Burn Away and Bulk Density for CLT

[Rebecca Lawson]

Hi, 

I am running some experiments for CLT slabs in FDS. I am trying to compare the burn away function with and without bulk density. However, I am struggling to achieve burn away in the models.

I have kept the size of the model extremely small so that I can achieve quick rudimentary results before I move on to a larger, more accurate model. However, I am struggling to actually achieve any burn away in any model and I am confused as to why. 

Prelimainary calculations of Total energy divided by HRR gives time to burn at approximately 300s. Naturally, I run my models for far longer than this time (2000s) but burn away still doesn't occur. The dimensions of the slab are 0.6mx0.6mx0.2m with a burner of the same area (0.36m2) releasing a HRRPUA of 5000kw/m2 or 1800kw/m2 in the model. 

In previous models for other projects I have easily achieved burn away and watched materials 'disappear' in smokeview so I am confused as to why I can't seem to get these models to work. 

I have attached my FDS codes for both models below (014 without bulk density, 015 with bulk density).

If there is some obvious criteria I am missing please let me know as I am struggling to understand how these models are working. 

Thanks!

[Kevin]

It appears that there is always some residue (NU_MATL) for the solid phase reactions. Thus, the material will never completely burn away. I have never used burn away with a complex solid like this, so I cannot confirm if it would or not even if the residue yield were zero.

Before worrying about BURN_AWAY, have you confirmed that your basic model agrees with a cone calorimeter measurement? Are you sure that the HEAT_OF_REACTION values are negative? In other words, are these reactions exothermic?

[Salah Benkorichi]

I will take a look

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[Salah Benkorichi]

I have played around with your cases. Let's focus here on the second (015.fds). 

You seems to have big mass that needs more time to get burned with your specific parameters. 

Your density was 480 kg/m3. This is a big figure to get it burned with your input, it would require more time. For 1 kg/m3 it would require you around 4s. Then 5kg/m3 would require around 24s, you can figure out then how it would take to get 480 kg. You might need at least 2300 s ~ 40 min to get the cells disappear ( I haven't tried it though). 

One thing also you should keep in mind. In order to see the cells disappear in smokeview, you need to close it and open it after the timing pass. If you leave it open until the time finishes and then reload it, you wont see the cells disappear. Close it and open it, then you will see the effect. 

Set time for 3000s to be in the safe side and rerun it and report what you get.

[Salah Benkorichi]

One thing also you might try is to double the nbr of cells, so you can see the burning taking place (which takes more time to finish). The reason for this long delay is that you have only one cell thick, which needs the burning of all mass before disappearing.

[RL]

Hi Kevin,

Thank you for your help! 

I reran the model without any char residue and the burn away function worked. It appears that you are correct in that for a complex solid, it won't be able to process burn away unless the entire material mass fraction has no residue specified (in this case, charring). 

[RL]

Thanks Salah!

I am aware that it is a large mass that needs to be burned. However, I was under the impression that if bulk density is specified on the obstruction, then FDS will not process the burn away cell by cell but rather as the entire material. Therefore, it should still burn away regardless of how large your cell size is. 

I have rerun the model without any char residue specified and the timber finally burned away in approximately 500 seconds. After I got it to work I have rerun the model with much smaller cell sizes as well to view the differences that a smaller cell size might yield. 

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[RL]

My more recent iterations have produced better results:

018 - run for 3000s with 0.2 cell size and no bulk density specified

017 - run for 3000s with 0.2 cell size and bulk density specified

021 - run for 2000s with 0.1 cell size and no bulk density specified

022 - run for 2000s with 0.1 cell size and bulk density specified

I have uploaded the HRR graphs from each simulation and it seems that the smaller cell size does indeed produce different results to the singular cell thickness previously used in 017 and 018. However, I am confused as to why the HRR is so low after the timber has burned away. I have specified a constant burner that should release 1.8MW and I would expect that it would remain at 1.8MW after the timber has burned away. In 021 and 022, the HRR never even reaches 1.8MW. 

[dr_jfloyd]

You have a domain that is 1.2 m^3.  A 1.8 MW fire will have a flame height of ~3 m and a flame volume ~5 m^3.  Your domain is too small for the fire to entrain enough air to burn the fuel.