Compressible laminar flow & Transport of diluited species: why there is no outflow of moles from the domain?

Hi everyone,

I have to model the transport of a chemical species dissolved in a gas mixture during an expansion of the gas.

I simulated a time-dependent gas expansion due to a reduction of pressure in a rectangular 2D domain with the Laminar Flow module.
I set:
- compressible flow (Ma<0.3),
-one boundary as the outlet ("Pressure, no viscous stress", with the pressure equal to a smooth ramp going from 800 torr to 760 torr in 3 sec);
- the other 3 boundaries as walls.

To model the transport of the chemical species I used the "Transport of diluited species module", with the "Outflow" boundary condition on the same outlet boundary of the laminar flow, and the others as No flux). The velocity field (u,v) is taken from the laminar flow.

The expansion of the gas is well simulated: there is an outward flow during the decreasing of the pressure.
Instead, there is no an outflow of the chemical species: the total content of moles in the domain is the same all over the time of the simulation. This is clearly unphysical.

Why is that?
Someone can help me?

Thank you so much.

J.