HRR Ramp Up Growth Coefficient Cap

[tjelse]

Hello everyone,

I’m working on a using FDS and PyroSim, and I’ve encountered an issue where the peak Heat Release Rate in my model does not reach the value I expected based on the input data.

I’ve carefully set up the fire scenario and ensured that the input for the peak HRR is correctly defined, but the simulation results show a much lower peak HRR than anticipated.

I’ve double-checked the following:

• HRR (200 MW)
• Area of burner (42 m^2)
• HRRPUA (4763.03882 kW/ m^2)

I first set up the burner surface ramp up time to t^2 at 800s, and in the first run it only reached to about 75 MW. After that I set up the custom ramp up table to reach a fraction of 1/1 at 800s, and the results were almost an exact match to the first run, reaching only about 75 MW.

Despite the ramp up selection, the peak HRR seems to act similarly each time. My first thought is that FDS might have a max growth curve coefficient that is being capped out at. I thought that by defining the peak time/ growth curve you could manually create the growth curve coefficient.

Another thought is that I am simulating a heavy goods vehicle fire in a small tunnel, so the enclosure the fire takes place in is relatively small. If not enough oxygen can reach the fire, could this also be why the fire growth is stalling less than inputted? The reaction is an SFPE Polyurethane_GM37 fire. The tunnel has a grade of 3.95 which makes me think oxygen would be sufficiently supplied from downstream.

Has anyone experienced a similar issue or can offer insight into potential reasons why this might be happening? Thank you in advance!

[marcosvanella]

Check the amount of fuel leaving the compartment through the tunnel exit. Does it correspond to the missing HRR?

[tjelse]

Is it possible to do this after its ran without setting up a species quantity 2D at the exit? I see that in the hrr csv the MLR of the fuel, which is SFPE POLYURETHANE_GM37_fuel defined at the fire source, follows the HRR curve.

I think because this curve drops after 800 seconds along with the HRR due to sprinkler activation that fuel is not escaping?

[marcosvanella]

I think you would need to add a FUEL mass flux area integral in the exit and run the case again (see User Guide Sec 22.2.3, Area Integral). If your computed HRR due to combustion (in the CHID_hrr.csv file) is lower than what you specify, you must have unburnt fuel leaving the domain.
Can't think of a different way to back that out from the calculation you just did.

[drjfloyd]

If the HRR in the hrr.csv file doesn't match your expectations it is generally one of three things:

1. You haven't defined the fire like you think you have. Check the MLR in the hrr.csv and multiply it by the fuel heat of combustion. Do you get the HRR you expect? If not then the issue is how you defined the fire.
2. You have a sealed domain and have used up the oxygen.
3. Fuel cannot mix with sufficient oxygen to burn before it leaves the domain. Look at the fire in smokeview. Do you see flames at OPEN VENTs that look like they extend outisde the domain?

[tjelse]

This makes sense.

1. When multiplying I do get the correct HRR I expected.
2. This I am pretty confident is not the issue, I can see the air entering and leaving the domain.
3. I believe this is the issue, you definitely can see flames at the downstream OPEN vent that look like they extend outside the domain.

I did not define a burner vent for the fire surface, I applied the surface to one of the surfaces of a geometry block obstruction. I'm assuming because the fuel can't mix with sufficient oxygen and the full HRR isn't reached, that the program is trying to make up for the missing fire elsewhere? Could you explain why this might be happening, and will applying the HRR surface to a vent be able to fix it?

[drjfloyd]

Fuel doesn't burn unless it is able to mix with sufficient air to combust. If 3 is happening, then this is not error in FDS. You simply have a scenario where the fuel is unable to burn inside the domain you have defined.