Species definitions

The following inputs are defined using the species prefix.

Description

Type

Default

solve

Specified name of the species or None to disable the species solver. The name assigned to the species solver is used to specify species inputs.

String

None

diffusivity

Fluid species diffusivity model.

  • constant a constant diffusion coefficient is used for all fluid species.

String

None

diffusivity.constant

Constant species diffusivity – required for constant diffusivity model

Real

0

specific_heat

Species specific heat model. This setting only applies if either fluid.specific_heat = mixture or solids.specific_heat = mixture

  • constant a constant specific heat is defined for each species and a mixture specific heat is computed based on the fluid or particle composition.

  • NASA7-poly the specific heat of each species is defined by two polynomials that are a function of temperature, then a mixture specific heat is computed based on the fluid or particle composition. One polynomial defines the specific across the low temperature range while the second defines the specific heat across the high temperature range.

    NASA7 polynomial format:

    \(c_p(T) = \sum_{i=0}^5 a_iT^i\)

String

None

ignore_discontinuities

MFIX-Exa asserts that the low and high NASA-7 polynomials are continuous at the 1000K temperature split. However, some polynomials do not have valid high temperature coefficients, for example, liquid water. By setting this keyword to 1, the simulation will only warn that the polynomials are discontinuous. Simulations that use discontinuous polynomials and cross between the 1000K temperature split may fail when computing temperature from enthalpy. Therefore, this option should only be used when the simulation is not expected to cross over the transition temperature.

int

0

The following inputs are for each species defined using the species.[species name] prefix.

Description

Type

Default

molecular_weight

Molecular weight of species. Required for mixture molecular weight model and when including chemical reactions.

Real

0

specific_heat.constant

Constant species specific heat. Required for all fluid (solids) species if the fluid (solids) specific_heat = mixture and the specific heat model is constant

Real

0

specific_heat.NASA7.a[i]

Species specific heat polynomial coefficients. Required for all fluid (solids) species if the fluid (solids) specific_heat = mixture and the specific heat model is NASA7-poly

Each polynomial is defined by six coefficients (a0 .. a5), and two values are required for each coefficient. The first value is the low temperature polynomial coefficient (T < 1000K) and the second is the coefficient for the high temperature polynomial (T > 1000K). A total of twelve coefficients are required.

Real

enthalpy_of_formation

Enthalpy of formation of species. This input is required only when the specific heat model is constant and chemical reactions are defined.

Real

0

Example inputs

Fluid species as passive scalars

In the following example, two species are defined and assigned to the fluid. We are required to define the species diffusivity and initial and boundary conditions. The IncompressibleFluid constraint is used, and the fluid density and species mass fractions are defined in the initial and boundary conditions. The fluid energy equation is not solved, and because we are not updating fluid density, the local species concentrations do not affect the fluid.

Listing 8 Snippet of inputs defining species as passive tracers. This is not a complete input file.
mfix.constraint = IncompressibleFluid

mfix.advect_density  = 0
mfix.advect_enthalpy = 0
mfix.solve_species   = 1


# Species model settings
# -----------------------------------------------------------------------
species.solve  =  N2  O2

species.diffusivity = constant
species.diffusivity.constant  = 1.9e-5


# Fluid model settings
# -----------------------------------------------------------------------
fluid.solve = fluid

fluid.viscosity.molecular = constant
fluid.viscosity.molecular.constant = 2.0e-5

fluid.species =  N2  O2

# Initial Conditions
# -----------------------------------------------------------------------
ic.regions = full-domain

ic.full-domain.fluid.volfrac   =  1.0
ic.full-domain.fluid.density   =  1.0

ic.full-domain.fluid.velocity  =  0.0  0.0  0.0

ic.full-domain.fluid.species.N2   =  0.77
ic.full-domain.fluid.species.O2   =  0.23

# Boundary Conditions
# -----------------------------------------------------------------------
bc.regions = inlet  outlet

bc.inlet = mi
bc.inlet.fluid.volfrac  =  1.0
bc.inlet.fluid.density  =  1.0

bc.inlet.fluid.velocity =  1.0e-8  0.0  0.0

bc.inlet.fluid.species.N2   =  0.77
bc.inlet.fluid.species.O2   =  0.23

bc.outlet = po
bc.outlet.fluid.pressure =  0.

Fluid as mixture

In this example, the fluid is treated as a mixture of two species and the IdealGasOpenSystem constraint is used. Because we are solving the energy equation, the fluid thermal conductivity is provided and the specific heats for both species are needed. The species specific heat model is set to NASA7-poly; therefore 12 coefficients (6 low-temperature range coefficients and 6 high-temperature range coefficients) are defined for each species. Additionally, species molecular weights are provided to compute the mixture molecular weights needed to evaluate the ideal-gas equation of state.

Fluid temperature is specified by the initial and boundary conditions, and unlike the previous example, density is no longer provided. Instead, the ideal gas equation of state is used to compute density from temperature, fluid composition, and the thermodynamic pressure. The thermodynamic pressure is taken as the outflow boundary condition pressure which is set here to 1 atmosphere.

Listing 9 Snippet of inputs defining fluid as a mixture with NASA7-poly specific heat model. This is not a complete input file.
mfix.constraint = IdealGasOpenSystem

mfix.advect_density  = 1
mfix.solve_species   = 1
mfix.advect_enthalpy = 1

# Species model settings
# -----------------------------------------------------------------------
species.solve  =  N2  O2

species.diffusivity = constant
species.diffusivity.constant  = 1.9e-5

species.O2.molecular_weight = 31.99880e-3
species.N2.molecular_weight = 28.01340e-3

species.specific_heat = NASA7-poly

# Oxygen
species.O2.molecular_weight = 31.99880e-3
species.O2.specific_heat.NASA7.a0    =  3.78245636E+00    3.66096065E+00
species.O2.specific_heat.NASA7.a1    = -2.99673416E-03    6.56365811E-04
species.O2.specific_heat.NASA7.a2    =  9.84730201E-06   -1.41149627E-07
species.O2.specific_heat.NASA7.a3    = -9.68129509E-09    2.05797935E-11
species.O2.specific_heat.NASA7.a4    =  3.24372837E-12   -1.29913436E-15
species.O2.specific_heat.NASA7.a5    = -1.06394356E+03   -1.21597718E+03

# Nitrogen
species.N2.molecular_weight = 28.01340e-3
species.N2.specific_heat.NASA7.a0    =  3.53100528E+00    2.95257637E+00
species.N2.specific_heat.NASA7.a1    = -1.23660988E-04    1.39690040E-03
species.N2.specific_heat.NASA7.a2    = -5.02999433E-07   -4.92631603E-07
species.N2.specific_heat.NASA7.a3    =  2.43530612E-09    7.86010195E-11
species.N2.specific_heat.NASA7.a4    = -1.40881235E-12   -4.60755204E-15
species.N2.specific_heat.NASA7.a5    = -1.04697628E+03   -9.23948688E+02


# Fluid model settings
# -----------------------------------------------------------------------
fluid.solve = fluid

fluid.viscosity.molecular = constant
fluid.viscosity.molecular.constant = 2.0e-5

fluid.thermal_conductivity = constant
fluid.thermal_conductivity.constant = 0.026

fluid.specific_heat = mixture

fluid.species =  N2  O2

# Initial Conditions
# -----------------------------------------------------------------------
ic.regions = full-domain

ic.full-domain.fluid.volfrac   =  1.0

ic.full-domain.fluid.velocity  =  0.0  0.0  0.0

ic.full-domain.fluid.species.N2  =  0.77
ic.full-domain.fluid.species.O2  =  0.23

ic.full-domain.fluid.temperature = 300.0

# Boundary Conditions
# -----------------------------------------------------------------------
bc.regions = inlet  outlet

bc.inlet = mi
bc.inlet.fluid.volfrac  =  1.0

bc.inlet.fluid.velocity =  1.0e-8  0.0  0.0

bc.inlet.fluid.species.N2   = 0.77
bc.inlet.fluid.species.O2   = 0.23

bc.inlet.fluid.temperature = 300.0

bc.outlet = po
bc.outlet.fluid.pressure =  101325.