Recirculation bubble stretching in laminar reacting flow simulation in reactingFoam
 

Dear all
I want to share an issue with you and asking for your suggestion. This might be helpful for the reactive flow simulation community as well.
I am trying to simulate a reactive flow through a flow tube and the flue gases are coming from a Mckenna burner, burning lean syngas/air. First I did not simulate the flame, rather the post flame hot gases, coming from the burner. No combustion happened in that case. There I got a very good jet and a recirculation bubbles (please see the attached post flame simulation figures) when it reached Steady state condition in 3 secs..

But when I simulated the flame+post flame with combustion happening, within a very short time (~1 sec), the bubble got stretched and the jet did not break (see the attached flame simulation figures). Since the mesh needs to be very very fine for capturing the flame, I took a shorter domain length, but should still be ok to assume a zero-gradient BC at the outlet.

Could anyone please tell me why is this 'bubble stretching' happening when I have combustion and not happening without any combustion.

Please suggest or comment
 

Anybody has any idea or suggestion or comment, or anyone faced similar kind of problem for other cases, PLEASE share.

Dear Foamers
Please share your thoughts about the issue.