# unit conversion of pre-exponential factor A of chemkin mechanism in Fluent

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 August 11, 2009, 23:54 unit conversion of pre-exponential factor A of chemkin mechanism in Fluent #1 New Member   Kevin Lee Join Date: Aug 2009 Posts: 6 Rep Power: 9 I have a question about the unit of pre-exponential factor "A" after a mechanism is imported into Fluent. After I imported a mechanism in chemkin format into Fluent, I found that A's value was changed, usually divided by 10^3 for bimolecular reaction, and 10^6 for termolecular reaction. I am confused with "A" value. CHEMKIN manual says the unit of "A" depends on the number of reactants. what is a correct unit of "A" for a mechanism that is imported into Fluent? Could someone please help clarify this out? Thanks so much.

 August 12, 2009, 12:38 #2 Member   Allan Walsh Join Date: Mar 2009 Posts: 55 Rep Power: 9 The units for "A" depend on the reaction. You can write out the complete reaction and see what the units have to be for A to get the units to cancel. And since the units for FLUENT are specific (ie. m³) and rate constants are often given in other units like mm and the species' concentration [mol/m³] may be used once or twice or more in a mechanism, the conversion of A from one unit system to another will be mechanism dependant. Not sure if this helps - and the bookkeeping can be tedious. My experience with coworkers is that each mechanism might take a week to implement and test. Good luck.

August 12, 2009, 17:01
#3
New Member

Kevin Lee
Join Date: Aug 2009
Posts: 6
Rep Power: 9
Allan,

I wrote the following to see if I understood that correctly.

The consumption rate of a reactant is d[C]/dt = -k * [C1]*[C2] = -A T^n Exp(-Ea/RT) * [C1]*[C2] assuming a biomolecular elementary reaction. And [C1], [C2] are concentrations of reactants 1 and 2. Let's assume the unit of concentration is [mol/cm^3]. then A's unit should be something like [s]^-1 [mol/cm^3]^-1. This is the unit in cgs. In SI, the unit is [s]^-1 [1e+3 kmol/m^3]^-1. For unimolecular and termolecular reactions, then the SI units would be [s]^-1 [kmol/m^3]^-1, [s]^-1 [1e+6 kmol/m^3]^-1, respectively. Are these correct?

Is it true that Fluent will automatically convert the units of "A" after a mechanism is imported into Fluent, assuming that the "A" units in the mechanism file are in cgs?

Thanks again.

Quote:
 Originally Posted by Allan Walsh The units for "A" depend on the reaction. You can write out the complete reaction and see what the units have to be for A to get the units to cancel. And since the units for FLUENT are specific (ie. m³) and rate constants are often given in other units like mm and the species' concentration [mol/m³] may be used once or twice or more in a mechanism, the conversion of A from one unit system to another will be mechanism dependant. Not sure if this helps - and the bookkeeping can be tedious. My experience with coworkers is that each mechanism might take a week to implement and test. Good luck.

Last edited by newbie; August 13, 2009 at 16:10.

 August 13, 2009, 12:55 #4 Member   Allan Walsh Join Date: Mar 2009 Posts: 55 Rep Power: 9 If you are entering reaction rate constants directly into Fluent, they need to be in the units specified by Fluent, not on the "cgs" basis. I have not read in a Chemkin mechanism to Fluent so I don't know what system the mechanism needs to be specfied in. But it looks like you have the concept down.

August 13, 2009, 16:08
#5
New Member

Kevin Lee
Join Date: Aug 2009
Posts: 6
Rep Power: 9
Thanks a lot.

Quote:
 Originally Posted by Allan Walsh If you are entering reaction rate constants directly into Fluent, they need to be in the units specified by Fluent, not on the "cgs" basis. I have not read in a Chemkin mechanism to Fluent so I don't know what system the mechanism needs to be specfied in. But it looks like you have the concept down.

 November 14, 2010, 06:44 #6 New Member   Ayan Join Date: Nov 2010 Posts: 9 Rep Power: 8 Thanks so much for this calculations & clear explanation.....this clears some doubts..

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