[User12]

Hello,

I am experiencing an issue in Fluent that is driving me crazy. I have a microchannel (small diameter, large length) in which a reaction occurs only in the wall (catalyst). The reaction is C2H5OH + H2O --> 4 H2 + 2 CO and is defined as direct reaction (no equilibrium) with reaction order 0.1 for C2H5OH and 1 for H2O.
My problem is that when I set a high pre-exponential factor so as to achieve fast kinetics - this is how I need it to be - I observe that CO and H2 keep on generating and H2O keeps on consuming even though C2H5OH is completely exhausted (Mass fraction=0). This creates an atomic imbalance from inlet to outlet, i.e., the mass of C in the inlet is notably lower than in the outlet. Global mass balance is respected, so H and O atomic balance also deviate in order to compensate for the deviation in C balance. The faster I set the reaction rate, the higher this deviation is. This imbalance grows as a straight line, so it makes me think of a numerical error that I cannot mitigate.

I am using pressure-based solver with coupled pressure-velocity scheme and second order discretization for all the variables. Residuals indicate that solution is converged <1e-8 and all the monitors I use are completely constant. I have tried using stiff chemistry solver, but it does not work either. I am using R15, but I have checked that the problem also occurs in R18.

Did anybody have similar problems? Do these observations suggest you where the problem might come from?

Any help is kindly acknowledged!

[User12]

Hello again,

I attach two pics in order to explain myself a little bit better. In the case of higher reaction rate, H2 and CO keep on generating despite C2H5OH is completely consumed, whereas for lower reaction rate they keep constant once C2H5OH is completely converted - as expected. As you can see, at lower reaction rate, c-mass is constant along the channel length, whereas at higher reaction rate, it is not. C-mass definition is in the picture too. This parameter should be constant, since atomic balance must be conserverd unless nuclear reactions take place, which is not the case.

Thanks for any help received!