Modeling chemical Equilibrium and non-Equilibrium

CFDGerCFDGer Member


I am modeling hypersonic flow around an object. Due to the high temperatures occuring in the shock layer I have to model chemical reactions which I implemented (chemical kinetics input file and thermodynamic database).

1) I want to start by modeling the reactions in chemical equilibrium and if that works out I want to try chemical non equilibrium.

In the user and theory guide I was able to find stuff regarding modeling of chemical equilibrium in combustion processes. But as I am modeling the dissociation and recombination of air (5 species 17 reaction model) this probably doesn't fit my case.

I tried to model Species Transport + Volumetric Reactions + Relax to Cehmical Equilibrium. But the results I get look like chemical nonequilibrium. But I am not sure.

If I model Transport + Volumetric Reactions + Stiff Chemistry Solver or Chemkin CFD Solver I think chemical nonequilibrium is solved.

Which chemistry solver/settings are needed to model equilibrium/non-equilibrium.

2) As far as I understand Fluent models vibrational/thermal equilibrium as standard, is that correct? And if yes, how can I model vibrational/thermal non-equilibrium (with an additional vibrational temperature).



  • DrAmineDrAmine GermanyForum Coordinator

    1/Relax to chemical equilibrium is just an additional option whenever a detailed reactions are unknown or to avoid very expensive and stiff detailed chemistry assuming equilibrium state

    2/Yes. I think it is beta feature for density based solver. This is important for hypersonic flows or when Damkoehler numbers are of order of unity. Please check beta feature manual for more information about usage and limitations.

  • CFDGerCFDGer Member

    1) Ok so if I use Relax to Chemical Equilibrium I still am modeling chemical non-equilibrium? Could you tell me how to calculate the chemical equilibrium or is it in the beta feature manual aswell?

    2) Where can I find that manual?


  • DrAmineDrAmine GermanyForum Coordinator

    1/No: only used if you really have chemical equilibrium or to avoid expensive detailed kinetics or a good extension of EBU model. It is not beta it is a full feature.


  • @DrAmine

    Which UDF would you recommend to force a simulation with chemical equilibrium? So far I tried to run the simulation with "Relax to Chemical Equilibrium" and "Finite-Rate/No TCI". In order to fasten the chemical reaction I didn't choose a Fuel Species for the Relax to Chemical Equilibrium parameters. In this way the characteristic time decreases (as the chemical reaction time scale is 0 and the only time scale remaining for the calculation of the rate of formation of chemical species is the Convection/diffusion time scale of the cell (see Ansys Problem #2041480).

    So my idea is to get the Rate of formation of chemical species "infinitely" high to "simulate" chemical equilibrium. I know that there is the UDF option DEFINE_VR_RATE in which I can specify a custom volumetric reaction rate for multiple reactions. Is this the right approach? To my understanding about what you said and due to my experience Fluent isn't able to model volumetric reactions in chemical equilibrium?

    Thanks for your support.

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