Surface thickness, Obstruction thickness and grid size.

[Danish]

Dear FDS User,

I have a query regarding the definition of surface thickness, in my model the at some location wall thickness is greater than cell size and at some location both are equal while the surface thickness is remain identical for both scenario. 

for example

Case1:

Surface thickness =0.75ft

Obstruction thickness=  0.5ft

Grid size =0.5ft

Case2 

Surface thickness =0.75ft

Obstruction thickness=  1.0ft

Grid size =0.5ft

 Is it correct to get the temperature variation inside solid, if I define the surface id with the thickness of 0.75ft for both cases.

Best Regards

[Wali Hasan]

Dear Users,

For above question, there is gas cell on both side in every case. i.e EXPOSED condition in surface id, then in that case above scenario is valid or not?

Thanks

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[Kevin]

If the obstruction is zero or one grid cell thick, FDS assumes that the THICKNESS on the SURF line is the actual thickness of the obstruction for the purpose of calculating the heat conduction. It also assumes that the back side of the obstruction is EXPOSED; that is, it is heated or cooled by the gases behind it. If the obstruction is at the edge of the computational domain, the back side exposing temperature is ambient (TMPA). If the obstruction is more than one grid cell thick, FDS assumes that the back side conditions are the same as if the obstruction were at the edge of the computational domain. There is no heat conduction through more than one grid cell.

On Wednesday, December 9, 2015 at 8:16:55 AM UTC-5, Danish wrote:

Dear Users,

For above question, there is gas cell on both side in every case. i.e EXPOSED condition in surface id, then in that case above scenario is valid or not?

Thanks

On Wed, Dec 9, 2015 at 6:01 PM, Danish <wali.th...@gmail.com> wrote:

Dear FDS User,

I have a query regarding the definition of surface thickness, in my model the at some location wall thickness is greater than cell size and at some location both are equal while the surface thickness is remain identical for both scenario. 

for example

Case1:

Surface thickness =0.75ft

Obstruction thickness=  0.5ft

Grid size =0.5ft

Case2 

Surface thickness =0.75ft

Obstruction thickness=  1.0ft

Grid size =0.5ft

 Is it correct to get the temperature variation inside solid, if I define the surface id with the thickness of 0.75ft for both cases.

Best Regards

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[Danish]

Dear Kevin,

 

Thanks for your quick response, so in that case when the obstruction thickness is more than the cell grid size the obstruction should be split into two parts placed one another. For that scenario, Is the heat transfer from one block to other?

 

So, in my work I have brick kiln model in which the bricks are arranged in particular fashion shown in Figure1.

Actual brick dimension: 0.75 x 0.375 x 0.25 (ft)

Cell grid Size that I have used 0.5 x 0.5 x 0.5 (ft)

I have some question regarding this:

1)      Which thickness should I use in surface id for the 1 D conduction and can I apply this surface id to all 6 faces since they have different dimension in all 3 axis.

2)      For some brick they are in contact each other ie they are not exposed to the gas cell in the domain. so for that face what surface id should I use so that heat transfer from one another via conduction. (Figure 2)

 

Best Regards

[Danish]

Dear FDS User,

Plz have a look in my issue
Thanks in advance

Best Regards

On Wednesday, December 9, 2015 at 6:01:47 PM UTC+5:30, Danish wrote:

[Kevin]

You are pushing FDS beyond its capability. FDS uses a 1-D conduction model through solids. This assumption is predicated on the fact that most solids in an FDS apps are walls, and the characteristic length and width are much greater than the depth. If you have a complicated arrangement of bricks, the 1-D assumption may not be appropriate.

My advise to you is not to use FDS for this application. The 1-D heat transfer model is inappropriate for the 3-D heat conduction that will occur in these bricks.

[Wali Hasan]

Thanks Kevin for your comments, but can you please clarify me the following points :

1) If the length or width of the blocks are large as compare to depth (i.e 1D conduction along the depth) then what happens when two blocks placed in contact like in Figure1 still heat transfer takes place from one block to another , since middle surface (the contact surface) is not directly exposed to the gas cell condition  so for that surface what surface id should I use 

2)For the same scenario, what surface id should I use for the surface having large length and width ie 4 surface other than exposed surface.(can I use inert surface id for these 4 surface and the exposed surface id for the depth along which the 1D conduction occurs)

Thanks once again 

Best Regards

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[Kevin]

No, heat transfer does not take place between two abutting obstructions. The heat is conducted in the direction normal to the surface through a distance of THICKNESS. Then the energy is lost to an ambient temperature void.

There is no need to specify a SURF condition for a surface that is not at the interface between gas and solid. FDS will ignore these SURF conditions.

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[Wali Hasan]

Dear Kevin

Plz also elaborate the second point..

Thanks

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[Randy McDermott]

SURF only applies to an exterior surface of an OBST.

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[Wali Hasan]

Dear Randy, 

Thanks for your comments but I am asking for my second point:

For the same scenario, what surface id should I use for the surface having large length and width ie 4 surface other than exposed surface.(can I use inert surface id for these 4 surface and the exposed surface id for the depth along which the 1D conduction occurs)

Best Regards

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[Randy McDermott]

We cannot answer this question for you.  You need to understand the model well enough to know what surface conditions to apply.  Please read the user guide.  If there is something in the guide that is unclear, please point to that specifically.

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[brookl...@gmail.com]

What is the surface thickness,Sorry ,i do not understand?

在 2015年12月9日星期三 UTC+8下午8:31:47，Danish写道：