Boundary conditions

[aishwarya sehra]

Hi, I am using THERM to understand thermal bridging in Wall and roof assemblies. While I am setting up my inputs for boundary conditions, I wanted to know if THERM takes into account the absorptivity of the material? For eg, the initial surface temperature of a window frame vs wall will be different, is there a way to account for this? 

Kindly suggest.

Thanks

[Collin Robinson]

Therm is a steady-state model, so I don't think it accounts for initial surface temp. If by "absorptivity" you mean thermal absoprtivity, it equals thermal emissivity according to Kirchoff's law. Each material and boundary condition in Therm in therm has its emissivity specified. Unless its a particularly shiny metal window frame, emissivity/absoprtivity is about the same for wall and window frame materials. WUFI ORNL will do dynamic sims but I don't think it will really do window frame wall interfaces. Curious what you find

https://web.ornl.gov/sci/buildings/tools/wufi/

[Dan Bettenhausen]

If by "absorptivity" you mean thermal absoprtivity, it equals thermal emissivity according to Kirchoff's law. 

Be careful with this assumption.  This is true for a given wavelength or frequency of radiation, but not necessarily true over a range of frequencies with varying magnitudes of incident radiation. Perhaps more context can be provided about the nature of the specific problem that is being solved?  I question if the author may have intended "thermal diffusivity" which is also commonly defined as "alpha".  

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[Rodrigo Borghino]

Diffusivity would be more relevant for dynamic thermal simulations. I am curious about what the original problem looks like. If what the original poster is trying to do is estimate a surface temperature for a window due to radiation, she could estimate a sol-air temperature as outside BC for the window surface given by its absorptivity and the radiation intensity at the given hour analyzed. 

AAMA TIR-A8 can also give guidance on estimated inside and outside surface temps for windows.