EXTINCTION model

[Khalid Moinuddin]

Until version 6.7.9, the default EXTINCTION model for VLES was EXTINCTION 2.  However, now (6.8.0), the default EXTINCTION model for VLES is EXTINCTION 1.  

What is the rationale behind such change? 

[Jonathan Hodges]

Extinction 2 relies on resolving high temperatures of the flame which generally will not be resolved in VLES. For the most part this change does not impact users. While the extinction model is technically active by default, the AUTO_IGNITION_TEMPERATURE is set at 0 K unless the user specifically sets it.

[dr_jfloyd]

Ignition in FDS requires the cell temperautre be over the A_I_T and that gas contents of the cell pass an extinction check. EXTINCTION 1 only uses the cell temperature. EXTINCTION 2 uses the cell temperature and the cell gas concentrations. For applications where you are well ventilated there should be little effect other than EXTINCTION 1 was not sensitive to high molecular weight fuels species without user defined thermophysical properties. For underventilated fires and coarse grids we noticed  that EXTINCTION 2 was resulting in too much extinction. 

[Khalid Moinuddin]

Thanks, Jonathan and Jason.

 

 

 

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Sent: Saturday, 9 December 2023 1:10 AM
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Subject: [fds-smv] Re: EXTINCTION model

 

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[Iqbal]

Hello, I am following the discussions and some more details is requested for the clarification regarding their implementation.

I have intend to employ Very Large Eddy Simulation (VLES) for modeling, taking into account an Auto-Ignition Temperature (A_I_T) set to 215 degrees Celsius for Heptane, utilizing a grid size of 50 mm or less, and with adequate/small ventilation. How the use of EXTINCTION 2 or EXTINCTION 1 is going to affect the suppression?

Considering Jonathan's discussion, as VLES does not resolve the high temperature of the flame, the use of EXTICNTION 2 will not give any further benefit.

Regards

Iqbal

[Jonathan Hodges]

Since the AIT of Heptane is relatively low, I expect you would get similar results between EXTINCTION 1 and 2, but it would be interesting to see in your case. Where I have personally seen a larger difference is in fuels with AIT in the ~500 C range and you get interesting interactions with compartment fire dynamics with flashover temperatures.

[H. M. Iqbal Mahmud]

Thanks Jonathan. 

I am planning to use the model EXTINCTION 2 in VLES to simulate fire extinguishment with AIT.

Let's see how it goes.

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[Iqbal]

Hi There, 

One more thing to clarify- If I use EXTINCTION 2 in SVLES mode, instead of the default EXTINCTION 1 model, how is it going to affect the suppression?
As mentioned by Jonathan Hodges in this link- "Since the AIT of Heptane is relatively low, I expect you would get similar results between EXTINCTION 1 and 2". Is this that this comment is also applicable for SVLES?

Thanks for your cooperation.

[Kevin McGrattan]

If you are using SVLES, use EXTINCTION 1. EXTINCTION 2 is intended for simulations with relatively fine resolution, like cells on the order of a few centimeters.