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[FAQ]

The Ray Trace Model is geared towards calculating solar gains on the inside of the building/car, as transmitted through the glazed (semitransparent) external surfaces. If the domain has an opaque external surface, the model will not calculate the incident irradiation on the external surface, and therefore apply the desired heat load on the opaque external wall. How to apply this external radiation source? A reasonable workaround is to calculate then apply the equivalent radiant temperature to the outside surface through the thermal conditions (radiation or mixed). Done manually this can be laborious, but it can be done semi-automatically using a 2-step approach in Fluent : Step 1: – For all of your external opaque walls, set the BC Type to be semi-transparent, then the absorptivity to 1 and transmissivity to 0 – Initialize the model (to calculate the incident solar gains) – Create a custom-field function for the blackbody temperature, i.e. (Solar Heat Flux/ 5.67e-8)^0.25. Call the function ‘radtemp’ – Write out profile of the blackbody temperature: Define -> Profiles -> write -> surfaces = all opaque external walls, and value = your blackbody custom field function name. Call the profile ‘blackbody.prof’ Step 2: – Now, read in the profile you just wrote out: Define -> Profile -> read -> blackbody.prof – Now change all of your ‘opaque’ walls to be opaque again – On each of these walls, using a ‘mixed’ thermal condition, change the external radiation temperature from constant to profile you have just read – from the drop-down list, apply the name that corresponds to the surface and the custom field function, e.g. for a surface called ‘westwall’ you should find a profile called ‘westwall radtemp). Remember to apply an appropriate external emissivity. – Initialize again and you’re ready to solve.