[fds-smv] Modelling a wooden crib. I have a problem with the material properties.

[Matt]

Hi all!

So i've recently started a new job as a fire consultant and part of
what I am doing involves specifying/building and testing wooden cribs
for fire tests. This is my first project and later on I know I am
going to have to do some FDS modelling, and therefore I thought I may
as well start learning now!

What I want to do is model one of the cribs I will be burning and
compare mass loss rate / video footage. I'm just doing it for fun
really.

So, the geometry of the cribs is not a problem, the problem is
specifying the material/surf to use. I get it to burn with a "foam"
example i found. I cant get a &REAC line to work at all. Whenever I
try to add in char the cribs just disappears. I've pasted the input
file, you can see some of the different materials/surfs i've used.
Foam works, but the two woods dont. I am going along the right way but
just need to pay more attention to the parameters (heat of combustion/
ignition) or am I doing something fundamentaly wrong?

Is there a "standard example" I could use? I'm using wood but any type
of wood would do, and the REAC line as well.

Also, the sticks have 6 faces, is the thickness applied to each face?
Or is it applied to each cell? Or each obstruction? What i mean is, if
you have a cube of 10cm lengths with a grid resolution of 10cm (so it
takes up one cell) and a surface thickness of 10cm, would that be
correct in modelling a real cube of 10cm? If you increase resolution
to 1cm and keep the thickness as 10cm, will it still be correct, or
will it be modelling a cube 1m square? I dont quite understand if
thickness acts from all sides of a obstruction or not.

Thanks for helping out a n00b!

&HEAD CHID='Crib-spruce', TITLE='My test FDS run' /
&MESH IJK=20,20,15, xb=0.0,0.5,0.0,0.5,0.0,0.4 /
&MESH IJK=10,10,12, xb=0.0,0.5,0.0,0.5,0.4,1 /
&TIME T_END=500. WALL_INCREMENT = 1/

---------------------------------------
Material------------------------------------------
&MATL ID = 'FOAM'
HEAT_OF_REACTION = 500.
HEAT_OF_COMBUSTION = 20000.
CONDUCTIVITY = 0.2
SPECIFIC_HEAT = 1.0
DENSITY = 20.
N_REACTIONS = 1
NU_FUEL = 1.
REFERENCE_TEMPERATURE= 200. /
&SURF ID = 'FOAM_SLAB'
COLOR = 'TOMATO 3'
MATL_ID = 'FOAM'
THICKNESS = 0.05
BURN_AWAY = .TRUE.
BACKING = 'EXPOSED' /


&MATL ID= 'birch_char',
SPECIFIC_HEAT=3.50,
CONDUCTIVITY=0.50,
DENSITY=94.00,
EMISSIVITY=1.00/
&MATL ID= 'birch',
SPECIFIC_HEAT=2.20,
CONDUCTIVITY=0.2200,
DENSITY=550.00,
EMISSIVITY=1.00,
HEAT_OF_COMBUSTION=1.4500000E004,
N_REACTIONS=1,
HEAT_OF_REACTION=300.00,
NU_FUEL=0.83,
NU_RESIDUE=0.1720,
RESIDUE='birch_char',
N_S=3.12,
A=7.5130000E011,
E=1.6100000E005/
&SURF ID= 'birch',
RGB=146,202,166,
BURN_AWAY=.TRUE.,
MATL_ID(1,1)='birch',
MATL_MASS_FRACTION(1,1)=1.00,
THICKNESS(1)=0.07/


&MATL ID = 'SPRUCE',
EMISSIVITY = 1.0,
DENSITY = 460,
CONDUCTIVITY = 0.1,
SPECIFIC_HEAT = 1.7,
N_REACTIONS = 1,
A = 1000000000000000,
E = 200000,
N_S = 3,
NU_RESIDUE = 0.21901997,
NU_FUEL = 0.78098003,
NU_WATER = 0.0,
RESIDUE = 'CHAR',
HEAT_OF_REACTION= 400,
HEAT_OF_COMBUSTION = 9000,/

&MATL ID = 'CHAR'
EMISSIVITY = 1.0,
DENSITY = 100,
CONDUCTIVITY = 0.7,
SPECIFIC_HEAT = 2. /

&SURF ID = 'SPRUCE',
MATL_ID = 'SPRUCE'
BURN_AWAY = .TRUE.
THICKNESS=0.01/
---------------------------------------------------- the fire
------------------------------------------
&SURF ID='FIRE',HRRPUA=50000.0, RAMP_Q='rampname1' /
&RAMP ID='rampname1', T= 1.0, F=0.0 /
&RAMP ID='rampname1', T= 2.0, F=1.0 /
&RAMP ID='rampname1', T= 9.0, F=1.0 /
&RAMP ID='rampname1', T=10.0, F=0.0 /
&OBST XB=0.1,0.4,0.1,0.4,0,0,SURF_ID='FIRE' /
----------------------------------------------------- the building
-------------------------------------

&VENT MB='XMIN', SURF_ID='OPEN' /
&VENT MB='XMAX', SURF_ID='OPEN' /
&VENT MB='YMIN', SURF_ID='OPEN' /
&VENT MB='YMAX', SURF_ID='OPEN' /

--------------------------------------------------------- cribs
-----------------------
&OBST XB=0.1,0.45,0.1,0.15,0.1,0.15, COLOR='RED', surf_ID= 'SPRUCE',/
&OBST XB=0.1,0.45,0.2,0.25,0.1,0.15, COLOR='RED', surf_ID= 'SPRUCE',/
&OBST XB=0.1,0.45,0.3,0.35,0.1,0.15, COLOR='RED', surf_ID= 'SPRUCE',/
&OBST XB=0.1,0.45,0.4,0.45,0.1,0.15, COLOR='RED', surf_ID= 'SPRUCE',/

&OBST XB=0.1,0.15,0.1,0.45,0.15,0.2 COLOR='RED', surf_ID= 'SPRUCE',/
&OBST XB=0.2,0.25,0.1,0.45,0.15,0.2 COLOR='RED', surf_ID= 'SPRUCE',/
&OBST XB=0.3,0.35,0.1,0.45,0.15,0.2 COLOR='RED', surf_ID= 'SPRUCE',/
&OBST XB=0.4,0.45,0.1,0.45,0.15,0.2 COLOR='RED', surf_ID= 'SPRUCE',/

&OBST XB=0.1,0.45,0.1,0.15,0.2,0.25, COLOR='RED', surf_ID= 'SPRUCE',/
&OBST XB=0.1,0.45,0.2,0.25,0.2,0.25, COLOR='RED', surf_ID= 'SPRUCE',/
&OBST XB=0.1,0.45,0.3,0.35,0.2,0.25, COLOR='RED', surf_ID= 'SPRUCE',/
&OBST XB=0.1,0.45,0.4,0.45,0.2,0.25, COLOR='RED', surf_ID= 'SPRUCE',/

&OBST XB=0.1,0.15,0.1,0.45,0.25,0.3 COLOR='RED', surf_ID= 'SPRUCE',/
&OBST XB=0.2,0.25,0.1,0.45,0.25,0.3 COLOR='RED', surf_ID= 'SPRUCE',/
&OBST XB=0.3,0.35,0.1,0.45,0.25,0.3 COLOR='RED', surf_ID= 'SPRUCE',/
&OBST XB=0.4,0.45,0.1,0.45,0.25,0.3 COLOR='RED', surf_ID= 'SPRUCE',/
------------------------------------------------------the outputs
--------------------------------------
&TAIL /

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[Kevin]

Wood cribs are seemingly very simple objects, and they are very useful
in the laboratory as a "standard" fuel. However, from a modeling point
of view, they are very complicated, both geometrically and thermo-
physically. It's not the place I'd want to start modeling. But if you
want to try, I suggest that you start with a simpler case. In the
"Examples" folder of the FDS download, there's another folder called
"Pyrolysis". There's a case in there called "charring_solid.fds". Run
this case as is and plot the various "DEVC" outputs in the file
"charring_solid_devc.csv". This case is a way that we test the solid
phase model only. It's like a cone calorimeter with no oxygen. It's
just a slab of sample that is exposed to a fixed heat flux. Study how
this case works, and when you feel you understand it (or if you don't
you can add questions to this thread), then you can replace the MATL
and SURF lines with those of your wood model. This is the necessary
verification step to make sure that your solid model makes sense. Are
you conserving mass? Is the peak burning rate reasonable, compared to
literature values? Do you end up with the right amount of char? Only
when you've convinced yourself that you have an adequate description
of the wood (and you have to be the judge of what "adequate" means),
then you move on to the crib geometry. It's way too complicated to
combine these two steps. Separate the pyrolysis from the gas phase
flow and radiation calculation.

By the way, we're currently developing techniques to model complicated
fuels like wood cribs using a lattice of solid particles rather than
grid-based solid obstructions. This will give us the freedom to define
realistic fuels like trees, office clutter, cables, whatever, in an
easier way. We will still use the MATL-SURF lines; but we'll apply
these properties to discrete particles rather than solid obstructions
like you're doing now. Even if you succeed in modeling this crib to
some degree of fidelity, you won't be able to use it in a large scale
simulation because you won't have the necessary grid resolution.

[JWilliamson]

I recall a group at UMD did some work with wood cribs that indicated
they could greatly simplify the methods of simulating the burning of
cribs. From what I recall, they performed experiments that
demonstrated a consistent burning behavior of identical wood cribs,
regardless of the surrounding fire environment. Pretty much all you
need after that is an ignition model.

Try this reference as a starting point.
Scale Modeling of Compartment Fires for Structural Fire Testing
Journal of Fire Protection Engineering, Vol. 18, No. 3, 223-240 (2008)

[R_Webster]

Matt

Following is a link to the paper by Andrew Perricone:

http://hdl.handle.net/1903/2977

Good Luck!
Robert Webster

[ivy]

Hello everyone, 

I just read this post recently. I wonder if anyone has successfully used particles to model wood cribs ? 

Should we use one particle to represent a wood stick and insert it into a volume regularly? Please kindly advise, thank you!

Kind Regards,

Ivy

[Kevin]

I haven't done a wood crib, but I've done electrical cables and vegetation (grass and pine needles). One particle per cell is fine, since FDS uses the exact same external boundary conditions for all particles within a cell. FDS just multiplies the mass and energy exchange between particles and gas to account for your desired mass per unit volume. Formulate particle properties to best match total mass, combustible mass, surface area, and thermal inertia. Remember that the particles occupy no volume, so don't be concerned that the size of your particle might not make physical sense. FDS just works with the surface area, not the volume.

[Rkj]

Hello,

I am trying to model a wooden crib using the particle model and the obstructions, this reply intrigues me. I have a few questions regarding the particle model. 

1. So does this mean that the particle we model is not represented as per the dimensions in Smokeview ? 

2. Are devices like WALL TEMPERATURE still applicable in the particle model ? 

3. Is the parameter SURFACE INTEGRAL applicable in the particle model for NET HEAT FLUX ? 

Br,
Rahul

[Kevin]

Solid particles can be cylinders, spheres or plates, and can be visualized in Smokeview as such.

'WALL TEMPERATURE' can be applied to a particle.

SURFACE_INTEGRAL does not apply to particles.

Word of caution on all this -- do very simple test cases before you attempt to model a wood crib. Modeling with solid particles is rarely done by FDS users, so I am sure there are going to be problems that pop up as you move along. Work on simple verification cases.

[Rkj]

Thank you. I'm starting with simple case of modelling a slab and checking the energy balance of the system.