Learning Forum

[uddin43]

Hi, I was trying methane-air combustion with hot air at 1000K and methane at atmospheric temperature, with an air mass flow rate of 2 kg/s and methane at 0.1 kg/s, using the default methane-air 5 species.

When using the finite-rate model, the direct source solver cannot predict combustion, whereas the relax-to-chemical-equilibrium solver can. I have also tried the eddy dissipation model, where both the direct source and equilibrium solvers can predict combustion. However, since the finite-rate direct source model is more accurate, I want to solve it solely with the finite-rate direct source. Is that actually possible? Also, why are these two chemical solvers behaving differently?

The attached picture shows the difference in results between the finite-rate direct source and the relax-to-chemical-equilibrium approaches. Why they are behaving so differently and please help in understanding what is happening in the background. I have read the theory guide and user guides properly. Thank you.

[vinaraya]

Hi,

Could you share the following information as this would help in my response:

a) Are you using the FR/ED model or FR/No TCI option under Turbulence chemistry interaction.

b) Have you tried increasing the air temperature to 1800K and check if it is triggering the reaction (just curious).

c) Relax to equillibrium is also kinetics independent and should predict the burned region as against the direct source option (depend on temperature to trigger reactions) 

Refer the below section from 24R2 Ansys Fluent theory Guide for details on Relax to equillibrium

7.1.1.2.7.The Relaxation to Chemical Equilibrium Model

Note: In general, Direct source should be good to use for single step/short chemistries. Stiff chemistry solver is recommended for detailed kinetics.

[uddin43]

I have used FR/ No TCI, raised temperature to 2000K. I understand that Finite rate (Direct source) is not suited for multi-step mechanisms. Even with two step methane-air mixer model it is very unstable. EDC is best for multi step mechanism with turbulence, as already mentioned in theroy guide. Thank you for your response. 

[vinaraya]

Hi,

Did the combustion initiate with FR/no TCI and with 2000K ? Could you give more details with regards to the unstability comment in your last response.