[newbie]

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.

[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.

[newbie]

Allan,

Thanks so much for your reply. It helps a lot.

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.

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.

[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.

[newbie]

Thanks a lot.

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.

[ayannath23]

Thanks so much for this calculations & clear explanation.....this clears some doubts..