Discrepancy between the analytical solution&modeli
 

Hi, has anybody tried in modeling first order reaction in Fluent? I set up a batch reactor in Fluent. And a UDS as a reactor. I shut off the velocity and turbulence in solution, just tried to calculate the UDS only. Of course it is unsteady problem, segregated solver, implicit. I set the time size to 1 sec. And found out that the reaction rate from the modeling result was lower that the one I set in source term. (modeling result: C=C0*exp(-0.0062t) source term: C=C0*exp(-0.0076t)). I decreased the time size to 0.5,0.1 sec, the difference even became bigger. I am confused. Can this problem caused by my mesh quality? Any advice is welcome. Thanks in advance.