Output mass fraction history of char yield at different depths of solid - pyrolysis modelling

[Chang Liu]

Hi all,

I recently came across a small question related to solid-phase outputs in FDS. 

Basically, I am in the process of modelling cone calorimeter tests on wood with complex pyrolysis model. I'd like to monitor the migration of charring/pyrolysis front inside the wood sample in FDS. I think the mass fraction histories of generated char/volume fraction of unreacted wood at different solid depths are better indicators for pyrolysis front progression than just solid in-depth temperatures. 

I've briefly gone through the relevant chapters in the User Guide. It looks like 'NORMALIZED MASS' or 'NORMALIZED MASS LOSS RATE' features might be of help? So I placed some point devices at different depths of the wood sample to output these quantities. However, I found that they all yielded identical results which seems that FDS output the overall mass fraction history of char yields of the whole wood sample instead of values at certain depth? So I was wondering if there is any feature available in FDS to help realise this function? Or maybe I should go with 'BNDF' rather than 'DEVC'?

Thank you very much in advance!

[Randy McDermott]

Have a look at the verification case

https://github.com/firemodels/fds/blob/master/Verification/WUI/ground_vegetation_load.fds

Near the bottom there are a series of PROF lines that will give the DENSITY of the material components in depth.  From there, you should be able to put together mass fractions.

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[Chang Liu]

Hi Randy,

Thanks for your quick response. I will give it a try on my models.

Many thanks,

Chang

[Chang Liu]

Hi Randy,

Apologies for contacting you again on the same issue.

I have followed your suggestions on implementation of &PROF lines in my model(s) (i.e. I placed them at 5mm below the sample front surface)

&PROF XYZ=0.0,0.0,-0.075, QUANTITY='CHAR', ID='Char_5mm', IOR=3 /
&PROF XYZ=0.0,0.0,-0.075, QUANTITY='WATER', ID='Water_5mm', IOR=3 / 

But then FDS reported an error message related to this after I run the job - 

ERROR: Reposition PROF No.1. FDS cannot determine which boundary cell to assign

The cell size for this cone calorimeter model is uniform 5mm cube throughout the whole domain:

&MESH IJK=44,44,104, XB=-0.11,0.11,-0.11,0.11,-0.14,0.38 /

The OBST line for wood sample is (100mm X 100mm X 35mm):

&OBST XB = -0.05,0.05,-0.05,0.05,-0.105,-0.07, BULK_DENSITY = 468.0, SURF_ID = 'wood', BNDF_OBST=.TRUE. /

Then after I read the relevant chapter in FDS User Guide, I was wondering maybe I should place he PROF devices on the surface of the sample because of this sentence "The next n numbers are the node coordinates, running from 0 at the surface to the full THICKNESS at the end." So I further modfied them as follows:

&PROF XYZ=0.0,0.0,-0.070, QUANTITY='CHAR', ID='Char', IOR=3 /
&PROF XYZ=0.0,0.0,-0.070, QUANTITY='WATER', ID='Water', IOR=3 / 

But the issue persists. I am a bit confused with the apporpriate locations to place PROFs. Intially I thought I needed to place them at the certain depth of interest below the front surface to output the quantities at the desired location. Then I wondered that maybe I just need to put them at the front surface and FDS will automatically output the in-depth quantities from the sample front to the back at the interval of 5mm (same as my cell size). But they all failed. So could I kindly ask where should I put them to get the densities of each material component inside the sample, please?

Many thanks!

[Randy McDermott]

You want to place the PROF devices just on the outside of the wall cell where you are interested in the in-depth profile.

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[Chang Liu]

Hi Randy,

Thanks for yoru quick response. 

I think this is what I did in my input file. For example, I am interested in the in-depth profile 5mm below the wood sample front surface. Thus, I place PROFs at this location -

&PROF XYZ=0.0,0.0,-0.075, QUANTITY='CHAR', ID='Char_5mm', IOR=3 /
&PROF XYZ=0.0,0.0,-0.075, QUANTITY='WATER', ID='Water_5mm', IOR=3 / 

FYI, -0.070m corresponds to the front surface of wood sample. Given that the cell size is 5mm cube, I believe these PROFs are also at the wall cell boundary as well? But why FDS reported it cannot decide which cell boundary to assign? The IOR direction is correct as well. Or maybe I should try the following lines to avoid the mesh boundaries (in real sims, I used multiple meshes to speed up the job)?

&PROF XYZ=0.001,0.001,-0.075, QUANTITY='CHAR', ID='Char_5mm', IOR=3 /
&PROF XYZ=0.001,0.001,-0.075, QUANTITY='WATER', ID='Water_5mm', IOR=3 / 

Thanks!

[Randy McDermott]

IOR should point *away* from the surface.  If these points are below, then you are pointing *toward* the surface.

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