positive feedback and negative species

Andrew Garrard

Hello everyone. I am attempting to write my own reaction model (because the kinetics in fluent do not cover what I need). I am using a UDS to model the species conc (in mol/m3 but could easily be mass fraction). The reaction rate is proportial to the species concentration like an arhenius rate. When I use the UDS however, I end up in a situation where the species concentration diverges. The reason is because the species concentration (modeled with UDS) goes negative. In a real life situation as the species concentration drops the reaction rate drops and hence the concentration can never go to zero.

My question is how does the FLUENT model stop the species mass fraction from going negative when the reaction rate may dictate that it should? The might be as a result of temperature, for example.

Any help deeply appreciated as I am at my wits end.