Learning Forum

[Chen Wei]

I'm trying to simulate laminar diffusion flame in ANSYS Fluent within radiation and soot models. Within the Moss-Brookes model, I always get an incorrect soot volume fraction profile, even I set the case almost the same, in comparison with a reference paper "On the relative contributions of soot to radiative heat transfer at different oxygen indices in ethylene – O2/CO2 laminar diffusion flames". Some differences are:

1. Due to the limited computational force. The mesh is not the fine (near 30,000 for 2D axis-symmetric)
2. The thermal and transport properties are obtained from GRI-Mech 3.0, because the properties used in the paper are not mentioned clearly.
3. I'm using the ANSYS Fluent Student version, while the reference paper uses 19.1.

The soot volume fraction profile in my case always has a peak in the axis of the flame, while the profile from others' experiment or simulation data should have a peak in the flame's fringe.

I even get a case from someone, who claims that the case could produce a correct soot volume fraction "shape". However, I still get the wrong shape with that case, even if I don't do anything but read the case and run it (300 steps for a stable field and then ignite by patching the field with 1,300 K).

I think I must miss something during the setup. But I really cannot find it out.

[Rob]

The version shouldn't matter as Student is exactly the same code, the only difference is there is a cell count limit of 512k. 

It's more likely that boundary conditions, model settings or mesh aren't the same, or aren't suitable. If you look at the results how do the other fields compare with the paper? How well converged is the case? 

[Chen Wei]

Thank you for your response and for taking the time to look into my issue further!

Here is some additional information for you.

 

In fact, the case I receive is just from the paper’s author. She gave me two files, case.dat and data.dat, but she didn’t give chem.inp, trans.dat and therm.dat. She said she create three files based on GRI-Mech 3.0 by removing the NOx part. So I create the files related to the chemical mechanism by myself based on her instruction.

 

Following the author's guidance, I attempted to replicate her work using the case.dat files she provided. My replication process included the following steps:

1. Importing the case.dat file into ANSYS Fluent.
2. Addressing an error that occurred due to the absence of the thermal and transport database by importing the chemical mechanism and database myself.
3. Running the simulation without making any modifications to the imported case.
4. Performing 1000 steps to achieve a cold stable field.
5. Igniting the flame by patching the entire mesh with a temperature of 1300 K.
6. Allowing the simulation to run until the flame reaches full coverage.

 

I was able to compare my results with the reference data provided by the author in the data.dat file. Overall, most of the fields in my simulation are similar to the reference, including some minor species that are crucial for soot formation, such as C2H2, C2H4, OH, and O. However, there are some discrepancies between my soot profiles and the reference data. Specifically, the shape of the soot volume fraction profile is not matching with the reference data, which has a peak in the flame's fringe rather than at the axis of the flame, as mentioned in the last reply.

 

We have the same Rate_of_Nucleation and Rate_of_Mass_Nucleation profiles, which is reasonable because we share the same temperature and pressure fields. However, there are some differences in the rest of the results. The source term, normalized nuclei concentration, and soot mass fraction all differ between my simulation and the author's simulation. Specifically, my results appear more diffusive compared to hers, which have much sharper profiles.

 

I suspected that the soot model parameters might be different in the two cases, which could explain the differences in the results. To test this hypothesis, I followed these steps:

1. Imported the case.dat and data.dat files.
2. Solved only the soot and nuclei equations.
3. Set the relaxation factor for both variables to be 1e-5.
4. Ran only one step.

If the two cases had different parameter sets, then the source terms would have changed after this step. However, I found that the source terms remained the same.

 

I am completely perplexed and unable to identify any other factors that could be contributing to the differences between my results and hers. I appreciate your prompt attention to this matter and eagerly anticipate your reply.

 

 

[Chen Wei]

Do you have any idea about the results?

[Rob]

Apologies, I missed this. Thanks for the reminder. 

If you set the relaxation factors that low not much will change, but the creation term is only part of the calculation: there's removal too. I'm not sure why it's different, but suspect there is a slight difference somewhere in the chemistry.