Gaseous fuel products from pyrolysis spread and exit from the computational domain?

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Alberto Prevedello

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Sep 14, 2022, 8:04:02 AMSep 14
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Hi everyone,

I am running an FDS file with a fire of several pallets, a great storage of paper. The total RHR available is more than 2000MW. The total surface of the aforementioned storage is more than 500m2 (total number of Grid Cells 1080000, physical dimensions 24x36x10m).

In the FDS file I used a "Simple Pyrolysis Model" (11.4 FDS User's Guide) with a specific HHRPUA [kW/m2], one for surface ("&SURF") named Burner and one for surface ("&SURF") named Paper.

Also, in the FDS file I used a "Single-Step, Mixing-Controlled Combustion", with reaction characteristics as follow:

&REAC ID='Carta',
FORMULA='C6H10O5',
CO_YIELD=0.004,
SOOT_YIELD=0.015,
RADIATIVE_FRACTION=0.35, /

and "Mass Fraction Transformation Matrix to Convert Species Mixtures (Columns) to Primitive Species (Rows)" extract from "CHID.out" as follow:

AIR Carta PRODUCTS

Carta 0.000000 1.000000 0.000000
NITROGEN 0.760861 0.000000 0.632995
OXYGEN 0.230510 0.000000 0.000000
CARBON DIOXIDE 0.000590 0.000000 0.263970
CARBON MONOXIDE 0.000000 0.000000 0.000672
WATER VAPOR 0.008039 0.000000 0.099841
SOOT 0.000000 0.000000 0.002521
HYDROGEN 0.000000 0.000000 0.000000

The ventilation consists to window and doors, the former broke up at 300°C before the 10th minute, and the latter open by fire figther at 20th minute.

The model suffers a low availability of Oxigen within the storage area. I suppose that the total of paper material gets to autoignition Temperature, and than all the paper material produce gaseous fuel due to pyrolisys which doesn’t burn due to the absence of Oxigen.

The gaseous fuel products from pyrolysis spread and exit from the internal area (paper storage) and from the computational domain. Is that plausible?

This idea comes from the following considerations:

  • the MLR_FUEL from CHID.hrr.csv, multiplied for the Heat of Combustion value (16090kJ/kg) is vastly major then HRR of computational domain;
  • the inlet of oxigen, measured whit a proper device ("QUANTITY='MASS FLOW', SPEC_ID='OXYGEN'"), multiplied for the Heat of Combustion value for oxygen (13600kJ/kgO2) is vastly minor of the RHR value of the computational domain;
  • the RHR value measured within the internal area (paper storage) is equal to the value of the previous point and is vastly minor of the RHR value of the computational domain.

Thanks

Alberto


dr_jfloyd

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Sep 14, 2022, 8:14:12 AMSep 14
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Fuel requires oxygen to burn. If there isn't enough oxygen inside a compartment for the fuel being generated there, then unburned fuel will leave the compartment where it can burn when it finally encounters oxygen. This is seen all the time in the real world. Think of large residential structure fires where you see flames leaving open doors and windows. That is unburned fuel exiting the compartment and burning outside.
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