Extinguishing Coefficient

[MahmoudZew]

Hello,

Normally, when I want to evaluate the effect of a sprinkler system on a fire's HRR, I simulate with sprinklers enabled to determine the sprinkler activation time. I then cap the HRR curve using a t² growth curve.

Recently, I have been reviewing the "E_COEFFICIENT" parameter in FDS. If I understand it correctly, this approach could be used to achieve a similar outcome by estimating the maximum HRR without explicitly simulating the sprinkler system, to only know the sprinkler activation time, and we can use "E_COEFFICIENT" to reduce the burning rate.
Do you have any insight into how this model has been validated, how accurate it is, and what sources or methodologies can be used to select appropriate values that are representative of real building scenarios?

[drjfloyd]

E_COEFFICIENT still requires that you model the sprinkler system. The amount the fire is suppressed is a function of the amount of water delivered to the burning surface which in FDS is obtained by integrating over time the mass of water droplets hitting the burning surface.

The challenge with this model is findiing data with which to define the E_COEFFICIENT. Madrzykowski and Vettori did some work in the 1990s with various office furniture fuel packages and different water delivery rates. https://nvlpubs.nist.gov/nistpubs/Legacy/IR/nistir4833.pdf

Whether or not this data is applicable to your scenario is a decision for you to make. There may be other data that you can find doing a literature search.

[MahmoudZew]

Sorry, just to make sure I understand it correctly, by modeling the sprinkler system, you mean to capture the lower HRR or just to see the other interactions of the water? Won't E_COEFFICIENT allow for reducing the HRR by reducing the burning rate?

[drjfloyd]

The outlined term is the FDS predicted delivered water per unit area. To use E_COEFFICIENT, you must have sprinklers in your calculation as that is the only way for FDS to predict that term.

[MahmoudZew]

Thanks, Jason.

I’m still working through the details and trying to form a clear approach. Does this need to be modeled complete with the full pyrolysis model, meaning the cell size should be refined to capture these effects (on the order of millimeters)? Or is that unnecessary, and would it be reasonable to proceed with the standard building-environment cell sizes?

Best regards,

[drjfloyd]

From the text in 15.7.2:

"If the surface has been assigned an HRRPUA...."

If you have detailed kinetics (i.e., A and E on the surface), then you do not have a specified HRRPUA.