Nitrogen oxide formation during combustion process in cfx

wjwj · February 14, 2017, 15:09

Hello everyone

It is my first post here, but i have one question about a combustion process of coal (exactly formation of nitrogen oxide during combustion process) in CFX. I have done a simulation of formation NOx in coal combustion mostly using a tutorial from Ansys CFX, and i have now a process of formation this NOx (look at my screen here: http://imgur.com/a/6Fdgm) If i think wrong please correct me.
Normally nitrogen oxides are transported with funes to the chimney. How can i change my simulation to show what is a NO concentration at output of my furnace? In this version a nitrogen oxides are created in higest temperature so it is true, but i want to show what is level of emmision at output of my furnace during transportation this NO to the chimney. Is it possible?
Thanks for all answers, and when my information is not complete i will complete it.

ghorrocks · February 14, 2017, 17:37

You don't need to change the simulation, you can do this in post processing. In CFD-Post use the function calculator to evaluate an expression, something like massflowAve(<NO mass fraction>)@Outlet - replacing the <...> term with the variable name of the NO mass fraction variable.

wjwj · February 15, 2017, 04:20

Thank you

It works

@edit
Sorry i missed a plane where i realised this.
I made another screen from tips what you gave to me: http://imgur.com/a/gb1r9
All emission of NO made in burner need to go to the chimney, in simulation is not a reduction atmosphere so NOx can't reduce to N2. If you know what i mean i want to show what is a sum of generated PPM of NO in furnace and how big is emission this pollutant to atmosphere. When i made this method that you gave me, emission on outlet are very small and it cant be a sum of generated NO.

ghorrocks · February 15, 2017, 06:05

This could be inaccuracy in your simulation: maybe your simulation is not converged sufficiently or run for enough physical time. Also it could be you have excessive dissipation so the NO is smeared out before it gets to the exit.

Or it could be a problem in your physics: Are you sure the NO is not reacting to something else as it travels in your simulation?

wjwj · February 15, 2017, 08:24

Im based mostly on tutorial to ansys cfx about coal combustion. On screen i show you one sentence from tutorial http://imgur.com/a/t5rZf. They are writing: "You can see that NO is produced in the high-temperature region near the inlet. Further downstream, the mass fraction of NO is more uniform."

In my furnace situation is the same, NO is produced where temperature is high so its true. To create a NO we need to have about 1500K or above. So i think that i have in my simulation a process of creation NOx without transport him to the chimney. In upper sides of furnace a temperature is lower than this 1500K so NO isnt produced in this area (it is true). Reduction of NO isnt possible because i have coefficient of excess air 1,8 or above.
Thanks for your attention

ghorrocks · February 15, 2017, 17:55

The tutorials show you how to do a model in CFX but they do not show you how to do an accurate simulation in CFX. One major issue is the mesh used in the tutorials is coarse so they run quick - but that means the results are not resolved and too diffusive to be accurate. You will need a finer mesh than the tutorials to get accurate results. Do a mesh sensitivity study to determine the mesh resolution you require in this case.

February 14, 2017, 15:09	Nitrogen oxide formation during combustion process in cfx	#1
wjwj New Member Wojciech Join Date: Feb 2017 Posts: 3 Rep Power: 9	Hello everyone It is my first post here, but i have one question about a combustion process of coal (exactly formation of nitrogen oxide during combustion process) in CFX. I have done a simulation of formation NOx in coal combustion mostly using a tutorial from Ansys CFX, and i have now a process of formation this NOx (look at my screen here: http://imgur.com/a/6Fdgm) If i think wrong please correct me. Normally nitrogen oxides are transported with funes to the chimney. How can i change my simulation to show what is a NO concentration at output of my furnace? In this version a nitrogen oxides are created in higest temperature so it is true, but i want to show what is level of emmision at output of my furnace during transportation this NO to the chimney. Is it possible? Thanks for all answers, and when my information is not complete i will complete it.

February 15, 2017, 04:20		#3
wjwj New Member Wojciech Join Date: Feb 2017 Posts: 3 Rep Power: 9	Thank you It works @edit Sorry i missed a plane where i realised this. I made another screen from tips what you gave to me: http://imgur.com/a/gb1r9 All emission of NO made in burner need to go to the chimney, in simulation is not a reduction atmosphere so NOx can't reduce to N2. If you know what i mean i want to show what is a sum of generated PPM of NO in furnace and how big is emission this pollutant to atmosphere. When i made this method that you gave me, emission on outlet are very small and it cant be a sum of generated NO. Last edited by wjwj; February 15, 2017 at 04:54. Reason: i made fault and i need to correct this

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February 14, 2017, 17:37		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	You don't need to change the simulation, you can do this in post processing. In CFD-Post use the function calculator to evaluate an expression, something like massflowAve(<NO mass fraction>)@Outlet - replacing the <...> term with the variable name of the NO mass fraction variable.

February 15, 2017, 06:05		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	This could be inaccuracy in your simulation: maybe your simulation is not converged sufficiently or run for enough physical time. Also it could be you have excessive dissipation so the NO is smeared out before it gets to the exit. Or it could be a problem in your physics: Are you sure the NO is not reacting to something else as it travels in your simulation?

February 15, 2017, 08:24		#5
wjwj New Member Wojciech Join Date: Feb 2017 Posts: 3 Rep Power: 9	Im based mostly on tutorial to ansys cfx about coal combustion. On screen i show you one sentence from tutorial http://imgur.com/a/t5rZf. They are writing: "You can see that NO is produced in the high-temperature region near the inlet. Further downstream, the mass fraction of NO is more uniform." In my furnace situation is the same, NO is produced where temperature is high so its true. To create a NO we need to have about 1500K or above. So i think that i have in my simulation a process of creation NOx without transport him to the chimney. In upper sides of furnace a temperature is lower than this 1500K so NO isnt produced in this area (it is true). Reduction of NO isnt possible because i have coefficient of excess air 1,8 or above. Thanks for your attention

February 15, 2017, 17:55		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	The tutorials show you how to do a model in CFX but they do not show you how to do an accurate simulation in CFX. One major issue is the mesh used in the tutorials is coarse so they run quick - but that means the results are not resolved and too diffusive to be accurate. You will need a finer mesh than the tutorials to get accurate results. Do a mesh sensitivity study to determine the mesh resolution you require in this case.