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Old   September 1, 2011, 02:50
Default Chemkin units
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Im using chemkin for combustion in dieselFoam and need to change the pre-exponential factor and activation energy for the reactions.
However trying to understand the default one is hard since there are no references to the literature for the values, there is also no unit for the pre-exp factor, and then I dont mean the orders 0.25 and 1.
The default value for A is 5E8 in the chem.inp, and the 1E8 for the chem.inp_15.
My reference value in Turns book is 5.1E11, but there is no straightforward scaling.
Does anyone know where these values come from ?
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Old   September 2, 2011, 04:59
Franco Marra
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Dear Rickard,

this is more a general question, concerning not only OpenFOAM. In order to understand the right way for translate coefficients of chemical reactions from one units to another, I would suggest you to write down the full balance equations in the form of ODE: d\rho Y/dt=omega.
By keeping consistent, while changing units, the dimensions of left and right hand side, you will find the rule for unit transformation of the reaction coefficients appearing in omega.

The following report by Andreas Lundstrom:
from the excellent web site of Prof. Nilsson, will give you further information and details.

Best regards,

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Old   September 2, 2011, 05:07
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Hi Franco, thanks for the reply.

I read the tutorial by Anders some time ago, and also checked up on how the chemkinReader for OF works. The problem for me, is when the default values are just taken out of thin air.
The values of 5E8 for C7H16, I can imagine are found from some reference, like Turns, who expresses the value in (gmol/cm3)^(1-m-n)/s and not (mol/cm3....)
What they probably did, was just to take the value 5.11E11 and divided it by 1000 to get mol/cm3, without taking into aspect the effect of the orders m and n.
For ethanol I just did as you said but considered the orders. It should work okay now. The activation energy value however, I dont know where they found it. It seems a bit low to me anyway, for my fuel its around 30 000.

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