transfer of species

Muhammad Shakaib

I want to compare two channels; one empty and other in the presence of rectangular bars in the center of the channel. The mass fraction of species at wall is 0.01 and the mass fraction of species in water is set to zero. When I take x-y plot of species at the wall the disruption of the conc. boundary layer is seen (i.e. the conc is around 0.001 direcly above the bar)but when I check the conc of species at the outlet (surface integral)it is less than for the case of empty channel. The reynold number for both the cases is same i.e. 200. The diffusivity is set to 1.67e-9 to have schmidt no. of 600.

What could be the problem.