Hi, there! This case is actually part of the preliminary research for my thesis project. I am trying to build a simple（an infinite oxygen supply and involving only a single reacting carbon particle） single carbon particle combustion case using MFIX-DEM to track the changes in the particle’s properties and state.However, I have encountered some issues and would really appreciate any help from the support team. Thank you so much in advance!
①I’m a complete beginner to MFIX, so this case is just an initial exploration. Looking at the current simulation results (background.csv and particle.csv), I noticed that no CO2 is generated (despite being set as a product in the Chemistry section), and the particle properties like radius and volume haven’t changed at all. What adjustments do I need to make to get the reaction working properly？I guess maybe the species settings?（UDF is not used for this result）
②When setting up monitors, why is the “value” option unavailable? (In the “Cell” settings, the “value” option is greyed out , while in the “Particle” settings, the “value” option is missing entirely)
7.2particlereaction_2026-07-10T225640.869851.zip (9.1 MB)
- You have two options to define the reaction rates: Either write the rates in a UDF, or do the automatic calculation by specifying Arrhenius model coefficients.
If you use a UDF, you must enable UDFs (Model setup pane) otherwise the UDF will be ignored. You also need to rebuild the solver so the UDF is compiled and linked to the solver.
If you use Arrhenius rates, you need to enable the Arrhenius model in the Chemistry pane, and set the Arrhenius coefficients.
Here, you have a UDF in your project directory (although you said it is not used), and you did not enable the Arrhenius model, so the reaction rates are zero and no reaction takes place. You should have gotten warning about having a UDF in the project directory if you did not rebuild the solver. Similarly, if you have both a UDF in the project directory and enable the Arrhenius model, you will geta a warning saying the UDF will not be used.
If you are new to MFiX, I would suggest removing the UDF, and enabling the Arrhenius model (for discrete particles in your case).
- For a cell monitor, the “value” type is only available for a point region. This will monitor the value of a single cell located at the point region. The “value” type is not available for particle data, because particles typically do not stay at a single point during a simulation. Now if you only have a single particle, you can define a region that covers the entire domain and use a monitor type of “average” (you can also use min/max or sum since you only have one particle, these are all equivalent).
I hope this helps, good luck with MFiX!
Thanks for your reply and inspiration, Jeff ! However when I enable the Arrhenius model, the default solver crashed immdiately once the mfix starts to run, and I just cant figure out the reason, could you give me some help?
7.2particlereaction_2026-07-11T120112.515072.zip (7.1 MB)
Hi,Jeff! There is no more crash right now, however the particle didnt change its mass,volume, and radius, could you please give me a hand? Thanks a lot!
7.2particlereaction_2026-07-11T215220.596713.zip (48.5 MB)
The reaction rate is about 1E-14 which is too small to transfer mass. You either need to increase the temperature, increase the preexponential factor or decrease the activation energy. I start seeing changes in the mass when the rates is around 1E-6
That’s it! I have also realized my assumptions by UDFs 
Thank you very much for your great help, Jeff!
