External Heat Flux in FDS5 vs FDS6 (repost)

[philip]

Hi All,
I have noticed that I cannot replicate cone calorimeter results of FDS5 in FDS (6).
I set up a simple cone calorimeter with plenty of space for combustion. However, with identical properties I get vastly different results.

The HRR chart attached, shows FDS5 in RED and FDS6 In BLUE. The light blue, is when I reduce the external_flux by half in FDS6.

I'm not sure what could be causing this. I prefer my FDS5 results, it seems like reactions happen to fast in FDS6. Did the EXTERNAL_FLUX parameter change from FDS5?
Thanks for all you advice once again.


FDS 6 code:
&HEAD CHID='FDS6 50KG', TITLE='Material'/
&TIME DT=.1, WALL_INCREMENT=1, TWFIN=100./

&MESH ID='Mesh', IJK=10,10,36, XB=0.0,0.1,0.0,0.1,0.0,0.6/

&REAC ID="nFUEL'',
    C=1
    H=4
    SOOT_YIELD=0.06/

&MATL ID                    = 'SAMPLE'
    SPECIFIC_HEAT         = 1.0
    CONDUCTIVITY          = 0.08
    DENSITY               = 50.0
    REFERENCE_TEMPERATURE=296
    N_REACTIONS=1,
    NU_SPEC=1,
    SPEC_ID='nFUEL',
    HEAT_OF_REACTION      = 970.
    HEAT_OF_COMBUSTION    = 2.9E+04 /
     
&SURF ID='UPHOLSTERY',
    RGB=135,97,89,
    MATL_ID='SAMPLE',
    THICKNESS = 0.030,
    EXTERNAL_FLUX=50,
    BACKING='INSULATED'/

&OBST XB=0.0,0.1,0.0,0.1,-0.06,0.0, COLOR='BROWN', SURF_ID='UPHOLSTERY'/

[dr_jfloyd]

Run your FDS5 and FDS6 cases like the one's in the Verification Guide (just enough grid cells for FDS to run and the the gas phase combustion turned off).  This will let you see if there are indeed significant differences in the solid phase model between 5 and 6.  Right now your domain is allowing for combustion to occur and the differences could be due to the gas phase.

Also you say you like FDS5 better because it looks better?   Do you have actual cone data to compare to?