Hi Kevin,
I found only one option to reply in that thread, therefore I am
replying through another relevant thread.
Thanks for your reply.
Now looking at the example of section 15.3.1 of the FDS5 User guide, I
feel that if we change the number of radiation angle from 100 to 300 -
the grid sensitivity related to the radiation loss can be improved. I
understand that it will computationally very expensive.
Regards,
Khalid
This information is very valuable, but unfortunately it is only I who
is getting it. Why not post this to the Group?
FYI, I am still trying to improve the grid sensitivity of FDS. The
problem is that all of these percentages are tied to the grid.
The real issue, and that which few have really talked about, is the
upper bound we use for the HRRPUV, which is 200/dx kW/m3. This is
needed because if we did not use a limit, then all combustion would
take place in the nearest cell to the boundary, because the rate of
oxygen and fuel diffusion is orders of magnitude greater on a coarse
mesh of say 10 cm, than it would be for an actual flame. All
practical CFD fire models have some sort of combustion limiter,
whether it be what I have said, or the Eddy Breakup, or whatever.
However, most non-modelers cannot understand these because of the
complicated mix of numerics and combustion physics.
Until we overcome this source of grid sensitivity, talking about
absorption coefficients is moot. It confuses the issue.
Thanks
Kevin
Kevin McGrattan
National Institute of Standards and Technology 100 Bureau Drive, Mail
Stop 8663 Gaithersburg MD 20899
Phone:
301 975 2712
Fax:
301 975 4052
----- Original Message -----
From: "Khalid Moinuddin" <
Khalid.M...@vu.edu.au>
To: "Kevin" <
mcgr...@gmail.com>
Sent: Thursday, January 24, 2008 1:07 AM
Subject: Re: Absorption coefficient
Hi Kevin,
I ran two ethanol_pan cases with FDS 5.1.0 (one with 5cm cell and the
other with 2.5 cm cell). I removed absorption coefficent and used
Radiation_Fraction=0.35
Radiation loss for 5 cm case is 38.5% and for 2.5 cm is 41% - roughly
the same as you got with your cases.
Integrated HEAT FLUX to the surface is larger for 5 cm case, hence the
higher HRR is obtained.
I have now run two more ethanol_pan cases with FDS 5.1.0 (one with 5cm
cell and the other with 2.5 cm cell). I removed absorption coefficent,
but used Radiation_Fraction=0.0
This time radiation loss for 5 cm case is 25% and for 2.5 cm is
30.5% . This time I have found that the integrated HEAT FLUX to the
surface is lesser for 5 cm case, hence the lower HRR (22%) is
obtained.
So in FDS5, higher radiation loss resulted in lower HRR when
Radiation_Fraction=0.35 and the opposite happened when
Radiation_Fraction=0.0
Now I looked back the simulations with FDS4 with
Radiation_Fraction=0.0 (though FDS4 does not have many features like
FDS5 and has a different combustion model). I find that Radiation loss
for 5 cm case was 21% and for 2.5 cm is 23.5% - which resulted in 5%
higher HRR for 2.5 cm case. So in FDS4, higher radiation loss resulted
in higher HRR when Radiation_Fraction=0.0 which is the same for the
case of FDS5.
I hope this information has some value to you.
Regards,
Khalid
On Nov 13 2007, 1:05 am, Kevin <
mcgra...@gmail.com> wrote:
> Khalid
>
> I have done some work on this problem last week. The ethanol_pan case
> is difficult to work with because there are so many parameters. So I
> decided to just look at a simple "burner" in FDS. Nothing more than a
> fixed HRRPUA, with no specified material properties. I had to add a
> new output to FDS in order to integrate the HEAT_FLUX to the burner
> surface. I noticed that for cases with grids of 10 cm, 5 cm and 2.5
> cm, the radiative loss (which is roughly proportional to the
> integrated heat flux to the burner) varied from 35% to 42%. The 35% is
> understandable in the 10 cm case, because FDS is just using the given
> RADIATIVE_FRACTION. In the 2.5 cm case, the sigma*T^4 term is also
> playing a role. In the ethanol_pan case, RADIATIVE_FRACTION=0, and the
> radiative loss is calculated purely from temperature and gas/soot
> composition. It appears that this is the leading cause of grid
> dependence in your case and in mine.
>
> Our goal is to eliminate as much as possible grid dependence in the
> gas phase calculation. We have had good success for total HRR and
> flame height. Now we need to turn our attention to the radiative loss.
> If we succeed in this, then we can be confident that the overall
> spatial distribution of the fire's energy is independent of the grid,
> and we can then turn our attention to the details of the pyrolysis. We
> are trying to decouple gas and solid phase phenomena. Results such as
> yours are a combination of the two. By looking at the burner only, we
> focus on the gas. On the other hand, we have simple examples of just
> the solid (or liquid) phase. Grid dependence is mainly in the gas
> phase. For the solid, we have as fine a grid as we need.
>
> I might bump this thread out into a separate one, to keep open the
> lines of communication, and to let others on the team know what we're
> up to. K
>
> On Nov 1, 11:31 pm, "KhalidMoinuddin" <
Khalid.Moinud...@vu.edu.au>
> wrote:
>
>
>
> > Hi Kevin and Simo,...
>
> > read more »
>
> > ethanol_pan_grid.doc
> > 78KDownload
>
> > I have run ethanol_pan case with three different cell sizes i.e. 50 mm,
> > 25 mm and 12.5 mm. The result is attached to this mail. You can see that
> > the results vary greatly.
>
> > I apologise that I did not do this during the beta testing. At that
> > time, I thought that the simulation result of fire in open space would
> > be grid independent at 50 mm cell as found with FDS4 which is shown in
> > reference [36] of FDS5 User's guide.
>
> > Regards,
>
> >Khalid
>