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[Swissi]

Hi there,
I have a really big problem with Pyrosim.
I created a model from a building with a burner inside it. Until today, the model was perfectly running.
But today, when I just wanted to move the burner to put it in another place, the burner is not more working!
And worse, I created the same model with the same burner, and it is alos not working. I don't really understand what is going on. Suddenly the burner doesn't work any more.
Could you please just help me? I'm totally lost...

[Charlie Thornton]

What is the burner doing? (e.g. FDS crashes, burner doesn't generate any heat, etc.)

Also, if you are able to share the model we may be able to find the problem directly.

[Swissi]

The burner doesn't generate any heat...

[Charlie Thornton]

I'm told that there are two likely causes of "no heat":

1. If mass is coming out but nothing is happening, there's probably something wrong in the gas-phase reaction.

2. If no mass is coming out, there probably something wrong with the surface definition.

You can test for stuff coming out of the burner using a velocity slice.

If you can't post the entire file, maybe you can post a few pieces? (e.g. SURF, MESH, and the VENT or OBST the surface is on)

[Swissi]

Hi everyone,

The problem is when I want to make the fire on the second floor. On the first floor, everything is alright, but on the second floor it doesn't work.

I have attached the FDS file.

I don't know how to resolve this problem... 

[Charlie Thornton]

If I had to guess, I'd say the problem is the size of your mesh cells relative to the sizes of your objects. In the end, everything gets snapped to the MESH. However, my attempts to get the burner working just keep getting me the "ERROR: VENT  14 must be attached to a solid obstruction" message. I'll ask someone who knows more than me to have a look.

[Bryan Klein]

The problem is when I want to make the fire on the second floor. On the first floor, everything is alright, but on the second floor it doesn't work.

When you place it on the first floor, you are setting the vent on the mesh boundary.  This is a stable location as it will not change when geometry is snapped to the nearest grid cell.

The problem comes on the second floor from such small geometric objects relative to the mesh cell size and how the vents and geometry surfaces are moving to fit the mesh.

I refined the mesh scheme as an example and reran the case, you can see that with 0.5 meter cells, the vent is resolved and heat is released from the vent on the second floor.  I have attached the PyroSim file with my refinements.

-Bryan

[Swissi]

Thanks a lot for your answer. I understand now the problem, but without your help I would never have thought that it was coming from the mesh. Thanks a lot! 

[Bryan Klein]

Thanks a lot for your answer. I understand now the problem, but without your help I would never have thought that it was coming from the mesh.

You are welcome.  It is good to keep in mind that FDS will simulate the model based on the resolution of the MESH.  Regardless of your actual geometry dimensions, everything in the model will be forced to fit to the MESH cells.

1/2 of a MESH cell width is the point where FDS decides to snap the object to the edge of the next cell.  I have attached an image that shows a red and a blue object (outline)  The filled lighter color is where FDS will snap the objects to.  For the red object, you can see that the shape moves down and to the left a little and fills 3 MESH cells.  With the blue object, it snaps down to a single edge of four cells.  This would result in a 'Thin Obstruction' and there are restrictions on what kind of surfaces can be applied to thin obstructions.  Which may result in certain vents or surfaces types not working as expected, or FDS reporting an error.

I am glad you have it working now, please let us know if you have other questions.

Best Regards,
-Bryan Klein
klein@thunderheadeng.com
Product Manager, PyroSim/Pathfinder
Thunderhead Engineering Consultants, Inc.

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