I am working on an internal flow model of a manure storage with a slotted cover that separates above airspace from contaminated air below. The initial gas concentrations are set up using an f(z) table with the space above the cover at 100% air, and the space below 156.9 ppm H2S gas. I set up an internal fan in the center of the slotted cover, with the outlet on the bottom, and the inlet at the top, so that fresh air is drawn in from above and forced into the pit beneath to evacuate the H2S. The model seems to work as far as flow vectors goes - i.e. it seems like flow lines go from the top of the fan through the bottom, and recirculation happens nicely. The problem I am having is that the internal fan seems to always be blowing in 156.9 ppm of H2S, not clean air from above, so the only fresh air that enters the pit comes in as a result of recirculation and vortexing through the slots.
Can anyone offer any tips? I see with the other types of fans that substance concentrations can be defined, but for the internal fan this option is not available (SW Flow Sim 2011). I have also tried lifting the internal fan higher, so that the outlet is above the hole but in the clean airspace above, thinking maybe it would push clean air in that way, but the f(z) initial conditions seem to override that for some reason. I've attached a copy of my model in case it helps.