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Author Topic: Simulating a fire by using a spread rate combined with a ramp  (Read 239 times)

razvan_costa

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Simulating a fire by using a spread rate combined with a ramp
« on: January 08, 2020, 04:19:32 am »

Hi,

I would like to start this topic by saying that there was a similar topic dating from 25 march 2013 and it was written by another member of the forum: Sofia. Sofia received an answer that was maybe helpful for her case, but I am not able to use the same answer as it does not solve my issue. The title of Sofia’s topic is ‘Simulate a fire by using spread rate’.

I would like to simulate a fire by using a combination between a spread rate and a ramp. The fire curve shall present 3 phases which are: an increasing phase, a constant phase and a decreasing phase.

The HRR has a peak value of 5000 kW and it is reached after approx. 326 seconds. From there I would like to 'keep' the HRR at its peak value for 60 seconds and afterwards, I would like to decrease the HRR until it reaches 1/3 of its peak value (the decreasing process shall take 60 seconds) and I would like to keep it at this value for the rest of the simulation.

The picture illustrating the theoretical fire curve, which I would like to obtain, is attached to the current post.

Reaching 100% HRR together with ‘keeping’ the HRR at 100% for 60 seconds by using a spread rate do not pose any problems, but decreasing the HRR from 100% to the desired 33.33% in the desired time span (60 seconds) is my main challenge, as I am not able to follow the theoretical curve.

If I use the spread rate and set the ramp values to match desired HRR values at specific time steps, I get a curve which does not match my theoretical curve. The picture illustrating both the theoretical fire curve and the actual fire curve (the one generated by FDS) is attached to the current post.

I would like to add that the following Ramp was used to generate the curve (the blue curve in the attached picture):
&RAMP   ID='FIRE_RAMP'   T=0           F=0   /
&RAMP   ID='FIRE_RAMP'   T=0.01   F=1.0   /
&RAMP   ID='FIRE_RAMP'   T=326.0   F=1.0   /
&RAMP   ID='FIRE_RAMP'   T=386.0   F=1.0   /
&RAMP   ID='FIRE_RAMP'   T=446.0   F=0.333   /
&RAMP   ID='FIRE_RAMP'   T=900.0   F=0.333   /

Summing up the pieces of information presented above: The challenge is to reduce the HRR in such a way that it matches the theoretical curve while using a spread rate.

My question is: Is it possible to combine the spread rate with a ramp and obtain the desired fire curve? For example the ‘theoretical fire curve’ that I have presented in the attached illustration.

I would like to add that if I use just a ramp (not combined with a spread rate) and ‘activate’ all the cells of the ‘fire vent’ from the start of the simulation (a distribution which is uniform across the entire surface), I can easily reproduce the desired curve, but the problem with using this approach is that I am generating a lot more smoke than the spread rate case. I know that this could be identified as being a conservative measure, but that is not in the scope of the current FDS simulation.

As I could understand from an earlier thread, it is possible to combine a spread rate with a ramp, but the overall HRR can’t be described easily. I would like to assess the difficulty of describing the HRR in this way on my own, but I do not have an example which I can use as a starting point. 

I am grateful for any and all the help I can get.

Thank you in advance.

Best regards,
Razvan Costa





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