Fluent is 2022 R2, but issue has been encountered in earlier versions as well. I am modeling a hydrocarbon combustion reaction using the Species Transport model with volumetric reactions. The form is Hydrocarbon + O2 -> CO2 + H2O(v). There is N2 in the modeling because the oxidizer is air, but no N2 reactions have been defined. After running calculations, if I plot N2 reaction rate, there is a non-negligible rate of N2 being consumed and/or generated, depending on the case being run. The reaction rate does reduce and change location with iterations, though I have never run enough iterations to eliminate the effect altogether. I thought this might be caused by the Fluent feature where the last component listed in the mixture material is used to fill in any imbalances, so I added Argon as a final species and ran per usual. For that case, Argon did indeed have significant reactions. However, there were also Nitrogen reactions, although they appear to be an order of magnitude lower and likely balanced between generation and consumption of N2. I am looking for confirmation that this would just be a matter of mass and energy imbalances that should resolve with more iterations, and that it is just a matter of Fluent including reaction terms in the calculations for these non-reacting species.

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October 30, 2022, 06:58		#2
alainislas Senior Member Alain Islas Join Date: Nov 2019 Location: Mexico Posts: 142 Rep Power: 6	Whate are your mixture species? Yes, in this case N2 should be the last specie defined in the species list, so Fluent treats it as the bulk. Which TCI model you using? Can you post a picture of your reactions panel?