Species Transprot

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Species transport model

The Species Transport model is used to simulate:

  • The transport of gaseous species.
  • The chemical reaction of the gaseous species.
  • The combustion process.

Whenever several gas species are used in the simulation in the form of different processes, the Species Transport model is used. These types of gases have different types depending on the type of process used in the model. There are several models for gaseous species including:

  • species transport
  • premixed combustion
  • non-premixed combustion
  • partially premixed combustion

The species transport model simulates the mixing and transport of gaseous species by solving conservation equations including transport, diffusion, and reaction sources for each species. With this model, multiple chemistry reactions can be simulated by the reactions occurring in the bulk phase (the same as volumetric reactions), on the walls, on the particle surfaces, and in the porous areas.


We use the Species transport model to define the combustion process inside the chamber. we calculate the continuity and momentum equations for all species in this model. The reaction usually is between air and fuel. Since we do the composition of fuel and air in a volume, so we use the Volumetric option.

Eddy-Dissipation should be used since the criterion of the combustion reaction is assumed to be based on the rate of mixing between fuel and air. Also, since the reaction contains energy sources, the Diffusion Energy Source option is used. Also, it is possible to produce contaminants during a chemical reaction. In some reaction, the created nitrogen oxides, including NO and NO2, are called NOx as reaction pollutants. One of the main causes of the growth rate of these nitrogen oxides is the excessive rise in temperature inside the combustion chamber. Also, the energy equation should be active.

Premixed Combustion

The premixed combustion model has the characteristic that fuel and oxidizer are mixed molecularly before combustion. The transfer and expansion of flame occur from hot products to cold reactants. Flame expansion rate, or flame velocity, depends on the internal flame structure, and the turbulence distorts the laminar flame shape and accelerates flame development. It should be noted that for the turbulent reactive flame simulation if used from a Mixture Fraction perspective, a pre-mixed combustion model must be used, also if the Reaction Progress variable is used, the premixed model should be used.

non-premixed combustion

The non-premixed combustion model has features such that the fuel and oxidizer enter the reaction zone from separate flow paths, meaning they are not pre-mixed before entering the chamber, such as diesel internal combustion engines and liquid coal furnaces, heat transfer or reactant diffusion from either side to the flame sheet, will distort the laminar flame shape and enhance mixing, and may simplify combustion to a mixing problem and eliminate problems associated with nonlinear average reaction rates. In this model, we define the Mixture Fraction, which denotes the mass fraction derived from the fuel flow (f symbol), which is the local mass fraction of the burnt and unburned fuel elements (such as C, H, and …) in various gaseous species (such as CO2, H2O, O2, …).

Boundary Condition

In the boundary section of the non-premixed model, it is necessary to define each of the gaseous species related to fuel and oxidizing flows as reactants in chemical reactions at the input boundaries, but the gas species related to the reaction products and reaction mediators are automatically obtained by the software. In fact, if fuels or oxidizers are a combination of different gaseous species, those gaseous species can be added to the list of gas species boundaries. Then we can define the amount of molar ratio or mass ratio of each of the fuel flows and input oxidizers. It is also possible to determine the amount of inlet temperature related to fuel flow and oxidizer in the chamber.

In the non-premixed model, there is no need to define the mass fractional value of the species at the boundary, and only the mean mixing fraction values and variance or deviation of the mixture fraction at the boundaries should be defined. For example, in the simulation of the reaction of a fuel with an oxidizing flow, the amount of the average mixture fraction should be considered at the input for the fuel flow equal to one and at the input for the oxidizing flow equal to zero. The amount of mixture fraction variance at the inputs is usually assumed to be zero.

Chemical equilibrium model

Chemical equilibrium mixing can be:

  • chemical equilibrium
  • close to chemical equilibrium (steady diffusion flamelet)
  • significantly different from chemical equilibrium (unsteady diffusion flamelet)

The chemical equilibrium model can have a more realistic prediction of flame temperature by incorporating the effects of intermediate species and the dissociation reactions. This model can also define the RFL value of the fuel flow in addition to defining the reaction pressure. It should be noted that if the secondary flow is used, in addition to defining the rich flammability limit of flow, the rich secondary flammability limit can also be defined, while using experimental or field fuel flow, It is not possible to define the rich flammability limit of the fuel flow. The Fluent software can calculate the composition in the rich range using equilibrium, but for fractions greater than this, the chemical calculates the equilibrium and suspends the mixture based on the mixture of fuel (not burning) with the rich composition. If the value of the rich combustion limit is equal to one, the equilibrium calculations are performed at a full interval of the mixture fraction, and if the value is less than one, the equilibrium calculations are suspended whenever the mixture fraction values of the fuel flows or secondary exceed the limits. The system operating pressure is also used to calculate the density using the ideal gas law. If in non-adiabatic mode, the system pressure changes significantly over time or in the workspace, the compressibility effects option must be enabled.

Adiabatic or non-adiabatic energy behavior

The non-adiabatic model is used for cases such as radiation or wall heat transfer, the entry of multiple fuels at different temperatures, the entry of multiple oxidizers at different temperatures, and for liquid fuel, coal particles, or heat transfer to Inert particles. However, if the adiabatic model is used, the energy equation does not need to be solved, and the system temperature is obtained directly from the mixture fraction and the inlet temperatures of the fuel and oxidizer.

Secondary Stream

The combination of a single fuel and a single oxidizer does not require the use of a secondary stream. Secondary flows can be used in cases such as two dissimilar gas fuels, mixed fuel from dissimilar liquid and non-similar fuels, mixed fuel from coal and liquid fuel, coal combustion, and combining a single fuel with two dissimilar oxidizers.

Empirical Stream

An alternative method of defining the composition of fuel or secondary stream that is used when the components of an individual species are unknown. For example, this option is used to simulate coal combustion or simulations containing complex hydrocarbon mixtures.

Defining the composition of streams

In the boundary section of the non-premixed model, it is necessary to define each of the gaseous species present in the fuel and oxidizing flows and the secondary flows (if any) existing as reactants, but the gaseous species reaction products and reaction intermediates are automatically obtained by the Fluent software. One can then define the molar ratio or mass ratio of each fuel stream and inlet oxidizer. It is also possible to define the inlet temperature value of the fuel and oxidizer flow and the secondary flow in the chamber.

Species Boundary Conditions

In the non-premixed model, the definition of the mass fraction of species at the boundaries is not needed and only the values of the mean mixture fraction (f) and the variance or variability of the mixture fraction (f´2) at the boundaries should be defined. For example, in the simulation of the reaction of a fuel stream with an oxidizing stream, the value of the mixture fraction shall be taken to be equal to one for the inlet for the fuel stream and for the inlet for the oxidizer or secondary stream equal to zero. The value of the variance of the mixture fraction at the inputs is usually assumed to be zero.

Pollutant Prediction

Advanced computational tools have made it possible to simulate heavy computing. Today, simulations such as combustion, turbulence, and so on are an integral part of Computational Fluid Dynamics studies. Multi-phase flows can be easily modeled and evaluated with different classifications, all thanks to the CFD. Among all different methods in computational software, transport equations are a holistic approach to simulate homogeneous two-phase flows. With this approach, in addition to molecular and combustion studies, pollutants can also be studied. The coupling of this method and the Large Eddy Simulation (LES) method in recent years has led to environmental, wind engineering, and architectural achievements. Simulation using the Species Transport method solves a Transport Equation for each component governing the two-component mixture. This equation contains diffusion and convection. Also, with the help of the source term, you can add user settings to the equations and communicate the equations.  A transport equation can be written for each component of the air in the pollution problem. As other differential equations, this equation also requires a boundary condition and it is necessary to specify the concentration for each component of the boundary.


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