Reply To: solar radiation

[Mark Owens]

Hi, there needs to be an opaque surface that has been set to participate in radiation that an incoming solar ray has a direct line-of-sight to.

There looks to be an issue with the search in the knowledge section in AIS. I have copied the article here.

Title
How to model external radiation with no external mesh?

Content
The built-in solar load models will only add a solar load to the fluid side of a wall boundary and so are not appropriate if the outside surfaces of a wall all belong to a boundary and there are no holes in the boundary to allow photons to enter.

The effect of the sun on external surfaces can be modelled by raising the external radiation temperature on the thermal tab of a wall boundary condition. The expressions below take account of the direction in which the external wall faces and only adds the solar load on wall faces facing the sun. Note that it will not take into account external walls that combine to have a complicated concave shape in which some external walls are in the shadow of other external walls. It only takes into account whether a wall is forwards facing or backwards facing.

The expressions make use of vector operations, which are available from Fluent 2021 onwards:
Expressions
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solar_emissivity = 0.5
thermal_emissivity = 0.5
external_temperature = 290 [K]
solar_direction = vector(1,-0.5,-0.5)
solar_flux = 250 [W/m^2]
wall_area = vector(XFaceArea,YFaceArea,ZFaceArea)
cos_wall_angle = max(0, -dot(solar_direction,wall_area)/(solar_direction.mag*wall_area.mag))
external_rad_flux = solar_emissivity*solar_flux*cos_wall_angle
external_rad_temp = (external_rad_flux/stefan/thermal_emissivity+external_temperature^4)^(0.25)
-----------------------------

In the wall boundary condition:
------------------------------
Set the external emissivity to thermal_emissivity
Set the external radiation temperature to external_rad_temp

Fluent will then compute:
------------------------
wall_flux = external_emissivity*stefan*(external_rad_temp^4 - wall_temp^4)
               = thermal_emissivity*stefan*(external_rad_flux/stefan/thermal_emissivity+external_temperature^4 - wall_temp^4)
               = external_rad_flux + thermal_emissivity*stefan*(external_temperature^4 - wall_temp^4)
               = solar_emissivity*solar_flux*cos_wall_angle + thermal_emissivity*stefan*(external_temperature^4 - wall_temp^4)