I'm not a combustion model expert. But most likely your combustion model is relying on flamelet libraries. In the simulation the mixture is obviously not igniting. This most likely can be achieved by setting in a suitable location the combustion progress variable from unbournt to burnt mixture. At least the mixture will not just ignite on itself. If there is no initial flame front, than the mixture in its combustable state will just flow through your geometry without getting burnt. It is one possible solution of your posed boundary condition determined CFD simulation, if there is no ignition source. Using an ignition model would be another possibility.

Regards,

Dr. Th. Frank.

Dear @Thomas Frank, thanks for your reply. But I have chosen Eddy dissipation model under LES. I have heard that, this eddy model doesn't need to patch the temperature inorder to start the combustion. Is there anything I might have missed?

Dear Sriram,

Kindly check the grid side at your excepted flame location. Also calculate mixing time at these location. It might be a case that your mixing time is too small to sustain combustion.

Dear @Syed,

Thanks for your reply. I have used non premixed combustion model. Is it necessary to calculate mixing time? Can you please brief what do you mean by grid size?

Why shouldn't any species transport model (like Eddy Dissipation) not require temperature patching? Table-based models like equilibrium or flamelets do not, but if you use some form of species transport, you will have to provide an ignition source at some point to initiate combustion.... otherwise, you'd only simulate mixing of fuel and oxidizer. What can happen is that after temperature patching, the flame will not stabilize and the hot exhaust products are transported out of the computational domain.This would indicate that your flow field is not as stable as you'd want. Sometimes it also helps to not only patch temperature, but also some intermediate species like CO or H2 (I assume you talk about the combustion of natural gas).

Be careful with nomenclature here, by the way: in FLUENT, non-premixed combustion models are mixture-fraction-based models. This is not to be confused with non-premixed combustion in general which just means that fuel and oxidizer are separately injected into the combustion chamber. Thus, you can simulate non-premixed combustion with some kind of species transport model, which is in fact done quite often.

Dear Sriram,

I am working on Extinction of non-premixed flame and I have found out that, Extinction a fundamental characteristic of non-premixed flame changes significantly with change in mesh size used for compution. My grid size I mean mesh size. For my case we have used grid size of 100 microns at and near vicinity of flame zone. However if we increase the mesh size to 400 microns for the same boundary condition, the flame front is not present.

Dear @Syed,

I used the mesh size at the fuel inlet as 1e-4

Ok than the only reason i can think of is that the mixing time smaller in comparison to reaction time. Hopefully that can be possibility.

Dear @Syed,

How to fix that mixing time and reaction time issue?

Dear Sriram Gnanasambanthan,

There are few checks you can do.

[1] Find the fluent tutorial on non-premixed combustion(it will not be LES but with steady state k-e turbulent model) . Follow the steps, and make sure you are getting the combustion.

[2] Once steady state combustion is achieved, then switch to LES

[3] Also check you overall equivalence ratio, a good starting point will be 0.85

Hopefully this be resolve your issue.

If problem still persist, you can share the case file. I will be happy to share my insight.

Dear @ shambhoo,

Thank you for your reply.

That a good question. If you flow field is fixed your mixing time will not change, what you need to check is your reaction time is smaller than mixing time or not. Strain rate parallel to flame surface is very good approximation of mixing time. For reaction time take any standard textbook on combustion.