HRRPUA and Fire Ramp function

[Raini D]

I was wondering what is the correct way in setting up a fire with HRRPUA using a burner ramp and a fixed fire surface area and whether there is any difference in the results if the F fraction goes >1.

For example, I want to set up a sprinkler controlled fire scenario. I have determined the burner ramp and maximum HRR. The burner/fire surface area is fixed at 2 m x 4.8 m.

Is the following set up correct? In this case to achieve the maximum HRR, F keeps increasing to >1 until the maximum HRR is reached.

 &SURF ID='FIRE D K',

      COLOR='RED',

      HRRPUA=521.0,

      RAMP_Q='FIRE D K_RAMP_Q'/

&RAMP ID='FIRE D K_RAMP_Q', T=0.0, F=0.0/

&RAMP ID='FIRE D K_RAMP_Q', T=10.0, F=3.752E-3/

&RAMP ID='FIRE D K_RAMP_Q', T=20.0, F=0.015/

&RAMP ID='FIRE D K_RAMP_Q', T=30.0, F=0.034/

&RAMP ID='FIRE D K_RAMP_Q', T=40.0, F=0.06/

&RAMP ID='FIRE D K_RAMP_Q', T=50.0, F=0.094/

&RAMP ID='FIRE D K_RAMP_Q', T=60.0, F=0.135/

&RAMP ID='FIRE D K_RAMP_Q', T=70.0, F=0.184/

&RAMP ID='FIRE D K_RAMP_Q', T=80.0, F=0.24/

&RAMP ID='FIRE D K_RAMP_Q', T=90.0, F=0.304/

&RAMP ID='FIRE D K_RAMP_Q', T=100.0, F=0.375/

&RAMP ID='FIRE D K_RAMP_Q', T=110.0, F=0.454/

&RAMP ID='FIRE D K_RAMP_Q', T=120.0, F=0.54/

&RAMP ID='FIRE D K_RAMP_Q', T=130.0, F=0.634/

&RAMP ID='FIRE D K_RAMP_Q', T=140.0, F=0.735/

&RAMP ID='FIRE D K_RAMP_Q', T=150.0, F=0.844/

&RAMP ID='FIRE D K_RAMP_Q', T=160.0, F=0.961/

&RAMP ID='FIRE D K_RAMP_Q', T=170.0, F=1.084/

&RAMP ID='FIRE D K_RAMP_Q', T=180.0, F=1.216/

&RAMP ID='FIRE D K_RAMP_Q', T=190.0, F=1.354/

&RAMP ID='FIRE D K_RAMP_Q', T=200.0, F=1.501/

&RAMP ID='FIRE D K_RAMP_Q', T=210.0, F=1.655/

&RAMP ID='FIRE D K_RAMP_Q', T=220.0, F=1.816/

&RAMP ID='FIRE D K_RAMP_Q', T=230.0, F=1.985/

&RAMP ID='FIRE D K_RAMP_Q', T=240.0, F=2.161/

&RAMP ID='FIRE D K_RAMP_Q', T=250.0, F=2.345/

&RAMP ID='FIRE D K_RAMP_Q', T=260.0, F=2.536/

&RAMP ID='FIRE D K_RAMP_Q', T=270.0, F=2.735/

&RAMP ID='FIRE D K_RAMP_Q', T=276.0, F=2.735/

&RAMP ID='FIRE D K_RAMP_Q', T=400.0, F=2.735/

Or the following is the correct way? Maximum HRR is reached when F is 1.0 (i.e. the value of F is always between 0 and 1, which is the fraction of the HRR at time t over the maximum HRR over the simulation time). In this case, the HRRPUA has to be increased to 1425.

.

&SURF ID='FIRE D K',

      COLOR='RED',

      HRRPUA=1425.0,

      RAMP_Q='FIRE D K_RAMP_Q'/

&RAMP ID='FIRE D K_RAMP_Q', T=0.0, F=0.0/

&RAMP ID='FIRE D K_RAMP_Q', T=10.0, F=0.001372/

&RAMP ID='FIRE D K_RAMP_Q', T=20.0, F=0.005484/

&RAMP ID='FIRE D K_RAMP_Q', T=30.0, F=0.01243/

&RAMP ID='FIRE D K_RAMP_Q', T=40.0, F=0.021938/

&RAMP ID='FIRE D K_RAMP_Q', T=50.0, F=0.034369/

&RAMP ID='FIRE D K_RAMP_Q', T=60.0, F=0.04936/

&RAMP ID='FIRE D K_RAMP_Q', T=70.0, F=0.067276/

&RAMP ID='FIRE D K_RAMP_Q', T=80.0, F=0.087751/

&RAMP ID='FIRE D K_RAMP_Q', T=90.0, F=0.111152/

&RAMP ID='FIRE D K_RAMP_Q', T=100.0, F=0.137112/

&RAMP ID='FIRE D K_RAMP_Q', T=110.0, F=0.165996/

&RAMP ID='FIRE D K_RAMP_Q', T=120.0, F=0.197441/

&RAMP ID='FIRE D K_RAMP_Q', T=130.0, F=0.23181/

&RAMP ID='FIRE D K_RAMP_Q', T=140.0, F=0.268739/

&RAMP ID='FIRE D K_RAMP_Q', T=150.0, F=0.308592/

&RAMP ID='FIRE D K_RAMP_Q', T=160.0, F=0.351371/

&RAMP ID='FIRE D K_RAMP_Q', T=170.0, F=0.396344/

&RAMP ID='FIRE D K_RAMP_Q', T=180.0, F=0.444607/

&RAMP ID='FIRE D K_RAMP_Q', T=190.0, F=0.495064/

&RAMP ID='FIRE D K_RAMP_Q', T=200.0, F=0.548812/

&RAMP ID='FIRE D K_RAMP_Q', T=210.0, F=0.605119/

&RAMP ID='FIRE D K_RAMP_Q', T=220.0, F=0.663985/

&RAMP ID='FIRE D K_RAMP_Q', T=230.0, F=0.725777/

&RAMP ID='FIRE D K_RAMP_Q', T=240.0, F=0.790128/

&RAMP ID='FIRE D K_RAMP_Q', T=250.0, F=0.857404/

&RAMP ID='FIRE D K_RAMP_Q', T=260.0, F=0.927239/

&RAMP ID='FIRE D K_RAMP_Q', T=270.0, F=1.0/

&RAMP ID='FIRE D K_RAMP_Q', T=276.0, F=1.0/

&RAMP ID='FIRE D K_RAMP_Q', T=400.0, F=1.0/

Or both methods above are acceptable?

We have run both and seems that there is a slight difference when we look at visibility slice. Should there by any difference?

[Randy McDermott]

Either way should work.  Might be slight differences in interpolation.

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[Raini D]

Thanks, Randy.

So the slight difference is expected and not an issue? 

[Randy McDermott]

Well, "slight" is a relative term.  Can you send plots of the HRR vs. Time for each?

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[dr_jfloyd]

Since you have the same T values in each RAMP, you can take the F values for each RAMP and multiply by their HRRPUA and compare the results. The maximum difference in interpolation won't be higher than the maximum difference between values at the same time T. Spot checking a couple values it looks like your two RAMPs differ by ~ 0.01 %. So the RAMPs are not identical, but they are very close. 

[Raini D]

Here are the HRR curves: 1st method on the left, 2nd method on the right

What I would like to confirm is what is best practice? Should method 1 (F>1) or 2 (0<F<1) be used? Or both methods are acceptable, depending on whether we want to set a specific HRRPUA for a specific scenario.

[Randy McDermott]

Both methods are acceptable and within any reasonable uncertainty you would see in a real experiment.  Turbulent flows are chaotic and very minor differences to inputs will result in numerical solutions which are not identical pointwise but are nearly identical from a statistical point of view.

If you ran two experiments that were nominally the same HRR, would you expect to have the exact same thermocouple time histories, matching each and every fluctuation perfectly?  Of course not.  But you would expect the statistics to be the same.  Numerical solutions behave similarly with nominally the same inputs.

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[Nandini Dandugula]

 I am very new to the fds software. How to give the values for the ramp function. how to get that values?

thanks in advance

[Robin]

Hi Nandini,

I very strongly suggest you take a day or two to look through the following resources:

Good FDS tutorials for beginners.

FDS wiki

FDS tutorials from Pyrosim (if you have Pyrosim)

FDS users guide

[Nandini Dandugula]

thank you for your technical support.