HDPE won't ignite (No sustained burning after removal of heat source)

[gforney]

FDS Version:6.1.0
SVN Revision Number: 4664
Operating System: Windows 7
Describe details of the issue below:

Dear community,
I am trying to burn a solid HDPE with a burner source (1000kw/m2).
The burner burns for 50 seconds, then stops. In this point in time, the HDPE block
should continue burning, but it won't.
I tried to play around with the burner duration, expanding it to 200sec, then stop.
However, the HDPE still won't burn.

I attach my .fds file, if anyone could take a look and see what's wrong
I be really grateful. All material properties were taken from the FAA thermoplastic
data (in the validation section of FDS)

All best,
David

Original issue reported on code.google.com by sirok4 on 2014-09-20 14:25:56

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• Attachment: HDPE-burning.fds

[gforney]

Could you attach a description of the experiment where a block of HDPE burns without
an external heat flux. I do not know if we should expect HDPE  to burn without it.

Original issue reported on code.google.com by mcgratta on 2014-09-20 19:48:16

[gforney]

Actually I have witnessed an experiment where a block of HDPE was sprinkled
with Diesel fuel that was ignited and burned for 30 mins. (HDPE was covered
with a layer of diesel because of this). The sprinkler was manually stopped
after 30 mins. Then the HDPE kept burning for another 3 hours with a medium
size flame.
בתאריך 20 בספט 2014 22:48, <fds-smv@googlecode.com> כתב:

Original issue reported on code.google.com by sirok4 on 2014-09-21 03:38:37

[gforney]

The fuel for your REAC is REAC_FUEL.  The gas your SURF emits is PROPANE.  Where have
you told FDS the reaction to use for the PROPANE that your SURF emits?

Also, once you fix that, if it took 30 minutes to ignite the HDPE in your experiment,
 are confident that a mere 50 s exposure will get the block going in your case?

Original issue reported on code.google.com by drjfloyd on 2014-09-21 11:50:19

[gforney]

You are right regarding the REAC_FUEL issue - I fixed that, but that didn't
help.
As for the exposure time, the experiment was conducted with diesel fuel
which emits ~110kw/m2, while the simulation is carried out
with an order of magnitude higher HRR, 1000kw/m2, so I assume this should
shorten the needed exposure time.
Moreover, I think the HDPE block doesn't even heat up, as far as I can see
from the thermocouples near the surface. The temperature
remains ambient 35 deg.

Original issue reported on code.google.com by sirok4 on 2014-09-21 17:19:08

[gforney]

Make a test case using your material inputs following the examples in the Pyrolysis
folder.

The 1000 kW/m^2 is  the HRRPUA of your burner. Not the heat feed back to the pool surface
outside of the burner.  

Test your inputs following the examples in the Pyrolysis folder.  Once you are satisfied
your inputs are sensible, then test them in your current input file.  Add a BNDF for
heat flux so you know what your ignition source is doing.

Original issue reported on code.google.com by drjfloyd on 2014-09-21 23:14:59

[gforney]

I agree. Create a simpler test case. For example, there is no need for 4 meshes and
some of the extra output. Use simple XB coordinates so that it is easier for us to
diagnose problems.

Original issue reported on code.google.com by mcgratta on 2014-09-22 13:52:50

[gforney]

Thank you, guys, I will try to simplify my case. Another question I have is
about estimating the burning duration of solid combustibles.
Suppose, Polyethyelene. I know that to calculate the burning duration I
need to divide the total mass of the solid by the mass loss rate under
burning conditions, to get the time to complete burn out. However, the mass
loss rate depends on external conditions, such as the applied heat flux
onto the sample. Thus, I don't understand how I can obtain the HRR or the
MLR of the self-burning (after ignition of the solid where no more external
heat is supplied). Is there a place of HRR or MLR values for materials
under self-burning conditions ?

It appears that this website
http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1805/s1/sr1805-sup1-vols-1-2.html

has a spreadsheet tool (Chapter 8 in the table) that uses the value of
1408kw/m2 for polyethylene, and can predict the burning duration of a block
of polyethylene given the mass, and a quantity called 'exposed floor area'
- which I am not sure to what it refers. If the material is 10 ft high or 2
ft high both with the same "area" of, say, 1m2 located on a floor, they
certainly will have different burning durations.

Any hints to this puzzle ?

Original issue reported on code.google.com by sirok4 on 2014-09-24 12:58:25

[gforney]

Those are the right questions to ask. The 1408 kW/m2 measurement is from the 3rd Edition
of the SFPE Handbook (or at least that is what is cited by NUREG-1805). I don't see
the value in the 4th edition of the Handbook, which is probably a good thing because
who knows what that value means. Does it mean that I took a pile of old electrical
cable and lit it on fire? Or does it mean that I took a tiny sliver of PE and burned
it in an open flame? It's basically meaningless. 

I suggest you get a sample of whatever plastic you are interested in and take it to
a lab and burn it under various heat flux exposures using a cone calorimeter or similar
device. Try a case where you ignite the sample, let it achieve a steady state burning
rate, and then remove the cone or exposing heat source. What happens?

Original issue reported on code.google.com by mcgratta on 2014-09-24 13:18:45

[gforney]

Well making experiments is for the experimentalists :) I hope the team
comes up with some new experimental results.
Anyway, I have found a nice summary of a simple burning ignition of
Polyethylene with a methane burner. The PE ignited after 30sec exposure and
kept burning.
Here is the file including some very nice photos
http://www.google.co.il/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CCUQFjAA&url=http%3A%2F%2Fhepwww.rl.ac.uk%2FCMSecal%2FSafety%2FTest%2520on%2520flammability%2520of%2520polyethylene%252021%2520Dec%252004.doc&ei=N_IjVK-vN8bB7AaqnIG4Dw&usg=AFQjCNFvz_naZc2PuX5cTX8laF-hHKagng&sig2=88l7xEqSM7Iu0ICpYmvH_w&bvm=bv.76247554,d.ZGU
So, I tried to reproduce that. Taking the PE block with 30, 50 and 80 secs
of burner (with a HRRPUA of 110kW/m2), but still
no ignition occurs.
I simplified the model, removed extra meshes and devices, so just the basic
necessary things are present.
I also refined the mesh, that also didn't help. (going from 20 cells per
dimension, to 40)
Plus, the input parameters regarding pyrolysis are taken from the HDPE FAA
Polymers example in fds-smv.

I can't understand what's wrong....All I want is to ignite the block and
produce self-sustaining burn.
Input file is attached (FDS version 6.1.1).

Original issue reported on code.google.com by sirok4 on 2014-09-25 10:52:35

[gforney]

1. FDS only has a 1-D heat conduction model. In your cited experiment, the block of
plastic is heated at a corner. FDS cannot replicate the internal heat conduction in
this case. 

2. The thermo-physical properties of the plastic in the experiment may be different
than what is listed in the FDS Validation Guide. 

3. What of the gas phase combustion? What is the fuel molecule? What is the soot generation
rate and the radiative feedback to the block? 

This experiment may not be well-modeled by FDS. This is not our target application.
It's perfectly fine to try to model this, but you have to look carefully at the assumptions
within FDS and ask whether or not it can model this case.

I don't believe that there is a specific bug in FDS. It might just be that that you
do not have the right properties, or the heat conduction in FDS is not detailed enough,
or the gas phase is not being modeled properly. Or maybe all three. 

We have no plans to modify the heat conduction in FDS, and we cannot measure all of
the properties of this specific kind of plastic. You may be doing nothing "wrong" --
this might just be a case of limited physical detail in the model.

Original issue reported on code.google.com by mcgratta on 2014-09-25 13:57:22

[gforney]

I understand your points, but it seems highly unlikely that under such high
heat fluxes (I have used up to 1000 kw/m2 on both sides of the block), the
material wouldn't ignite - regardless the geometry. I even tried to model
PMMA with validated physical parameters from the FAA polymers validation
guide, same thing - no sustained burning. So, I tried to play with the
thickness and geometry, and even lowering the activation energy for
pyrolysis, or even turning the walls to adiabatic instead of open, but that
didn't help either. I just can't reach sustained burning after the burner
is removed !
As a matter of fact, I even googled every possible message in the mailing
list dealing with sustained ignition but found that no one could achieve
this phenomena after removing the burner - the materials just quench.

Could anybody manipulate the attached input file just so that sustained
ignition is achieved ? I don't care if you change the physical parameters
to unrealistic values, I just want to see the block self-burning after the
burner is removed. It could really be a bug in fds-smv, not allowing the
materials to burn on their own.

*desperate face*

David

Original issue reported on code.google.com by sirok4 on 2014-09-27 16:30:22

[gforney]

Run the attached input file. It is a simulation of a 1 cm thick, 10 cm by 10 cm slab
of HDPE whose properties are listed in the FDS Validation Guide, Burning Rates chapter,
FAA Polymers. I impose a 50 kW/m2 heat flux on the sample for 300 s and then linearly
reduce this heat flux to zero over the next 100 s. The burning ceases by 400 s.

The only data I have for HDPE in the guide is the mass loss rate of a sample exposed
to a heat flux of 50 kW/m2 in an oxygen-depleted environment. I urge you to do a cone
calorimeter experiment similar to my simulation, where you impose a heat flux on the
sample (say 50 kW/m2 for 300 s) and then remove the external heat source to see if
the sample continues to burn. I do not know if the properties of HDPE vary significantly
from manufacturer to manufacturer. To do this right, you have to measure the properties
of your HDPE, perform cone calorimetry at various heat fluxes, and compare to numerical
simulations. I have no data for this kind of plastic burning or not burning without
an external heat source.

Original issue reported on code.google.com by mcgratta on 2014-09-28 16:59:35

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• _Attachment: HDPE_Free_Burn.fds_