Air gap boundary condition for surface backing

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Imran Ali

Sep 16, 2022, 4:45:28 AMSep 16
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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

Sep 16, 2022, 6:18:06 AMSep 16
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3

fde

Sep 16, 2022, 8:20:56 AMSep 16
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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?

dr_jfloyd

Sep 16, 2022, 8:51:25 AMSep 16
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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

Sep 16, 2022, 9:12:53 AMSep 16
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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

dr_jfloyd

Sep 16, 2022, 9:21:05 AMSep 16
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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.