Kinetic properties of CSIRO grass fire

[Khalid Moinuddin]

For the validation of CSIRO grass fire cases, the following kinetic properties are used:

      A                     = 36300.
      E                     = 60300.

A value seems to be extremely low and E value is quite high. I am wondering what is the source of these values. I can't find any source for these values in the FDS validation guide. This guide mentions Pyrolyis Temperature of 200oC with a reference to Morvan and Dupuy (2004). However, I can't find this  200oC value in Morvan and Dupuy (2004). 

Secondly, there is no parameter named 'Pyrolyis Temperature'. Does it mean REFERENCE_TEMPERATURE? If so, what are the values of REFERENCE_RATE and HEATING_RATE?

[Randy McDermott]

The 200 C is the first step of pyrolysis from Fig 1 of Morvan and Dupuy (2004).

A more complete analysis of the TGA data from that paper is found in the FDS Verification Guide:

12.8.5 TGA of various Mediterranean vegetation (Morvan_TGA)

But, back to your original point, yes, the A and E may need to be updated.  We were hoping to work things into a more generic vegetation model.  The A and E version of the Morvan TGA is found in WUI/Morvan_TGA_2.fds.  The A value is two orders of magnitude higher in that case.  At one point not too long ago were were trying to reconcile the various "vegetation" models used in CSIRO and NIST Douglas Firs.  I think we got sidetracked, but hope to return to this soon.

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[Khalid Moinuddin]

Thanks, Randy.

 

By putting all fire properties (kinetic, flammability and thermal) of Morvan_TGA_2.fds, reducing NU to 0.17 and setting  DRAG_COEFFICIENT to 8 ( = 1/0.125) – I could get results for C064_BFM close to Moinuddin et al (IJWF, 2018) and hence close to experimental results.

 

Further fine-tuning of fire properties can lead to a better agreement. As the fire properties of the actual grass species were not measured, fine-tuning can be a sensible option (unless we measure properties for one type of kerosene grasses).

 

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[Kevin]

The values for A and E come from 

B. Porterie, J.L. Consalvi, A. Kaiss, and J.C. Loraud. PredictingWildland Fire Behavior and Emissions
Using a Fine-Scale Physical Model. Numerical Heat Transfer, Part A, 47:571–591, 2005.

The other parameters are listed in Chapter 19.1 of the FDS User's Guide.

[Khalid Moinuddin]

I have found that with  fire properties (kinetic, flammability and thermal) of Morvan_TGA_2.fds, it is difficult to obtain grid converged results. With the following properties, I had to use 0.167 m grid to obtain grid convergence. I adjusted  NU_MATL=0.3 to get results for C064_BFM  close to  the experimental results. Not sure how Vanella et al got grid convergence with such course resolution.

&REAC FUEL='FUEL VAPOR', C=3.4, H=6.2, O=2.5, SOOT_YIELD=0.008, HEAT_OF_COMBUSTION=16400., IDEAL=T /

&SPEC ID='WATER VAPOR' /

&MATL ID                    = 'GENERIC VEGETATION'
      DENSITY               = 512.
      CONDUCTIVITY          = 0.1
      SPECIFIC_HEAT_RAMP    = 'c_v'
      A                     = 3.88E+06
      E                     = 8.79E+04
      NU_SPEC               = 0.7
      SPEC_ID               = 'FUEL VAPOR'
      NU_MATL               = 0.3
      MATL_ID               = 'CHAR'
      HEAT_OF_REACTION      = 418. /

&MATL ID                    = 'CHAR'
      DENSITY               = 150.
      CONDUCTIVITY          = 0.052
      SPECIFIC_HEAT_RAMP    = 'cp_char'
      N_S                   = 1.
      N_O2                  = 1.
      GAS_DIFFUSION_DEPTH   = 0.001
      SPEC_ID               = 'PRODUCTS','AIR'
      NU_SPEC               = 8.18,-7.2
      MATL_ID               = 'ASH'
      NU_MATL               = 0.02
      HEAT_OF_REACTION      = -32000.
      A                     = 2.25E+01
      E                     = 5.30E+04 /

&MATL ID                    = 'ASH'
      DENSITY               = 5.
      CONDUCTIVITY          = 0.1
      SPECIFIC_HEAT        = 2.0 /

&RAMP ID='c_v', T=  0., F=1.0 /
&RAMP ID='c_v', T= 25., F=1.0 /
&RAMP ID='c_v', T=125., F=2.0 /
&RAMP ID='c_v', T=175., F=2.4 /

/&RAMP ID='cp_dry_veg', T= 0.,    F=1.18 /
/&RAMP ID='cp_dry_veg', T= 1000., F=4.81 /

&RAMP ID='cp_char', T=   27, F=0.715 /
&RAMP ID='cp_char', T= 1724, F=2.04 /

[Ruddy]

The validation needs, in addition to the spread rate, the fire depth to agree with observation (ideally all along the fire perimeter). For these AU grassland experiments, the head fire depth is approximately 10 m. The FDS results from Vanella et al. have a head fire depth that is approximately a factor of 4 too small (of course the definition of fire depth needs to be given and consistent). With this small head fire depth the heat release rate associated with the head fire will be 4 times larger than in the experiment, assuming the FDS head fire rate of spread matches observation.