Reactions occur, but the number of cells with reaction is zero!

wuming · September 29, 2020, 9:05am

Hi, everyone

As shown in the figure below, reactions in my simulation were going on, but the message showed that there is no cell with homogeneous or heterogeneous reactions. Could you please tell me what may be wrong with my simulation?

wuming · October 1, 2020, 4:47pm

Could anyone please give me some advice? Now i am not sure whether my simulation is right or not.

onlyjus · October 1, 2020, 6:43pm

Did you try saving the reaction rates in a vtk file to make sure they are non-zero?

wuming · October 2, 2020, 8:26am

Hi, Weber

I didn’t try to saving the reaction rate in a vtk file. But i added a sentence into usr_rates_des file to make sure whether this files had been incorporated into my simulation. In addition, i monitored the change of volume fraction of productions. From the figure posted above, the reactions really occured. And, honestly, i don’t know how to try your way, could you please give me some steps? I will try it as soon as possible.

jeff.dietiker · October 2, 2020, 2:35pm

The stiff chemistry message is coming from the TFM side, not PIC. If you do not have any gas-gas reaction, then the message looks OK. We should make it less confusing though.

wuming · October 2, 2020, 4:01pm

Hi, Dietiker

Do you mean that for the cases with only heterogeneous reactions, the number of cells with reaction will not be displayed? Recently, i try to implement the same simulation by using MFIX-TFM. Configurations of two cases are almostly the same and there are still only two gas-solid reactions. But the result shows that the number of cells with reaction is still zero!

wuming · October 3, 2020, 3:55am

Hi, Jeff

For my TFM simulation, the number of cells with heterogeneous reaction is displayed and not zero again.
This time, i checked the boundary condition in my simulation, and found there was no reactant gas into the column. After modification, it looks like it is solved.
But for the same simulation using PIC method, it is still not solved.

mariam.yehia33 · February 22, 2021, 12:38pm

Hello, How do I save the reaction rates in a vtk file?? and how to show the data in vtk file?

mariam.yehia33 · February 22, 2021, 1:01pm

Did you solve this issue yet?

cgw · February 22, 2021, 3:05pm

Hi Mariam. There hasn’t been any activity on this thread in a while, as you can see.

To save reaction rates to VTK file: go to Output/VTK/Reactions and check the box as indicated. You can also provide a custom label.

Once you’ve created the VTK file you can use the “VTK” pane on the right to visualize it.

– Charles

mariam.yehia33 · February 23, 2021, 8:30am

Hello Charles

Thank you very much for your reply. Unfortunately this option is not available for me as you can see in the attached photo. I’m running a DEM model not TFM. What can I do instead?

Thanks again for your continuous support.

Mariam

kececi.ahmet · February 23, 2021, 9:26am

If you don’t see a reactions tab, it seems you have a problem with the chemistry tab. Are you sure reactions occur? I think the system is going to stable case. I don’t know your conditions but I think %100 gas(1) is in your initial condition and the others in boundary conditions.

cgw · February 23, 2021, 2:44pm

Hi -
It’s a source of confusion when items in the MFIX GUI are disabled without a clear explanation of why. I’ve been adding messages to disabled items to reduce the confusion.

In this case - the Reactions subtab in Output can be disabled for several different reasons. If you have no reactions defined, as Ahmet mentioned. But there’s a more obscure reason - if nrr (Number of Reaction Rates) is set to 0. This control is in a different location, under Model Setup

It’s more than a little confusing that the Output/Reactions subtab has a dependence on this other setting in a far-away place. I’ll try to improve this to make it less confusing. I had to look at the source code to see why the tab was getting disabled! - we don’t want our users to have to do that

– Charles

jeff.dietiker · February 23, 2021, 4:06pm

A few comments on this thread:

The underlying issue is coming from a confusing message (see first reply to this thread):
Reactions occur, but the number of cells with reaction is zero! - #5 by jeff.dietiker
Outputting the reaction rates requires users to write some code so the rates are saved in the ReactionRate array. It is not sufficient to set nrr and the vtk keywords. The actual quantities stored into ReactionRate must be coded inside a udf . Note that ReactionRate is cell-based, not at the particle or parcel level, so one would need to accumulate the particles/parcel rates within every cell.

mariam.yehia33 · February 25, 2021, 1:13pm

What about this message?
Number of cells with Homogeneous/Heterogeneous reactions: 0/5695

Is it only solving Heterogeneous reactions?
Note: I’m using DEM model and I have two UDF’s (usr_rates.f and usr_rates_des.f)

Thanks,

– Mariam

mariam.yehia33 · February 25, 2021, 1:18pm

Thanks a lot. However, I don’t understand how the nrr in the Model Setup option because I can set it to more than the reactions that I have (for ex.: I have total homogeneous and heterogeneous reactions of 7 and I can set this value with 10) and it doesn’t give an error. Also how can I know which reaction rate is which? because the label is set manually so I can’t identify them.

jeff.dietiker · February 25, 2021, 1:45pm

We are looking into it. The stiff chemistry solver only reports stats for continuous phases. It was originally implemented only for TFM. Now it looks like there is a bug in the stats with DEM or PIC. We are trying to address this. I hope to fix this for 21.1 release, but it may need to wait till 21.2. From what I can tell, this is only a statistic gathering issue on the number of cells, not a calculation issue (reactions are occurring where they should).

jeff.dietiker · February 25, 2021, 1:47pm

Please take a look at the bottom of the silane pyrolysis usr_rates.f to see an example on how to output reaction rates.

cgw · February 25, 2021, 2:49pm

When you build the custom solver, a file species.inc should be created (it might be in the build subdirectory). It defines indices into the reaction_rates array -

      INTEGER, PARAMETER :: RX1F = 1
      INTEGER, PARAMETER :: RX1R = 2
      INTEGER, PARAMETER :: RX2F = 3
      INTEGER, PARAMETER :: RX2R = 4
      INTEGER, PARAMETER :: RX3 = 5
      INTEGER, PARAMETER :: RX4 = 6

But this is just the order that the reactions appear in your input file - the first reaction is 1 etc.