FLUENT 6: How to rename species during the calculation

In some (mostly transient) cases it can be useful to rename one or more species after some time of calculation and then continue with computing. An example is fire and smoke modeling in houses or tunnels. At the beginning of the fire, flue gases start to fill the domain. After some time, the gases reach the smoke detector and the fire doors are shut automatically. At this time you can stop the calculation and rename the species, for example flue_gas_old, air_old, etc., and then continue the calculation. So at the end you can differentiate the flue gases which have been present in the domain before closing the fire doors and the flue gases that have been formed after the closing.
How to proceed:

First, define as many User Defined Scalars (UDS) as species to rename. Then, create the same number of Custom Field Functions (CFF). Every CFF must be defined as the mole fraction of one of the species to be renamed. After this, create the same number of CFFs as above and assign one of the UDSs to every function. Additionally, define as many new species as you want to rename with the properties of the original species. For these species, the boundary condition values must be zero. Be careful to give the new species and the CFFs reasonable names, for example flue_gas_old, air_old. etc for the species and CFF_flue_gas_old, CFF_UDS0_flue_gas_old, etc. for the CFFs.

Now you can start the calculation. You should deactivate the equations for the UDS to save computing time.

After stopping the simulation for renaming, first patch the UDS with the CFFs assigned to the computed mole fractions (flue_gas, air, etc.): by doing so, the secondly defined CFFs (assigned to the UDSs) will get the value of the computed species. Then patch the mole fractions of the first species (flue_gas, air, etc) with zero (the computed values are stored in the UDSs). (The mole fraction of the species defined as the last one in the material panel can not be patched, the value is 1.0 minus all other fractions.) Now patch the mole fractions of the additional species (flue_gas_old, air_old, etc.) with the CFFs assigned to the UDSs. By doing so, the computed values of the mole fractions are transferred to the additional defined species.

Now the calculation can be continued, you can track the distribution of all species.

If you have chemical reactions in your model, you will have to define the reaction models also for your additional species.

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