Discussion Forum

Hi,

Check following discussion:

www.comsol.com/community/forums/general/thread/23717/

Best regards

Tero

Thank you, Tero. I looked over the guidelines that you wrote. I know that you used species transport for reaction/mixing, but can you define a multi-step reaction mechanism here?
I understand that turbulence is 3D, but have you ever tried 2D modeling? Also, do you have any additional tips or models I could reference. Again, thank you for your help!

Lindsey

Hi Lindsey

Eddy Dissipation Model can handle also two step reaction. For example in methane combustion:

CH4 + 1.5*O2 => CO + 2*H2O
CO + 0.5*O2 => CO2

Both reactions need own reaction rate according EDM.

My work was done in 3D due to mixing problems in 2D (Comsol ver. 4.2a). In 2D fuel and oxygen weren't mixing. In new version of Comsol 4.3, turbulence mixing is taken account and now combustion is working in 2D and 2D-axi. Problem is that convergence is not working properly.

So far I don't have good reference models (which are working properly). My own models are from other type of combustion and I cannot share them. What kind of tips do you need?

Tero

Hi Tero,

I'm having trouble just using some of the physics, such as transport. Do you have an example of how to set up the constraints? I'm having trouble adding the reaction rates and concentration of each species. Maybe I need to define each one first? I'd also like to use the reaction module instead but I'm having trouble getting it to run.

Lindsey

Hi Lindsey,

I don't have Reaction Engineering Module but I have tested it couple times and I had lot of problems to get it working. Another problem was that Arrhenius type functions (reaction rates are coming with these) and convergence, which were difficult with highly reactive flows. Due to this I started use turbulence based reaction rates (Eddy Dissipation Model (EDM) and Eddy Break-Up Model (EBU)).

Basic idea goes like this:

Reaction: CH4+2O2->CO2+2H2O

Reaction rate for EBU: R_EBU=3.5*ep/k*c_ch4
Reaction rate for EDM: R_EDM=4*ep/k*min(c_ch4,min(c_o2/2,0.5*(c_co2+c_h2o)/3))

Under "Transport of Diluted Species -> Reaction" you define:

c_ch4: -R_EDM
c_o2: -2*R_EDM
c_co2: R_EDM
c_h2o: 2*R_EDM

Now you have to define how many moles comes to system (fuel and oxygen) and divide it with volume flow to get inlet concentration (mol/m^3).

Note that EDM needs initial values for co2 and h2o or reaction doesn't start.

Best regards

Tero

Thanks a lot for the input!
I am currently trying to use the Level Set method to simulate mixing between the two fluids and capture the moving interface. Do you know how I could work in these reactions? Would it work if I used both Transport of Diluted Species and Two Phase Flow, Level Set? Please let me know if you have any thoughts.

Lindsey

Hi Lindsey,

Sorry, I don't have experience about Two Phase Flow so I cannot give you any advice.

Best regards

Tero