[george_brickhouse]

Hi everyone,

I am modelling combustion of methane-air premixture (1:10 by volume) inside the enclosure with internal pressure 7 kPa and nitrogen gas inside. Models: EBU, k-e standard, radiation DO.

I have 2 cases:

1. enclosure is sealed, Ideal gas inside.
Result: combustion proceeds, pressure in enclosure rises adequately, initial flame temperature is adequate, but it drops with time (by about 10% within 2 min). This is not understandable for me - pressure and ambient temperature grows but the flame temperature drops.

2. enclosure has pressure outlet with gauge pressure 7 kPa. Incompressible ideal gas is in enclosure.
Result: combustion proceeds, pressure in enclosure almost does not rise (this is reasonable), initial flame temperature is adequate and it does not drop.

What to do with compressible problem? Why flame temperature drops?

I hope somebody could help me. Thanks.
George.

[Edeluc]

Hello George,

Taking your stated boundary conditions into consideration you are having a rich combustion environment (because the stoichiometric Air-to-Fuel ratio for methane is about 17.2).
The EBU model in it's original form has difficulties in dealing with rich/lean combustion. Therefore I would suggest to change to a different combustion model if possible. If you only have the original EBU available I would change to a stoichiometric mixture and see if the temperature drop still occurs.
Furthermore could you explain what you mean by "nitrogen gas inside"? Do I understand correctly that you have your premixed air/methane flame burning into an enclosure with nitrogen? (meaning some sort of jet flame).

Regards,
Lucas

[george_brickhouse]

Thank you Lucas.

By "nitrogen gas inside" I meant that the surrounding gas is nitrogen. Yes, you are right, the ch4-air mixture burns within the sealed enclosure.
What model would you recommend to switch to?

Regards,
George