I am modeling jet mixing in a tank and need help choosing an appropriate outlet boundary condition. The jet will be modeled using a constant velocity inlet and I need to define an outlet condition (ideally just a mass flow outlet) which is needed to keep the volume of fluid in the tank constant. Any suggestions will be appriciated.

If you use a pressure-outlet then you will see a check box where you can turn on a target mass flow rate. Be careful using the velocity-inlet BC on this problem. I don't know your exact problem but it seems like a mass-flow-inlet might be better. GOOD LUCK

If the flow is incompressible then Velocity inlet and mass flow inlet are identical. Velocity inlet and Pressure Outlet BC's should be sufficient. Regards,

The flammable fluid jetting out of a container needs the pressure difference and stochiometric fuel constants as input or as source terms.

So use the Pressure outlet boundary, mass flow rate also should govern actually.

Go through the tutorials on Combustion problem using Chemical reaction rates.