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Reaction Flow_Reaction Rate Unit Problem
Posted Feb 1, 2021, 3:59 a.m. EST Computational Fluid Dynamics (CFD), Chemical Reaction Engineering Version 5.6 2 Replies
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Hi,
We have a concentrated reaction flow (transportation of concentrated species + turbulent flow) model to simulate. Because of a keypoint in simulation we've encountered 2 main problems. First one is defining the reaction rate constant. According to Arrhenius equation, rate constant (k) in unit of "cm^3/(molecule.s)" but COMSOL wants us to define this in a different form. The form comes from R = k[A][B] rate law and, it uses R as rate constant in unit of "mol/(m^3.s)". Our model consists of chain reactions and we couldn't find any way to define our rate constant (k) as intented (note: We tried to define it as "m^3/(mol.s)" but given results were wrong.) Second problem is also about reaction rate constant. After encountring with a problem in unit definition, we tried to define it with the option of activating "Use Arrhenius Expressions" and then selected reaction rate calculation to automatic. Unfortunately two problems came up with this solution. First, in documentation of Comsol it says A (forward rate constant, in Arrhenius form) is dimensionless but our A (pre-exponential rate constant) is in units of "cm^3/(molecule.s)". Second, when we select "Use Arrhenius Expressions" if we don't define any initial concentration in volume (except mass flows from inlets and inflows) reaction is not working. But initially there shouldn't be any species in our volume. Is this a problem of solver for initial iterations? To sum up, We have questions as listed below; 1) How can we convert mentioned reaction rate units to intended versions for COMSOL? 2) To simulate model without initial concentrations in our volume, what do we have to change in our model? (solver or reaction rate definition?)
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COMSOL converts everything to SU units. In the uppermost level you can, however, define the unit system, but I do not see a problem. You can type k = 0.1 cm^3/(mols) and COMSOL writes 10^-10 m^3/(mols). It does not, naturally, effect on the calculations, as the concentrations are expressed in mol/m^3.
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Dear Lasse,
Thank you for the response. It was a problem that I encounter when I create a model without using Reaction Engineering interface. Somehow I confused about the unit of Af (forward frequency factor) because it is unitless (in COMSOL Help documentation). When I started to use reaction engineering interface I saw the unit of Af as m^3/mol.s. Defined it that way then it worked. Thank you so much.