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Reaction Rate constant at the wall

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Old   December 29, 2016, 16:18
Default Reaction Rate constant at the wall
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At this site:
and the 2 follow ups the reaction at walls is described...
I'm wondering though in which units k_f is?!?
Or even the gas phase concentrations [G] and site species [S]
On the follow up pages it is talking about surface density [S] and thus i presume it is sth like kmol/m^2.
[G] on the other hand I guess is the molar concentrations in kmol/m^3 but in the cells at the wall, right?
Furthermore the molar reaction rate R_i has to be sth like kmol/m^3/s and a reaction like
would then imply k_f being in units m^2/kmol.
On the other hand I'm not sure how to interpret this reaction constant.
I do know the formula
where gamma is the reaction probability upon interaction with the wall and n the molar concentration in [kmol/m^3] and v the mean thermodynamic velocity of the molecules at some given temperature.
Gamma is in units kmol/m^2/s and therefore gamma*v/4 is in units m/s. Gamma cannot be compared with R_i as the units are wrong (is R_i in a surface correction indeed in kmol/m^3/s???)
In the picture above n would correspond to sth like [G] though remember [G] was only defined directly at the walls.
So the question now is how do I relate this to to k_f
Often one can relate Gamma to a volumetric reaction rate constant including the geometry, i.e. surface A and volume V
the mean reaction constant in units 1/s would be
gamma*v/4 * A/V.
This however would give me a mean value for the concentration in the entire volume.
In the ANSYS surface reaction mechanism this value should be probably much higher, since the region of interest is directly at the wall!?!?
So yes, can anyone tell me how to proceed here?

PS: I'm thinking of sth like I would need the mean ratio dA/dV (surface to volume ratio) of the cells at the wall...Is there sth like this?

Last edited by Diger; December 29, 2016 at 17:50.
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