Problem description

There appears to be an inadequate upstream boundary condition in IonFlow. For charged species, the upstream boundary condition uses a von Neumann (zero gradient!) assumption, where the following rationale is provided:

There are obvious concerns about the physicality of this boundary condition; mathematically, this appears to impose von Neumann conditions for both inlet and outlet, which is inherently ill defined. Finally, the short-cut does not accommodate a case where a user could specify a non-zero upstream concentration of a charged species (where a convective term added in Boundaries1D::eval would be quite problematic).

While upstream gradients may be sufficiently small for this ad-hoc assumption to work for a free flame (i.e. ion_free_flame.py), it appears that this assumption is inadequate for burner-stabilized flames. However, the assumption does not appear to be a root cause for poor convergence (see Additional Context).

System information

• Cantera version: 2.5
• OS: Ubuntu 18.04
• Python/MATLAB/other software versions: 3.6

Additional context

The example ion_burner_flame.py exhibits extremely poor convergence with multiple errors thrown (recovered in auto solution mode)

***********************************************************************
CanteraError thrown by OneDim::timeStep:
Took maximum number of timesteps allowed (500) without reaching steady-state solution.
***********************************************************************

As this happens during stage 1 of the solution (i.e. using IonFlow::frozenIonMethod) where m_flux of charged species is set to zero (and the code section above is bypassed), the boundary condition is not a root cause. Commenting out the lines where the BC residual is set in IonFlow::evalResidual does indeed not change the behavior.

Poor convergence is also observed for Cantera 2.4, i.e. the version where the example was initially introduced.