Losing reaction when time step size is increased.

slaughter4622 · June 30, 2012, 07:06

I am pretty new to fluent so this may be a simple problem.

I am running an unsteady combustion model with a hydrogen-air mixture. A reaction mechanism of 9 species and 31 reactions has been setup. I am initially running an adiabatic case where no heat from the reaction zone is lost to the adjacent walls. I have run the case for 2000 time steps at a size of .0001s and get what looks like a good solution. However, when I try to run for more time with an increased time step size to .001s, the plots show that the reaction is lost and the temperature of the gas is constant at the inlet value. I have a few questions:

1) For an adiabatic case do I need to run it for a long period of time, for instance further than the .2s that results from the initial run.

2)I have experienced discrepancy in my other tests with this change in time step and I'm not sure what is causing it. Any ideas?

Thanks for the help

eng_s_sadeghi · June 30, 2012, 09:12

Most of the time in combustion problems you should use a low time step in the order of 1e-6
So it is natural for the flame to be vanished by increasing the time step.

June 30, 2012, 07:06	Losing reaction when time step size is increased.	#1
slaughter4622 New Member Scott Slaughter Join Date: Jun 2012 Location: College Station, TX Posts: 1 Rep Power: 0	I am pretty new to fluent so this may be a simple problem. I am running an unsteady combustion model with a hydrogen-air mixture. A reaction mechanism of 9 species and 31 reactions has been setup. I am initially running an adiabatic case where no heat from the reaction zone is lost to the adjacent walls. I have run the case for 2000 time steps at a size of .0001s and get what looks like a good solution. However, when I try to run for more time with an increased time step size to .001s, the plots show that the reaction is lost and the temperature of the gas is constant at the inlet value. I have a few questions: 1) For an adiabatic case do I need to run it for a long period of time, for instance further than the .2s that results from the initial run. 2)I have experienced discrepancy in my other tests with this change in time step and I'm not sure what is causing it. Any ideas? Thanks for the help

June 30, 2012, 09:12		#2
eng_s_sadeghi Member Join Date: Mar 2011 Posts: 64 Rep Power: 15	Most of the time in combustion problems you should use a low time step in the order of 1e-6 So it is natural for the flame to be vanished by increasing the time step. __________________ Saeed Sadeghi Ansys Fluent CFD Consultant

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