Using the fire spread rate to mimic a t^2-fire

[Steffen Sæle]

Hi. I wanted to simulate a specific t^2- fire by using the fire spread rate- code. These fire- curves are suggested in our national standards/codes, and I wanted a way to simulate a slightly more realistic version of these curves, since specifying this as a time-ramped fire on a vent with a constant area causes, for instance, the boyancy forces to be disproportionate with the area of the vent for quite some time. 

However, I read in the FDS User's Guide this should not be done. I was wondering why this is. The Guide continued to say that the whole point of the radially spreading fire was to mimic this curve (t^2-fire- curve) in a more natural way. I'm probably just misundarstanding something, since English is not my first language, but is the Guide saying that this t^2- fire is less "accurate"/"ralistic" than the alternative- to use a time ramp on a vent with a constant area? Or is there a better way to simulate such a fire?

I've used this version of a t^2- fire for some of my school projects and it works (mostly) just fine. Regrettably this use of the code, occationally causes a numarical instabilty when nearing the end of the simulation time. Might this be part of the answer to my question? In these cases I've been looking for divergencies in the simulation, but haven't been able to find substancial evidence for one.

Thank you in advance. I really appreciate this gruop and I've used it a lot through my studies to become a FSE (still in school). I could't, however, find a "thread" in this group asking about this issue.

[Kevin]

Cut and paste the text from the User's Guide that you do not understand.

[Steffen Sæle]

Note that the RAMP_Q is used to turn the burning on and off to simulate the consumption of fuel

as the fire spreads radially. It should not be used to mimic a t-squared fire growth rate – the whole point of

the exercise is to mimic this curve in a more natural way. Eventually, the fire goes out as the ring grows past

the boundary of the rectangle.

Is this referring to the use of the RAMP_Q and not, as I thought, the use of the radially spreading fire to mimic a t^2- fire?

[Kevin]

If you use SPREAD_RATE, do not use RAMP_Q. By allowing the fire radius to increase linearly in time, you will cause the fire HRR to increase quadratically, i.e. a t^2.

[Steffen Sæle]

Ok thank you :) I see what I wasn't understanding before. 

Den man. 4. jun. 2018, 22:36 skrev Kevin <mcgr...@gmail.com>:

If you use SPREAD_RATE, do not use RAMP_Q. By allowing the fire radius to increase linearly in time, you will cause the fire HRR to increase quadratically, i.e. a t^2.

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