Air gap boundary condition for surface backing

[Imran Ali]

Hi,

I have a question about the air-gap boundary condition for surface backing in FDS. How does the heat flux through this boundary modeled/calculated? Based on my understanding, heat flux through a wall can only be specified as either:

1. Explicit specification of the heat flux itself
2. Wall temperature
3. Convection and radiation through the specification of heat transfer coefficient, emissivity, and external temperature (beyond the analysis domain).

Which one of these conditions is imposed implicitly by the "air-gap" BC?

Any insight about how this is computed will be highly appreciated.

[dr_jfloyd]

3

[fde]

If the convective heat transfer coefficient is not set and the wall is at the mesh boundary, then how is the convective heat transfer coefficient is calculated for air gap backing?

https://groups.google.com/g/fds-smv/c/eXEajebMhXc/m/a2htHu0PBQAJ

[dr_jfloyd]

A natural convection heat transfer correlation is used:

HEAT_TRANSFER_COEFFICIENT = 1.31_EB*ABS(DELTA_TMP)**ONTH

where DELTA_TMP is back wall temperature - TMPA

[fde]

Thank you @dr_jfloyd.  I did not derive it but it seems like a simplified equation because the equations in the technical guide are more complex.

h = 1.31 * dT^(1/3)

By searching the line you shared, I found it in the source code for those who are interested. 

HEAT_TRANSFER_COEFFICIENT = 1.31_EB*ABS(DELTA_TMP)**ONTH ! Natural convection for vertical plane

https://github.com/firemodels/fds/blob/80cb7a5b0a2da7dc44cc4c350732f55248f97086/Source/wall.f90#L4268

[dr_jfloyd]

It is a different and simpler correlation than used for a cell exposed in the domain. I see that Randy just did a commit to add this to the guides.  

[fde]

I think I found where the equation comes from:

Table 7-2

Heat transfer coefficient on turbulent flows over vertical plane or cylinder:

https://www.researchgate.net/file.PostFileLoader.html?id=579747fc3d7f4b9edf22872b&assetKey=AS:388060144521217@1469532156767