I really need help

student85 · June 20, 2011, 12:43

Dear all
I am really stuck... I have to simulate fire propagation in a tube. i created the mesh. i filled it with methane oxygen (stoichiometric) and now i am using the eddy dissipation model. The problem is that i want to see a gradual consumption of CH4 in the tube and also i want to see a gradual increase in the reaction rate. BUT using the methane-oxygen WD1 reaction the only thing i can see is that it reacts (without putting energy) simultaneously... Can anyone explain why this is happening??? i would like to see a gradual increase in temperature due to exothermic reaction after i ignite it with heat source at the inlet.
Please help me.
thanks

ghorrocks · June 20, 2011, 18:45

You would have to explain what you are doing better. How big is the chamber? What is your timestep size?

I would imagine the "gradual" increase in temperature you expect to be extremely rapid if the mixture is initially stiochiometric. You will need very small time steps to capture this.

student85 · June 20, 2011, 20:30

Thank you so much for your reply.
My task is to study the propagation of flame in a closed tube (inlet is a wall) two meters long with diameter of 50mm. The outlet is "opening" and the initial values are 298K and 1 atm. My time step is 0.005 sec and the simulation runs for about 2sec. The problem is that the consumption of methane in the tube is happening simultaneously everywhere and the temperature is rising simultaneously at every point in the tube as well. So i can not see propagation of the flame. i also can not make a chart of the reaction rate Vs time...The problem is that the reaction (methane-oxygen WD1) starts even without the ignition source. (i putted the mixture as reacting mixture (is that a mistake????). Should i change the reacting mixture with variable composition mixture? Because i did that and i could not ignite it at all

In that case do i have to change the CCL code??? because i do not know how to do that...

Any suggestions????????? Ps my temperature is 5000k and i think its very high.. i really appreciate your help because i run out of ideas. thank you again.

ghorrocks · June 21, 2011, 20:07

I have no experience in combustion modelling so cannot help you with the setup, but I suspect the entire combustion event is over in 5ms. You will need a MUCH smaller timestep.

student85 · June 22, 2011, 09:51

thank you for your reply. I made simulations with very small timestep (0.000001) and the problem remains... Firstly i have ignition without putting energy at all and i really do not understand why...

Secondly the temperature rises at the same time at every point in the tube simultaneously... Any suggestions????????????????????

ghorrocks · June 22, 2011, 19:12

The time step required to resolve it may be far smaller than that.

But if you are getting combustion without ignition then something is not right in your physics. Better fix that first. I have no idea what, I have not done combustion modelling.

June 20, 2011, 12:43	I really need help	#1
student85 New Member Student85 Join Date: Jun 2011 Posts: 10 Rep Power: 14	Dear all I am really stuck... I have to simulate fire propagation in a tube. i created the mesh. i filled it with methane oxygen (stoichiometric) and now i am using the eddy dissipation model. The problem is that i want to see a gradual consumption of CH4 in the tube and also i want to see a gradual increase in the reaction rate. BUT using the methane-oxygen WD1 reaction the only thing i can see is that it reacts (without putting energy) simultaneously... Can anyone explain why this is happening??? i would like to see a gradual increase in temperature due to exothermic reaction after i ignite it with heat source at the inlet. Please help me. thanks

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June 20, 2011, 18:45		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,690 Rep Power: 143	You would have to explain what you are doing better. How big is the chamber? What is your timestep size? I would imagine the "gradual" increase in temperature you expect to be extremely rapid if the mixture is initially stiochiometric. You will need very small time steps to capture this.

June 20, 2011, 20:30		#3
student85 New Member Student85 Join Date: Jun 2011 Posts: 10 Rep Power: 14	Thank you so much for your reply. My task is to study the propagation of flame in a closed tube (inlet is a wall) two meters long with diameter of 50mm. The outlet is "opening" and the initial values are 298K and 1 atm. My time step is 0.005 sec and the simulation runs for about 2sec. The problem is that the consumption of methane in the tube is happening simultaneously everywhere and the temperature is rising simultaneously at every point in the tube as well. So i can not see propagation of the flame. i also can not make a chart of the reaction rate Vs time...The problem is that the reaction (methane-oxygen WD1) starts even without the ignition source. (i putted the mixture as reacting mixture (is that a mistake????). Should i change the reacting mixture with variable composition mixture? Because i did that and i could not ignite it at all In that case do i have to change the CCL code??? because i do not know how to do that... Any suggestions????????? Ps my temperature is 5000k and i think its very high.. i really appreciate your help because i run out of ideas. thank you again.

June 21, 2011, 20:07		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,690 Rep Power: 143	I have no experience in combustion modelling so cannot help you with the setup, but I suspect the entire combustion event is over in 5ms. You will need a MUCH smaller timestep.

June 22, 2011, 09:51		#5
student85 New Member Student85 Join Date: Jun 2011 Posts: 10 Rep Power: 14	thank you for your reply. I made simulations with very small timestep (0.000001) and the problem remains... Firstly i have ignition without putting energy at all and i really do not understand why... Secondly the temperature rises at the same time at every point in the tube simultaneously... Any suggestions????????????????????

June 22, 2011, 19:12		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,690 Rep Power: 143	The time step required to resolve it may be far smaller than that. But if you are getting combustion without ignition then something is not right in your physics. Better fix that first. I have no idea what, I have not done combustion modelling.