Setting of chemical reactions

After all chemical species are entered in the Species tab of the Species dialog, the chemical reactions between them can be defined in the next tab Reactions . The same species identifiers that were entered and accounted in the Species tab, now are accessible in the reactions tab.

There are homogeneous and heterogeneous chemical reactions. The homogeneous reactions are localized in the inner volume of regions. They produce sinks and sources in the transport equations for the species transport. The heterogenous reactions are localized in opposite at the interfaces between the specified materials. They contribute to the balance boundary conditions for the species boundary concentrations.

Dialog for definition of chemical reactions 

Figure 48. Dialog for definition of chemical reactions

reactions list

After all reactions are defined, they appear in the drop-down reactions list. The single reaction can be chosen in the list. Then all reaction data are indicated in the dialog window. The reaction data can be modified or the reaction can be deleted from the dialog list.

account change

Analogously to the Species tab the changes in the reaction data will be accounted in the graphical user interface after the button account change is pressed. The data in the computational chemical model will be modified only after the button Apply or OK will be pressed. The data of the already defined reaction can be modified. The new added reaction data will be accounted in the graphical user interface also only after confirmation by pressing the account change button.

The mnemonic description of the currently selected reaction in the bottom part of the dialog window is also updated after the button is pressed.

After the button account change is pressed the consistence of the entered reaction data is controlled. In case of inconsistency the storage of the reaction data will fail and the error message will be indicated. Examples of possible mistakes are shown below.

Error message in case if same species are present in the reactant and product part of the reaction. 

Figure 49. Error message in case if same species are present in the reactant and product part of the reaction.

Error message if storage of the reaction data failed because of wrong user input. 

Figure 50. Error message if storage of the reaction data failed because of wrong user input.

Error message in case if the same materials for identification of the interfaces for the heterogeneous chemical reaction are entered. 

Figure 51. Error message in case if the same materials for identification of the interfaces for the heterogeneous chemical reaction are entered.

Error message in case if a pair of the same reacting species is entered. 

Figure 52. Error message in case if a pair of the same reacting species is entered.

add new reaction

Adds a new reaction to the list. The reaction will appear with default parameters e.g. "reacting species 2" etc. Then all reaction parameters should be specified. According the data of the new reaction should be confirmed by pressing the button account change . The number of considered chemical reactions is not limited. For the maximum possible number of independent reactions, Nr, one should consider the number of the chemical elements Nel, the number of the chemical compounds in the liquid phase Ncom and the number of the condensed media Ncon:

Nr = Ncom + Ncon - Nel + 2

delete reaction

This button deletes the currently selected chemical reaction from the reaction list.

reaction name

The reaction name serves for identification of the reactions. The name of each reaction should be unique and not occur twice or more in the reaction list.

Spaces are allowed in the reaction names. The same is valid for the species and transport media names which are copied from the materials list of the CrysMAS model.

reaction type

The dialog box reaction type selects between two available reaction types: homogeneous and heterogeneous. By selection of a particular type the dialog window is adjusted, some dialog items will be hidden and some items appear according to the number of required reaction items to be entered. The definition of a homogeneous reaction requires an input of the single transport media and heterogeneous reaction is described by means of two materials treated formally as medium.

Number of reacting species

Either one single reactant or a pair of reactants are available. In case of the single reactant the dissolution of the solid phase species at the phase interface or the homogeneous decomposition reaction in the single medium can be considered.

Number of reaction products

Either single or pair of reaction products are available at the moment.

Tmin

The activation of this option allows to define the zero reaction rate everywhere in the transport medium where the absolute temperature is below the specified threshold value of the activation temperature. In the rest volume with temperature higher then the activation temperature the reaction rate is computed according to the reaction expression entered in the dialog field rate expression.

Reaction Enthalpy

If heat is produced or consumed in the homogeneous chemical reaction, this requires the back coupled consideration of the heat transfer with the chemical model. The consideration of the reaction heat should be avoided if its contribution into the global heat balance is negligible because otherwise the computational effort increases significantly.

The exothermal or endothermal reaction can be specified by checking the checkbox Reaction Enthalpy . The reaction enthalpy is entered in W/m^3. This energy density corresponds to a reaction rate and concentrations of reactants equal to unity.

reacting species

The reacting species are entered in this dialog box. The single reacting or two reacting species are selected from the drop-down lists where all chemical species defined in the previous dialog tab Species are placed. The reacting species may not coincide with any of the reaction products.

valence

The valence of the reacting species and of the created reaction products is entered in the dialog frames of the reacting species and of product species as positive numbers according to valence in chemical reactions. The amount of reacted species is proportional to the corresponding valence. The default value for the valence is unity. The reaction rate in the rate expression dialog field corresponds to the reaction rate where reactants are entering with the unity valence.

product species

The product species are entered in this dialog box. The single reaction product or two products are selected from the drop-down lists where all chemical species defined in the previous dialog tab Species are placed. The product species may not coincide with any of the reacting species.

transport medium 1

This medium is the material name. The name identifies the material where the species transport and homogeneous reaction will be considered provided the reaction is defined as homogeneous.

Otherwise the first transport medium is one material from the materials pair which identify the interface where the reaction takes place. The heterogeneous reactions will be computed everywhere at the region boundaries where two defined materials (transport media) stay in direct contact.

If the same material name occurs in several heterogeneous reactions as a transport medium, then correspondingly several different chemical reactions should defined.

transport medium 2

This is a counterpart of the transport medium 1 . The second transport medium is enabled only for the heterogeneous reactions. Together with the first medium it identifies locations where the reaction is running.

reaction rate expression

Defines the reaction rate as a constant value or a function of absolute temperature in Kelvin. The reaction rate expression refers to unity concentration and unity valence of the reacting species.

The species amounts are given by the mass fraction in the transport model formulation. The reaction rate expression is multiplied with the molar weight of the species in the reactive species source term according to the transport model formulation.

reaction order

The reaction order is the power in which the mass fraction of the reacting species occurs in the reactive source term. The default value for it is unity. It means then, that the total reactive species consumption in the reaction with single reactant is directly proportional to its mass fraction.

The reaction order is defined for the second reacting species in the same sense for reactions with a pair of reacting species. The reactive consumption rate of each reacting species is then calculated as product of the molar weight with the reaction rate and with the mass fraction of each participating species in power given by the reaction order for the corresponding reacting species.

saturation

The activation of this option in the dialog box saturation driven sets the reaction rate additionally in product with the excessive concentration of the single reacting species. This option is available only for reactions with single reactant. The total reaction rate is then a product of the reaction rate expression with positive difference between the threshold value of the mass fraction and the actual mass fraction value. The threshold mass fraction can be defined also temperature dependent.

The resulting reaction rate is zero in case of the saturation driven reaction if the mass fraction is less than the threshold (saturation) value.

Reaction mnemonics

The last two rows of text on the bottom of the dialog window is the mnemonic description of the actual selected reaction. Here the formal description of the defined reaction is assembled. The mnemonic expression serves as an auxiliary mean to facilitate the user control of the reaction data.

The reaction mnemonics text is generated each time after the button account change is pressed and the reaction data is stored within the graphical user interface.

Related procedures

Definition of chemical reactions .