Learning Forum

[abtharpe42]

Hey, all:

I'm wanting to conduct ammonia combustion simulations to see if I can match the NOx trends that two papers reported that I'm currently following. Due to the size of the mechanism that I'm using, I need to use the FGM model to speed up this study. My problem with using the FGM model is its treatment of the laminar flame speed when none of the fuel species native to the software is included in the tabulation. Even though I'm using pure ammonia fuel, the model uses methane characteristics to calculate the flame speed, which is a massive discrepancy. Is there anyway that I can tell the model to use ammonia's characteristics for the flame speed calculations instead of me manually inputting those values?

Thanks in advance,

[Ren]

You can try the "laminar-flame-speed-computed" method for lamianr flame speed property input in the mixture material dialog box. Fluent will then compute the laminar flame speed data based on the input for the FGM model.

 

[abtharpe42]

Is "prepdf-polynomial" what you are refering too? This is in Fluent 2023 R1 by the way.

[Ren]

No, not "prepdf-polynomial". I just checked 23R1 and I could see "laminar-flame-speed-computed" (see screenshot below). It's also described in the Theory Guide:

8.3.2. Partially Premixed Combustion Theory

It's not clear why you are not getting the option. Could you send a screenshot of the "Chemistry" tab of the "Species Model" dialog box? Also please double check the release numer?

 

[abtharpe42]

Over the weekend I figured out what was going on, and it might be a bug in 2023 R1. I'm having the same issue in 2024 R1 on a different device, so the bug might be there too.

If I generate the flamelets in the FGM model, generate the lookup table, and then check the PDF mixture material properties, the "laminar-flame-speed-computed" (LFSC) option was available as shown below:

When I switched to LFSC and then recalculated the properties in the FGM model under the "Properties" tab, the resulting flame speed numbers were corrected for ammonia combustion. However, if I imported a set of pregenerated flamelets into the FGM model OR duplicated a project where a FGM simulation was already completed without LFSC enabled, the LFSC option in the pdf mixture material properties was gone. Also, if a FGM simulation was ran after generating the flamelets and without enabling LFSC before running the calculation, the LFSC option disappears. 

This has been a massive headache, but I figured it out. The major problem with all of this is that the LFSC option disappears entirely beyond the initially setup and I don't know what to do to bring it back. I could find no information about these challenges regarding the FGM laminar flame speed property online or in the User/Theory Guides. I will say that I ended up trying the "laminar-flame-speed-library" (LFSL) option as it was consistently available throughout all of this, and it also fixed the flame speed values for ammonia combustion, but I currently don't know if it's able to handle a mixture of different fuels.

Thanks for the help, I just wish I could reenable the LFSC option after the fact.

[Ren]

Thank you for the detailed feedback about the behavior of Fluent involving the LFSC method. I'll check it in the latest release (2024 R2) and report to the development team for improvement if the issue still exists.

 

[Ren]

I've checked 2024R2 and can confirm the following:

1. if I import a set of pregenerated flamelets and then set up the FGM, I do not see the option for LFSC. I think the reason is that, in this scenario, the flamelet-mixture material does not exist and hence Fluent does not have the informaiton to automatically calculated the flame speed.

2. if I duplicate a project where a FGM simulation was already completed without LFSC enabled but with flamelet generated, the LFSC option in the pdf mixture material properties is available. I think this works because the flamelet-mixture material exists.

3. if a FGM simulation was run after generating the flamelets and without enabling LFSC before running the calculation, the LFSC option is still available for the same reason as above.

You might be observing a slightly different behaviour in older releases that no longer exist in the latest release.

 

[abtharpe42]

Okay, so the takeaway is that the flamelet mixture has to be present for the LFSC option to be available, which only occurs when the flamelets are generated instead of imported. Thanks for the clarification!