[User Tip]Using Mixing Rules in the CFD-ACE-GUI Database Manager

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Mixing rules can be used to define the properties of a system. Mixing rules state the method which the solver will use to eval‎uate the species/analyte properties in conjunction with the local species/analyte composition to determine the computational cell properties. Mixing rules are used only when the 'Chemistry' module is activated. There are three types of mixing rules in the Chemistry database:

1. Gas mixing rule
2. Liquid mixing rule
3. Bio mixing rule

Gas Mixing Rule:
Gas mixing rules are used only when Gas Phase chemistry is activated under Model Options / Chem. tab as shown in Figure 1.

Figure 1. Gas Phase Chemistry

A gas mixing rule can be used to define the following properties of a mixture of gases:

• Physical - Density
• Fluid - Viscosity
• Chemical - Diffusivity and mobility
• Thermal - Specific heat, thermal conductivity
• Electric/Magnetic - Electrical conductivity, relative permittivity, relative permeability
• Plasma - Electron collision frequency, electron diffusion coefficient, ion diffusion
• Kinetic - Diffusion coefficient, convective velocities

Thermal, Electric/Magnetic, Plasma and Kinetic properties are required only when Heat, Electric/Magnetic, Plasma and Kinetic modules are activated respectively.

Below are the steps to define a gas mixing rule in the Database (Figure 2):

1. Open Database and select Gas Mixing Rule in the Chemistry section
   Tools -> Database -> Chemistry -> Gas Mixing Rule
2. Click on Local Folder if you want the mixing rule to be available for all future simulations.
3. Click on File / New / Rule
4. Fill in all the required information, e.g. rule name and all settings under the various tabs. Click Apply button to save.

Figure 2. Gas Mixing rule in the Database

Note: Remember, a Mixing Rule does not set the property of a fluid, it determines the method used to eval‎uate the properties of the fluid. For example as shown above, you can use "Ideal Gas Law" option for calculating the density of a gas mixture. This tells the solver to use ideal gas law (Equation 1) to calculate the density of the mixture using the species molecular weight from the species database.

where

The mixture molecular weight is the calculated using the species mass fraction information in that cell in conjunction with the molecular weight of the specie in the database. The solver will use this information and Equation 1 to calculate the density of this cell.

Mixing Rules are similarly applied for the other properties, e.g. viscosity, thermal conductivity, specific heat, etc.

You can perform the same steps to create a Mixing Rule for Liquid Phase chemistry or Biochemistry, only the required inputs may vary. If you often use a specific method for eval‎uating your mixture properties, try creating a Mixing Rule next time to make you setup up quicker.