How to... model two gases flow on Ansys Fluent

Often, in CFD analysis modeling, we deal with more than one gas. Then in Ansys Fluent we follow the path of modeling using Multiphase. This is the right path when we need to define phase transitions. However, if we want to model a simple analysis with two gases that do not interact significantly with each other, then we can use the Species model (figure below).

Species Model in Ansys Fluent 

This model is primarily intended to define the combustion of gases and devices where burners are installed. It is an excellent tool for modeling the shape and characteristics of the flame. And the default settings of this model is very good for solving simple problems with the flow of two gases.

After selecting two gases of interest from the material library and turning on the Species model, a default mixture of selected media is generated. We can edit the properties of the created mixture and choose the type of the analyzed model: compressible or incompressible. Editing of the created mixture is presented in the drawing below.

Default mixture created when Species Model is activated

Thanks to the created mixture, it is possible to define the percentage of a given gas (one of two gases)  for each boundary condition. For example, on Inlet we can define the percentage of a first gas. The percentage of the second gas will be the difference in the value we defined for that first gas.

In the figure below, we have defined that in Inlet the percentage of nitrogen is 14%, then the percentage of the second gas will be 86%.

Example of mass fraction definition in Ansys Fluent. 

If we want to define the mass fraction of gases as the initial condition for the domain, then after the initialize command we can set the percentage of the gas from the Patch level. The figure below shows the window in which we define just such an initial condition.

Initial Condtion of mass fraction on domain in Ansys Fluent 

Also on Outlet, if we have backflows, we can define mass fraction of defined mixture. The figure below shows how to define such a boundary condition on an Outlet.

Defined mass fraction on Oultet Ansys Fluent