Thermally thin or thick

[AmirrezaHBSH]

Hello, your help and consideration would be greatly appreciated.

I have a question regarding Lagrangian particles that may be treated as thermally thin or thermally thick. I was wondering how this distinction is made when defining a PART_ID as a Lagrangian particle in FDS. Specifically, does FDS require any explicit command or parameter to designate a particle as thermally thin or thermally thick when using the Lagrangian particle model?

Thank you very much for your time and guidance.

[drjfloyd]

All liquid droplets (PART with SPEC_ID) are treated as thermally thin and represented with a single temperature.

All solid paricles (PART with SURF_ID) will be noded for in-depth heat transfer. It is possible, depending on the particle diameter and thermal properties, that there will only be one wall node making the particle thermally thin.

[mcgratta]

Use the void-free value.

[ericvmueller]

I think it should be the opposite - that is, when we input typical values of 500 kg/m3 for wood that includes internal void space. We don't input any information on the porosity of the wood, so the only way we would recover the right mass for the total object volume is if the voids are already included in the density that is input.

[AmirrezaHBSH]

Thank you for your help and detailed replies.

When void information for the solid fuel (e.g., pine needles) is unavailable, the intrinsic (void-free) true densities of the constituent components are typically on the order of 1000 kg/m^3. In a multi-step pyrolysis formulation using these intrinsic component densities, the LAYER_DENSITY is calculated based on the mass fractions of each component. As a result, the solid-phase density used in the reaction rate for each component is reduced to values on the order of 300-500 kg/m(^3).

In contrast, for a single-step pyrolysis model, if the true density is specified directly as 500 kg/m^3, the component mass fraction is unity, and therefore the same value (500 kg/m(^3)) is used directly in the reaction rate. Do you think that, if internal voids are incorporated into the multi-reaction pyrolysis model through the component densities, the corresponding mass fractions of each component should also be adjusted to remain consistent with that void treatment?

[mcgratta]

I agree with Eric. I thought that typical wood densities of approximately were 500 kg/m3 was "void-free". It is not. So the DENSITY that one finds on the MATL line when describing a piece of wood is that which you would measure with a scale and a ruler.

[ericvmueller]

For the multi-component approach, unless you are specifying some special 'void' material, the mass fractions of your solid components should sum to 1. If they all have roughly the same 'true density' then the total LAYER_DENSITY would be no different from the 'true density' (and the total mass would be too high using true density because no voids are included).