Modeling Bulk Ion Chemical Reactions

Simcenter STAR-CCM+ allows you to model chemical reactions that involve electrochemical species within the bulk fluid volume of the electrolyte. Currently, the ability to model equilibrium reactions is provided.

Follow this procedure to model bulk chemical reactions within a fluid volume.

Note	A double precision version of Simcenter STAR-CCM+ is required to model the transport of electrochemical species when coupling the electrochemical species model with an electric potential model.

Set up the physics continua.

For the physics continuum that represents the electrolyte, select the following models:

Note	Make sure that Auto-Select recommended models is activated.


Group Box	Model
Space	Select one
Time	Select one
Material	Liquid
Optional Models	Electrochemistry
Electrochemistry	Electrochemical Species
Enabled Models	Electromagnetism (selected automatically) Electrodynamic Potential (selected automatically) Reacting Gradients (selected automatically)
Optional Models	Bulk Ion Chemical Reactions
Flow	Segregated Flow
Equation of State	Select one (Constant Density is the most appropriate choice for liquids with very small temperature changes)
Viscous Regime	Select one

To include modeling electrochemical surface reactions, select the Electrochemical Reactions model from the Optional Models group box. For more information on modeling electrochemical surface reactions, see Modeling Electrochemical Surface Reactions.
Select any extra Optional Models that you require for the physics continuum.
- If you want to set a constant temperature without solving an additional transport equation, select the Segregated Fluid Isothermal model.
- To account for temperature changes due to electrochemical reactions, select the Electrochemical Reaction Heating model.
- If no temperature model is selected and the Electrochemical Species model is selected, the Electrochemical Species model runs with a constant temperature of 293.15K.
Click Close.

Set any model Properties that are required. In particular, select the Models > Bulk Ion Chemical Reactions node and specify the necessary properties. See Bulk Ion Chemical Reactions Model Properties.
When using the Bulk Ion Chemical Reactions model, the only option that is available for the Electrochemical Species model property, Electrochemical Species Solver Option, is Coupled. The Segregated Electrochemical Species Solver option is not available with the Bulk Ion Chemical Reactions model. The Coupled setting ensures the best results for solving the electrochemical species concentrations in the bulk volume.
Define the material species.
See either:
- Managing Single-Component Materials
- Managing Multi-Component Materials
If a rate exponent is specified for any mixture components that are in the Bulk Ion Chemical Reactions, the rate exponent is not used to compute the equilibrium reactions. It is the stoichiometric coefficients, $v_{i}$ in Eqn. (4175), that are used to compute the equilibrium reaction source terms. If a non-zero value is entered for the rate exponent, upon initialization, the rate exponent is zeroed and a message is displayed in Simcenter STAR-CCM+ saying that the rate exponents are not used.

Specify the chemical reaction species components.

Right-click the Electrochemical Species > Electrochemical Species Components node and select Select Mixture Components.
Select all of the electrochemical species that represent the reactants and products in the bulk chemical reactions. If you cannot find a complex electrochemical species component, you can either:
- Close the dialog, add electrochemical species to the database, then reopen the Select Mixture Components dialog, and select the species. See Modifying a Copy of the Material Database.
- Select a similar electrochemical species component and modify its material properties to change the charge, and the type and quantity of atoms.
Click Apply, then Close.
To modify the composition or properties of an electrochemical species component, expand the Electrochemical Species Components > [Electrochemical Species Component] node and edit the Material Properties as required. See [Electrochemical Species—Material Properties and Methods].

Define the reactions that occur in the bulk phase. See Defining Chemical Reactions.

Right-click the [Continuum] > Models > Reacting > Reactions node and select New Reaction.
1. Define the Reactants and Products, making sure that the charges and proportions of elements balance.
  You can select electrochemical species components and liquid components as reactants and products in each reaction. However, although the concentrations of the liquid components are used to compute the equilibrium constants, it is assumed that these values are constant, and no source term is added to the liquid components.
  
  Since the equilibrium reactions are solved as a coupled set of equations, each reaction must have linearly independent sets of stoichiometric coefficients. For example, the following reactions are independent from each other:
  $A + B \to C$
  
  $B + C \to D$
  
  However, if the following equilibrium reaction is also added:
  
  $A + 2 B \to D$
  
  this equilibrium reaction is a sum of the two previous reactions and is therefore linearly dependent on the previous two reactions. In Simcenter STAR-CCM+, if equilibrium reactions have linearly dependent sets of stoichiometric coefficients, an input error occurs upon initialization.
2. Expand the Reactions > [Reaction] > Properties > Equilibrium Constant node.
3. Select the Equilibrium Constant > Keq Equilibrium Constant node and specify the equilibrium constant.
  The dimension of the Keq Equilibrium Constant Eqn. (4176) changes depending on the number of reactants and products. Therefore, the Equilibrium Constant Unit System property allows you to specify the Keq Equilibrium Constant value using units of either (kmol, m, s, K) or (mol, cm, s, K).
Specify the material properties.
1. Expand the Models > [Material] > Material Properties node.
2. Select the Electrical Conductivity node and set Method to Electrochemical Species.
3. Set any other material properties that are necessary.
Define initial conditions for the molar concentration.
For electrochemical species components which have concentrations of zero—but are involved in equilibrium reactions, set a small molar concentration, such as 1E-20 kmol/m³.
To define initial conditions for the electric potential, do one of the following:
- If you want to specify the electric potential manually, select the [physics continuum] > Initial Conditions node and set the values for the electric potential.
  Simcenter STAR-CCM+ uses values that you set for the electric potential initial conditions everywhere.
- If you want to let Simcenter STAR-CCM+ automatically initialize the electric potential, select the Solvers > Electric Potential > Expert Initialization node and set Method to Presolve.Prior to running the electric potential presolver, Simcenter STAR-CCM+ initializes the electric potential using a parts-based or region-based approach—dependant upon the set-up. For more information, see Expert Initialization.
Define electrochemical species concentrations at boundaries and interfaces, as the simulation requires. For wall boundaries, if you are setting concentrations directly, set Wall Electrochemical Species Option to Specified Value.
Bulk reactions require equilibrium conditions on boundaries. As a non-equilibrium concentration profile adversely affects the solution, Simcenter STAR-CCM+ automatically computes equilibrium profiles based on the input concentrations that you specify. You are not required to pre-compute equilibrium concentrations yourself; in each iteration, Simcenter STAR-CCM+ applies its own computed concentrations to the boundary. These computed concentrations are guaranteed to conserve the charge and elemental composition specified in the original profile.
- Avoid zero molar concentrations for any species that participates in electrochemical reactions. Instead, set a very small value (for example, 1E-20 kmol / m^3) in lieu of a zero molar concentration.
Define the region types.
You can specify a fluid or porous region for the electrolyte continuum. However, when using the Electrochemical Species model, porous baffle interfaces are not allowed.
Specify any necessary Solver settings.
Set the Stopping Criteria.
Set up Scenes and Plots to visualize the solution.
For example, you can visualize the following field functions, among others, in a scalar scene or plot them on an x-y plot:
- bulk ion production rate
- electrochemical species residence time
- charged species mobility of specific electrochemical species
- electric current density
- electric potential
- migration flux of specific electrochemical species
- molar concentration of specific electrochemical species
- number density of specific electrochemical species
- molecular diffusivity of specific electrochemical species
Run the simulation.