Simcenter STAR-CCM+ Difficulty Reaching Mass Flow Rate Balance Of A Multi-Component Gas When A Very Small Mass Flow Rate At An Inlet Boundary Is Specified

Consider a computational domain with two inlets and one outlet. In the Continua sub node of Simcenter STAR-CCM+, Multi-Component Gas and Non-reacting models are enabled. The gas comprises of two scalars: Air and H2. A snapshot of the computational domain and its inlet and outlet boundary and the Physics selections in the Continua are shown below:

The mass flow rate balance between the inlet and the outlet boundaries at 100K iterations is -0.0599 g/s. The plot of the mass flow rate balance with respect to iteration is shown in the picture below. As you can see that the simulation has reached convergence (running further iteration will not change the solution). And yet the mass flow rate balace is far from being close to 0.0 g/s. In fact it is significantly larger than the inlet mass flow rate of the Hydrogen.

For this type of situation the solver is not designed to handle a very small inlet momentum velocity. The small inlet momentum velocity is caculated from the small value of inlet mass flow rate of Hydrogen and the large area of the inlet boundary. Instead of setting the boundary type to Mass Flow Inlet, it is advisable to change the boundary type to Wall. Under Physics Conditions, set Wall Species Option from Impermeable to Specified Flux. The details of the changes to the boundary setups are shown in the pictures below. Note that you may need to re-execute the meshing operation so that the Wall boundary, which was originally set to Mass Flow Inlet boundary type, will now have prism layers on the boundary.

After applying the above suggested changes (NOTE since the inlet mass flow boundary is set to wall, the meshing was re-run to ensure proper prism layers on the boundary), the mass flow rate balance at iteration 100K is 1.7x10^-13 g/s, which is the small value that we expect to see. The plot of the mass flow rate balance with respect to iteration is shown in the picture below:

So for steady state simulation involving multi-component gas (e.g., Air and H2) and a very small mass flow rate of one of the gas components, the mass flow rate balance for that gas component at all inlet and outlet boundaries may not approach 0.0 g/s. In fact its value may be larger than the specified inlet mass flow rate. In our example above the inlet mass flow rate of one of the gas components is 4.0 x 10^-5 g/s. And the mass flow rate balance for that gas component at all inlet and outlet boundaries is -0.0599 g/s, which is significantly larger than the specified inlet mass flow rate. When you encounter this type convergence problem, you may change the boundary type from Mass Flow Inlet to Wall. And within that boundary type Wall settings, you may set Specified Flux for Wall Species Option and they specified the mass flux for that gas component.  

The SIM file included in this article contains the model mentioned above with the correct settings (Wall boundary type for the Hydrogen inlet boundary and steady state). You can use this model to reproduce the values reported in this article.