Suppression=T issues about Heat of Combustion

[Han-Seokhyeon]

Hello,

I am currently designing a fire simulation related to suppression effects on an electrical cabinet fire, where thermoset cables are installed.(FDS Ver. 6.7.7)

As a prerequisite, I have set "SUPPRESSION = T" in the simulation.

The combustible material in question is "XPE/Neoprene", and its formula is defined as 'C3H4.5Cl0.5' based on data referenced from NUREG-1934.
For the Fire RAMP, I have applied a t-squared growth curve, ensuring that steady-state burning is reached after 12 minutes.
The HRRPUA is set to 1000, and the Fire VENT area is 0.2 m², making the total HRR of the fire 200 kW (based on NUREG-2178 Vol.1, Table 4-1, "3-Power Inverters - TS/QTP/SIS 98th% HRR Value").

Issue:
The known heat of combustion for the cable is 10,300 kJ/kg, but when this value is applied, the fire does not ignite at all.
If I set SUPPRESSION = F, the fire ignites as expected. However, this approach prevents me from analyzing the intended suppression effects in the model.

Interestingly, when I artificially increase the heat of combustion to 15,000 kJ/kg or 20,000 kJ/kg, the fire ignites and behaves normally.
However, modifying the heat of combustion arbitrarily may undermine the validity of the simulation results.

The issue is that even though the mass file indicates sufficient oxygen is available, combustion does not occur when the heat of combustion is set to 10,300 kJ/kg.
Additionally, Auto-Ignition Temperature is set to 0K, so combustion should theoretically take place.

Why does the fire fail to ignite when the heat of combustion is low?

[Vojta-Salek]

Hello, can you provide your input file? Also, is there a reason to use an old FDS version?

[Han-Seokhyeon]

There is no specific reason for using the old version. I was simply using the version I had been working with.

I am attaching the file below for verifying this issue
(Due to GitHub's file attachment restrictions, I have changed the extension of the FDS file to .txt and attached it.
TS_SUPP_T.txt

[drjfloyd]

Have you read the Technical Reference Guide discussion of the default exintcion model for FDS 6.7.7?

[drjfloyd]

Your conclusion that you cannot have 10500 kJ/kg and SUPPRESSION=T is correct but the reason is not quite correct.

The default extinction model in 6.7.7 is a thermal model where enough energy must be released during combustion to heat the resulting products above the fuels critical flame temperature. With your fuel and REAC you need 3.05 moles of O2 per mole of fuel which means also heating 11.46 mols of N2 or you are heating ~8.2 kg of gas per kg of fuel. The default critical flame temperature is 1427 C. To heat 8.2 kg of gas from 20 C to 1427 C is going take at least 11600 kJ (8 kg * 1407 C * 1 kJ/kg/K). 10500 < 11600.

[Han-Seokhyeon]

I don't fully understand why the product needs to have enough energy to raise its temperature above the Critical Flame Temperature (CFT).

Looking at the explanation of the Extinction Model, in the case of Extinction Model 2 (the default), wouldn't burning still occur as long as there is a sufficient O₂ volume fraction, even without the product reaching CFT? (Referring to the trapezoidal burning region at the top of the triangle in Fig. 5.2.)

I'm confused about where my understanding is incorrect. Could you clarify?

[drjfloyd]

The default extinction model in FDS 6.7.7 is EXTINCTION 2. This model has two requirements that must be met before combustion is allowed in a grid cell:

1. The pre-combustion cell temperature is above the auto ignition temperature. This defaults to 0 K so unless you increase it, this requirement will always be met.
2. The post-combustion temperature of the mixture must exceed the CFT.

EXTINCTION 2 makes no explicit check on the O2 concentration. That is the EXTINCTION 1 model. In EXTINCTION 2 the local O2 concentration of the mixture is used to determine homw many kg of all the gas that isn't fuel needs to be heated. For your fuel defintion per the quick hand calc I gave above, at ambient O2 levels the amont of mass that must be heated to the CFT takes more energy than the heat of combustion.

[Han-Seokhyeon]

So according to your explanation,

if the heat of combustion is 10,300 kJ/kg, applying EXTINCTION 2 would mean that the fire cannot occur due to insufficient combustion energy.

However, if EXTINCTION 1 is applied, would the fire be able to occur?

Since there would be no need to heat the surrounding gases up to the CFT, right?
(Assuming that a sufficient amount of oxygen is provided.

)

[drjfloyd]

EXTINCTION 1 only looks at the O2 concentration and the cell temperature. Baked into this method are assumptions about the fuel EPUMO2 and the fuel CFT. Your fuel EPUMO2 is about half of typical value is why the EXTINCTION 2 method with the default parameters is failing.