User's Guide SOOT definition

[Vojta-Salek]

Hello,
I noticed a small inconsistency in the FDS documentation regarding soot composition in the Simple Chemistry Parameters section (13.1.1).

In older FDS versions, soot composition could be adjusted using the parameter SOOT_H_FRACTION (default value 0.1). The documentation still refers to this approach, including the equations shown below.

In current FDS versions, soot is treated by default as pure carbon, while hydrogen content can be specified using FORMULA. Either way, there is no need to specify the volume fraction of hydrogen as currently stated in the Guide.

This is only a minor issue, but it may be slightly confusing for users — it confused me as well, since I remembered that soot used to be defined differently in older FDS versions.

Thanks for considering this clarification.

[drjfloyd]

Good catch. Thanks.

[Vojta-Salek]

Happy to help. As always, thank you for your work.

[drjfloyd]

On second thought the text is technically correct only now like the preferred approach for fuel chemistry it is defined using the SPEC; however, think a reminder of that here would be appropriate.

[Vojta-Salek]

It is technically correct. What I find confusing is that "You need only specify...". I would understand it as: I have to somehow enter the amount of H in the soot. That is not correct. It would make more sense to me to mention that the composition of the soot here is taken from the FORMULA defined for SOOT (pure carbon by default and it can be adjusted at &SPEC).

What do you think, would that be more accurate?

[drjfloyd]

The C,H,O.N and SOOT_H_FRACTION inputs have been decprecated to encourage people to use SPEC (and thus hopefully think about things like specific heat) for defining the fuel species. I will rework this section to refer back to the species definition section and I think add a footnote about the deprecation. This is also a good reminder to scrub the current V&V files to use SPEC with FORMULA or C,H,O,N instead of REAC to be consistent with what we would now prefer users to do.

[drjfloyd]

Edited that section

and added an example to the SPEC chapter

[Vojta-Salek]

Sounds good to me. Since you are talking about another example, I have the impression that the example is not in Sec. 12.1.6 Soot. Wouldn't that be the primary section to search for examples on soot definition?

[drjfloyd]

good suggestion. I had it a couple subsections up but that is a better location.