Question about Radiation Option

[ManuelOsburg]

The LES standard radiation option “Optically thick” includes a correction algorithm to achieve the preset radiation fraction. The correction factor C has limits between 1 and 100. What is the practical reason why the default lower limit is 1 and the RTE term can only increase? Why is the correction factor constrained at all?

[drjfloyd]

That has been recently changed; in the current source the default C_MIN is 0.1.

The original idea behind using 1 was we didn't want to reduce the radiative output if temperatures in cells with combustion resulted in a larger radiant fraction than input (for example a user had an fairly resolved input file with a sooty fuel but gave a 10 % radiative fraction and FDS predicted the radiant fraction to be 25 %.) Now the thought is more along the lines of we should give the user what they requeset for a radiant fraction. If the user wants to preserve the ability to get a larger one, they can set C_MIN=1.

[ManuelOsburg]

Many thanks for the quick and detailed clarification.

Can I assume that the lower and upper limits of 0.1 and 100 were chosen arbitrarily small and large respectively, so that they are generally not reached?

[mcgratta]

yes

[ManuelOsburg]

Note: The current user´s guide for FDS 6.10.0 does not yet contain the changed default value for C_MIN (see Chapters 14.1.3 and 23.27).

[mcgratta]

You're right. I thought that I changed it, but I guess not.

[mcgratta]

I updated the guides.