Issue with Custom FUEL in FDS: Unrecognized SPEC and Missing Properties

[GabboGuidi]

Good morning everyone,
As has happened to many others, judging from the posts in the group (e.g., #12099 and others), I am encountering the message:

"SPEC SFPE WOOD_OAK_fuel is not in the table of pre-defined species. Any unassigned SPEC variables in the input were assigned the properties of nitrogen."

I understand what this means — that my FUEL is understood to be a gas with the properties of nitrogen, except for the molecular weight, which will be calculated from the formula.

At this point, however, I have a few questions:

If I understand correctly, not specifying viscosity, diffusivity, and conductivity may lead to problems due to the assumption of gamma = 1.4, which is not suitable for high molecular weights. How high is "high"? Is there a threshold?

I haven’t understood whether directly specifying the Heat of Combustion (HoC) allows me to bypass all these issues.

Finally, where can I find values for viscosity, diffusivity, and conductivity for reactions like wood or polyurethane?

Thank you in advance!

[mcgratta]

Unless you are doing a DNS (direct numerical simulation), it may not matter what values of coefficients of viscosity, conductivity, diffusivity are because these terms are modeled. Also, the fuel gas is a small fraction of the overall gas species, in which case the properties of the air and products matter more. But again, the diffusive terms in the governing equations are modeled and the precise values of the coefficients do not matter much. There are ways to set them, and you can do this to see if it matters.

[drjfloyd]

The fuel Cp value matters most with EXTINCTION 2 (Default for DNS and LES modes but not VLES [FDS default] or SVLES) and with higher grid resolutions where you might have resolvable volumes with high fuel concentrations. Fuel vapors generally have a Cp around 2 (methane is 2.2, hexane is 1.7). For methane a MW=16 you would get Cp=1.8 assuming gamma=1.4 or about 20 % low. Not horrible. For heptane you would get Cp=0.34 assuming gamma=1.4, a factor of 5 low.