HRR Curve Discontinuity - Device Froze Time but HRR Continued Growing (at a much slower rate)

[nikunjpatelnz]

Hi All,

Currently reviewing a model using the growing-area fire methodology. It has produced a HRR curve that is discontinuous following sprinkler activation.

This action (sprinkler activation) has been programmed to freeze a device (which records time) with a control linked to the activation of various sprinkler links.

I have found one item that may have resulted in this issue (but I am unsure, hence, the post)

The Ramping Function Values associated with each fire, leave 1 second in between the beginning and the end of another fire spawning

"Fire 10"

"Fire 11"

Between Fire 10 and Fire 11 (184 s - 185 s), there is no registered fraction for the ramping function to call.

Now in this case, the sprinkler activated at 184.57 s, right smack bang in the middle.

I speculate that this may be the reason for the HRR discontinuity, we are going to rerun the simulation closing each of these gaps to eliminate this as a possibility.

In the meantime has anyone seen anything like this?

I will respond back in this thread whether or not was the cause of the issue.

Regardless, I am keen to understand why it seems the device freezing time has activated but the HRR has continued to grow (to what seems another 160 kW, which is the size of one additional fire grid cell at max HRR). While we're here it is probably useful to mention these details:

Fire Obstruction Size = 0.4 m x 0.4 m (X x Y)
Fire Max HRR = 1000 kW/m2

Pyrosim Screenshot:

[Vojta-Salek]

Hello, can you please post your input file? Ideally simplified as much as possible to isolate your problem. I am not sure I understand your concern. The RAMP functions always have a value to call. They should maintain their final value forever unless another point is specified.

[nikunjpatelnz]

Unfortunately this documentation is sensitive so I cannot post the entirety of the input files. Although here are the records for the the fires before and after the sprinklers activate:

&SURF ID='Fire 10',
COLOR='RED',
HRRPUA=1000.0,
RAMP_Q='Fire 10_RAMP_Q',
HEAT_TRANSFER_COEFFICIENT=0.0/
&RAMP ID='Fire 10_RAMP_Q', T=176.0, F=0.07984, DEVC_ID='HRRTime'/
&RAMP ID='Fire 10_RAMP_Q', T=177.0, F=0.183313/
&RAMP ID='Fire 10_RAMP_Q', T=178.0, F=0.287372/
&RAMP ID='Fire 10_RAMP_Q', T=179.0, F=0.392018/
&RAMP ID='Fire 10_RAMP_Q', T=180.0, F=0.49725/
&RAMP ID='Fire 10_RAMP_Q', T=181.0, F=0.603068/
&RAMP ID='Fire 10_RAMP_Q', T=182.0, F=0.709472/
&RAMP ID='Fire 10_RAMP_Q', T=183.0, F=0.816463/
&RAMP ID='Fire 10_RAMP_Q', T=184.0, F=1.0/

&SURF ID='Fire 11',
COLOR='RED',
HRRPUA=1000.0,
RAMP_Q='Fire 11_RAMP_Q',
HEAT_TRANSFER_COEFFICIENT=0.0/
&RAMP ID='Fire 11_RAMP_Q', T=185.0, F=0.032203, DEVC_ID='HRRTime'/
&RAMP ID='Fire 11_RAMP_Q', T=186.0, F=0.140953/
&RAMP ID='Fire 11_RAMP_Q', T=187.0, F=0.250288/
&RAMP ID='Fire 11_RAMP_Q', T=188.0, F=0.36021/
&RAMP ID='Fire 11_RAMP_Q', T=189.0, F=0.470718/
&RAMP ID='Fire 11_RAMP_Q', T=190.0, F=0.581812/
&RAMP ID='Fire 11_RAMP_Q', T=191.0, F=0.693493/
&RAMP ID='Fire 11_RAMP_Q', T=192.0, F=0.80576/
&RAMP ID='Fire 11_RAMP_Q', T=193.0, F=1.0/

My concern is that when 'Fire 11' is scheduled to activate at 185 s, the device measuring time (which this fire responds to in Ramping Function Values) has already activated (at 184.57). Therefore, the HRR should not rise more than 1600 kW (which is 10 fires multiplied by the registered HRR per unit area [10(quantity of fires) x 1000(kw/m2) x 0.16 (m2)]. Instead, it seems that the HRR of the fire keeps growing (albeit at a slower rate) until about 300 s and then remains 'constant'. 300 s does not signify the start of any controls registered in FDS.

Devc Ctrl Log:

HRR Curve:

Activation Control for Freeze Device:

[Vojta-Salek]

Ok, now I get it. Just two quick thoughts - they probably won’t solve the issue, but hopefully they won’t hurt either:

1. Have you checked the MLR of your fuel? Is it still increasing after 184.57 s? HRR is influenced by oxygen mixing, so not all the fuel necessarily burns immediately in the domain.
2. If I understood correctly, your Fire 11 is already burning from the start of the simulation, even if just at F = 0.032203. To delay its ignition, you could modify the RAMP like this, adding a line to hold it at zero until 184.9 s:

&RAMP ID='Fire 11_RAMP_Q', T=184.9, F=0, DEVC_ID='HRRTime'/
&RAMP ID='Fire 11_RAMP_Q', T=185.0, F=0.032203/
&RAMP ID='Fire 11_RAMP_Q', T=186.0, F=0.140953/
&RAMP ID='Fire 11_RAMP_Q', T=187.0, F=0.250288/
&RAMP ID='Fire 11_RAMP_Q', T=188.0, F=0.36021/
&RAMP ID='Fire 11_RAMP_Q', T=189.0, F=0.470718/
&RAMP ID='Fire 11_RAMP_Q', T=190.0, F=0.581812/
&RAMP ID='Fire 11_RAMP_Q', T=191.0, F=0.693493/
&RAMP ID='Fire 11_RAMP_Q', T=192.0, F=0.80576/
&RAMP ID='Fire 11_RAMP_Q', T=193.0, F=1.0/

[drjfloyd]

Double check to make sure you in fact have the freeze applied to all of your fires.

[nikunjpatelnz]

Hi drjfloyd,

I have checked the freeze is applied to all the fires.

The Burner Surfaces and their associated Ramp-Up Time custom values have been copied over from another sim where the fire has grown and 'froze' like what you would expect.

We will rerun it filling in the gaps between the fires and their adjacent Ramp-Fraction times and report back.