Modeling the surface of the large scale pool fire

1.

Modeling the surface of the large scale pool fire

2.

Features of the large scale pool fire
• Large square
• High power
• Turbulence and intensive heat and gas exchange
• Burning with comparatievly continuous heat release rate
through over the square occupied with fuel

3.

Plain surface as a burner.
• Chemical reaction takes place in the thin slice above the pool surface.
Realistic modelling demands oxygen supply through over the surface.
But there is not a crack for fresh air to squeeze inside.
• Large amount of air is needed to supply uprising hot products. But
fresh air can not reach inner regions of the burning surface because
of the strong uprising flux at the boundary of the burning place. It
acts like a solid wall.
• Extremely hi cell resolution is required which may lead to dramatically
expensive modelling from the point of view of the machine time and
memory.

4.

Scheme of gas exchange

5.

To improve computation one can model fire surface as an array of unit
patches. It is sort of porous structure.
Streams of fresh air can freely plunge into the space between unit
patches.
Burner
Domain

6.

Each unit patch is a box with almost zero thickness. Effective modelling
would be achieved with vertically oriented unit patches. Both sides of each
unit patch work as the sources of gaseous fuel.
SnappyHexMesh permits building arrays of square unit patches with sides
of one or two cells long and with distance of two cells between the nearest
unit patches counting at the horizonal and vertical directions.
Here are the fragments of the construction of the burner.

7.

Fragments of SnappyHexMesh file (total number of patches here is about
1500)
fire1
{
type searchableBox;
min ( 1.50 1.50 0.00 );
max ( 1.51 1.60 0.1000 );
}
fire2
{
type searchableBox;
min ( 1.70 1.50 0.00 );
max ( 1.71 1.60 0.1000 );
}
fire3
{
type searchableBox;
min ( 1.90 1.50 0.00 );
max ( 1.91 1.60 0.1000 );
}
fire4
{
type searchableBox;
min ( 2.10 1.50 0.00 );
max (2.11 1.60 0.1000 );
……………………..
"fire.*"
{
level ( 1 1 );
}
convertToMeters 1; vertices
(
( -0.0 -0.0 0 )
( 9 -0.0 0 )
(960)
( -0. 6 0 )
( -0.0 -0.0 11.5 )
( 9 -0.0 11.5 )
( 9 6 11.5 )
( -0.0 6 11.5 )
);
blocks (
hex (0 1 2 3 4 5 6 7) (90 60 115 )
simpleGrading (1 1 1)
);

8.

Such a structure has not much influence to the shape and size of the
large scale fire. Here are the brief results of modeling
Center temperature field
General view

9.

Contours of velocity and temperature at 8 m
above the fire surface

10.

Thank you very much indeed.
The end