Material Property
Material properties are volume conditions in the Multicomponent Mixing module that refer to selected volumes.
The material properties appear in the Properties panel when you select a fluid domain under Domains in the Flow Analysis Tree.
Density
You can specify the density of a mixture under both the Multicomponent Mixing and Common modules in the Properties panel.
Density of a mixture is computed from the densities of the individual components using the following methods:
• Mass Fraction Average—Calculates fluid density for the mixture in a selected volume by averaging the density of the components, based on their respective mass fractions.
• User Specified—Directly sets the average fluid density for the mixture in a selected volume based on a fixed value or user-defined function using the Expression Editor. This occurs regardless of the density of the individual components.
Density of the components is specified only under the Common module when you select one of the following options: Constant Density, Ideal Gas Law, Isentropic Gas Law, Compressible Liquid, or User Specified.
Viscosity
Viscosity is computed from the viscosities of the individual components of the mixture using the following methods:
• Mass Fraction Average—Calculates viscosity of the mixture in a selected volume by averaging viscosity of the components, based on their respective mass fractions. This sets the diffusivity of each component and sets the mixture value under Multicomponent Mixing to Constant Schmidt Number.
• Kinetic Theory—Requires input of the Lennard-Jones Parameters for the individual components.
• User Specified—Directly sets the viscosity of the mixture in a selected volume based on a fixed value or user-defined function using the Expression Editor, regardless of the viscosity of the individual components.
Viscosity of the components is specified only under the Flow module as one of the following: Constant Dynamic Viscosity, Constant Kinematic Viscosity, Sutherland Law, Bingham, Herschel-Bulkley, User Defined Function (NonNewtonian), or User Specified using the Expression Editor.
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If you set Viscosity for the mixture to Kinetic Theory, then Viscosity for the components is also set to Kinetic Theory by default.
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For Multicomponent flow problems that involve heat transfer, the following conditions for Enthalpy Model and Conductivity are specified:
• Enthalpy Model—Computed from the enthalpies of the individual components of the mixture by using one of the following methods:
◦ Mass Fraction Average—Calculates fluid enthalpy of the mixture in a selected volume by averaging the enthalpy of the components, based on their respective mass fractions.
◦ User Specified—Directly sets the enthalpy of the mixture in a selected volume based on a fixed value or user-defined function using the Expression Editor, regardless of the enthalpy of the individual components.
You can set Enthalpy Model of the components only under the Heat module as one of the following: Constant Capacity, JANAF Table, User Defined Function (NonNewtonian), or User Specified using the Expression Editor.
When you set Conductivity of the mixture to Kinetic Theory, then Enthalpy Model for the components in the Heat module is also set to Kinetic Theory. This allows you to specify the Degrees of Freedom whose default value 3 represents the three translational directions. A value equal to 6 includes the three rotational directions in addition to the translation directions. A value more than 6 includes as many vibrational modes as you choose in addition to the translational and rotational modes.
• Conductivity of a mixture is computed from the conductivities of the individual components using one of the following methods:
◦ Mass Fraction Average—Calculates conductivity of the mixture in a selected volume by averaging the conductivity of the components, based on their respective mass fractions. This sets the Diffusivity of each component and sets the mixture under Multicomponent Mixing to Constant Schmidt Number.
◦ Kinetic Theory—Sets Diffusivity of each component and the mixture under the Multicomponent Mixing to be calculated based on Kinetic Theory. Also requires you to enter Lennard-Jones Parameters for the individual components.
◦ User Specified—Directly sets conductivity for the mixture in a selected volume based on a fixed value or user-defined function using the Expression Editor. This occurs regardless of the conductivity of the individual components.
You can set Conductivity of the components only under the Heat module using one of the following options: Constant Prandtl Number, Constant Conductivity, Kinetic Theory.
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If you set Density for the mixture to Kinetic Theory, then Conductivity is also set to Kinetic Theory by default.
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Diffusivity
Diffusivity allows you to specify the method to calculate diffusivity. Except when using kinetic theory, the diffusivity is currently only a function of the individual components, regardless of the mass fractions of the surrounding components. You can specify Diffusivity of a component using the following methods:
• Constant Schmidt Number—Specifies diffusivity using the value set for Schmidt Number. You can make the input value a function using the Expression Editor.
• Constant Diffusivity—Specifies diffusivity directly. You can make the input value a function using the Expression Editor.
• Kinetic Theory—Specifies diffusivity using the values set for Lennard-Jones Parameters as explained in the kinetic theory.
Model
Model appears under each component for a volume under Multicomponent Mixing, when you activate the Turbulence module.
Turbulent Schmidt Number is specified under Model. The value of Turbulent Schmidt Number cannot be greater than 1.
Lennard-Jones Parameters
Lennard-Jones Parameters are specified when you set Diffusivity for the individual components in a selected volume to Kinetic Theory. The parameters that you specify are Diameter and Energy.