Thermal degradation model for Vegetation

[buildingd...@gmail.com]

Dear Colleagues

I am presently in the process of modelling in FDS a compartment fire using timber pallets (pine) as my fuel source with a burner as the fuel ignitor but having difficulties with the following:

(a) the timber crib (fuel) is not igniting

(b) not sure if I have set up the Pyrolysis Reactions in materials tab correctly as per the 8.6 - Thermal Degradation Model for Vegetation (FDS User Guide)

Would appreciate any comments / direction to help me.

I have attached the FDS file.

Thank you very much in advance.

BD LAB

[dr_jfloyd]

Have you tested your solid phase reactions by doing a virtual cone calorimeter test (Section 11.7 in the User's Guide)?

You have a very coarse grid and you may not be seeing high enough heat fluxes to ignite your fuel.  Have you looked at what your wood surface temperature is getting too (are you seeing high enough temperatures to ignite the wood)?

[o...@aquacoustics.biz]

Hi folks at the BD LAB,

when trying to overcome challenges with FDS modelling a useful approach is to simplify the problem to focus on the particular issue.  Try experimenting with a single combustible object above a significant user-controlled heat source.

I have had the briefest look at your model (no time for simulations just now) but if you add up the heats of reaction for WOOD_MATL you have a net endothermic reaction - your staged combustion process absorbs more heat than it generates.  Compare this directly with the three phase reaction for vegetation in the FDS Users Guide (around page 110) where the decomposition from char to ash generates 32,000 kJ/kg (and note that +/- signs are really important here).

NIST kindly provides a whole heap of example code in the Users Guide, Validation and Verification model.  Note that some of this is totally fictitious and stated to be so.  I encourage you to 'steal' this code with cut and paste, get a single wooden object burning from your igniter, modify the combustion reaction and material properties based on peer reviewed research, experimental measurements, or at a pinch what works, and then start extending your model.

t.

[Nigel O'Sullivan]

dr_jfloyd &  o...@aquacoustics.biz Thank you for your comments, very much appreciated. I will try your recommendations now, might revert if I have more queries if that is ok? Regards BDL

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[Tim O'Brien]

I'd be very happy to assist where I can Nigel.

 

t.

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[Nigel O'Sullivan]

I have set up the Virtual Cone Calorimeter as per FDS User Guide Section 8.7, but I getting an error message: ERROR: SPECies REAC_FUEL corresponding to MATL PINE is not a tracked species (CHID: Pine_Cone_Calorimeter)

Not sure what I am missing here?

Comments appreciated.

Regards

Nigel

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[Randy McDermott]

FDS is expecting REAC_FUEL to be defined somewhere, but it has not been.  You have asked FDS to track METHANE (which FDS understands) and PRODUCTS (which it defines itself for simple chemistry).

I recommend you scan the user guide on the combustion model and how it interacts with the solid phase model.  But the simple solution to your current problem is to remove the SPEC lines, add a REAC line with FUEL='METHANE', and set the SPEC_ID on the MATL line to 'METHANE'.

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[Nigel O'Sullivan]

Randy

Thank you for that. I have updated the accordingly as you have suggested, it appears that the fuel is not burning.

Pine_Cone_Calorimeter.fds
Generated by PyroSim - Version 2018.2.0730
13-Aug-2018 14:44:54

&HEAD CHID='Pine_Cone_Calorimeter', TITLE='Black PMMA at 50 kW/m2'/
&TIME T_END=600.0, DT=0.05, WALL_INCREMENT=1/
&DUMP RENDER_FILE='Pine_Cone_Calorimeter.ge1', DT_DEVC=5.0/
&MISC SOLID_PHASE_ONLY=.TRUE./

&MESH ID='Mesh01', IJK=3,3,3, XB=-0.15,0.15,-0.15,0.15,0.0,0.3/

&SPEC ID='METHANE'/

&DEVC ID='temp', QUANTITY='WALL TEMPERATURE', XYZ=0.0,0.0,0.0, IOR=3/
&DEVC ID='MF', QUANTITY='MASS FLUX', SPEC_ID='METHANE', XYZ=0.0,0.0,0.0, IOR=3/
&DEVC ID='thick', QUANTITY='WALL THICKNESS', XYZ=0.0,0.0,0.0, IOR=3/

&MATL ID='PINE',
      SPECIFIC_HEAT=1.0,
      CONDUCTIVITY=0.05,
      DENSITY=360.0,
      EMISSIVITY=1.0,
      HEAT_OF_COMBUSTION=1.2E4,
      N_REACTIONS=1,
      HEAT_OF_REACTION=1000.0,
      SPEC_ID(1,1)='METHANE',
      NU_SPEC(1,1)=1.0,
      A=1.0E20,
      E=1.6E5/

&SURF ID='Pine',
      RGB=239,204,102,
      BACKING='INSULATED',
      MATL_ID(1,1)='PINE',
      MATL_MASS_FRACTION(1,1)=1.0,
      THICKNESS(1)=0.05,
      EXTERNAL_FLUX=52/

&VENT ID='Vent #1', SURF_ID='Pine', XB=-0.05,0.05,-0.05,0.05,0.0,0.0, RGB=255,59,106/

&SLCF QUANTITY='TEMPERATURE', VECTOR=.TRUE., CELL_CENTERED=.TRUE., PBY=0.025/
&SLCF QUANTITY='VOLUME FRACTION', SPEC_ID='METHANE', VECTOR=.TRUE., CELL_CENTERED=.TRUE., PBY=0.025/

&TAIL /

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[Randy McDermott]

Nigel,

Please read the user guide.  You need to remove the SPEC line and add

&REAC FUEL='METHANE'/

Assuming you have enough heat flux to the surface, it will burn.

Since you are new to this, I also recommend starting from a working example.  See the FAA_Polymers cases in the FDS validation suite.

https://github.com/firemodels/fds/tree/master/Validation/FAA_Polymers

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[Nigel O'Sullivan]

Randy

Thank you for your time and response.

Will review the guide again now and start with those examples

Regards

Nigel

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[buildingd...@gmail.com]

Thank you all for your assistance with my queries.

I would like specially to thank Tim O'Brien for his help and direction, it proved to be invaluable.

Thank you

Nigel