under ventilated fires - reac

[Ioanna Koromila]

Dear all,

I want to simulate an under-ventilated fire in a long and narrow geometry (ship car deck) and I was wondering whether I should consider different chemistry due to low ventilation. After discussing it with fire experts they suggested considering the increase in the production rate of CO and soot due to low ventilation through REAC. How can I consider such an increase?

Thank you, 

Ioanna

[Randy McDermott]

Search the user guide for N_SIMPLE_CHEMSITRY_REACTIONS.  You can set N_SIMPLE_CHEMSITRY_REACTIONS=2 on COMB.

Also, review the validation cases for underventilated compartments: e.g., NIST_RSE_2007, NIST_FSE_2008

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

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[Ioanna Koromila]

Hi Randy, 

Thank you.  As I understand, it is enough to set the one-step chemistry yields, but does this result in the calculation of increased CO due to underventilation?

Ioanna 

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[dr_jfloyd]

As discussed in Section 15.1.3 of the User's Guide, the two step model first converts the fuel to CO and C and then converts the CO and C to CO2 with the second step limited by available oxygen.  In underventilated areas of the domain this will mean some CO and C will not become CO2. In these regions you will see higher CO and C than that expected from just the CO_YIELD and SOOT_YIELD input. Once that CO and C moves towards more well ventilated portions of the domain it will once again react with oxygen to make CO2.

[Randy McDermott]

If you set yields with one-step chemistry, you are assuming you know the global equivalence ratio for the under ventilated compartment, and you should be setting the yields based on that.

But now the preferred approach is to set post-flame yields on the REAC line and then set N_SIMPLE_CHEMISTRY_REACTIONS=2 on COMB.  In this way, FDS will automatically generate higher CO and Soot yields in regions with low O2 because there will not be enough O2 to complete the reaction to CO2.

We need to clarify the user guide on this issue---it currently states that N_SIMPLE_CHEMISTRY_REACTIONS=2 should only be used if you are interested in "in-flame" species compositions, but we have now demonstrated this approach works well for under-ventilated compartments and it is used in all such validation cases.

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[dr_jfloyd]

The validation cases for compartments are a single compartment with an opening where we are looking at gas measurements inside the compartment.  Excess CO leaving the compartment would have been in very hot flow that would burn once it mixed with ambient air.

In the case of a compartment fire where combustion products with excess CO enter a long hallway or a duct, the gasses might cool before they encounter oxygen. In FDS, with the default settings the CO will always react once it encounters ambient air. If you think in your case that this is important then you will need to change the default extinction settings.

[Ioanna Koromila]

Thank you all for your kind assistance and suggestions, they really helped me. I will consider N_SIMPLE_CHEMSITRY_REACTIONS in my study and see the differences.

Ioanna  

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[Ioanna Koromila]

Dear all, 

I'm back to ask if there is any official document (a paper or a guide) that I can refer to, which demonstrates that using the two-step simple chemistry approach can be applied to consider CO increase under-ventilated compartments since the FDS guide states that " The two-step simple chemistry option should only be invoked when you have an interest in near-flame phenomena where the increased concentration of CO and Soot play an important role in the flame chemistry and radiative emission."

Thank you, 

Ioanna 

[Kevin]

I sent you a copy of this paper

A Two-Step Combustion Scheme for Predicting CO in Under-Ventilated Compartment Fires
Kevin McGrattan, Randall McDermott, and Jason Floyd

that I presented at this year's Fire and Explosion Hazards meeting in Oslo Norway.

[Ioanna Koromila]

Kevin, thank you so much. I really appreciate it.

 

Ioanna

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