Setting of boundary conditions

The chemical model requires boundary conditions for the transported chemical species. Usually the boundary condition of Poincare type is automatically associated with the heterogeneous chemical reaction where the considered species is involved either as the reacting agent or as the reaction product. Such reaction boundaries are identified by the solution algorithm automatically based on the description of the heterogeneous chemical reactions and on the materials distribution in the model.

If no reaction can be found at some region boundary, the chemical species is allocated and no boundary condition is defined explicitely for the chemical species. The boundary condition of the impermeable wall is applied then as the default one.

In some cases additional boundary values should be prescribed. For example this is the case at the flow inlet where the gas mixture is entered into the reactor. The mass fractions of the partial gases should be set by means of the Dirichlet boundary conditions for all species there. Also the reaction products which are absent in the gas mixture before entering the reactor and will be generated in it should obtain Dirichlet boundary conditions with zero value.

The dialog tab special boundaries serves for setting of special boundary conditions for single species.

Dialog for definition of special boundary conditions for chemical species 

Figure 53. Dialog for definition of special boundary conditions for chemical species

The items of the special boundaries dialog window are described below.

line selected

A first line of the drawing should be selected where the boundary conditions shall be defined. This procedure is identical with the line selection in the Boundaries dialog. The symbols for the defined boundary conditions can be inspected in the main CrysMAS window. A new boundary condition can be imposed or the existed boundary condition can be modified on the selected line(s).

Hide/ Show for selected species/ Show all

The main CrysMAS window shows special boundary conditions for species. Selecting Hide hides all boundary conditions for all species. Checking the checkbox Show for selected species shows special boundary conditions for the single species selected in the drop-down species list.

In order to refresh the indication after another species is selected from the species list one should click again on the checkbox Show for selected species or Show all. If the checkbox Show all is checked then all special boundary conditions for all species will be indicated.

The indication of special boundary conditions includes the identifier of the chemical species and the symbol of the type of the special boundary conditions in round parentheses.

Indication of the special boundary condition for chemical species 

Figure 54. Indication of the special boundary condition for chemical species

Boundary condition type

The boundary condition of Dirichlet, Neumann or Poincare type can be inserted for single chemical species. The chosen species should correspond with the defined single phase liquid materials.

The species derived from the solid phase materials are preserved for treatment as not transported solid species accumulated and consumed at the defined interfaces. No special boundary conditions can be prescribed for them because such species don't build a continuum field.

If several lines are selected where the same type of boundary condition with the same parameters is defined for species selected in the dialog window then the boundary condition type will be set for the checkboxes in the dialog window and entry fields for numerical parameters of the boundary conditions appear. Otherwise if boundary conditions don't much for selected species, no special boundary checkbox is checked and the entry fields will be hidden.

After several boundary lines and any species in the list are selected, the boundary condition can be defined at once for all selected lines by checking the checkbox for the type of the boundary condition.

Checking the checkbox no special boundary and pressing Apply deletes all special boundary conditions at selected lines for the selected species.

Species list

The drop-down species list consists of the defined identifiers of chemical species. The list is dynamically adjusted on the actual state of the chemical model each time changes in chemical species were accounted. One should take care to define the boundary conditions only for liquid chemical species. Solid chemical species are associated with the materials of the same name which consist of a single solid state phase.

Boundary Value

The same data is entered for the Dirichlet, Neumann and Poincare boundary conditions as for the temperature into the Boundary value dialog.

The Dirichlet value means here the mass fraction of the particular species.

The Neumann value is the flux density for the mass fraction of it.

The Poincare boundary conditions defines the flux density which depends linearly on the mass fraction.

All parameters can be entered also as functions of temperature.

Apply/ OK

The special boundary conditions are stored simultaneously in the graphical user interface data and into the computational chemistry model. No account change button is available for special boundary conditions. The complete chemical model which was treated first only in the graphical user interface will be checked for its consistency and then transferred to its computational counterpart.

Therefore first all inputs associated with chemical species itself and all chemical reactions in first two tabs of the Species dialog should be completed and then the boundary conditions specified.

Press Apply every time after a new boundary condition is entered into the dialog window or the existing boundary condition is modified or deleted.

The description of the chemical model will be available in the computational part and can be written later into the crys - file only after its complete mnemonic description is transferred from the graphical user interface level by pressing the Apply or OK button.

Related procedures

Definition of special boundary conditions.