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Setting correct boundary condition for external walls in normal solar radiance

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Old   March 13, 2023, 01:49
Question Setting correct boundary condition for external walls in normal solar radiance
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Tapasvi Bhatt
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I am trying to simulate the effect of normal irradiance of sun on a roof and an external wall of a 2D domain - on the internal temperature of a room using ANSYS Fluent 2020. I have a general variation of outside temperature with time for a typical day in a typical city shown by continuous red line in the figure attached - I wish to analyze the heating of the room due to this external higher temperature with time.

For the external wall and the roof, FLUENT provides various thermal options such as Heat Flux, Temperature, Convection, Radiation, Mixed, etc. I've tried all of these but my room is getting heated up instantly, (by instantly, I mean within 5 minutes, the temperature within the room increases or drops by 5 to 10 degrees) which is not expected. When boundary condition is set as:

(i)Constant Temperature - I set it equal to external temperature from the red curve in the graph.
(ii)Heat Flux - I assumed 150 W/m^2 which is a typical heat flux I found from some sources online.
(iii)Convection - I used 'h' as 12 (I found one source which mentions heat transfer coefficient between concrete and air is 12, which may slightly vary depending on speed of air), and used free stream temperature as the external temperature from the red curve in the graph.
(iv) Radiation - I set emissivity as 0.95 and set external radiation temperature as external temperature from the red curve in the graph. (This leads to opposite effect - temperature inside the room drops by 5-10 degrees very very quickly).

I was expecting a gradual rise in the temperature of the room that we generally observe in typical houses. I wanted to know where I am going wrong, and what can be a possible remedy for the same.

Last edited by bhatt01tapasvi; March 13, 2023 at 02:43. Reason: image was not uploaded correctly
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boundary condition, convection, radiation

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