flame propagation problem, help

[VincenzoG08]

Hello everyone,
I'm simulating the pyrolysis of a piece of cotton, I have included all the necessary properties however in the simulation only in the first seconds there is the propagation of the flame, then it seems to stop completely. Is it possible that there is something I am not considering? For example, some properties or options to insert?
Below I leave you the code:

&HEAD CHID='cotone', TITLE='Cotone' /

&MESH IJK=25,25,24, XB=0.0,0.25,0.0,0.25,0.0,0.24, MULT_ID='mesh' /
&MULT ID='mesh', DX=0.25, DY=0.25, I_UPPER=1, J_UPPER=1 /

&TIME T_END=60. /

&DUMP NFRAMES=3000, DT_HRR=5., DT_DEVC=5. /

&SPEC ID='Cotton',FORMULA='C6.3H10.5O5.3'/

&REAC FUEL='Cotton', SOOT_YIELD=0.022, CO_YIELD=0.013, HEAT_OF_COMBUSTION=8927. /

&MATL ID = 'FABRIC'
FYI = 'Properties'
SPECIFIC_HEAT = 2.3
A = 2.16E16
E = 2.21E5
CONDUCTIVITY = 0.178
DENSITY = 254.
NU_SPEC = 1.
SPEC_ID = 'Cotton'
REFERENCE_TEMPERATURE = 360.
HEAT_OF_REACTION = 385.
HEAT_OF_COMBUSTION = 8927.
EMISSIVITY = 0.9 /

&MATL ID = 'GYPSUM PLASTER'
CONDUCTIVITY = 0.5
SPECIFIC_HEAT = 1.0
DENSITY = 500. /

&SURF ID = 'Cotone'
FYI = 'fabric'
BACKING = 'VOID'
COLOR = 'PURPLE'
BURN_AWAY = .TRUE.
MATL_ID = 'FABRIC'
THICKNESS = 0.000500 /

&SURF ID = 'WALL'
DEFAULT = .TRUE.
RGB = 200,200,200
MATL_ID = 'GYPSUM PLASTER'
THICKNESS = 0.012 /

&OBST XB= 0.15, 0.23, 0.24, 0.2402, 0.01, 0.21, SURF_ID='Cotone', BULK_DENSITY=254. /

&PART ID='ignitor particle', SURF_ID='ignitor', STATIC=.TRUE. /
&SURF ID='ignitor', TMP_FRONT=1400., EMISSIVITY=1., GEOMETRY='CYLINDRICAL', LENGTH=0.015, RADIUS=0.01 /
&INIT XB=0.175,0.205,0.23,0.25,0.01,0.02, PART_ID='ignitor particle', N_PARTICLES_PER_CELL=1, CELL_CENTERED=T /

&VENT XB=0.1,0.4,0.0,0.0,0.0,0.2, SURF_ID='OPEN' /

&BNDF QUANTITY='RADIATIVE HEAT FLUX' /
&BNDF QUANTITY='CONVECTIVE HEAT FLUX' /
&BNDF QUANTITY='NET HEAT FLUX' /
&BNDF QUANTITY='WALL TEMPERATURE' /
&BNDF QUANTITY='BURNING RATE' /

&DEVC XYZ=0.19,0.2402,0.11, QUANTITY='MASS FLUX', SPEC_ID='Cotton', IOR=2, ID='Cotton_MassFlux' /

&SLCF PBX=0.15, QUANTITY='TEMPERATURE',VECTOR=.TRUE., CELL_CENTERED=.TRUE. /
&SLCF PBX=0.15, QUANTITY='HRRPUV', CELL_CENTERED=.TRUE. /

&TAIL /

[mcgratta]

I'll take a look.

[VincenzoG08]

thank you very much, I will wait

[drjfloyd]

In addition to Kevin's comment it looks like your OBST input is not consistent with your SURF for the cotton. Each face of your OBST is defined with a SURF that contains 5000.0005 = 0.25 kg/m2 of cotton. Your OBST will have two faces (top and bottom) each of which is 0.08 x 0.2 m or a total of 20.080.20.25 = 0.008 kg of cotton. Your OBST volume is 3.2E-6 m3. Based on your SURF this is a BULK_DENSITY of 0.008/3.2e-6 = 2500 kg/m3.

[VincenzoG08]

Thank you very much for pointing out this inconsistency! If anything else comes to mind, please let me know

[mcgratta]

First, why do you specify a backing condition of VOID. The default is EXPOSED and that seems appropriate here. Second, you have specified kinetic parameters A and E, but also REFERENCE_TEMPERATURE. You should specify A and E OR REFERENCE_TEMPERATURE, not both. All that being said, I see nothing particularly wrong here. Sometimes when one holds a swatch of fabric and ignites it with a match, it burns completely. Sometimes it just burns in the vicinity of the match. Depends on all these parameters. I did thicken your sample to 1 mm and it spread a bit further, but still stopped when the heat from the particles was no longer able to sustain ignition and spread. Also, you have a grid resolution of 1 cm. This may or may not be enough to resolve the various physical phenomenon.

[VincenzoG08]

The goal of this simulation carried out on FDS is to make a comparison with a real experiment.
In reality, the piece of cotton, which has the same dimensions as those specified in the simulation, burns completely (in a box with the dimension defined in the code).
I am therefore trying to achieve the same behavior in the simulation as well.

As for the mesh, I refined it, but it didn’t make much of a difference. The pyrolysis stops in any case.

Thank you very much for the suggestions. I will keep you updated on any developments.
If you happen to think of any other ideas on how to allow complete pyrolysis, please let me know.

[drjfloyd]

How was this ignited in the experiment? Presumably you did not have a series of tiny, hot heaters at the bottom of the sheet. Are you adequately representing the ignition source?

[VincenzoG08]

Ignition in the experiment is carried out using a lighter at the base of the piece of cotton. I am attaching an image of the actual experiment