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Topics - Lars

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PyroSim / Radially spreading circle instead of ring
« on: April 11, 2019, 08:45:33 am »
Hi everyone,

I'm modelling a fire with a radial spread rate and I'm looking for some help.

The burner has a HRRPUA of 1.5 mW/m2 applied to a vent with a size of 42 m2. With the given spread rate of 0.003 s, the fire should reach the bounderies of the (circular) vent at 1200 s, which means at that point there should be a HRR of aprrox 63 mW that should stay at 63 mW for the remainder of the simulation (1800 s).

However, at 1200s, when the fire reaches the bounderies of the vent, the HRR cuts off at 25 mW and stays 25 mW for the remaining 600s. For some reason, the fire does spread radially, but instead of creating a widening circle, it forms a widening ring. The cells in the centre of the fire seem to go out at the same rate as the fire widens, but this stops when the fire reaches the vent boundery, resulting at a ring of (25 mW / 1.5 mW =) aprrox. 16.6 m2, that stays like this for the rest of the simulation.

Initially I did not model any ramps into the burner, that would result in this behaviour.
Trying to solve this problem, I did add a ramp:
T=0, F=0.0
T=1, F=1.0
T=1799, F=1.0
T=1800, F=0.0
The output files, both the visual as the HRR plot are exactly the same, so this did not solve anything. Still a cut off at 25 mW and still a widening ring.

In the FDS manual on p.95, an example is given to create this widening ring with a ramp that turns the cells on and off in 30 secondes. How do I create a widening circle in which the inner cells stay 'on' and the HRR reaces 67mW instead of 25mW?
I believe I've modelled these kinds of fire spreads before and I can't find out where i'm going wrong.


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