Mass loss in FDS vs. Cone Calorimeter

[ChrisDagenais]

Good afternoon,

My question is a follow-up from a previous discussion in the forum (https://groups.google.com/g/fds-smv/c/3dt5QgN0qWo).

When using the SURF_DENSITY throughout the modeling period, I get significant differences and cannot seem to identify why.

As an example, the initial surf_density is 14.527921 kg/m². My thickness is 25 mm, which converts to a density of 581 kg/m³.

In my material, I defined it as being 0.91 of wood and 0.09 moisture content (water).

With density of 558 kg/m³ (wood, dry) and 1000 kg/m³ (water), this would give an initial density of 598 kg/m³. For now, it's just 3% difference and am satisfied with it.

Right at the 2nd time step (each step = 1.0s), the surf density drops to 13.229469 kg/m² (529 kg/m³). After 1 sec, it seems to be impossible that the water has already been evaporated?!?!

Why such sudden important drop in the surf_density?

I am attaching images from the results in FDS. The HRR and THR predictions are reasonable (at least for me), but the mass loss is clearly not. The trend seems correct, but the drop after the 1st time step shifts down the entire results. I also monitored the thickness and I would have expected a mass loss following a similar trend - which is not the case.

Thanks in advance for your help and guidance!

Christian

[Kevin McGrattan]

We need a simple case to work from. Very simple. Post to the Issue Tracker.

[ChrisDagenais]

I submitted my case to the issue tracker.

Thanks.