Volumetric Heat source in FDS

[zxz zxz]

Hello,
Can I build up a Volumetric Heat source in FDS?
Since I need compare the result to other CFD code (using Volumetric
Heat source to simulate the fire).

[zxz zxz]

2. Has anyone use the volumetric heat source to simulate the fire ?
What is the important setting for that?

3.Since when I use the volumetric heat source to simulate the fire in
another CFD code, and compare the result to FDS.
When I use the coarse grid in another CFD code, the result has a good
agreement with FDS. However using the find grid, the result is bad.

[dr_jfloyd]

A volumetric heat source is not a particularly good way to simulate a fire.  You see this done with CFD codes when there isn't an adequate capability to have combustion occur.   I would expect FDS results differ from a volumetric heat source.  When you have a very coarse grid and do not capture the dynamics of the fire (i.e. the fire looks like a unmoving blob in Smokeview) then you probably have something that is close to a volumetric heat source due to how poorly resolved the calculation is.

With FDS 6 you probably could do this by seeding a volume with STATIC particles that release heat.  Again though, this is a poor way to simulate a fire with a CFD model.

[Chris]

As I know both codes, I don't get the point about why volumetric heat source is not a good way. Could you give some more insight why you think this is not a good approach? This would be a strong argument to use FDS!

Cheers

Chris

[Kevin]

Why don't you explain to us how you choose the volume over which to
release the energy. How do you distinguish between a low and high fire
Froude number (Q*)?

[Chris]

I just can tell you how it works at Star-CCM+: You define the Fire
Volume and then the HRR. No Froude number needed at this point.

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>
> - Zitierten Text anzeigen -

[Kevin]

If you believe this to be an adequate description of fire, why don't
you just continue to use this program?

[Chris]

Actually my question was why is a Volumetric Heat Source not a
appropriate approach. I use both programs. I don't want to discredit
either of this programs.

> > > - Zitierten Text anzeigen -- Zitierten Text ausblenden -

[Kevin]

How do you determine the "Fire Volume"?

[zxz zxz]

Thanks for your suggestion,
My based setting (Called based setting A ) in FDS just use the
boundary
heat flux to present the fire, and it is no combustion, just
convection.
I just done the simulation for volumetric heat source in FDS6. And
the
problem is same. When the grid size is coarse , it has a good
agreement
with  the based  setting  A.  However the grid is very fine, and the
agreement is bad and the temperature is double to the  based  setting
A.

[dr_jfloyd]

The volume enclosed by the flame is highly dependent on entrainment which is influenced by the geometry. For very simple fires (axisymmetric in a large space) you could use a correlation to estimate the volume and yes, you will probably do OK in predicting the temperature of a plume. Anything beyond that and it will be very difficult to specify exactly what the volume should be.
A fire does not have a constant heat release rate over its entire volume.
Specifying a constant heat release rate of a volume ignores the dynamics of a fire (the pulsing one sees in the flame height).
Specifying a constant heat release rate of a volume means you are not going to account for vitiation of the fire (how are you producing products/depleting oxygen with no combustion model that depends on entrainment?)

[dr_jfloyd]

Why do you expect to get the same results as you decrease grid size? If your first grid is not resolved, you should expect to see large changes as you change grid size. Also, comparing one run against another or even against another CFD code isn't going to tell you if your answer is correct. You are best off comparing to an experiment.

[zxz zxz]

Do you face a problem for the grid size setting for this kind
simulation.

I found when use a coarse grid to do the simulation for Volumetric
Heat source , it has a good agreement. Using the fine grid is bad.
So do you also face this problem?
And how do you chose the grid size?

[zxz zxz]

Yes , I am doing a fire axisymmetric in a large space simulation. And
also try to compare the result for the similar experiment.

And for FDS (boundary heat flux setting, no combustion) can get the
good agreement with experiment, so I expect FDS (Boundary heat flux
setting) has a good result.

Today I am do two set simulation in FDS6 And compare the plume
temperature.
1. The fire is boundary heat flux (no combustion). When the gird size
decrease, the plume temperature will converage to 100 C.
2. The fire is Volumetric Heat source, also in FDS6. When the gird
size is coarse, the plume temperature will be 110 C. However when the
grid size is decrease, the plume temperature also increase and for
very fine grid it is 500 C.
So I think in coarse grid has a good agreement for Volumetric Heat
source.

[dr_jfloyd]

Why not just simulate the fire?  Neither boundary heat flux nor a volumetric heat source are going to allow FDS to reproduce the dynamics of a fire plume in the same manner as it can when the combustion is simulated.

If you are doing CFD, and you change your grid size and the answer changes drastically then that means there is some grid sensitive effect in either your input or in the mode.  Simply choosing a coarse grid because it works well for a particular output quantity is really not appropriate use of CFD.

[shostikk]

I think you just demonstrated one weakness of the Volumetric Heat Source method.

[Stefano Capra]

Hi everyone,

I am really interested in volumetric sources of heat and humidity

The reason is I am planning to test the performance of FDS as CFD solver for HVAC systems

I know there are a lot of drawbacks but I am fascinated by some interesting pros:

-validated jet fans

-validated evaporative cooling

-nice HVAC modelling 

Is there any way to set a volumetric source

Thanks

Ste

[dr_jfloyd]

Initialize the volume with static Lagrangian particles that emit heat. 

[Randy McDermott]

User guide p. 51.

&INIT XB=..., HRRPUV=.../

also works.

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