Definition of special boundary conditions

Boundary conditions for the chemical species are defined mostly automatically. Either no heterogeneous chemical reaction is prescribed for the considered interface treated as the external boundary of the computational domain for the chemical model. Then no flux boundary condition will be set automatically for all transported species. The interface is then the impermeable wall. Or at least one heterogeneous reaction is defined for the considered interface and transported chemical species. Then the diffusive flux of the species is computed at the interface as a summary result of all reactions with this species. Reactions rates and mass fractions of reacting and produced species are utilized for computation of the diffusive flux of each species in the transport model.

If no heterogeneous chemical reaction is defined for any species and no additional special boundary condition is formulated at the whole boundary of the computational domain, then the transport problem of the species is the convective-diffusive transport with the Neumann boundary condition at the whole boundary. The stationary solution of the transport problem is then defined with accuracy up to arbitrary integration constant. The numerical solution cannot be found for such conditions because the system of equations is under-determinated. Additional boundary condition is required in order to set the number of equations equal to the number of unknowns.

For the opened system the flow inlet and the flow outflow boundary should be defined. At the outflow boundary again no user defined boundary conditions for species are necessary. The zero diffusive flux will be set for all transported species there.

At the inlet boundary in opposite a Dirichlet values should be set for all transported chemical species. If some species are not available in the gas stream incoming into the reactor zone from outside and then the species is produced inside of the reactor zone as a result of chemical reactions, a zero mass fraction should be set as a Dirichlet boundary condition for this species.

The prescribed values or fluxes of the species at another interfaces like one between crucible and melt is also possible. In this case one should check that this boundary condition stays not in contradiction with any heterogeneous chemical reaction defined for the same interface.

A special dialog tab special boundaries in the dialog window Species can be used for definition of the additional boundary conditions listed above. The dialog tab special boundaries is used in the same manner as the dialog window Settings -> Boundaries for the variables other than of the chemical model. One selects a line or a group of lines, the chemical species in the species list, the type of the boundary condition and presses the Apply button.

Three types of boundary conditions are available: Dirichlet, Neumann and Poincare. The Dirichlet boundary condition refer to the mass fraction of the species. The other conditions refer to the molar diffusive flux in mol/mē/s. The diffusive mass flux of the species at the interface due to the chemical reaction can be recomputed by multiplying of the molar flux (considered by the code) by the molar weight value of the species.

After all selections for the boundary condition are done and the button Apply is pressed, the new or the modified old boundary condition will be stored into the mnemonic part of the chemical model.

Note

There is no button apply changes in the special boundaries tab which can be applied in two other tabs after any model modification was done. Therefore the full entered chemical model is stored on the mnemonic level each time after the boundary conditions are changed or added and confirmed by the Apply button. Therefore first finish all settings for species and reactions and only then proceed with special boundary conditions.

Related Procedures

Setting of boundary conditions