Re: Re-ignition occurs when fire extinguishing with nitrogen gas

[sssq ss]

Here are some simulation pictures

在2023年11月22日星期三 UTC+8 15:48:09<sssq ss> 写道：

Dear expert, I am simulating an experiment in extinguishing a flame with nitrogen, but the flame re-ignites after the nitrogen is extinguished. Is this a problem with the setting of ait_exclusion_zone, or is it because it is inextinguishable?

[dr_jfloyd]

Did you set an AUTO_IGNITION_TEMPERATURE on REAC?  If that is not the issue then we would need to see your input file.

[Kevin McGrattan]

There is an AIT but also an EXCLUSION_ZONE. If the O2 level is high enough, the fire will continue burning.

[dr_jfloyd]

Your VENT for the NITROGEN is commented out in this input file. Is this the exact file you used? If so, then the issue is with the VENT commented out you have no NITROGEN being added.

If you did run a version of this without the VENT and its CTRL commented out a few comments:

-You have a lot of OPEN VENTs in the domain. Gasseous suppression requires that a sufficient concentration of the agent be maintained to keep the fire suppressed. OPEN VENTs are going to allow NITROGEN to leave the domain and AIR with oxygen to enter the domain. 

-The EXTINCTON 2 model in FDS says combustion is allowed as long as there is fuel and oxygen present above the AUTO_IGNITION_TEMPERATURE with enough fuel and oxygen to reach the CRITICAL_FLAME_TEMPERATURE. You have an AIT of 215 C which means all it takes is some gas cell somewhere to be above that temperature while there is fuel and oxygen present to reach the CFT to reignite the fire.  You also have an AIT exclusion zone where it all it takes is having fuel and oxygen present to reach the CFT for the fire to burn. If the fire reignites somewhere that heat addition can result in more cells reaching the AIT and being able to burn

-Your WOOD surface has an IGNITION_TEMPERATURE and an HRRPUA. One a wall cell reaches the IGNITION_TEMPERATURE it will emit HRRPUA of fuel for the remainder of the simulation. With these inputs, there is no coupling between heat feedback and the size of the fire once the IGNITION_TEMPREATURE is reached. 
-You may want to add slice files for NITROGEN, OXYGEN, and REAC_FUEL plus EXTINCTION. This will let you see where exticntion is occuring and the conditions that are present in those locations. It may help you better understand what is happening in your simulation.

On Thursday, November 23, 2023 at 2:02:16 AM UTC-5 10034...@qq.com wrote:

Yes, I have set up AIT and also set up AIT_EXCLUSION_ZONE. Is this kind of resurgence bound to happen? Or am I missing some settings? Could you please help me take a look?

[sssq ss]

Dear professor, maybe my file has some modifications that were previously used in pyrosim. That's right. I want to simulate an experiment in which the flame is completely extinguished by using nitrogen, but I set ait_exclusion_zone, which will cause re-ignition. I would like to ask if you have any good suggestions to completely extinguish the flame?

[dr_jfloyd]

I suggested you add more outputs so you can make an effort to understand what is happening in your simulation. You may also wish to start with simpler test cases so that you can more easily develop an understanding of how extinction in FDS works.